ASTRON-2: precision stabilization system

In 1989-1991, right up to the collapse of the USSR, SKB Granit carried out work to create a precision guidance system for the large T-170 orbital telescope under the promising SPECTRUM-UV project, which is a natural continuation of the ASTRON project.

Currently, this telescope is being created at the NPO Lavochkin (Russia) and is the basic instrument of the international observatory WSO.

The main characteristics of the telescope are as follows:

• Ritchie-Chretien optical system,
• diameter of the main mirror 1.7 m,
• secondary mirror diameter 0.5 m,
• equivalent focal length 17 m,
• distance between mirrors 3.5 m.
• field of view 40 arcmin.

The telescope must provide exceptionally high image quality in the focal plane – close to diffraction in the center, and in the field no worse than 1 arcsec. In this regard, very high demands are placed on the precise guiding system of this telescope.

At SKB Granit, for this telescope, a version of a two-loop iterative system with an original structure was developed and tested. The image position in the system is measured using a star sensor in the focal plane, either directly or offset.
The executive element of the first low-frequency tracking circuit with a bandwidth of about 0.5 Hz is a movably mounted secondary mirror of the telescope (similar to the guiding system in the ASTRON telescope). The secondary mirror is tilted around the neutral point of the telescope.

The second, high-speed precision circuit had a bandwidth of up to 15-20 Hz and was built using an optical image motion compensator. This compensator is a flat diagonal mirror controlled by a biaxial piezoelectric drive.

Main characteristics of the compensator:

• mirror diameter 100 mm,
• maximum deviation angle for each axis ±3.5 arcmin,
• operating frequency range 0…25 Hz

When developing this system, maximum use was made of the experience accumulated during the design and long-term operation in orbit of the precision guidance system of the ASTRON space station.

The system was tested as part of a ground-based technological sample of the ASTRON telescope at SKB Granit. Below are the results of ground tests illustrating the operation of the developed system.

Oscillogram of the tracking error and the corresponding image movement in the focal plane of the telescope in the guiding mode with only the secondary mirror (ASTRON system). Tracking error – no more than ±0.3 arcsec.	Oscillogram of the tracking error of the developed iterative system and the corresponding image movement in the focal plane of the telescope. Tracking error no more than ±0.05 arcsec