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The ZIMLAT Telescope
General Requirements
The 1-meter Laser and Astrometric Telescope ZIMLAT, installed in 1997, should allow state-of-the-art laser ranging to satellites (SLR) and should also serve as astronomical telescope for the optical observation of positions and magnitudes of near-Earth objects, such as space debris, using digital (CCD) cameras.
This dual use of the system asked for a high complexity of the design. Some reductions in the performance, e.g., in the imaging quality or the density of the observations, had to be accepted.
The telescope, its additional equipment (camera tables, filter wheels, deflection mirrors, etc) as well as all components for laser ranging are remotely controlled by the station computer.
The applications request from the telescope a high accuracy in positioning and tracking:
Absolute: 1-2 arc seconds, important for the laser tracking with a narrow laser beam.
Relative: A few 1/10th of an arcsecond for "smooth" optical tracking of slow objects during the exposure time.
During daytime and dawn the system operates in SLR mode only. During nighttime the available observation time is shared between SLR and CCD using negotiated priorities. The switching between the modes is done under computer control and needs less than half a minute.
The goal is a fully automated combined SLR/CCD operation.
Astronomical telescope (CCD)
- High-precision tracking of
1. stationary objects
(geostationary satellites and space debris)2. slow-moving objects (e.g., minor planets)
3. fast-moving objects (low-orbiting satellites)
4. two tracking modes
- 0-1 arc minutes/second with exposure times of minutes
- 0-1 degrees/second with exposure times of a few 1/10th of a second
- High image resolution: about 1arc second per CCD pixel
- Derotation of the field of view according to various criteria.
- Fast switching between the tracking modes, i.e. several camera mounts with individual reduction optics
Satellite Laser Ranging (SLR)
- Ranging to all satellites included in the roster of the International Laser Ranging Service (ILRS) (altitudes between 300 and 23'000 km)
- Accuracy per single shot: A few millimeters to one centimeter
- Pointing accuracy and noise suppression sufficient for observations in daylight
- Fast switching between satellites (10-20 seconds)
- Remote control (e.g., from the university)
- Automated procedures up to fully automatic operation
- Two-color observations, i.e. additional use of the primary (infrared) wavelength of the laser
- 10 to 20 degrees minima elevation
- visual control of the tracking during night time with a wide-field camera