The prime focus autoguider is a Peltier cooled frame transfer CCD, similar to the units at the Cassegrain and UES focii; see Section 2.2 for more details. The CCD must be flushed with Nitrogen when the cooler is in operation to prevent the formation of condensation and ice crystals on the CCD surface. If this is not done the CCD could be badly damaged. There is an automatic sensor to detect loss of nitrogen flow, but if in any doubt the user should bring the telescope to access park and check the flowmeter on the PFIP instrument platform.
The limiting magnitude of the autoguider, for guiding at 1 Hz, is V=15-17, depending on the seeing and sky brightness. Guide stars (down to V 14.5) can be found automatically using the ICL GSSEARCH procedure, which searches the on-line HST Guide Star Catalogue for the brightest star in the autoguider field, and moves the guide probe to that position (see Section 4.6 for more details). When the autoguider is guiding successfully, the observer or TO then ``closes the loop" by telling the autoguider to send its errors to the telescope control system. When the telescope is servoing on these errors, the telescope tracking errors are reduced to the 0.1 arcsec level.
The prime focus autoguider is provided with a filter wheel with six filter positions. The available filters are: CLEAR (UBK7), EMPTY (no filter; different focus), OPAQUE (blanking disk), B (BG 28), V (BG 38), and I (RG 630). These filters do not give a standard photometric system and the Johnson letters are provided for guidance only. The autoguider can be focussed independently with the PGDFOC command to compensate for different filter thicknesses. The PSETGDFOC command may be used to do this focussing automatically, once the autoguider filter names and focus offsets have been set up using the PCHANGE command (see Section 6.1 for more information).