We have performed various tests to measure the image quality and quantify the distortion introduced by the bad collimator position. For each of these tests we provide results for the two different collimator positions. Position 1 is from 2 Aug 2002 - 19 Oct 2002. Position 2 is from 20 Oct to the present.
There is a pinhole mask in the ISAAC slit mask wheel which has several pinholes regularly spaced across the field of view. These are effectively point sources at the slit mask wheel. Images of the pinhole mask show the amount of distortion introduced by the collimator. By taking spectra of a continuum lamp or, at longer wavelength, the thermal background, through the pinhole mask we can look at the spatial profile of the spectra. Similar spectra of arc lamps give us a measure of the amount of line tilt (this only gives useful results for SW (0.98-2.5m) spectroscopy. In the L and M bands there are not enough arc lines.)