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CCDPHOT: CCD Photometry Made Fast and Easy (1Sep92)
(from KPNO, NOAO Newsletter No. 31, 1 September 1992)

CCD photometry has been practiced at the 0.9-m for the better part of a
decade. However, even the observation of a single standard star may result
in ten million numbers, rather than the one number (the magnitude!) that
the astronomer wants.

In order to make CCD photometry faster and more accessible, the IRAF
group has developed in collaboration with Kitt Peak staff astronomers a
software package called CCDPHOT, which is now available for general use
at the 0.9-m telescope.  The CCDPHOT package combines telescope and
instrument control, CCD data acquisition, star list management, CCD
reductions, and photometric reductions in an efficient manner so that
CCDs can be used for all-sky photometry of single stars, or two- or
three-star differential photometry. (A brief overview of the software
can be found in a paper presented by Tody and Davis in Astronomical
Data Analysis Software and Systems I, ed. D.M. Worrall, C.
Biemesderfer, and J. Barnes, p. 484).  CCDPHOT allows the observer to
define a filter system, select the size and position of the region of
the chip to be read out, set various CCD and photometric reduction
options, program an observing sequence, and see instrumental magnitudes
and quick-look reduced magnitudes in "real-time." The program keeps a
detailed log of these operations which appears both on the terminal and
in a file. There is also a summary of the results which is recorded in
a separate file. These files can be edited, printed out on the site,
and sent to a home institution by e-mail.

The current instrumental configuration includes the T2KA CCD normally
used at the 0.9-m telescope, a small, fast, and accurate shutter, and
special filter holders to install 2 x 2 filters in the large filter
wheel used at the 0.9-m. The T2KA CCD provides relatively good UV
response, and the small shutter allows precise photometry of bright
standard stars with short integration times. A corner of the CCD is
used in order to obtain the shortest possible readout time and minimize
system overhead. The maximum size of the subregion read out and
processed by CCDPHOT, currently set at 512 x 512 pixels, is dictated
both by the requirement for real-time processing and by the need for a
small, fast shutter, not the software. The duty-time for readout and
changing filters has been kept short, but has not yet been tested in
this final instrumental configuration.

The CCDPHOT package will be tested with real observations during three
observing runs in December. We hope that the improved sky subtraction
available with CCDPHOT will allow more photometric observations to be
carried out during grey or bright time. With the 0.9-m telescope and
this chip, users can expect to achieve 1% statistics on a 15th mag star
(U=B=V=R=I=15) in 45 sec each in U and I, and 15 sec each in B, V, and
R. For a moon phase of 10 days, these times increase only slightly: 60
sec in U and I, and 25 sec in B, V, and R.  Brighter stars will of
course be affected less by moon light, and there is no increase in
exposure times for stars brighter than 13th even in full moon.

The advantages of CCDPHOT over the standard photoelectric photometer is
the high DQE of the CCD and the fact that the sky is measured
simultaneously. In addition, the ability to use the system as a two- or
three-star photometer means it is possible to do comparative photometry
relative to other stars in the field under marginal or non-photometric
conditions. All stars must fit within the 512 x 512 pixel region
mentioned above; this corresponds to a region 5.8 arcmin on a side.

CCDPHOT is available for proposals for observations in the spring
semester, and a user's guide will be available shortly.

Caty Pilachowski, Tom Kinman, Lindsey Davis

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