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Photometry may always be carried out using the CCD imaging facilities described in Chapter gif, rather than with one of the dedicated photometers. The sky subtraction with a CCD image is better than with the photometers, and it is possible to measure all the stars in the field at once, albeit only in one band. Bright stars will saturate a CCD, but this can be avoided by defocussing the telescope. There is a time overhead in clearing and reading a CCD chip but this can be reduced by windowing the chip so that only a reduced area is used. ADAM procedures exist to allow exposures to be made quasi-continuously. Reducing CCD photometry generally takes much longer than reducing the corresponding amount of material from a photometer but the available reduction packages are continually being improved. If the stars you want to observe are fainter than about twelfth magnitude, you should seriously consider using a CCD camera rather than one of the photometers. For photometry of ultra-high accuracy (0.1%) there are additional problems with CCD photometry, caused by undersampling and sub-pixel response variations, which need to be treated carefully.

Currently the only common-user photoelectric photometer supported on La Palma is the Peoples' Photometer (PP), and this is only supported on the JKT. It is described in section gif. The Multi-Purpose Fotometer (MPF) is not currently available as a common-user instrument, nevertheless it may be available by arrangement with the instrument provider (Dr. J. Tinbergen at Roden), and is described in section gif.

The main characteristics of the PP and MPF instruments are summarised in Table gif.

Table: Comparison of photometers

Tue Aug 15 16:42:46 BST 1995