2.3. Instrumental Profile

During ISO's performance verification phase, several objects showing strong, narrow, emission lines (mostly Planetary Nebulae) were observed 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 on the instrumental profile (IP). These measurements were based on AOT 2 and 6 observations, but should also be applicable to AOT 7 SW grating observation, and an example IP can be seen in figure 1. Preliminary results can be summarised as follows:

1

The SWS grating instrumental profile is almost Gaussian.

2

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. The only parameter this seems to depend on is the position of the source in the slit.

3

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.

4

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

5

The shape, central wavelength and the line strength can vary drastically as the source moves further away from the aperture center.

  
Figure 1: An example of a measured SWS line (crosses) for an unresolved source. The measured IP is compared to a Gaussian fit to the data (solid line), a Gaussian with the expected resolving power for a point source (lower dotted line) and a Gaussian with the expected resolving power for an extended source (upper dotted line).