5.11 Grating instrumental profile and resolution

The document `Grating instrumental profile and resolution', by M.E. van den Ancker, R. Voors & K. Leech, 1 July 1997, discusses investigations of the grating instrumental profile and describes possible instrumental effects on emission lines profiles observed with SWS.

Ground-based tests showed the SWS Instrumental Profiles (IPs) to have a Gaussian shape up to accuracies of a few percent. To verify that this would also be the case in-flight, several objects showing strong, narrow, emission lines (mostly Planetary Nebulae) were observed in ISO's performance verification (PV) phase to investigate the influence of parameters such as the position of the source in the aperture, the size of the target and the SWS AOT band.

The conclusions (based on AOT 2 & 6 observations, should be applicable to AOT 7 SW grating observations but are not applicable to AOT 1 observations) are:

• The SWS grating instrumental profile is very close to Gaussian.

• The only systematic differences from a Gaussian profile are a slight ``shoulder'' on the blue side of the base of the profile and a depression on the red side as compared to a purely Gaussian profile. The only parameter this seems to depend on is the position of the source in the slit.

• The grating resolving power of SWS for all investigated sources is better than expected for extended sources at all wavelengths. This could either mean that none of line-emitting regions in any of the sources fully filled the SWS aperture, or that the SWS resolving power for extended sources is better than expected.

• In AOT bands 2 to 4 the resolving power for point-like sources is slightly better than predicted from the ground.