Goto (telescope)
Go to or Goto (Engl. Go to ... ) refers to a property in the Mount astronomical telescopes that a semi- or fully automatic detection of objects under observation allows. It can be used to supplement existing servomotors with control electronics or a computer control integrated into the telescope . A mount equipped with a Go-to does not increase the accuracy of the tracking of the apparent star orbits, but only makes it easier to find the stars.
Types
Many manufacturers already offer ready-made mounts for amateur telescopes, but also retrofit kits. In the second case, the mount only needs to be able to attach servomotors to both axes . In the simplest case, these motors can compensate for the rotation of the earth and also adjust the telescope mount without contact. If you now replace the simple power circuit for the motors with a special control computer, you already have a go-to control. This small computer can be held in one hand and is smaller than a paperback book. A PDA is also suitable for Goto controls (assuming a corresponding program) if additional equipment provides the high currents required by the servomotors.
Goto mounts are available for both equatorial (parallactic) and azimuthal mounts . In the first case, the tracking takes place around the hourly axis aligned with the celestial pole , in the second case both servomotors are activated to compensate for the rotation of the earth. In the process, however, the observed field is twisted, which causes the stars on photos that have been exposed for a longer period to become more and more lines towards the edge. To compensate, azimuthal mounts can be retrofitted with a polar cradle .
function
The control computer is either
- in the control box or
- its own housing or
- inside the mount near the servomotors.
A model of the starry sky is programmed into the control computer, which the user can adapt to the actual circumstances by setting up the mount correctly. If the computer model and reality match, the built-in database of objects can be accessed. Usually, all objects that can be observed with amateur astronomical means are already stored. This includes all planetary orbits and, as a result, their current positions, all objects in the Messier catalog , the New General Catalog and its indices. In addition, some computers also have catalogs from Abell , Collinder etc. and catalogs about the 88 constellations of antiquity up to the SAO catalog with its 259,000 objects. If the desired object is not included in one of the catalogs, its position can also be entered directly by specifying the coordinates . If the object is below the observer's horizon , a corresponding warning is issued or, during a search, only objects are suggested that are also above the horizon. After the input, the mount is moved to the target position. The speed of movement depends on the strength of the servomotors, but speeds of 6 ° per second can certainly be achieved.
Minor planets or comets are mostly not recorded and have to be transferred from other databases. For this purpose, the computers of the Goto mounts have their own input, usually a serial interface . Via a special protocol, e.g. B. the Ascom protocol or LX200 can now take over the control of the mount and supplement and expand the astronomy program of a computer - ideally a notebook in the field .
If the control computer is inside the mount, there is also a hand-held device, the so-called control box. The entire device is operated via them. The mount can be moved at various adjustable speeds using four directional keys, and further inputs can be made using a number field or similar keys.
The power supply is usually 12 volts DC with a cigarette lighter plug from the automotive sector. Medium- sized telescopes up to about eight inches (20 cm lens or mirror size) are usually also equipped with a battery compartment for 8 baby or mignon cells . For larger telescopes there are systems with up to 40 volts and correspondingly stronger currents in order to be able to move heavy mounts at a reasonable speed.
service
First, the mount is aligned with the Pole Star (the so-called " northing "). If not equipped with a GPS module, the date, time and the latitude and longitude of your own location must be entered (once). These data are also saved when the device is switched off, and in some cases the date and time are continued. If the mount has clamps for uncoupling the motors so that it can be swiveled by hand, these clamps must no longer be released for any of the following steps. The mount is only moved electrically via button controls using the servomotors.
First, one to three celestial objects are controlled and adjusted via a finder scope and in a further step with a crosshair eyepiece in the main tube and their position is communicated to the computer ( alignment ). All objects visible to the naked eye can be selected. Adjustments to bright planets, the moon and - with particular caution - the sun can even be carried out during the day. From these positions, the control computer now calculates the position of the mount in relation to that of the sky model. With an azimuthal mount, a single object is sufficient, but the accuracy for other objects is then low. With an equatorial mount two objects are sufficient, but here it is recommended to control a third object in the same meridian .
Now the user can search for the desired object to be observed from the lists of the mount or enter its position directly. The motors of the mount now move the telescope to the calculated position of the object. If this is then not exactly in the center of the field of view , the mount can be informed of a correction position and the objects in the area are then approached more precisely. This is often necessary for very faint objects that are no longer visible to the naked eye. A bright star that is as close as possible is used as a basis for calculation and then the (invisible) object is approached and made visible with a long-term photographic exposure.
The mount immediately begins to compensate for the rotation of the earth by means of the motorized counter-movement after the journey to the respective object. However, a gear backlash can delay this tracking by up to a minute (the so-called backlash ). It is therefore possible to program a bridging run of the motors that minimizes this delay. When moving from west to east, some mounts deliberately go beyond the target and approach the object from the east at a reduced speed; So they move again with the direction of travel west and reduce the speed more and more until the target object is reached. With this method, the gear backlash is already balanced.
Some models also have an automatic celestial alignment method built in. It corresponds to the classic Scheiner method (also called "Einscheinern") and calculates the rotation matrix between instrument and celestial coordinates from mostly 2 set stars . With precise servomotors, this increases the tracking accuracy considerably, since an exact adjustment to the celestial pole is required for astrophotographic recordings .
An even higher accuracy is only achieved through a tracking correction, e.g. B. with a guide telescope . This can also be done automatically using an additional computer; Goto devices often have a so-called autoguider input for this purpose . If this is not available, it can also be corrected via the data input with restrictions.