Tracking (astronomy)
In astronomy, tracking is the compensation of the earth's rotation while observing or photographing astronomical objects. The mechanics of the tracking is part of the mount and should ensure that each set celestial object remains exactly in the center of the field of view of the telescope or camera. Without such tracking, a star would quickly move out of the field of view of the eyepiece at higher magnification .
The tracking technology for equatorial mounts was developed in the 18th century and partially automated in the 19th century. For photographic recordings it is absolutely necessary for longer exposure times, for which the hour axis of the mount must be precisely adjusted .
The tracking can be done manually or by motor. The motorized tracking can be more precise, but requires considerable technical effort. Their quality can be increased significantly through ongoing monitoring (guiding). In the case of amateur telescopes, this monitoring is carried out either via the main instrument using an off-axis guider or a parallel guide telescope .
Tracking without tracking control
Tracking does not always require separate monitoring for shorter exposure times in relation to the focal length and resolution of the image sensor. When using a barndoor mount, for example, you usually work without guiding.
Barndoor
The barndoor mount (from English barn door ) is the simplest form of the equatorial mount. In the simplest case it consists of two wooden boards, which are connected congruently at one end with a hinge. At the other end, a threaded rod is attached in the radius of the boards so that turning the thread removes the two boards from each other. If the hinge axis is aligned with the celestial pole, the course of the stars can be followed over a certain period of time, depending on the length of the threaded rod. In order to be able to target the entire sky with the instrument, there is a tripod head on the upper board , preferably a ball joint head . A light telescope or camera is attached there. The barndoor mount can even be driven by a motor. It was featured by GY Haig in the April 1975 Sky and Telescope magazine. Dave Trott described a multi-arm mount in the same magazine in 1988.
Observed tracking
Parallactic mount
With the equatorial mount , the guide star is observed with a crosshair eyepiece . One of the two threads is aligned with the right ascension axis of the mount . The star is held in the crosshairs by constantly turning the right ascension axis. If the mount is not perfectly aligned, the declination axis must also be rotated in between.
Azimuthal mount
With an azimuthal mount , both axes must be continuously tracked. This can also happen automatically with a Goto mount . When tracking objects with a Dobsonian mount , one speaks of pushing , since observation is carried out until the object threatens to disappear from the field of view. Shortly beforehand, the extremely smooth-running mount is moved just enough with a slight push that the object is back on the opposite edge of the picture. With this type of tracking, only extremely briefly exposed photographs of z. B. the sun or bright planets possible.
Semi-automatic tracking
In the simplest case, the semi-automatic tracking takes place by means of a mechanical clockwork, which is built with a corresponding reduction so that it compensates for the sidereal day . The Purus movement tracking is an example of this.
A semi-automatic tracking only makes sense on an equatorial mount. The polar cradle is an exception ; With it an azimuthal mount can be tilted so that an equatorial mount is created. A motor can be coupled, or the mount is coupled to the constantly rotating motor. The motor now drives the right ascension axis. It is observed again with a reticle eyepiece. Now, however, only the errors in the declination axis have to be compensated. If a servomotor is also available on the declination axis, corrections are made at low speed in order to avoid movement beyond the target. If the motor speed cannot be changed, the declination axis can only be used to correct manually. Usually only corrections on this axis are necessary anyway, since the motor speed on the right ascension axis is sufficiently precise.
An electric stepper motor or servo motor (rarely also a synchronous motor ) can track the mount with even higher accuracy. A quartz movement is normally used here, which delivers results that are accurate to the second. If the mount has been sheared in , an astronomical object can be observed over a longer period of time.
Limits of the semi-automatic tracking
There are limits to precision due to manufacturing tolerances and play in the mechanics. Stars commute z. B. constantly, often over a period of 8 minutes, back and forth. This is the periodic auger error. In many mounts, the axes are surrounded by a toothed ring, each with a worm . One revolution of this worm creates this recurring error. It can be compensated by training the mount control, the so-called PEC correction .
The play between the teeth delays the synchronism of the mount by up to one minute ( backlash ); only when the backlash of all gears has been overcome does the mount turn evenly.
Fully automatic tracking
If it has to be tracked very precisely, there is also the method of having star movements compensated for by a computer. A video camera is used instead of the crosshair eyepiece. This camera is connected to a computer and a program evaluates the images from the camera and sends control signals back to the tracking system for correction. This can be done either directly via a second line from the computer to the mount or indirectly, as in the picture; In this case, the camera (blue) on the guide scope also receives the control signals from the computer and sends them on to the mount via a second cable (here black). However, the camera on the guide scope must be able to capture at least one star. The user selects the star (s), the program first checks the alignment of the camera and then monitors the position of the star. If this moves from its original position, the program sends correction commands to the mount until the star is back in its original position. The second camera (on the right in the picture), which takes the actual photos, now always exposes the same section of sky and the objects in it remain immovable and appear "razor sharp" on the actual pictures.
With equatorial mounts, this method is precise for hours, only when crossing the meridian does the disadvantage of the equatorial system arise and the tracking must be interrupted.
In the case of azimuthal mounting, the problem of field rotation arises after longer observation . All celestial objects rotate from east to west and change their angle in the process. Two stars go in the east z. B. one after the other and stand steeply on top of each other, but at their highest point in the south they are already next to each other and when setting in the west the first upper star is now below. If a long-exposure astrophotography is made with an azimuthal mount , stars on the edge of the image are distorted into lines after just a few minutes. To avoid this, you can use a polar cradle and with large telescopes there is another motor in front of the camera, which turns the camera against the rotation of the image field.
Tracking speeds
With automatic, parallactic tracking, the speed of the declination motor can almost always be set to:
- Sidereal is intended for tracking stars,
- lunar is suitable for observing the moon; this apparently wanders "backwards" across the starry sky. 360 ° in 28 days. This means that it is already moving over 32 arc seconds per minute , which would already produce a visible movement at sidereal speed.
- Solar : The sun also has its own motion compared to the stars, since it revolves once every year and the earth's orbit is also slightly elliptical ( equation of time ). When observing the sun for a longer period with a series of images over a whole day, a shift would therefore also be observed here, which can be compensated for using the "solar speed" setting.
For comets, a computer control system must be able to move both axes, as these move neither with the stars nor in their direction. For other moving objects, such as satellites, special tracking is usually necessary, since they usually move at too high speeds for conventional tracking to be able to follow.
See also: Goto (telescope)
literature
Günter D. Roth, Wilhelm J. Altenhoff, Rainer Beck: Handbook for Star Friends . 4th edition. Springer, 1989, ISBN 3-540-19436-3 , pp. 74, 148 .
Web links
Individual evidence
- ↑ tfrank.de
- ↑ a2p.at
- ↑ rudolf-reiser.de
- ↑ astronomie.de ( Memento of the original from April 26, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
