Hubble Space Telescope

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Hubble Space Telescope (HST)
Hubble Space Telescope (HST)
Type: Space telescope
Country: United StatesUnited States United States Europe
Operator: National Aeronautics and Space AdministrationNASA NASA ESA
European space agencyESA 
COSPAR-ID : 1990-037B
Mission dates
Dimensions: 11,600 kg
Size: Length; 13.1 m
diameter: 4.3 m
Begin: April 24, 1990, 12:33 UTC
Starting place: KSC , LC-39B
Launcher: Space Shuttle Discovery
Status: in operation
Orbit data
Rotation time : 95.4 min
Orbit inclination : 28.5 °
Apogee height 549 km
Perigee height 545 km

The Hubble Space Telescope ( English Hubble Space Telescope , HST for short ) is a space telescope that was jointly developed by NASA and ESA and which is named after the astronomer Edwin Hubble . It works in the range of the electromagnetic spectrum from the infrared range through the visible light to the ultraviolet range . The mirror diameter is 2.4 meters.

The HST was launched on April 24, 1990 with the space shuttle mission STS-31 and released the next day from the hold of the Discovery . It was the first of four space telescopes planned by NASA as part of the Great Observatory program . The other three space telescopes are Compton Gamma Ray Observatory , Chandra X-Ray Observatory, and Spitzer Space Telescope .

After exposing the telescope, it quickly became apparent that the image quality did not meet expectations. A fault in the main mirror resulted in images that were practically unusable. Three years later, in 1993, the error was successfully corrected with the help of the COSTAR mirror system (Corrective Optics Space Telescope Axial Replacement). After this first repair mission STS-61 , further maintenance missions took place: STS-82 , STS-103 , STS-109 and STS-125 . With the last maintenance mission , the COSTAR correction became superfluous because all instruments had their own system for correcting the mirror error.

In 2021, the planned James Webb Space Telescope could succeed the Hubble Space Telescope. It is currently being tested and is a joint project between NASA, ESA and the Canadian Space Agency (CSA).

Mission objectives

The Hubble Space Telescope was created primarily to circumvent the restrictions imposed by the earth's atmosphere. The molecules in the atmosphere limit the resolving power of telescopes on the earth's surface, and various spectral ranges are blocked. The space telescope should achieve a resolution that had not been achieved before . The mission objectives are therefore extremely broad and include practically all essential objects and phenomena of the universe :


Lyman sharpener

The first serious concept of a scientific telescope in orbit was presented by Lyman Spitzer , then a professor at Yale University , in 1946. In the scientific publication Astronomical Advantages of an Extra-Terrestrial Observatory (German: "Astronomical advantages of a space observatory") he described the then inevitable disturbances by the earth's atmosphere , which limited the resolving power of any earth-based telescope. In addition , the atmosphere also absorbs all X-rays , which makes it impossible to observe very hot and active cosmic events. As a solution, he proposed a telescope in orbit outside the atmosphere.

Some time later, the National Academy of Sciences approached Spitzer, who was now teaching at Princeton University , in order to hire him as head of an ad hoc committee for the design of a Large Space Telescope . During the first meeting in 1966, extensive studies were made for the use of such a telescope. Three years later, a paper entitled Scientific Uses of the Large Space Telescope was published, in which the committee called for the construction of such a telescope, as it was “an essential contribution to our knowledge about cosmology ”.

In order to realize this project, they turned to NASA, as no other organization had the means and skills to carry out such an ambitious project. This had already carried out several internal studies, among others also under the direction of Wernher von Braun , on space telescopes, which, however, were all planned with smaller mirrors. The decision to build the space shuttle in the mid-1960s gave the company the flexibility it needed to further develop the existing designs. In 1971, the then NASA director George Low created the Large Space Telescope Science Steering Group (English: "Scientific Steering Committee for the Large Space Telescope"), which was supposed to carry out the first feasibility studies.

OAO-1 , a forerunner of the Hubble telescope launched in 1966

Meanwhile, the Orbiting Astronomical Observatory satellites achieved significant success, which gave a boost to the proponents of a large space telescope. The satellites worked primarily in the ultraviolet range and had telescopes with 30.5 to 97 cm main mirrors. In 1983, IRAS, a telescope with a 60 cm mirror diameter, was launched for infrared observation. The KH-11 Kennan spy satellites of the National Reconnaissance Office are considered the technical forerunners of the Hubble telescope ; they were launched from 1976 to 1988 and have a primary mirror comparable to the Hubble telescope.

The next step was to secure government funding for the project. Due to the high price of 400 to 500 million (according to today's value about two billion US dollars) , the first application was rejected by the budget committee of the House of Representatives in 1975 . As a result, intensive lobbying began under the leadership of Spitzer, John N. Bahcall, and another leading astronomer from Princeton. In addition, the one turned to finance solar cells to the ESRO (a predecessor of the ESA), which was offered observation and scientific cooperation in return. In the same year she announced her consent. By additionally reducing the size of the main mirror from 3.0 to 2.4 meters, the price could be reduced to around 200 million US dollars. The new concept was approved by Congress two years later so that work on the new telescope could begin.

The most important orders were awarded in 1978: PerkinElmer was to design the optical system including the main mirror, Lockheed was responsible for the structure and the satellite bus , with the solar cells and an instrument (the Faint Object Camera ) coming from European production. Due to the importance of the main mirror, PerkinElmer has also been instructed to use a subcontractor to manufacture a backup mirror in case of damage. The choice fell on Eastman Kodak , which opted for a more traditional manufacturing process (Perkin-Elmer used a new laser and computer-aided grinding process ). Although both mirrors passed what later turned out to be faulty quality control, according to some scientists, the Kodak make was the better one. Even so, PerkinElmer decided to use his own mirror. The telescope was originally supposed to be launched in 1983. This date could not be kept due to delays in the construction of the optics, and the final readiness for launch was achieved in December 1985. In the meantime, the Space Telescope Science Institute was founded at Johns Hopkins University in 1983 , which was to take over the operation of the new telescope as part of the Association of Universities for Research in Astronomy . In the same year it was renamed the Hubble Space Telescope (HST for short) after Edwin Hubble , the discoverer of the expansion of the universe .


Hubble is launched into space from Discovery's payload bay

After internal problems had delayed the start by two years, the new start date for October 1986 could not be met either. This was due to the Challenger disaster on January 28, in which all seven astronauts due to material failure on one of the solid - Booster died. Since Hubble was to be transported with the space shuttle, the start was delayed by another four years due to the extensive improvement measures on the other space shuttles.

On April 24, 1990 at 12:33 UTC , the space shuttle Discovery finally took off with the telescope on board from launch complex 39B of the Kennedy Space Center in Florida . The mission, named STS-31 , went smoothly despite the record height of 611 km, the telescope was successfully launched the next day and was activated as planned.

The primary mirror defect

The first picture of the WFPC instrument. The star in the middle should be mapped as a point without stray light.

Although after the production of the primary mirror measures for quality assurance have been taken, it was discovered already at the first light massive image errors (see picture). According to the specifications, a point target (such as a star ) should concentrate 70% of the light within 0.1 ″ (0.1  arc seconds ). In fact, it was spread over 0.7 ″, which massively lowered the telescope's scientific value. Subsequent measurements with the help of the Wide Field / Planetary Camera , the Faint Object Camera and the wavefront sensors of the three Fine Guidance Sensors showed with a high degree of certainty a strong spherical aberration due to unevenness on the primary mirror.

When it was established that it was a large and complex error, the NASA Director Richard Harrison Truly ordered the formation of a committee of inquiry (Hubble Space Telescope Optical Systems Board of Investigation) , which should further isolate and correct the error. The investigations focused on an instrument that was used in manufacturing for quality control and which should have displayed the spherical aberration : the zero corrector , a fairly simple optical instrument that projects a special wavefront onto the mirror, which, if it is correctly ground was reflected as an exactly circular pattern. On the basis of deviations from this circular pattern, you can see whether and to what extent polishing and grinding work is still necessary. For reliable results, however, the lenses built into the zero corrector must be precisely aligned and adjusted. When examining the original corrector, which had been stored after the main mirror had been delivered, it was found that one lens was 1.3 mm too far from another. Computer simulations were then run to calculate the effects of this error on the primary mirror. The type and extent of the results matched the observed aberrations of the telescope in orbit very precisely , so that the incorrect lens spacing in the corrector was ultimately responsible for the primary mirror error.

A picture of the Messier 100 galaxy before (left) and after (right) the installation of COSTAR

In the further course of the investigations, a large number of failures and obstructive structures were uncovered in the area of ​​quality assurance:

  • Persons responsible for quality assurance were not integrated into the project team.
  • There were no independent reviews by neutral branch offices.
  • There were no documented criteria for the tests that could be used to differentiate between failure and success.
  • Quality assurance employees had no access to large parts of the proofreading department.

