Tip-tilt Camera

This camera is normally to be preferred. It gives tip-tilt capability. The camera is on an x-y stage and views the field though a dichroic. This allows one to choose guide stars over a 5.6 arcmin field centered approximately on the slit of the IRS. The camera has an acquisition (panoramic) mode that permits viewing a field approximately 1 minute in diameter. The dichroic permits guiding on the science target. The stage is controlled by the telescope control program to permit precision offsetting of the telescope under control by the guider (follow on).

This camera can be operated in two modes:

Panoramic Mode:

Used for acquisition of objects and guide stars in the 5.6 arcmin field of the tip-tilt guider box. This greatly simplifies acquisition and guiding for objects that are visible optically. On the 4m under full moon one can usually see objects down to v$\approx$19 ?? under moderate seeing ( $\approx 1^{''}$) using long integration times.

Guiding/Fast-guiding (tip-tilt) Mode:

Used for guiding and pointspread function correction. Once the guide star is acquired, the guide star image is moved to the corner of the chip (this is accomplished by an automatic offset of the stage which carries the camera), a subraster image is sampled at a frequency (under operator control) of 10-50 Hz and the secondary is tilted to minimize the shift of the image centroid. As the mean tilt of the secondary increases, the telescope is guided to keep the secondary near zero mean tilt.