IRAS Explanatory Supplement
VI. Flux Reconstruction and Calibration
D. Comparison of IRAS Observation with Ground Based Observations
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In Table VI.D.1 a comparison is given between the IRAS measurements of selected stars made in the pointed mode and the results of ground based measurements by Tokunaga (1984) and Rieke et al. (1984). At 12 µm, the IRAS measurements agree with the ground based magnitudes within 1%. The difference is largely due to
-Dra.
| [12 µm] | [25 µm] | |||||
|---|---|---|---|---|---|---|
| IRAS |  Tmag |
R mag |
IRAS | T mag |
R mag |
|
 -Tau |
-3.00 | -0.01 | -0.01 | -2.97 | +0.07 | +0.07 |
| -Aur |
-1.91 | -0.01 | -0.01 | -1.91 | -0.03 | -0.02 |
| -CMa |
-1.36 | -0.04 | -- | -1.32 | -0.01 | -- |
| -CMi |
-0.74 | -0.00 | -0.02 | -0.72 | -0.04 | -0.01 |
 -Gem |
-1.20 | -0.02 | -0.01 | -1.18 | -0.02 | -0.04 |
| -Boo |
-3.15 | -0.00 | -0.00 | -3.10 | -0.02 | +0.06 |
| -Dra |
-1.43 | -- | -0.05 | -1.47 | -- | +0.04 |
| -Lyr |
0.02 | 0.00 | +0.00 | -0.18 | +0.23 | +0.23 |
If
Lyr is omitted, the average difference (and the uncertainty in the mean)
between the IRAS magnitudes and those of Tokunaga
(1984) at 25 µm is -0.01 ± 0.02 while the difference
between the IRAS magnitudes and those of Rieke
et al. (1984) is 0.04 ± 0.0l. The 0.02 mag difference in zero
point between the IRAS magnitudes and those of
Tokunaga
has been incorporated in these differences as well as the difference of
0.03 mag assumed in the 12 to 25 µm color. A corollary of this good
agreement is that the excess in
-Lyr
does not start until longward of the long wavelength cutoff of the filters
used in the ground based observations.
 |
|
Figure VI.D.1 Comparison of measurements of Uranus at infrared wavelengths
with those of IRAS. The observations have been normalized to a distance
of 17.75 A.U. larger largest |
The most accurately calibrated measurements to compare with the IRAS 60 and 100 µm observations are those of the planets Uranus and Neptune by Hildebrand et at. (1984) using NASA's Kuiper Airborne Observatory (KAO). Hildebrand et al.'s measurements of Uranus are compared to observations taken early in the IRAS mission in Figure VI.D.1. It is seen that the flux densities obtained by IRAS are ~ 20% lower than those obtained by Hildebrand et al. The cause of the discrepancy is not understood at this time.
It should be noted that the temperature of Uranus is ~ 60 K. The planet thus has an energy distribution quite different from that used in the calibration procedure. Although a short wavelength leak in the 60 µm filters as large or larger than 20% would account for this discrepancy, the good fit of the asteroid and stellar models at 25 and 60 µm and the near constant stellar colors with effective temperature appear to rule out a short wavelength leak of this magnitude. It should be noted, however, that all the measurements from the KAO derived their absolute calibration from observations of Mars and a single model thereof. Thus the internal agreement of the KAO observations at different wavelengths does not reflect independent calibrations. An analysis of the final IRAS survey flux densities, as contrasted to those obtained from the pointed observations, is given in Chapter VII.
T
and R
are the differences between the IRAS [12 µm] and [25 µm]
magnitudes and the corresponding values from Tokunaga
(T)
and Rieke et al. (R).
Differences are in the sense =
[IRAS] - [ground-based]. In displaying these differences, the
10.1 µm
magnitudes of Tokunaga have been adjusted by 0.02 mag to account for a
different zero point convention, and the 20.1 and 25 µm magnitudes
of Tokunaga and Rieke
et al. have been similarly adjusted by 0.05 and 0.03 mag to
agree with Eq. (VI.C.2).
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