Since Perkin-Elmer used some new and largely untested computer-based techniques to manufacture the primary mirror, NASA had commissioned Kodak to manufacture a reserve mirror made with more traditional means. Since the spherical aberration at Perkin-Elmer was not discovered before take-off, it remained on earth. After the error was discovered, it was therefore considered to recapture Hubble with a shuttle and exchange the mirror for the Kodak make. However, this turned out to be extremely complex and expensive, which is why a correction system was developed that corrects the primary mirror error before the collected light reaches the instruments. It is called Corrective Optics Space Telescope Axial Replacement (short: COSTAR) and was installed two and a half years after it was launched during the first service mission. Only after this mission was the telescope able to start its scientific operation without any problems worth mentioning. However, COSTAR occupied one of five instrument bays that was actually intended for scientific systems (specifically, the High Speed ​​Photometer (HSP) had to be removed during installation). For this reason, all of the following instruments were equipped with their own internal correction systems so that they could obtain the light directly from the main mirror without having to go through COSTAR. During the last fourth “service mission” , the system was removed and replaced with a scientific instrument, the Cosmic Origins Spectrograph (COS). In today's operation, the main mirror defect is practically no longer a factor.

The service emissions

begin SM 1 SM 2 SM 3A SM 3B SM 4
date Apr 1990 Dec 1993 Feb 1997 Dec 1999 Mar 2002 May 2009
Track height
618 km 590 km
+ 8 km
596 km
+15 km
603 km 577 km
+ 6 km
567 km
Instr. 1 WF / PC WFPC2 WFC3
Instr. 3
(axial pos.)
Instr. 4th FOC ACS ACS (R)
Gyroscopes 6th 4 (R) 2 (R) 6 (R) 2 (R) 6 (R)
Photovoltaics SA1 SA2 SA3

The Hubble telescope was designed for maintenance in orbit right from the start, making a total of five space shuttle missions for repair and upgrade possible. These are listed and described below; the exact technical modifications can be found in the corresponding linked sections.

Service mission SM 1

COSTAR (above) is installed.
  • Mission number: STS-61
  • Time period: December 2, 1993 (09:27 UTC) to December 13 (05:25 UTC)
  • Number of EVAs : 5
  • Total EVA time: 28.5 hours

The primary objective of the first service mission was to correct the optical error of the primary mirror. For this purpose, the high-speed photometer instrument was removed and replaced by the COSTAR lens system, which was able to provide all other instruments with a correct and error-free image. The also new Wide Field and Planetary Camera 2, which replaced its predecessor, already had its own correction system and was therefore not dependent on COSTAR. This should be removed in the long term in order to be able to use the space scientifically again, which is why all of the following newly installed instruments were equipped with their own construction to correct the primary mirror aberration.

In addition, some other technical systems were replaced, modernized and maintained. So new solar wings were installed because the old ones deformed too much under the frequent temperature changes. In the area of ​​position control, two magnetic field sensors , two measuring systems for the gyroscopes and their fuses were replaced. In addition, the main computer received an additional coprocessor system .

Service mission SM 2

Two astronauts inspect the isolation of Bay 10.
  • Mission number: STS-82
  • Period: February 11, 1997 (08:55 UTC) to February 21 (08:32 UTC)
  • Number of EVAs: 5
  • Total EVA time: 33.2 hours

The primary goal of the second service mission was to replace two sensors. On the one hand, the Goddard High Resolution Spectrograph was replaced by the Space Telescope Imaging Spectrograph; on the other hand, the Faint Object Spectrograph was expanded to accommodate the Near Infrared Camera and Multi-Object Spectrometer. This made it possible to massively increase the resolution and the spectral accuracy, and it was possible for the first time to carry out observations in the infrared range.

Extensive modernization and maintenance work was also carried out on the technical systems. In the area of ​​position control, a fine guidance sensor was replaced by a newly certified and calibrated model, the OCE-EK system was retrofitted to better maintain alignment accuracy, and one of the four reaction wheel assemblies was replaced. In addition, two of the three tape storage systems have been serviced, the third has been replaced by a much more powerful solid state recorder . Furthermore, a data interface unit and the alignment system for one of the two solar wings were replaced. In the end, the insulation of the telescope was repaired unscheduled during the last spacecraft mission , after significant damage had previously been discovered. Reserve materials were used that were actually intended for a possible repair of the solar wings.

Service mission SM 3A

Two astronauts secured at the end of the shuttle arm change gyroscopes.
  • Mission number: STS-103
  • Period: December 20, 1999 (00:50 UTC) to December 28 (00:01 UTC)
  • Number of EVAs: 3
  • Total EVA time: 26.1 hours

Originally there should only be one mission called “SM 3”, during which improved scientific instruments should be installed again. However, the RWAs necessary for alignment turned out to be unexpectedly unreliable. After the third of a total of six gyroscopes failed, NASA decided to split the mission in two. In the first SM-3A mission, new gyroscopes were to be installed, in the second SM-3B mission, the installation of the new instruments was planned. On November 13, 1999, a good month before the planned start of the first mission, the on-board electronics put the telescope in a safety state that only guaranteed the operation of the most important technical systems. The reason was the failure of a fourth gyroscope, which meant that only two of them were still functional. However, at least three copies were necessary for proper operation, so scientific operation of the telescope was no longer possible.

During the first outboard operation, all three Reaction Wheel Assemblies and a Fine Guidance Sensor were immediately replaced with new models, making Hubble operational again. In addition, other technical systems were later maintained or upgraded. The old DF-224 central computer was replaced by a much more powerful model and another tape drive was replaced by an advanced solid state recorder . Voltage / Temperature Improvement Kits were also installed on the batteries to improve the charging process. A defective S-band transmitter was also exchanged for a new one, which was a very time-consuming and complex operation, since such an exchange was never intended and was not part of the ORU concept. Finally, the improvised thermal shielding of Mission SM 2 was removed and replaced by two newly manufactured devices .

Service mission SM 3B

An astronaut is working on replacing the power control unit.
  • Mission number: STS-109
  • Period: March 1, 2002 (11:22 UTC) to March 12 (09:32 UTC)
  • Number of EVAs: 5
  • Total EVA time: 35.7 hours

After only repair and maintenance work had been carried out on the SM 3A mission, the telescope also received a new scientific instrument with the SM-3B mission: the Advanced Camera for Surveys. It replaced the Faint Object Camera and extended the spectrum range from Hubble into the far ultraviolet range. In order to restore the capacities in the infrared range, the NICMOS instrument was equipped with an additional cooling system that works permanently and does not become ineffective after a certain period of time. With the installation of new, significantly more efficient solar wings , the telescope also had around a third more electrical energy available, which enabled four instead of two scientific instruments to work in parallel. To make this possible, the Power Control Unit , which is used for central power distribution, also had to be replaced. In addition, another RWA was replaced and another device to isolate the telescope was attached.

Service mission SM 4

COSTAR is being expanded.
  • Mission number: STS-125
  • Period: May 15, 2009 (18:01 UTC) to May 24 (15:39 UTC)
  • Number of EVAs: 5
  • Total EVA time: 36.9 hours

During this last service mission, extensive measures were taken to upgrade and extend the service life in order to ensure the operation of the telescope for as long as possible. The Wide Field Planetary Camera 2 was replaced by a modernized model called Wide Field Camera 3, which meant that the COSTAR system could be removed, since all instruments now had internal methods for correcting the mirror aberration. The Cosmic Origins Spectrograph was installed in its position, which means that the telescope again has a dedicated spectrograph. In addition, repairs were necessary on two other instruments: the Advanced Camera for Surveys, which had been virtually unusable since July 2006 due to a failure in the internal electronics, and the Space Telescope Imaging Spectrograph, whose power supply system failed in August 2004. Both instruments could have been easily removed as a whole, but the decision was made to attempt repairs in space, even if this was not intended during construction. Despite the complex processes - at the ACS alone, 111 screws had to be loosened, some with specially made tools - both repairs were successful so that the instruments can work again (whereby one of the three ACS sensors was not repaired and is still defective).

In addition to the instruments, many technical systems were maintained. All six gyroscopes and all three battery modules were replaced by new models. Finally, the last three remaining NOBL protective panels and a soft capture mechanism were installed on the outer skin . The latter is located at the rear of the telescope and enables another autonomous spacecraft to be docked easily. In this way, a targeted and safe re-entry into the earth's atmosphere should be made possible after the telescope has been switched off at the end of its life.


Hubble's planned successor: the James Webb Space Telescope

The James Webb Space Telescope is currently planned to replace the Hubble telescope , and its launch is scheduled for March 2021. It has a mirror that is more than five times the size and, especially in the infrared range, has considerably greater capacities than Hubble, with which objects behind particularly dense nebulae or at extreme distances can be better examined. In return, the visible and ultraviolet spectral range is no longer covered. In order to be able to investigate these areas in the future, the Space Telescope Science Institute, which is currently responsible for the operation of Hubble, has presented a concept called Advanced Technology Large-Aperture Space Telescope (ATLAST); this has meanwhile been further developed into LUVOIR . This is a space telescope with an 8 to 16 meter mirror with instruments for the visible and ultraviolet spectral range. In addition to cosmological research, it is to be used primarily for research into exoplanets . The period from 2025 to 2035 is targeted as the starting date.

