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The entries below provide a little background on some of the calibration issues with the IRS we've uncovered.
A comparison of spectra of bright and faint standard stars reveals that the shape of the LL spectrum depends on the brightness of the source. We calibrate using sources which have fluxes of ~300 mJy at 24 µm, and for sources with fluxes of ~50 mJy, the effect becomes significant (~5%). It is likely that an imperfect correction for non-linearities in LL produces this effect. Until we can improve the non-linearity correction for LL, it may be necessary to have separate spectral corrections for bright and faint sources if an accuracy better than 5% in continuum shape is desired.
Analysis of repeated low-resolution spectroscopy and peak-up photometry of multiple standards reveals that the system responsivity has decreased over time. As measured in LL, the decrease is slightly more than 5%. As measured in the Red PU array, the decrease is smaller, only about 2%. The simplest model is that the responsivity has been constant from Campaign P to 14, then changed, and has been constant from Campaign 15 to 29.
Beginning with Camp. 21, eta1 Dor became our main faint standard, replacing HR 7018 and HR 5467. Compared to these two other A dwarfs, though, eta1 Dor shows a weak excess at the longest wavelengths.
Last modified 9 May, 2006. © The IRS Team.