IRAS Explanatory Supplement
VII. Analysis of Processing
D. Photometric Accuracy
D.1 Absolute Calibration Uncertainty Checks
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There are a number of checks which have been made to ensure that the absolute calibration, which was established with respect to the pointed observations, was carried consistently through the survey processing. Three specific checks include a comparison of the IRAS 12 and 25 µm observations of selected stars with ground-based observations of the same stars; a comparison of the quoted flux densities for the secondary standard NGC 6543 with those inserted into the processing; and a comparison of the flux density ratios between different wavelength bands with the relations assumed in Eq. (VI.C.2).
Table VII.D.1 lists the flux densities quoted in the catalog of a subset of bright stars observed by Rieke et al. (1984) and by Tokunaga (1984) at 10 and 20 µm. The selected stars showed no obvious excess at the IRAS wavelengths, and thus the IRAS flux densities were color-corrected assuming their input energy distributions followed a 10,000 K blackbody. The flux densities are represented as magnitudes following the formulation of Eq. (VI.C.1). As in Section VI.D, for the purposes of easier comparison, the magnitudes of the ground-based, observations were adjusted in their zero points to correspond to the IRAS magnitude and color conventions. It is seen that the agreement between the IRAS measurements and the ground-based measurements is excellent.
The second check of the photometry, that of comparing the quoted catalog values of the flux of NGC 6543 with those which were inserted in the processing is important in order to verify that no systematic errors were introduced in the complex processing procedure. In fact, the ratios of the quoted catalog flux densities to those entered in the processing are: 1.02 ± 0.02, 1.04 ± 0.01, 1.01 ± 0.01, and 0.97 ± 0.01 in the four wavelength bands. The quoted flux densities were the result of 18 hours-confirmed measurements of the source and the uncertainties quoted above are in the mean value derived from these observations. Thus the processing did, in fact, introduce some systematic bias into the calibration procedure, but this bias is less than 5%. The origin of this bias is not understood.
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Figure VII.D.1 The 12 and 25 µm flux densities for cataloged sources are plotted. The line shows photospheric models
(see Section VI.C). larger largest |
The final check on the photometric accuracy of the IRAS calibration is provided by Figs. VII.D.1, D.2 and D.3. In these figures, the 25, 60 and 100 µm flux densities are plotted versus the 12, 25 and 60 µm flux densities for all point sources in the IRAS survey at Galactic latitudes |b| > 50° each of the plots, the locus followed by hot stars is apparent as the top envelope of the plots. At the longer wavelengths the regions defining these stars becomes more spread out because of the presence of cooler envelopes around many of the stars. The color relations defined by Eq. (VI.C.2) and
(VII.D.1)
are included on the figures. Equation (VII.D.1)
is based on the solar flux densities discussed in
Section VI.C. The color relationships
are seen to agree with the observed colors of the presumed hot
stars, thus confirming the validity of the calibration and processing
procedures at the 10% level.
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Figure VII.D.2 The 25 and 60 µm densities for cataloged
sources are plotted. The line shows Photospheric models
(see Section VI.C). larger largest |
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Figure VII.D.3 The 60 and 100 µm flux densities for cataloged
sources are plotted. The line shows photospheric models
(see Section VI.C). larger largest |
| Star | IRAS mag1 | 12 µm Band  T2 | R3 | IRAS mag | 25 µm Band T | R |
|---|---|---|---|---|---|---|
 -And | -2.11 | -0.09 | -- | -2.14 | -0.10 | -- |
 -Ari | -0.70 | -- | +0.08 | -0.83 | -- | -0.03 |
| -Tau | -3.08 | -0.07 | -0.07 | -3.02 | +0.02 | +0.02 |
| -Aur | -1.90 | +0.02 | +0.02 | -1.95 | -0.07 | -0.02 |
| -CMa | -1.36 | +0.04 | -- | -1.38 | -0.07 | -- |
| -CMi | -0.71 | +0.03 | +0.01 | -0.72 | -0.04 | -0.01 |
| -Gem | -1.21 | +0.01 | +0.00 | -1.19 | -0.03 | +0.03 |
| -Hya | -1.46 | -- | -0.04 | -1.37 | -- | +0.07 |
| µ UMa | -0.98 | +0.03 | -- | -1.07 | -0.04 | -- |
| -Boo | -3.22 | -0.07 | -0.07 | -3.09 | -0.01 | +0.07 |
 -Dra | -1.44 | -- | +0.04 | -1.50 | -- | +0.81 |
| -Aql | -0.68 | +0.08 | +0.07 | -0.81 | -0.04 | -0.04 |
| Avg Population = | -- | -0.00 | +0.00 | -- | -0.04 | +0.01 |
 = | -- | 0.06 | 0.06 | -- | 0.04 | 0.04 |
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1 IRAS mag is the magnitude obtained from the survey using
(Eq. VI.C.2). 2 T is the diference between the survey magnitudes and those of
Rieke et al. (1984).3 R is the difference between the survey magnitudes and those of
Rieke et al. (1984).
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