Regardless of the specific successor, Hubble's mission is limited by its steadily decreasing orbit. Unless the orbit is raised again by another spacecraft, this will cause the telescope to Template: future / in 4 yearsre-enter the earth's atmosphere and burn up around the year 2024 . There are no concrete plans to prevent this, as the James Webb Telescope should already be fully operational by then, and new earth-based telescopes such as the Extremely Large Telescope and the Thirty Meter Telescope should be put into operation around this time.

Technology and structure

The following exploded view illustrates the main structure of the Hubble telescope. The graphic is link-sensitive , a click on the respective component leads to the corresponding section. Brief quick information is displayed when the mouse rests over the object for a short time.

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About this picture

General structure

The components of the Support Systems Module

The Hubble Space Telescope is generally a cylindrical construction with a length of 13.2 m, a diameter of up to 4.3 m and a weight of 11.11 tons. Most of the volume is taken up by the optical system , at the end of which the scientific instruments are housed in the Focal Plane Structure (FPS). These two components are enclosed by several interconnected cylinders, the so-called "Support Systems Module" (SSM). This also includes a hollow ring in the middle of the telescope, which houses the majority of all technical systems for its control. The required electrical energy is generated by two awnings , which are also installed in the middle. Two cantilevers, each with a high-performance antenna, are attached to the SSM for communication .

At the front end of the Hubble there is a flap with a diameter of 3 m, with which the opening of the optical system can be completely closed if necessary. It is in the aluminum - Honeycomb executed construction method and is externally provided with a reflective coating for protection against sunlight. This is continuously monitored by several sensors, as too high a level of incident light could damage the highly sensitive scientific instruments. If the sun is less than 20 ° from the alignment axis of the telescope, this system automatically closes the flap within less than 60 seconds, unless it is switched off manually by the ground control.

The flap itself is attached to a 4 m long light protection cylinder (baffle) . This consists of magnesium in the form of corrugated iron , which is protected from the strong temperature changes during an orbit by an insulation layer. On the outside there are handles for the astronauts and the fastening elements for securing in the cargo bay of the space shuttle, the following components: a low-gain antenna , two magnetometers and two sun sensors .

The next cylinder is also 4 m long, made of aluminum and stiffened by additional struts and support rings. As with the light protection cylinder, there are several devices for attaching the telescope, whereby a particularly stable mechanism is attached to which the robot arm of the space shuttle can dock. On the outside there are four magnetic gate crossers and the brackets for the two arms with the high gain antennas . In this section, too, insulation materials are attached to the surface in order to reduce the thermal load.

The next component is the most important of the entire Support System Module: the equipment section. This is a donut-shaped ring that completely encloses the telescope. In it about 90% of all technical systems in a total of ten individual equipment bays ( English bays ). Each of these bays is approximately 0.9 m × 1.2 m × 1.5 m and is easily accessible from the outside through a flap. These are designed in a honeycomb design and each have their own insulation on the surface. The individual bays are assigned as follows:

  • Bay 1: Data processing ( central computer and DMU )
  • Bay 2: Power supply ( accumulator module and two timing oscillators)
  • Bay 3: Energy supply (accumulator module and a DIU )
  • Bay 4: Power distribution ( PCU and two PDUs )
  • Bay 5: Data storage and transmission ( communication system and two E / SDRs )
  • Bay 6: Position control ( RWA )
  • Bay 7: mechanical systems for solar sail alignment and a DIU
  • Bay 8: Data storage and emergency systems (E / SDRs and PSEA )
  • Bay 9: Position control (RWA)
  • Bay 10: Data processing ( SI C&DH and a DIU)

The telescope is closed by a last 3.5 m long cylinder at its rear. As with the previous section, this one is also made of aluminum and stiffened by struts. Between this cylinder and the equipment ring there are also four bays for the installation of the three FGS and the radial scientific instrument (No. 5). The other four instruments are located behind maintenance hatches within the construction in an axial position. At the end of the cylinder there is a final aluminum honeycomb plate with a thickness of 2 cm. A low-gain antenna is attached to it, which has openings for several gas valves and electrical connectors. The latter enable the operation of internal systems via charging cables from the space shuttle during service emissions if the solar cells' own electricity production has to be deactivated.

power supply

Close-up of an awning after the SM 3B service mission. Note the conductor tracks for the panels.

All of the electrical energy for operating the telescope is generated by two wing-like solar modules developed and built by ESA . The originally silicon- based modules delivered an output of at least 4550 watts (depending on the orientation towards the sun), each measuring 12.1 m × 2.5 m and weighing 7.7 kg each. Since the telescope itself, like the space shuttle's payload bay, has a round cross-section, the two wings could not simply be folded in as usual. Instead, the individual panels were applied to a surface made of glass fibers and Kapton , the cabling was implemented using an underlying silver thread matrix, which was then protected by another layer of Kapton. This combination was only 0.5 mm thick and could be rolled up onto a drum, which in turn could be folded up to save space.

However, problems quickly became apparent due to the high bending forces caused by the intense thermal stress when entering and exiting the earth's shadow. Due to the rapid change between light and shadow, the panels were quickly heated from −100 ° C to +100 ° C and also cooled down again, which led to undesirable twisting and deformation and thus vibrations of the entire telescope. For this reason, during the SM 1 service mission, they were replaced with newer models that no longer had this problem.Advances in solar cell technology enabled the installation of better, gallium arsenide -based solar modules for the SM 3B service mission nine years later , which provide around 20% more energy despite a 33% reduction in area. The smaller area of ​​the wings also ensures less atmospheric drag, so that the telescope does not lose altitude as quickly.

An opened battery module. The total of 66 cells can be clearly seen.

Due to the low orbit of the telescope, the solar modules are only illuminated about two thirds of the time, as the earth's shadow blocks the solar radiation. In order to supply the systems and instruments with energy also during this time, six nickel-hydrogen accumulators were integrated, which are charged as soon as sunlight hits the solar modules, whereby the charging process uses around a third of the electrical energy generated. The accumulators can each store around 75  Ah , which is sufficient for uninterrupted operation for 7.5 hours or five full orbits (the telescope consumes around 2,800 watts). This excess capacity is required because some objects to be observed are positioned in such a way that the solar sails do not have a good orientation to the sun and accordingly deliver less power. The accumulators have their own systems for charge, temperature and pressure control and consist of 22 individual cells . Each three accumulators are organized in a module, which were built in such a way that they can be safely replaced by astronauts in open space. Such a module is approximately 90 cm × 90 cm × 25 cm and weighs 214 kg.

In order to compensate for the natural aging of the accumulators, they were equipped with a Voltage / Temperature Improvement Kit (VIK) for the service mission SM 3A , which reduces the thermal load and overcharging problems in particular through improved systems for charging control. In the SM 4 service mission, all six batteries were replaced by improved models. Thanks to new manufacturing processes, these are significantly more robust and have a capacity increased to 88 Ah, of which only 75 Ah can be used due to thermal limitations. However, this excess capacity offers greater wear reserves, which ensures an even longer service life (the old accumulators had already been in operation for 13 years).

The energy is distributed centrally by the Power Control Unit (PCU), which weighs 55 kg and is installed in bay 4 of the equipment section.Connected to this are four Power Distribution Units (PDUs), each weighing 11 kg and to which the bus systems of the instruments are connected. They also contain monitoring instruments and overcurrent protection devices . In the SM 3B service mission, the PCU was replaced by a new model in order to be able to fully utilize the increased energy production of the new solar cells. The entirety of all power supply systems is known as the Electrical Power Subsystem (EPS).

Electronics and data processing

The DF-224 computer
The SI-C & DH system in the clean room

All systems for data processing and storage are organized in the Data Management Subsystem (DMS).Up until the SM 3A service mission, its heart was a DF-224 central computer , which was responsible for the higher-level control of all technical and scientific systems. It contained three identical, at 1.25 MHz clocked 8-bit - processors , always only one was used, the other two served as a reserve in case of failure. The memory is organized in six modules, each with a capacity of 192 kBit. The internal bus is designed triple redundant, the connection to the external systems is double redundant. The computer measures 40 cm × 40 cm × 30 cm, weighs 50 kg and was programmed in an assembly language specific to it.

Just a few years after the start, two memory modules failed (three are at least necessary for operation), so that an additional coprocessor system was installed for the SM 1 service mission. This consists of a dual redundant combination of an Intel 80386 x86 CPU and an Intel 80387 - coprocessor , eight shared storage modules with a capacity of 192 Kibit and 1 MiB memory exclusively for the 80386 CPU. The coprocessor of the system programming was done in C .

