Is the Hale-Bopp [OI] Asymmetry an Instrumental Artifact?
The flatfield corrections made to the WHAM Hale-Bopp images are small, so there is no chance that the flatfielding process could induce such a large distortion. The point spread function is a potential source of systematic distortion. Below are four images of stars which measure the point spread function of WHAM. The WHAM point spread function changes as a function of telescope look direction since it is primarily due to distortions in the two flat mirrors that comprise the WHAM siderostat. As the mirrors rotate relative to each other, their distortions either reinforce (second image) or cancel (other images). The third image was taken with the telescope pointing within a few degrees of the March 5 WHAM observations. As you can see, the distortion in the point spread function is small compared to the distortion seen in the Hale-Bopp [OI] image (pixel size is the same in both cases). Click on image to download a postscript file. The next three images show fields with several stars. The
