Previous Article
Next Article
Table of Contents

CCDPhot - The KPNO CCD Photometer (1Sep95)
(from KPNO, NOAO Newsletter No. 43, September 1995)

The KPNO 0.9-m telescope is used most often for wide-field imaging
(23' field of view) with T2KA, a Tektronix 2048 x 2048 thinned CCD.
The optional 2 element corrector provides an extremely uniform
point-spread function across this field, so this telescope has
applications for crowded field photometry (e.g., in globular clusters)
as well as for studies of large, low surface brightness objects (e.g.,
galaxy halos, galaxy clusters).

The telescope has one alternative instrument capability---CCDPhot. The
CCDPhot system is a CCD-based photometer intended to replace the Mark
III photoelectric photometer that has, until recently, been available
on the 1.3-m telescope. CCDPhot consists of a thinned
back-side-illuminated Tektronix CCD (designated T5HA) with 512 x 512
27um pixels producing an image scale of 0.77"/pixel and a field of
view of 6.6' x 6.6' at the f/7.5 focus of the 0.9-m. The CCDPhot
software is an IRAF program designed to do multi-aperture,
multi-filter stellar photometry in real time. It performs all of the
functions of a conventional photoelectric photometer, using a CCD in
place of the photomultiplier, and software apertures in place of the
aperture wheel.

The CCD is read with a fast microcode, which when combined with a user
defined subregion (say, 128 x 128 pixels) where the program stars are
measured, results in a readout time of only a few seconds. CCDPhot
then subtracts the DC offset using the overscan region, subtracts a
user-derived bias frame, and divides the resultant image by a
user-derived flatfield exposure for the appropriate filter. CCDPhot
then reports an instrumental magnitude in each user-defined aperture,
subtracts the sky contribution, and writes the result into a text
file. In addition, CCDPhot has an option to save the CCD images to
disk for further analysis. The entire process, including the CCD
readout, image processing, and photometric measurement, requires
approximately 10 seconds to complete. The observer takes home only the
text file containing all of the relevant information needed to convert
the instrumental magnitudes to standard ones.

In the case of BVRI photometry, we achieve photometric precision of 1%
or better using simple linear color transformation terms for stars
over a B-V color range of 2 magnitudes. In the U band, however, a
linear color term yields a precision of only about 4% over a U-B color
range of 3 magnitudes. Some observers report obtaining much better
precision by restricting observations to a narrow color range, or by
using higher order color terms and more standards.

If you are primarily interested in high precision photometry of
individual stars, CCDPhot has a number of advantages over direct
imaging on the 0.9-m telescope. First, the full-well capacity of the
T5HA CCD is approximately 2.5 times greater than that of T2KA, allowing
the CCDPhot system to measure brighter stars than possible with direct
imaging. Secondly, the smaller CCD used with CCDPhot and the smaller
shutter mean that no shutter timing correction is needed for exposure
times as small as one second. Lastly and most importantly, in the case
of single star photometry, CCDPhot combines short readout times with
near real-time image processing and photometric measurement to
significantly enhance observing efficiency.

Those interested in reading about results obtained with CCDPhot should
refer to the paper by Sarajedini and Milone (1995, AJ, 109, 269). Any
questions about the CCDPhot system should be directed to Ata
Sarajedini (ata@noao.edu).

Ata Sarajedini, Lindsey Davis,
Ed Carder, Tom Kinman, George Jacoby

Previous Article
Next Article
Table of Contents