During the SM 3A service mission, the entire computer system including the coprocessor was removed and replaced by the significantly more powerful Advanced Computer . It has three 32-bit Intel 80486 processors that have a clock frequency of 25 MHz and are about 20 times faster than the DF-224 computer. Each CPU is housed on its own circuit board with 2 MiB SRAM and a 1 MiB EPROM . The entire system is 48 cm × 46 cm × 33 cm and weighs 32 kg.

The central element for distributing data within the computer is the data management unit (DMU). In addition to the routing , the approximately 38 kg DMU is responsible for distributing the time used throughout the system, for which it is connected to two redundant, high-precision oscillators .Most systems are connected directly to the DMU, ​​but some components are only connected to them via four 16 kg Data Interface Units (DIUs).

The Science Instrument Control and Data Handling Unit (SI C&DH) installed in bay 10 is responsible for controlling the scientific instruments . This is a complex of several electronic components that control the instruments, read out their data and format them. The core element of this system is the Control Unit / Science Data Formatter (CU / SDF). It formats commands and inquiries from the ground station in the appropriate format for the target system or instrument. In the opposite direction, it also translates data streams from the connected components into a format suitable for the ground station. The NASA Standard Spacecraft Computer (NSCC-I) is responsible for interpreting the formatted data and commands . It has eight memory modules with a capacity of 148 kbit each, in which command sequences can be stored. This allows the telescope to work even when it is not in contact with the ground station. The commands generated or called up by the NSCC-I itself are then transmitted to the CU / SDF again via direct memory access . In addition, all components of the SI C&DH are designed redundantly so that in the event of a failure an identical reserve module is available.

Three Engineering / Science Data Recorders (E / SDRs) are available for storing data that cannot be transmitted to Earth in real time . At the start, these were tape drives with a capacity of 1.2 Gbit each, a weight of 9 kg each and the dimensions 30 cm × 23 cm × 18 cm.Since magnetic tapes have to be moved by means of electric motors for reading and writing, one copy was already replaced by a flash- based memory known as a solid state recorder (SSR) during the SM 2 service mission. This has no mechanical components, so that it is much more reliable and has a longer service life. In addition, the SSR has a capacity of 12 GBit that is around ten times as high and enables parallel read and write access.

In addition to the redundant design of important components, there is a software and hardware security system for the operational safety of the telescope. The software system is a series of programs that are executed on the central computer and monitor various operating parameters. If any, but not highly dangerous, malfunction is discovered, all scientific instruments are switched off and the telescope is kept in the current orientation. This mode can only be canceled by intervention of the ground control after correcting the error. However, should serious deviations occur in the energy system, the telescope is aligned in such a way that the sun sails are optimally illuminated by the sun in order to produce as much electricity as possible. In addition, measures are taken to keep all components at their operating temperature in order to ensure a quick resumption of the scientific investigations after the release of the safety mode.

In the event of highly critical system failures or malfunctions, there is another safety system called Pointing / Safemode Electronics Assembly (PSEA). This is a 39 kg complex of 40 special circuit boards on which programs are located that are solely intended to ensure the survival of the telescope. In contrast to the software-based safety system in the central computer, these are permanently wired in the PSEA hardware , which means that they are considerably more robust against interference. The PSEA system is activated when one or more of the following situations occur:

After activation, the wired programs ensure that the sun sails are optimally aligned with the sun and that all components that are not essential for survival are switched off. The temperature control is controlled in such a way that all systems are kept above their temperature necessary for survival. In order to remain able to act even in the event of severe damage to the main systems, the PSEA complex is connected to the critical telescope components with its own data lines. To compensate for a failure of the RGAs, there are also three reserve gyroscopes, which, however, are much less precise and can only ensure a rough alignment, which does not allow scientific operation. The PSEA system can thus work completely autonomously; a connection to the ground station is only necessary for troubleshooting.


On this picture, the two HGAs can be easily recognized by the arms.

Hubble has two high-gain and two low-gain antennas (referred to as HGA and LGA) for communication. The two high gain antennas are designed as parabolic antennas in honeycomb design ( aluminum honeycomb between two CFRP plates) and mounted on two separate 4.3 m long arms, which also serve as waveguides due to their box-shaped construction . They have a diameter of 1.3 m and can be swiveled by up to 100 degrees in two axes, so that communication with a TDRS satellite is possible in any position. Since the HGAs have a high data rate due to their strong directivity , this property is important in order to transfer the very extensive scientific image and measurement data in an acceptable time.The signals to be sent are generated by the S-Band Single Access Transmitter (SSAT). This transceiver has a transmission power of 17.5 watts and uses phase modulation to achieve a data rate of up to 1 Mbit / s. In total, around 120 GBit of data is sent to the ground station per week in this way, using the frequencies 2255.5 MHz and 2287.5 MHz. A second, identical SSAT is available as a reserve, which had to be put into operation after the failure of the primary transceiver in 1998. In December 1999 this was replaced by a functioning model during the SM 3A service mission.

Two low-gain antennas are available for the transmission of technical data and for emergencies. These are immobile and have a very broad antenna pattern . In combination, communication with the telescope is possible even if its HGAs are not correctly aligned. The low directional effect, however, severely limits the data rate, so that only short technical control commands and status data can be transmitted. The frequencies here are 2106.4 and 2287.5 MHz. Two redundant transceivers, which are referred to as multiple access transmitters (MAT), are used to generate signals. Commands are received at 1 kBit / s, data can be sent with up to 32 kBit / s.

Position control

Personnel practice its later installation in space with a newly certified FGS.

Since Hubble is supposed to observe objects with a very high resolution, the entire telescope must be aligned and tracked extremely precisely . The system responsible for this, called the Pointing Control Subsystem (PCS), can align the telescope with an accuracy of 0.01 ″ and track an object for 24 hours with an accuracy of at least 0.007 ″. If Hubble were in San Francisco , it could use a narrow beam of light to illuminate a moving 10-cent coin over Los Angeles , some 600 km away . In order to achieve such a high-precision alignment, a total of five different sensor complexes are used.

A total of four Coarse Sun Sensors (CSSs), two of which are located at the bow and stern, determine the orientation to the sun,Two Magnetic Sensing Systems (MSSs) on the telescope cover use measurements of the earth's magnetic field to determine the orientation relative to the earth and three star sensors , known as Fixed Head Star Trackers (FHSTs), record the orientation towards a specific guide star .The movements in the three spatial axes are recorded by three rate gyro assemblies (RGAs). Each RGA has two gyroscopes ( Rate Sensing Unit, RSU) that can detect and measure the acceleration along their respective axes. Hubble has a total of six gyroscopes available, with at least three being necessary for operation. Since these showed a high degree of wear and tear relatively quickly after starting, two to six of them were replaced with each service mission.

The actual core system that enables the high precision of the telescope is the complex of three Fine Guidance Sensors (FGSs). They draw their light from the edge areas of the illumination area of ​​the main optics and thus work coaxially and in parallel with the scientific instruments. Since the optical aberrations are greatest in the edge area , every FGS has a large field of view , so that the probability is high to find a suitable guide star anyway. If one is found, it is precisely recorded and focused using a complex system of small electric motors, prisms and mirrors in order to direct its light onto two interferometers , which in turn consist of two photomultipliers . These complexes capture the phase of the incident light, which is exactly the same when the guide star is exactly in the center of the field of view . If the telescope moves towards the edge of the image, there is a phase shift between the two interferometers, which is recorded by a computer system. This calculates the necessary alignment correction and sends the appropriate commands to the position control system. Since the complex is able to detect deviations from 0.002 8 ″, the corrective maneuvers can be initiated before significant deviations (from 0.005 ″) occur. However, an FGS can only record the deviation in one spatial dimension, which means that at least two of them are required for correct alignment, the third system also measures the angular position of the star. Each FGS is 1.5 m long, has a diameter of 1 m and weighs 220 kg. During the service missions SM 2, SM 3A and SM 4, one sensor each was replaced by a newly calibrated and certified model.In addition, a system called Optical Control Electronics Enhancement Kit (OCE-EK) was installed during the SM 2 mission . It allows minor adjustments and calibrations of the FGSs without external intervention, whereby their accuracy can be maintained to a certain degree without new service emissions.

The movements requested by the control systems are primarily implemented by four Reaction Wheel Assemblies (RWAs). These contain two reaction wheels each , which transmit an angular momentum to the telescope when their rotational speed changes, thereby realigning it. Each wheel is 59 cm in diameter, weighs 45 kg and can rotate at a speed of up to 3000 revolutions per minute. Hubble has a total of six of these wheels, only three of which are necessary for operation, the rest is kept as a reserve.In addition, four magnetic gates are used for position control . These are electromagnets that interact with the earth's magnetic field and thus control the speed of the inertia wheels by means of pulse transmission. In the event that the RWAs fail completely, the telescope can use these torquers to reach a position in which it can align the solar modules with the sun so that electricity continues to be generated.

