1.0 Overview of the Tip-Tilt Corrector in UnISIS

The tip-tilt correction requirements for UnISIS are more complex than for simple natural guidestar adaptive optics or active optics systems. The tip-tilt system operates in closed loop feedback (inner loop) and can be used in 2 distinct ways with other variations planned for the near future. All of the operating modes are controlled by a TI C30 DSP/ PC operating in conjunction with a camera controller. The first mode of operation is an focal surface mode, with either no high order corrections or in conjunction with the a laser guidestar. In this mode the tip-tilt signal is derived using a centroiding algorithm and a PID control loop. In the second mode, a pupil imaging mode, the wavefront is analyzed with a Shack-Hartmann lenslet array and the average x and y slopes are determined and a correction term derived using a PID controller.

One additional feature of the tip-tilt system is that in addition to the inner loop operation just described there is an outer loop control that is used to bump or reposition the telescope slightly should the tip-tilt control system be running to the edge of its stroke.

2.0 Focal Plane Tip-Tilt

This is the simplest mode of operation for the tip-tilt controller. A star is imaged onto a coronagraph mirror and the image is relayed with some magnification onto the fast readout CCD camera (Sciemeasure, Inc.). The CCD is readout at rates varying from a 800 frames per second to about 30 frames per second selected based on the brightness of the tip-tilt star.

The centroid of the star's position on the CCD in a 16x16 pixel area is determined and fed into a PID controller. The proportional gain and the exposure time are adjustable while the camer is running and locked on a star. It is anticipated that guidestars fainter than 15th magnitude (V-band) can be used. In conjunction with the laser guidestar providing the high-order correction the system will provide improved sky coverage compared to natural guidestar systems.

3.0 Pupil Plane Tip-Tilt

In this mode of operation, the star is focused on the coronagraph mirror, however, the reimaging optics reimage the telescope pupil onto a Shack-Hartmann lenslet array and the slope of the wavefront is determined by quadcells on the fast readout CCD camera (Sciemeasure, Inc.). The slopes are averaged in both the x and y directions and the overall slopes are forced to zero by the PID controller.

The wavefront is imaged over a 64x64 array of pixels, greatly reducing the magnitude limit that can be used with this variation. It is anticipated that the system will provide reasonable operation with guidestars down do about 10 magnitude (V-band). This magnitude limit is one of the inherent problems of using the natural guide star adaptive optics approach.

4.0 System Images

The following are some images of the tip-tilt system in place on the UnISIS optical bench (images will appear soon!).
 

Plate 1. The PI tip-tilt mirror.	Plate 2. The PI control electronics rack.
Plate 3. Closeup of coronagraph mirror system.	Plate 4. Computer/ DSP and Xinetics Deformable Mirror racks in control room.
Plate 5. The Scimeasure CCD and camera optical system.	Blank for the moment.

5.0 System Status

System is now operational and available for use.