Spitzer observers and archival researchers will undoubtedly wish to plot measurements from multiple instruments on the same flux scale. While the calibration strategy for each instrument was derived independently (see the Instrument Handbooks for details), checks of the consistency of the absolute calibrations were coordinated by the SSC. The requirement was that the absolute calibrations be consistent to within 10%.
A separate cross-calibration observing program was not implemented for Spitzer. Instead, the individual instrumentís calibration programs included targets common to two or three instruments. These targets included both dedicated cross-calibrators, and objects selected from the individual teamsí lists. Cross- calibration observations were carried out during IOC/SV, and every month or two during nominal operations.
Sensitivity and saturation limits are critical constraints in the choice of cross-calibrator targets. Many traditional infrared calibrators were too bright for Spitzer instruments. For example, Alpha Boo was too bright for all bands except MIPS 160 microns. Planets and the best-understood asteroids are generally even brighter.
5.2 Stellar Cross-Calibrators
Dedicated stellar cross-calibrators were selected in the Continuous Viewing Zone (CVZ). The steeply-falling SEDs of most stellar photospheres in the infrared make it difficult to use just one type of star for cross-calibration. Hence, the stellar cross-calibrators were roughly divided into IRAC/IRS cross-calibrators and IRS/MIPS cross-calibrators.
The stars were chosen from the Hipparcos catalog. Stars of three spectral types were selected: A dwarfs, K giants, and solar-type stars. The K magnitudes of the stars range from about 4 for the K giants to 7 for the A dwarfs and G stars. In addition to these dedicated cross-calibrators, still fainter stars from the IRAC team's list were used for IRAC/IRS cross-calibration. Additional stars common to both IRS and MIPS, but not in the CVZ, were also used.
5.3 Red Cross-Calibrators
The absolute calibrations of all three instruments are mainly based on stars. Since the stars are blue in the infrared, cross-calibrators with much redder spectral energy distributions were observed as an additional check not covered by the stellar cross-calibrators.
Since planets and well-known asteroids were too bright for most Spitzer bands, galaxies with few infrared spectral features have been selected as red cross-calibrators for IRAC and MIPS. Type-1 (radio-quiet) AGNs lack strong features in the IRAC, IRS, and MIPS 24 micron bandpasses. Galaxies with a 24 micron flux density of a few hundred mJy were suitable. Mrk 279 and IRAS 23060+0505 were among the galaxies being observed for this purpose.