Optical system

Construction of the optical system
Fields of view of the instruments
The primary mirror during polishing

The optical system (referred to as Optical Telescope Assembly, OTA for short) is the actual heart of Hubble, as it collects the light required for scientific investigations and distributes it to the individual instruments. It is a Ritchey-Chrétien-Cassegrain construction that consists of only two mirrors. The first is the primary mirror, which is responsible for collecting the light. It has a diameter of 2.4 m and is hyperbolically shaped, so that the incident light is thrown onto the 30 cm large secondary mirror. This reflects on the scientific instruments and the three FGSs. A special feature of the Hubble telescope is that all instruments receive a fixed part of the collected light and can therefore work at the same time. Otherwise, it is common to “switch” between different sensors so that only one measurement can be active at a time. The optical construction is 6.4 m long and has a focal length of 57.6 m with an aperture of ƒ / 24.

The Hubble main mirror was manufactured by the Perkin-Elmer company (now part of Raytheon ), using a special type of glass from the Corning company , which hardly deforms when the temperature changes and thus preserves the imaging performance. A 3.8 cm thick front surface was made from it, which was additionally stabilized by a honeycomb structure also made of this glass with a thickness of 25.4 cm. Thanks to this construction, the weight could be reduced to a moderate 818 kg, a conventional, solid glass body would have weighed around 3600 kg to achieve the same performance. In order to guarantee that the body was absolutely free of tension, it was cooled very slowly from its casting temperature (1180 ° C) to room temperature over a period of three months before it was brought to Perkin-Elmer for final production. There, the front surface was first brought into an almost hyperbolic shape using diamond-set grinding machines , with about 1.28 cm of material being sanded off from the front surface. Experienced opticians then removed another 7.6 mm using manual tools. Finally, a computer-aided laser system was used that formed the desired surface profile with a deviation of less than 31.75 nm (if the mirror had the size of the earth, a deviation of no more than 15 cm would be high). Despite the precise manufacturing and quality control, there was a significant deviation that was only detected in orbit and made the telescope practically useless ( details above ). Only the installation of a special correction system called COSTAR in the SM 1 service mission three years later made the planned scientific investigations possible. The actual reflection properties of the mirror are determined by a 100 nm thick aluminum layer, which is protected from environmental influences by an additional 25 nm magnesium fluoride . In addition, this layer increases the reflectance of the mirror (to over 70%) in the area of ​​the Lyman series , which is of great importance for many scientific studies. In the visible spectrum, the reflexivity is more than 85%. Behind the primary mirror there is a special beryllium support structure that contains several heating elements and 24 small actuators . The former ensure that the mirror is kept at its optimal temperature of around 21 ° C, with the help of the actuators, the shape of the mirror can be readjusted minimally by means of a control command from the floor. The entire construction is in turn held in position by a 546 kg, hollow titanium support ring with a thickness of 38 cm.

The primary mirror is shaped so that all of the collected light hits the 30 cm secondary mirror. Its reflective coating also consists of magnesium fluoride and aluminum, but Zerodur glass was used for the even more hyperbolic mirror body. This is held in place in the center of the telescope near the opening by a highly stiffened construction made of CFRP. This is additionally coated with a multilayer insulation in order to further minimize deformations due to temperature differences. This is very important for the correct operation of the telescope, since a position deviation of more than 0.0025 mm is sufficient to cause serious aberrations. As with the primary mirror, there are also six actuators with which the alignment can be corrected to a small extent. The light is then directed to the instruments through a 60 cm hole in the center of the primary mirror.

To protect against stray light , which mainly comes from the earth, the moon and the sun, there are three baffles . They are elongated, cylindrical constructions, the inner wall of which is provided with a deep black, finely and coarsely ribbed structure. This absorbs or disperses light that comes from objects that are in the vicinity of the targeted target and could thus interfere with the examinations. In terms of purpose, it is similar to a lens hood , but the structure, which is often found in many commercially available cameras in the area around the sensor and, more rarely, on the front part of the lens, is located inside the telescope. The largest primary baffle is attached to the edge of the primary mirror, is made of aluminum and extends to the opening of the telescope, resulting in a length of 4.8 m. Another 3 m long central baffle is attached in the center of the mirror to shield the light reflected from the secondary mirror, on which such a construction was also mounted.All parts of the optics are connected and held together by a skeletal structure made of CFRP. This is 5.3 m long and weighs 114 kg.

Isolation and temperature control

A look at the ring-shaped equipment section (lower half of the picture). The old FOSR film without protection in the middle and the four new NOBL panels on the right and left are clearly visible.

Due to the low orbit, the telescope passes the Earth's shadow very frequently and for a long time. This creates very high thermal loads when it emerges from the shade again and is immediately exposed to intense sunlight. To reduce this load, the entire surface of Hubble is surrounded by various insulation materials.With a share of 80%, the multilayer insulation (MLI) is the most important component. This consists of 15 aluminum- vaporized Kapton layers and a final glued-on layer of so-called"Flexible Optical Solar Reflector" (FOSR). This is a stick-on Teflon film that is either vapor-coated with silver or aluminum, which gives Hubble its typical shiny appearance. It was also used to protect surfaces that were not additionally protected by an MLI layer, the largest areas being the front cover flap and the side surfaces of the telescope (these are less intensely illuminated by the sun than the upper and lower parts ). Since the scientific instruments have different optimal temperature ranges, insulation materials are also provided between the four axial instrument bays in order to create individual temperature zones.

Although Teflon is a very elastic and robust material, small cracks in the FOSR material were already apparent during the inspection as part of the first service mission. By the time of the next mission, SM 2, these had expanded massively within just three years; over 100 cracks with a length of more than 12 cm were counted. During the mission itself, the first improvised repairs were carried out using FOSR adhesive tapes that were carried along. In order to solve the erosion problem of the FOSR film safely and finally, a new cover was developed:the New Outer Blanket Layer (NOBL). This is a construction made of a specially coated stainless steel panel that is inserted into a steel frame. This frame is individually adapted to a specific bay in the equipment section, where a NOBL module is installed over the old, damaged insulation to protect it from further erosion. In addition, some modules are also equipped with a radiator for improved cooling. This was necessary because, with the ongoing modernization of the telescope, more and more powerful electronics were installed, which produced more heat than their predecessor systems, which in turn adversely affected Hubble's heat balance. A total of seven of these protective panels were installed during the outboard operations on missions SM 3A, SM 3B and SM 4.

In addition to the passive insulating materials, the telescope has a system for actively regulating the temperature. This is recorded internally and externally by over 200 sensors, whereby an optimal thermal environment can be created specifically for each important component. This is done through the use of individually placed heating elements and radiators.

Scientific instruments


The following five instruments are currently installed and are used for scientific research except for the defective NICMOS . As no further service missions are currently planned (as of December 2013), all instruments will remain on board until the end of the mission.

Advanced Camera for Surveys (ACS)

The sensor of the WFC channel

This instrument is designed to observe large areas of space in the visible , ultraviolet and near infrared spectrum . This generally enables a wide range of applications. In particular, galaxies are to be investigated that were formed shortly after the Big Bang and thus show a high redshift . The instrument was installed at the SM 3B service mission, displacing the faint object camera from instrument bay no. Three different subsystems are available for examinations: a high-resolution channel for detailed measurements ( High Resolution Channel, HRC), a channel for wide-angle recordings ( Wide Field Channel, WFC) and a special channel for the ultraviolet spectral range ( Solar Blind Channel, SBC). In addition, 38 different filters are available to enable targeted examinations as well as special optics to correct the primary mirror error without the help of COSTAR. Due to electronic failures in July 2006 and January 2007, the HRC and WRC channels were not operational until service mission SM 4. During the maintenance, only the WRC channel was repaired, the damage to the HRC channel was too profound, which is why it is no longer usable.

The WFC channel has two back-illuminated CCD sensors based on silicon . Each has 2048 × 4096 pixels and is sensitive in the range from 350–1100 nm, with the quantum yield up to 800 nm being around 80% and then falling evenly to below 5% at 1100 nm. With a pixel size of 225 µm² and a field of view of 202 ″ × 202 ″, the channel achieves a resolution of 0.05 ″ / pixel. The high-resolution HRC channel, on the other hand, has a much narrower field of view of 29 ″ × 26 ″ and, despite a smaller CCD sensor with 1024 × 1024 pixels, achieves about twice as high a resolution of 0.027 ″ / pixel. In addition, it already has a quantum yield of around 35% from 170 nm , which increases to up to 65% from 400 nm and, like the WFC channel, continuously decreases from around 700 nm to 1100 nm. Both sensors are otherwise identical and work at a temperature of −80 ° C. A special feature of the HRC channel is the ability to observe weakly luminous objects in the vicinity of strong light sources. For this purpose, a special mask ( coronograph ) is inserted into the beam path so that light from the bright source is blocked. For observations in the ultraviolet spectrum, the SBC channel is available, which also uses the optical construction of the HRC channel. The cesium iodide -based sensor is a spare part for the STIS instrument. It has 1024 × 1024 pixels with a size of 25 µm² each, which achieve a quantum yield of up to 20% in the 115–170 nm range. With a field of view of 35 ″ × 31 ″, the channel achieves a resolution of 0.032 ″ / pixel.

