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CCD imaging at auxiliary focus

A simple `no frills' adaptor has been built to mount one of the AAO CCD cameras with a manually-operated filter wheel at the Cassegrain auxiliary focus. Since it is possible to swap between the main and auxiliary foci at the push of a button (in ~20  seconds), this may be useful for programs requiring both spectroscopy and `quick look' imaging, and also enable the best use to be made of periods of photometric weather. Since the filter wheel is not remotely controlled, the present system is best suited to programs requiring photometry through only one filter (which can be any of the standard UBVRI 2 inch square filters). The filters need to be placed in the filter wheel before the CCD is mounted on the telescope, and cannot be changed without dismounting the entire CCD dewar. Thus observers should specify their filters at least one day in advance of their run.

With the THX CCD the scale is 0.128"  per pixel and the field size 1.31' , and with the RCA CCD the scale is 0.202"  per pixel and the field size 102" x 64". On-chip binning is available to match the pixel size to seeing conditions. The total efficiency is about 20% less than for the same chip at prime focus, due to the two extra mirrors in the light path. The autoguider cannot be used with this system because the chip is not at the nominal telescope focus.

The auxiliary focus CCD will not be mounted unless specifically requested by observers, and prospective users should first contact one of the AAO staff members to determine its availability. If there is sufficient demand, a more general-purpose instrument with remote control (and possibly a focal reducer to give a larger field) may be provided in the future.

Note also that there are currently only 2 CCD controllers. This means quick changes from RGO+FORS spectroscopy to auxiliary focus imaging are not possible.

Some information information on f/8 imaging is given in the AAO CCD Imaging Manual.



This Page Last updated: Mar 5, 1996, by Chris Tinney.