Photometry may always be carried out using the CCD imaging facilities
described in Chapter , 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 . 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
.
The main characteristics of the PP and MPF instruments are summarised
in Table .
Table: Comparison of photometers