Wide Field Camera 3 (WFC3)

The WFC3 in the clean room

The Wide Field Camera 3 (WFC3) enables the observation and imaging of an extensive spatial area with high resolution and a large spectral bandwidth (200–1700 nm) at the same time. In the visible and infrared range, its performance is only slightly below the level of the Advanced Camera for Surveys, so that if it fails, the WFC3 can be used as an alternative. In the ultraviolet and visible range, however, it is clearly superior to all other instruments in terms of field of view and bandwidth, which makes it ideal for large-scale investigations in this spectral range. The observation goals are accordingly diverse and range from the investigation of nearby star formation regions in the ultraviolet range to extremely distant galaxies using infrared. The instrument was installed during service mission SM 4 in the axial instrument bay no. 5, where the Wide Field / Planetary Camera 2 was previously located.

The WFC3 has two separate channels for imaging in the near infrared (IR) and ultraviolet / visible (UVIS) range. For the latter, two combined 2051 × 4096 pixel silicon- based CCD sensors are used, which are kept at a temperature of −83 ° C by four-stage Peltier cooling . They achieve a quantum efficiency of 50 to 70%, the maximum being around 600 nm. By combining 225 µm² pixels with a field of view of 162 ″ × 162 ″, this channel achieves a resolution of approx. 0.04 ″ / pixel in the spectral range from 200 to 1000 nm. In contrast, the square HgCdTe - CMOS sensor of the near-infrared channel is only 1 megapixel in size and, despite its smaller field of view of 136 ″ × 123 ″, only delivers a resolution of 0.13 ″ / pixel. On the other hand, its quantum yield of almost continuous 80% over the entire spectrum (900–1700 nm) is significantly better. Since infrared detectors react particularly unfavorably to heat, it is also equipped with a more powerful six-stage cooling system, which enables an operating temperature of −128 ° C. Both channels also have a large number of filters (62 for UVIS and 16 for IR) in order to be able to investigate specific properties of the observed region. Three grating prisms (one for UVIS, two for IR) are of particular interest here , as they enable both channels to produce classic spectra for an object in the center of the image. Although these are only slightly resolved (70–210), they combined extend over the spectrum from 190–450 nm and 800–1700 nm.

Cosmic Origins Spectrograph (COS)

COS shortly before loading onto the space shuttle

The COS is essentially a spectrometer , so it usually does not deliver images, but measured values ​​for a single targeted point. In this way, the structure of the universe and the evolution of galaxies, stars and planets are to be explored. The measuring range (90 to 320 nm) overlaps with that of the STIS instrument, whereby it is about ten times more sensitive for point targets. For examinations, you can choose between a far-ultraviolet ( far-ultraviolet, FUV) and a near-ultraviolet ( near-ultraviolet, NUV) channel. Both sensors are preceded by one of a total of seven special optical grids that split the incident light and deflect it to different degrees according to its wavelength . Parts with a short wavelength hit the downstream CCD sensor more in the middle, while long-wave components hit the edge area more. From the position and charge of the pixels, an intensity spectrum can be produced depending on the wavelength, which in turn allows conclusions to be drawn about the chemical structure of the observed object. The instrument was installed during the SM 4 service mission and replaced the COSTAR system, as all other instruments were equipped with internal correction mechanisms at this point and it was no longer needed.

In the FUV channel, two adjacent CCD sensors based on cesium iodide with a combined 16,384 × 1024 pixels are used for measurements . A quantum yield of up to 26% at 134 nm is achieved, the spectral resolution and bandwidth of the spectrum is mainly determined by the optical grating used. Two of them are optimized for a high resolution (about 11,500 to 21,000 in the range 90 to 178 nm), while the broadband grating can work on a large wavelength range from 90 to 215 nm, but only has a low resolution of 1500 to 4000. The situation is similar in the NUV channel, here there are three narrow-band, but high-resolution gratings (16,000 to 24,000 with a bandwidth of around 40 nm) and one broadband grating that achieves a resolution of only 2,100 to 3,900 in the 165 to 320 nm range. However, a different CCD chip is used in this channel. It is based on a cesium - tellurium compound and has 1024 × 1024 pixels, which achieve a quantum yield of up to 10% at 220 nm. The square structure also enables an imaging measurement mode for this channel, with which a resolution of 0.0235 "/ pixel is achieved with a field of view of 2". Since strong vignetting already occurs from a viewing angle of 0.5 ″ away from the image center , only small and compact objects can be reliably observed.

Space Telescope Imaging Spectrograph (STIS)

The CCD sensor (~ 9 cm²) of the STIS

The STIS instrument is a spectrograph that covers a wide range from ultraviolet to infrared radiation (115 to 1030 nm). In contrast to the COS instrument, which specializes in single targets, STIS can be used to create spectra at up to 500 points in a recording, which enables extensive objects to be examined quickly. However, the measurement results are less accurate than with the COS instrument, but are particularly suitable for the search and analysis of black holes and their jets . In total, three channels are available for observations: the CCD channel with a large bandwidth (ultraviolet to infrared) and the NUV and FUV for the near and far ultraviolet spectrum. The spectra are generated by means of optical grids, analogous to the COS instrument. The instrument was installed during service mission SM 2 in instrument bay no. 1, where it replaced the Goddard High Resolution Spectrograph . Between August 2004 and May 2009, STIS was inoperative due to a failure in the internal power supply. Since the installation of a new circuit board during the SM 4 service mission, the instrument has been working again without any malfunctions.

The STIS has two similarly structured MAMA sensors to generate spectra . They each have 1024 × 1024 pixels with a size of 625 µm². A field of view of 25 ″ × 25 ″ results in a resolution of 0.025 ″ / pixel. The difference between the two sensors lies in their spectral bandwidth and quantum efficiency. The CsI sensor in the far ultraviolet (FUV) channel is sensitive in the range from 115 to 170 nm and has a quantum efficiency of up to 24%, the CsTe sensor in the far ultraviolet (FUV) channel operates at 160 to 310 nm with an efficiency of only 10%. A large number of optical gratings are available for the formation of spectra. These achieve a resolution of 500 to 17,400 with a bandwidth of around 60 or 150 nm. Using Echelle gratings and special data processing techniques, resolution values ​​of over 200,000 can be achieved with a similar bandwidth. In addition to the two MAMA sensors, a CCD chip is available for measurements. This is also one megapixel in size, but its spectrum is much wider at 164–1100 nm and offers a wider field of view (52 ″ × 52 ″). In addition, the quantum efficiency is almost always over 20%, with 67% reaching its maximum at 600 nm. The total of six optical gratings enable a resolution of 530 to 10630 with a bandwidth of 140 to 500 nm.

Near Infrared Camera and Multi-Object Spectrometer (NICMOS)

Elevation of the NICMOS. The large dewar can be seen here in the middle of the instrument.

The NICMOS is a relatively highly specialized instrument, which is mainly due to its focus on the near infrared spectral range (800-2500 nm). In return, all three existing measurement channels (with slightly different viewing areas) can be used at the same time, so there is no need to switch internally for different examination methods. Another unique feature is the complex cooling system. For the observation of the near infrared spectrum, the lowest possible temperature of the sensors is of decisive importance, since their own thermal noise would otherwise superimpose almost all signals collected by the main mirror. Therefore, these are housed in a complex, four-way insulated Dewar vessel , which takes up a good half of the available volume within the instrument. The cooling only took place by means of a supply of 109 kg of solid nitrogen . A closed cooling system was installed during the SM 3B service mission, as the nitrogen was used up after almost two years of operation. After a good six years of operation, it could no longer be started reliably after a software update, so that the operation of the instrument has been suspended since the end of 2008 due to the sensor temperature being too high. Before the failure, the instrument was particularly well suited for observing objects inside or behind dense clouds of dust and gas due to its spectrum, which reached very far into the infrared, as these short-wave radiation in the visible and ultraviolet range, in contrast to infrared light, absorb very strongly. The NICMOS was already installed in the instrument bay no. 2 during the service mission SM 2, where it replaced the faint object spectrograph .

Each of the three measurement channels (NIC 1 to 3) has an identical HgCdTe-based sensor with 256 × 256 pixels each. The channels therefore only differ in a few aspects:

channel Field of view
(″ / pixel)
NIC 1 11 × 11 0.043 Polarization measurement at 800–1300 nm
NIC 2 19 × 19 0.075 Polarization measurement at 1900–2100 nm, coronograph with 0.3 ″ radius
NIC 3 51 × 51 0.20 3 grid prisms
Cross section through the dewar. The CFRP construction with the sensors can be seen in the center.

