The Prime Focus Instrument Platform

The Prime Focus Instrument Platform (PFIP) is designed primarily to allow direct CCD imaging at prime focus. Facilities provided are:-

• A seven position filter wheel, each position being able to accomodate filters up to 125mm in diameter. This filter wheel can be driven remotely under computer control to change filters. The filters are held in cells which are attached to the periphery of the wheel mechanism by a 4 mm allen screw. There is a facility for reading a barcode on the filter cell, to allow automatic detection of which filter is in the beam. The filter wheel is shown in Figure 3.
• A standard CCD Autoguider head and its Peltier cooling mechanism.
• A guide probe containing a 45 pickoff prism, a transfer lens, and a flexible fibre light guide. The output end of the fibre light guide is reimaged onto the CCD autoguider by a further transfer lens. The focus adjustment of the output end of the fibre onto the autoguider is manual, and should be set correctly by technical staff and then not adjusted. The input end of the fibre and its transfer lens can be moved under computer control to effect focus adjustments; the guide star image must be correctly focussed onto the input end of the fibre bundle.
• A remotely operated six position filter wheel between the output end of the fibre bundle and the CCD autoguider. This filter wheel normally carries colour filters.
• A remotely driven, precisely encoded X-Y carriage which carries the guide probe. The carriage lies to one side of the optical axis, and has a travel of 110mm in the X direction (tangential at the centre of its travel) and 20mm in the Y direction (radial in the centre of the X travel. The field covered by the autoguider is 32 x 5.9 arcmin, although the field of view at any one time is limited by the size of the coherent bundle to 25 arcsec square. The autoguider field is almost entirely in the partially vignetted region of the corrector field, although the vignetting does not exceed 45%. Because of the vignetting, when selecting guide stars it is advisible to choose stars which give as high a value of the autoguider probe Y co-ordinate as possible, and as near a value of X to the centre of the range as possible.

The layout of the Prime Focus Instrument Platform is shown in Figure 2.

  
Figure 3: Plan view of the WHT Prime Focus Main Filter Wheel

EEV P88300 and Tek 1024 CCDs are offered at prime focus, and the field and pixel sizes that they give are listed in Table  1.

  
Table 1: CCD pixel and field sizes at the prime focus