Principal Guider

The principal guide camera images an 105 arc-sec field of view at 0.10 arc-sec/pixel, re-binned 2x2 for 0.20 arc-sec/pixel. The detector is a 1024 x 2048 x 13 $\mu$m pixel EEV CCD-47 frame-transfer CCD, packaged with a thermoelectric cooler. The principal camera is used for object acquisition, and in conjunction with the Secondary Guider camera, is used for guiding and correction of field rotation. A pick-off mirror ahead of the f/11 focus samples the field of view approximately 12 arc-minutes from the optical axis and redirects the sample beam into the camera's re-imaging optics. The camera and feed optics are mounted so as to allow them to traverse a 45 degree sector of the field of view. The Principal Guider will move only in the angular coordinate so as to maintain focus for all positions, and will be tilted so as to be tangent to the focal surface at the position. The sampled area of $\sim$ 15 arc-min² is sufficient to find at least one R=19 mag star, even at the Galactic poles, for guiding at approximately 1 Hz.

The Principal Guider will include an infrared filter that will allow the guider to be used during considerable moonlight. This and other possible filters will be focus-compensated by a glass dummy filter when no filter is desired, so that the camera requires no refocusing mechanism after its original setup. (The focus of the telescope is held fixed to the focal plane where the apertures are installed.)