In total, NICMOS has 32 filters, 3 grating prisms and 3 polarization filters to enable specific examinations. All these components are mounted on a CFRP construction in the innermost part of the Dewar vessel. This complex was together with a supply of frozen nitrogen in an envelope, which was kept at a temperature of about 60 K by its cold gases. To further improve the insulation, this complex is surrounded by two Peltier-cooled shells before the dewar is closed by an external pressure vessel.

The stock of frozen nitrogen was originally intended to ensure sufficient cooling of the sensors for around four and a half years. However, during its melting process, ice crystals formed and an unexpectedly strong deformation occurred, so that the deep-frozen CFRP support structure came into contact with the innermost shell of the dewar. This led to a significantly increased heat flow, which on the one hand led to even greater deformations and in turn caused an increased need for nitrogen cooling. The result was the halving of the instrument's mission time and a strong defocusing of the three measuring channels due to the deformations that occurred. The latter could be reduced to an acceptable level, at least for NIC 3, through an internal compensation system.

In order to make all channels of the NICMOS operational again, a closed cooling system was installed by Hubble in the rear area of ​​the SM 3B service mission. This has a powerful air conditioning compressor that works with neon as a coolant. The heat generated is conducted via a pump to a radiator on the outer structure of the telescope, where it is radiated into open space. The compressed neon, on the other hand, is expanded in a heat exchanger , whereby it cools another neon gas circuit via the effect of the heat of vaporization . This leads to the innermost part of the dewar via a special interface that was originally intended to continuously cool the instrument during floor tests, which ultimately cools the sensors. The complex is only operated periodically because it requires a lot of energy with 375 watts of electrical power. Since the dewar is still very well insulated despite its deformation, the cooling lasts for a long time, so that the system only rarely needs to be activated, whereby the sensor temperature is kept at a stable 77 Kelvin.

After an observation and cooling break in September 2008, the cooling system could surprisingly no longer be put into operation. The cooling compressor worked, but the closed neon gas circuit of the dewar required an additional coolant pump, which no longer started. The reason is assumed to be an accumulation of water ice in their housing. In order to liquefy this again, the instrument was not cooled for several weeks. On December 16, this measure proved successful, as the pump could initially be put back into operation. However, it failed again just four days later. Further attempts in 2009 were also largely unsuccessful, which is why it was decided to shut down the instrument completely for an indefinite period of time.


The following instruments were removed during the service missions and brought back to earth with the help of the space shuttle. Most are now on public display.

Corrective Optics Space Telescope Axial Replacement (COSTAR)

Structure of the optical and mechanical systems from COSTAR

COSTAR is not a scientific system in the actual sense, but a correction system to neutralize the main mirror error . For this purpose, small correction mirrors have been developed which are also not perfectly shaped and which reflect the incident light unevenly. However, the deviations have been calculated in such a way that they are exactly the inverse of those of the main mirror. Thus, after the light has been reflected by two uneven mirrors, it is again in the correct form and can be used for scientific research. In principle, the system is similar to conventional glasses , but mirrors are used instead of lenses . After installation during the service mission SM 1, these were brought into position by three mechanical arms in front of the entry openings of the following instruments: Faint Object Camera , Faint Object Spectrograph and Goddard High Resolution Spectrograph . Since these instruments have more than one measuring channel, a total of ten correction mirrors had to be used, each with a diameter of around 1.8 to 2.4 cm. With the SM 4 service mission, COSTAR was expanded again, since all new instruments now have their own correction mechanisms. It is on public display today at the National Air and Space Museum in Washington .

The entire development, production and verification of COSTAR took only 26 months, whereby in many areas a single task was assigned to two completely separate teams with different approaches in order to rule out further errors such as in the construction of the main mirror. The measurement of its error was determined on the one hand by examining the production plant that was still completely intact, and on the other by calculations based on distorted images that Hubble transmitted. Both groups came to practically identical measurement results, so this step was carried out correctly with a high degree of certainty. The correction mirrors produced afterwards were also checked by two independent teams to ensure that they were free of errors. For this purpose, COSTAR was first built into a special test system called the COSTAR Alignment System (CAS), which checked these mirrors through special tests. The Hubble Opto-Mechanical Simulator (HOMS) was developed to rule out that errors in the CAS lead to incorrect results . This simulated the deviations of the main mirror so that the correction mirrors could be verified according to their output image. The HOMS system was also tested by two independent groups, with ESA also getting involved by providing the engineering model of the faint object camera. A final comparison of the test systems and COSTAR with images from Hubble finally showed the correctness of the correction mirror.

Faint Object Camera (FOC)

The FOC in the Dornier Museum

This camera was the Hubble telephoto lens because it achieved the highest image resolution of any instrument. It covered a large part of the ultraviolet and visible spectrum with high sensitivity. In return, however, the field of view had to be greatly reduced so that a picture can only depict a small area of ​​the room. This profile makes the instrument particularly interesting for examining small objects and fine structures. The field of view and the associated resolution can be influenced by choosing between two separate measuring channels, whereby the detectors are structurally identical. Due to the good performance values, the FOC stayed on board Hubble for a long time and was only replaced with the Advanced Camera for Surveys during the penultimate SM 3B service mission. The instrument was a major contribution from ESA to the project and was built by Dornier . After it had been removed and transported back, it was therefore given to the Dornier Museum in Friedrichshafen , where it is now on public display.

Both measuring channels are optically designed in such a way that they enlarge the image from the main mirror by twice or four times. This focal length extension reduces the f-number, which is why the two channels are named: ƒ / 48 for double magnification and ƒ / 96 for four-fold magnification (main mirror f-number: ƒ / 24). With the installation of COSTAR the optical formula was changed significantly, the f-numbers are therefore ƒ / 75.5 and ƒ / 151 in real terms. The fields of view vary accordingly by double with 44 ″ × 44 ″ or 22 ″ × 22 ″. The detectors, however, are identical in both channels and are sensitive to a spectrum from 115 to 650 nm. In order to also be able to register weak signals, the FOC has three image intensifiers connected in series, which increase the original electron current generated by the magnesium fluoride window by around 10,000 times. The electrons are then converted back into photons through a phosphor window, which are then directed onto a plate with silicon diodes through an optical lens system. These are then read out by an electron beam and interpreted in such a way that a 512 × 512 pixel image can be saved at the end. Resolutions of up to 0.014 ″ / pixel can be achieved in the ƒ / 96 channel.

Faint Object Spectrograph (FOS)

View inside the FOS

This highly sensitive spectrograph was used for the chemical investigation of distant and faint objects. The instrument proved to be particularly helpful when researching black holes, as it could be used to precisely measure the speeds and movements of the surrounding gas clouds, which enabled conclusions to be drawn about the black hole itself. Two independent measurement channels are available for examinations, which only differ in terms of the spectral ranges they cover. Combined, both can cover a range from 160 to 850 nm (far ultraviolet to near infrared). The instrument was ousted by NICMOS in the SM2 service mission and is now on public display at the National Air and Space Museum in Washington.

The two detectors are referred to as blue and red channels according to their spectral ranges. Both have line sensors with 512 silicon photodiodes each , which are "bombarded" with electrons by different photocathodes . In the blue channel, Na 2 - K - Sb is used as the cathode material, in the red channel cesium was also added (results in Na 2 -K-Sb-Cs). This variance has significantly changed the spectral sensitivity: the blue channel is highly sensitive in the 130 to 400 nm range (quantum efficiency 13–18%) and loses efficiency at around 550 nm, while the red channel works best in the 180 to 450 nm range ( 23–28% efficiency) and only reaches its upper limit at 850 nm. Regardless of this, both detectors achieve a resolution of up to 1300 with a field of view of 3.71 ″ × 3.66 ″ (after installing COSTAR, before that 4.3 ″ × 4.3 ″). Due to the main mirror error and errors in the construction of the instrument (one mirror was dirty and the shielding of the photocathodes was insufficient), initial observations were only possible with significant restrictions. It was only through the installation of COSTAR and a complex recalibration that the capabilities of the instrument could be used almost fully.

High Speed ​​Photometer (HSP)

This instrument specializes in the study of variable stars , especially Cepheids , and is therefore relatively simple (no moving parts). Using five separate detectors, the brightness and polarization can be measured up to 100,000 times per second, which means that extremely high-frequency fluctuations can also be recorded. The stars in question are mainly in the far UV spectrum, but measurements can be made up to the near infrared range. Since the HSP was unable to make a significant contribution to many of the mission's research objectives due to its strong specialization, it was expanded during the first service mission to make space for the COSTAR correction system. It has been on public display in Space Place at the University of Wisconsin-Madison since 2007 .

