Acquisition and Guiding at Cassegrain

 

• Acquisition. A Westinghouse extended S20 TV camera views a 1.81.8square of the 20diameter unvignetted Cassegrain Field (on or off axis). The TV display includes labels giving orientation and scale. It is possible to store TV images, which is useful during good seeing for recording high spatial resolution images of the target. A filter wheel containing up to 4 circular filters is loaded. The filters currently available are green(BG28), blue(BG38), red(RG630) and clear (UBK7). The effective magnitude limit for the acquisition camera is 21.5 magnitude in integrating mode.
• Slit Viewing. A simple movement of the flip mirror allows viewing of the light reflected from the slit jaws using the acquisition TV. This allows visual confirmation of the positioning of the the target on the entrance aperture of the instrument, as well as an estimate of the seeing profile.
• Autoguider. The autoguider allows offset guiding using stars anywhere in a 90 square arcmin field (see Figure for available area). Guide stars should have magnitudes in the range 714, and should ideally be selected in advance of the observing run. Care must be taken to avoid selecting a guide star so positioned in the field that the autoguider probe obstructs the on-axis star-beam.

  Alternatively, an overlay showing the IDS slit and the area within which guide stars may be selected can be superposed on the TV image of the finder field. A guide star is then selected visually and the A&G unit autoguider probe directed to that position. There is a filter wheel in the autoguider beam allowing up to four circular filters green(BG28), blue(BG38), red(RG630) and clear (UBK7).