Figure 1: Distribution of POSS-E Internal Residuals Using Various Photometric Parameters.
Figure 1: PS 27 Kb
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PS reprint
J. Doggett, M. Postman, B. M. Lasker, M. Meakes
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218
The Digitized Sky Survey (DSS) is based on scans of an all-sky set of plates taken with the UK and Palomar Schmidt Telescopes. (See Lasker and McLean (1994) for more information about the DSS.) The initial distribution of the DSS included a set of astrometric calibrations. Here we present the current status of the photometric calibration of the DSS.
The DSS-CS software is implemented as an IDL subsystem layered on top of the Special Plate Analysis and Measurement program (SPAM; McLean 1994). SPAM contains tools and data structures designed to facilitate interactive image analysis and calibration from within IDL. The DSS-CS software supplements SPAM with the abilities to non-interactively drive calibration pipelines, and to archive and retrieve calibration data.
The reference star catalog used for calibrating the DSS is the the Guide Star Photometric Catalog (GSPC; Lasker et al. 1988). The GSPC is particularly well-suited because it is an all-sky photometric catalog whose sequences are located at the centers of the plates of the DSS. The thresholding algorithm of Lutz (1980) as implemented in SPAM is used to detect objects on the plate and compute image parameters for the objects including size, area, and integrated signal.
Three analyses of the photometric calibration of the DSS are presented here. The first explores which image parameter is best suited for performing the photometric calibrations. The second illustrates the consistency of the calibration function for the plates among a single photographic survey. The third examines the calibration of an individual plate. Only analyses of the E-bandpass plates of the Palomar Observatory Sky Survey (POSS-E) are presented here. The results from the analyses of plates from the other photographic surveys in the DSS are similar to that of the POSS-E plates.
Different image parameters may be used for deriving photometric calibrations. Trial calibration runs were performed to select the image parameter that yielded the smallest internal rms magnitude residuals.
Three image parameters which are computed by the Lutz algorithm were selected for these trials: integrated signal, ellipse area, and pixel area. The integrated signal is the sum of the counts above the first threshold in an object. The ellipse area is based on using the approximate major and minor axes of an object to compute the object's area. The pixel area is the number of pixels above the first threshold of an object.
Figure 1 shows the distributions of the internal residuals from the trial calibrations of the POSS-E plates for each of the image parameters. The modes of the distributions show that on average the integrated signal gives the smallest residuals.
Figure 1:
Distribution of POSS-E Internal Residuals Using Various
Photometric Parameters.
Figure 1: PS 27 Kb
The global calibration curve, log
(integrated intensity)
vs. magnitude, for 2941 GSPC stars from the POSS-E plates is
shown in Figure 2. This global calibration curve has an rms
scatter of about 0.5 magnitudes. Because the calibration curve is
well-behaved, it may be used to derive a single coarse calibration for
the entire POSS-E survey.
Figure 2:
Global Calibration for the POSS-E Plates.
Figure 2: PS 155 Kb
The calibration of the POSS-E plate XE 322 is show in
Figure 3. The GSPC stars used to calibrate XE 322 are
plotted with 
. The internal rms magnitude residual of a
second order polynomial fit (solid line) to the GSPC stars is
0.09 magnitudes. Also plotted are stars from the composite
photometric catalog of Mermilliod and Mermilliod (1994; +
symbols) which, unlike the GSPC stars, are distributed across the
whole plate. The external rms residual from the polynomial fit of
the Mermilliod and Mermilliod stars is 0.17 magnitudes.
Figure 3:
Photometric Calibration of the POSS-E Plate XE 322.
Figure 3: PS 40 Kb
Global calibration functions for each photographic survey of the DSS will be available by the Spring of 1996, and more precise plate-by-plate calibrations will be ready in the the Fall of 1996. Announcements will be posted on the DSS WWW home page (http://www-gsss.stsci.edu/img_cdrom/img_top.html) as the calibrations become available.
This work was supported in part by NASA grant NAG W-2166.
Lasker, B. M., Sturch, C. R., et al. 1988, ApJS, 68, 1
Lutz, R. 1980, Computer Journal, 23, No. 3, 262
McLean, B. J. 1994, private communication
Mermilliod, J.-C., & Mermilliod, M. 1994, Catalogue of Mean UBV Data on Stars, (Springer-Verlag New York, Inc.)