Four of the five detectors are used to measure the brightness, two of which consist of Cs - Te -based photocells and magnesium fluoride photocathodes and a further two of Bikali photocells (similar to those from the FOS ) with quartz glass cathodes. The former cover a spectral range of 120 to 300 nm from the latter the range 160-700 nm Three of the detectors, as well as a. GaAs - photomultiplier , for photometry used, the remaining is used for polarimetry , wherein the quantum efficiency with only 0.1 to 3% is extremely low. The opening of the optical system can be reduced to up to an arc second in order to focus the measurement as precisely as possible by masking out the background and neighboring objects. In order to precisely limit the wavelength to be measured, 23 filters are also available, the filter effect of which is very strong according to the purpose.

Wide Field / Planetary Camera (WFPC)

Construction of the WFPC

This camera system was designed for the multispectral recording of relatively large spatial areas and is therefore suitable for a large number of scientific investigations. The broad spectrum from the far UV to the near infrared range is particularly useful here . In addition, there are also some filters and optical gratings with which, to a limited extent, spectrographic measurements can be carried out. The instrument has two measuring channels: the wide-angle channel (Wide Field), which has a particularly large field of view at the expense of the resolution, and the planetary camera, which has a smaller field of view, but uses the resolution of the main mirror to the full can. At the start, the WFPC was housed in the only axial instrument bay (No. 5), but was replaced with an improved model ( WFPC2 ) during the second service mission . After returning, the instrument was dismantled in order to be able to recycle structural parts for the third generation of cameras ( WFPC3 ).

Both channels have four back-exposed CCD sensors each with 800 × 800 pixels. These are 15 µm in size and use silicon as the semiconductor material, with an additional layer made of corons that converts UV light into visible photons and thus makes it detectable. The measurable spectrum ranges from approx. 130 to 1400 nm, the quantum efficiency is generally 5 to 10% close to these limits, but increases constantly in the 430 to 800 nm range and reaches the maximum of 20% at 600 nm. A two-stage cooling system has been integrated to reduce the dark current . The sensor is cooled by means of a silver plate and a Peltier element , which then transfers the heat via a heat pipe filled with ammonia to an externally mounted radiator, where it is radiated into space. In this way, a sensor can be cooled down to −115 ° C. Due to the different areas of responsibility of the channels, they have a different optical configuration. While the wide-angle channel has a field of view of 2.6 '× 2.5 ' (arc minutes) and an aperture of f  / 12.9, these values ​​are 66 ″ × 66 ″ and f  / 30 for the wide-angle channel . Thus a resolution of 0.1 and 0.043 ″ / pixel is achieved. In order to be able to observe particularly bright objects without symptoms of overload, there are several light-weakening filters that are mounted on a wheel. In addition, spectra can be generated using a total of 40 optical gratings and grating prisms .

Wide Field / Planetary Camera 2 (WFPC2)

The WFPC2 is an improved version of the WFPC, which it replaced in the SM 3B service mission in the only radial instrument bay No. 1. The research objectives of the instrument remained unchanged: The investigation of relatively large spatial areas with good resolution and a broad spectrum. In return, the camera is relatively insensitive to extreme UV and infrared radiation and does not achieve any peak values ​​in terms of resolution.

The most important improvement over the previous camera is an integrated correction system to compensate for the main mirror error. Thus the WFPC2 is no longer dependent on COSTAR, which brought its expansion one step closer. Due to a tight budget, the design could not be significantly improved. The detectors are based on the same design, but have been manufactured differently. There were only significant increases in performance in the areas of dark noise (eight times lower), readout noise ( about two times lower) and dynamic range (a good twice as large). To save costs, only four instead of eight CCDs were produced, which halved the recording area. In addition, the sensors are no longer illuminated from the rear, which worsens the signal-to-noise ratio somewhat and reduces the resolution . The other parameters are essentially identical to those of the WFPC.

Goddard High Resolution Spectrograph (GHRS)

The GHRS during the expansion

This instrument is the first spectrograph of the telescope. It works exclusively in the ultraviolet range from 115 to 320 nm, as the measuring range has been clearly limited by the COSTAR correction system. The spectra are generated with optical ( Echelle ) gratings and then measured by two detectors with a resolution of up to 80,000. The instrument can also produce images in the UV range, but it is not optimized for this task, so that the performance values ​​are rather low. The GHRS was expanded in service mission 2 and replaced by the STIS , which has improved performance values.

Two Digicon detectors with different materials serve as detectors . In the first model as designated D1 is a cesium iodide - photocathode behind a lithium fluoride used window, when D2 detector takes a cesium cathode behind a magnesium fluoride window is used. This results in a measuring range of 110–180 nm (D1) and 170–320 nm (D2). The electrons generated behind the windows are then accelerated and electronically mapped onto a CCD array with 500 measuring diodes; another 12 diodes are used for calibration.

Five optical gratings and two echelle gratings are available for creating spectra. The former have a bandwidth of 800 to 1300 nm and achieve a resolution of 15,000 to 38,000. The Echelle gratings cover a larger bandwidth (up to 1500 nm) with higher resolution (up to 80,000), but the signal strength is very low, so that only very bright objects can be effectively observed or very long exposure times are necessary. With the help of the four focus diodes on the edge of the digicons, rudimentary images can also be created. At 0.103 ″ / pixel, these are indeed high-resolution, but the field of view of 1.74 ″ × 1.74 ″ is extremely small, which limits the scientific benefit to very specific investigations and target objects.

Tasks and results

Hubble telescope image of a 40,000 light-year area of ​​the Andromeda Galaxy . In the picture with a resolution of 1.5 billion pixels, over 100 million stars and thousands of star clusters can be seen.
Multiple colliding galaxies captured by the Hubble Space Telescope

The operation of a telescope outside of the earth's atmosphere has great advantages, since it does not filter certain wavelengths in the electromagnetic spectrum, for example in the ultraviolet and infrared range. There are also no disturbances from air movements ( scintillation ), which can only be compensated with great effort in terrestrial telescopes.

With its complex instrumentation, the Hubble space telescope was designed for a variety of tasks. Particular attention was paid to a program to determine the exact distance of these galaxies by observing Cepheids in nearby galaxies (up to a distance of about 20  Mpc ). By comparing it with the radial velocity of the galaxies, it should be possible to calculate the Hubble constant , which determines the extent of the universe , and thus also the age of the universe. After solving the initial problems, the HST was successful in this and other areas. Well-known results are:

The Hubble Telescope in the media

  • Use of measurement results:
    • Some of the pictures taken by the Hubble telescope were the science fiction series Star Trek: Voyager provided and served as background images of the universe . Many of the nebulae shown there were not created on the computer, but rather arise from reality.
    • The Google Sky program uses the images from the Hubble telescope.
  • Use as a dramaturgical element:
    • As a result, when aliens attack the Futurama series , the Hubble telescope is mistaken for an enemy spaceship and destroyed.
    • In the film Mystery Science Theater 3000, the Hubble telescope burns up after being rammed by a space station.
    • In the film Armageddon , the Hubble telescope is used to take the first images of an asteroid.
    • In the movie Gravity one is Space Shuttle crew behavior during repair work on the Hubble Space Telescope by a hail of space debris hit and destroyed, among other things, the telescope.

Visibility from Earth

Like other large Earth satellites, the Hubble Space Telescope is visible to the naked eye from Earth as a star-like object moving from west to east. Due to the slight incline of the orbit and the moderate orbit height, this is only possible in areas that are no more than about 45 degrees north or south of the equator. Thus, for example, it is not visible in Germany, Austria and Switzerland because it does not rise above the horizon . The Hubble Space Telescope can achieve a maximum brightness of 2  mag .

See also


  • Daniel Fischer , Hilmar Duerbeck : Hubble: A new window to space . Birkhäuser Verlag Basel, Boston, Berlin, 1995, ISBN 3-7643-5201-9
  • Daniel Fischer , Hilmar Duerbeck: The Hubble Universe: New Images and Insights . Approved licensed edition by Weltbild Verlag, Augsburg, 2000, copyright Kosmos Verlagsgesellschaft (formerly Birkhäuser), ISBN 3-8289-3407-2
  • Lars Lindberg Christensen, Davide de Marin and Raquel Yumi Shida: Cosmic Collisions - The Hubble Atlas of Galaxies , Spectrum Academic Publishing House, Heidelberg 2010 ISBN 978-3-8274-2555-3
  • Robert Noble and Sarah Wrigley, New York: Expanding Universe . Verlag Taschen, Cologne 2015. For the 25th anniversary of the Hubble Space Telescope, text by Owen Edwards , Charles F. Bolden and others, among others. a. in English, German, French, numerous large, sharp “Hubble images” ISBN 978-3-8365-4922-6 .
  • David J. Shayler, et al .: The Hubble Space Telescope - From Concept to Success. Springer, New York 2016, ISBN 978-1-4939-2826-2 .

Web links

Commons : Hubble Space Telescope  - collection of images, videos and audio files


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