Mid-infrared spectrophotometry (AOT P40) is considered to be one of the best understood and calibrated mode of ISOPHOT. However, a number of systematic instrumental artifact have not been analysed so far.

New beam profiles

In a dedicated project first we re-determined the beam profiles of each of the 128 individual pixels of the ISOPHOT-S subinstrument. The results provide improvements in two fields:

(i) we get a better surface brightness calibration not affected by a spectral artifact around 8.6 micron characteristic of the earlier data products; and

(ii) we can determine more reliably the spectra belonging to sources observed with a spatial offset from the detector's centre.

The amplitude of the latter correction can be as high as 50% of the total signal.

Correction for memory due to preceeding bright source observation

The second effect we investigated is the appearance of artificial spectral features in some spectra due to memory effects from a preceeding bright observation. In order to identify affected spectra we studied the short (32 sec) dark measurements performed before each P40 observation. Reducing a large sample of these measurements we determined their typical signal levels and worked out criteria to find signatures of the memory effect. We found that subtracting the signal of the short dark from the main sky observation can correct for the memory effect.

Background subtraction

There are a number of spectrophotometric observations where the observer hasn't performed a separate sky background measurement. We created a simple model which is able to predict the spectrum of the sky background towards a given direction and on a given date using COBE/DIRBE photometry as input.

Empirical photometric correction

The routines were applied on 58 observations of standard stars, and comparing the results with the stars' model predictions empirical correction curves were derived.