Acquisition and guider unit for the f/8.06 focus

The A&G unit is part of the Wide-Field Photographic Camera (WFC), and can only be used with that instrument. See the ``1-m camera users manual'' (La Palma Users' Manual V) for more details.

The following facilities are provided:

• Acquisition. There is no acquisition TV camera for viewing the field direct. Instead, a guiding probe views a quadrant of the field about 0.6 square degrees in size (the area of the sky recorded on the plate is approximately 1.77 square degrees). The quadrant sampled is determined by the orientation of the instrument turntable. The probe is mounted on (x,y) slides which have a maximum travel of 120 mm. Each axis is encoded in units corresponding to a linear motion of 0.1 mm (2.6 arcsec). For visual acquisition and guidance an eyepiece with double crosswires and red illumination is available to view the field through the guiding probe. The eyepiece can be rotated with a gooseneck to different positions for ease of viewing. It is also possible to view the full 90 arcmin field through a field eyepiece which can be mounted in place of the plate-holder.

• Autoguider. Once a suitable guide star has been acquired, its image can be directed onto the autoguider by means of a movable prism. The autoguider is equipped with two filter wheels, each having three positions plus clear, for colour (U, J, F) and neutral density (ND = 0.5, 1.0, 2.0) filters respectively. The detector is an FW-130 image dissector with an S-20 photocathode.

  It is possible to find guide stars at the telescope using an eyepiece as described above. However, this is a very inefficient use of observing time, and probe positions should therefore always be computed in advance, preferably with GSS.

• Comparison lamps. A 25 step spot-sensitometer is used for calibration purposes. During an exposure light is projected continuously through the calibration wedge and the filter onto one corner of the plate. A series of apertures is available to optimize the intensity of the calibration source according to the exposure time and plate/filter combination in use. Each plate is identified by projecting onto an unexposed corner the information carried in the top left-hand corner of the corresponding observing card, consisting of: (i) telescope identification; (ii) plate number; (iii) plate--centre coordinates.

• Filters. The following broad-band filters are available: UG1, GG 385, GG 395, GG 455, GG 495, RG 630 and RG 715. The filters are 250 200 4 mm and are coated with single-layer broad-band anti-reflection coating. Relevant transmission curves, supplied by the manufacturers for uncoated filters, are collected in Appendix . Additionally, sub-frame assemblies are provided to hold 100 mm 100 mm interference filters. Two filters centred at 5007 Å and 6563 Å are currently available.