Derivation of a complete set of in-orbit RSRFs is needed to obtain an optimal flux calibration accuracy. However, this calibration is still ongoing. Until this has been done, the RSRFs measured in the lab have to be used for most bands. For bands 1a and 2a, a correction of the overall shape was applied to compensate for the leakage problem previously mentioned. This correction was done by polynomial fitting of in-orbit and lab data and multiplying the lab data with the ratio of the two fits.
For OLP V6.1 the Cal-G 25_x files containing the RSRF's were updated leading to a significantly improved calibration of the spectra. The few problems found in the new calibration files were, however, almost always present in the previous set of RSRF's.
These problems were:
are most likely due to responsivity
correction - see section 6.6.1.
for some
sources - see section 5.13.
If the dark current subtraction was not correct, and therefore incorrect offset correction applied, the Derive-AAR respcal step (which is a gain correction) can introduce incorrect shapes of the continuum or false features.
A way to check if the dark current subtraction has been done correctly is to overplot the AAR spectrum after respcal with the RSRF and checking to see if any small residual RSRF features remain.
Examples are given in figures 6.2 and 6.3.
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The following section gives an overview of the RSRFs as seen in the lab versus the RSRFs as measured in orbit. These graphs can be used to determine in which regions the RSRF calibration is more uncertain. This is mostly at some band edges. They help us to understand inconsistencies as in figure 6.4.
After rebinning the spectrum shown in this figure, the continuum level in band 1e will be in agreement with band 2A. However, the line flux at 4.05 microns will be about 30% lower. One should keep in mind that we're talking about a continuum which is virtually zero - errors in a gain correction (like dividing by RSRF or the absolute flux conversion) will be almost invisible on the continuum, but very clear in the line fluxes.
This means one has to be careful when interpreting line fluxes of spectra with a zero-continuum: if the line is in a region where the lab RSRF is different from the in-orbit RSRF, the flux of the line is less accurate. Thus in the example shown in figure 6.4, the line flux in band 1e has to be believed.
It should be noted that (1) these problems are less likely to occur with spectra with a high continuum because the errors in the gain correction will be applied to the continuum as well (thus the line/cont ratio will be correct) and (2) the differences in line fluxes in the example do not represent the current uncertainties of the RSRF flux calibration in that region: this observation was an early PV observation with the AOT logic not fully optimised.
In detector band 3 large fringes are present. These fringes can shift with the position of the source in the slit. A small mispointing (within the specs of ISO) can shift the fringes over 1/4 of their period. Before measuring line fluxes of unresolved lines in this band, it has to be checked if the fringes in the SPD are in phase with the fringes in the RSRF. If not, the fluxes of unresolved lines in band 3 might be off by 25% since the relative size of the fringes is about 50%.
This also means line fluxes measured with fast AOT 1 observations that cannot resolve the fringes in band 3 are only reliable for less than 25%.
