There are some uncertainties in the results, associated with the intrinsic variability of hot pixels and the time span between darks. We have recently started a program to obtain dark frames daily, resulting in better warm pixel information. These darks are taken as a service to users, and they are not currently part of our delivered calibration files. Users should propagate this information by the appropriate method, which will depend on the specifics of the task. Figure 26.6 shows the impact of cosmic rays in an 800 second exposure with WFPC2.
Cosmic rays are noticeable even for very short exposures.
As a result of the undersampling of the WFPC2 PSF by the WF and PC pixels, it is very difficult to differentiate stars from cosmic rays using a single exposure. If multiple co-aligned images are available, cosmic rays can be removed simply and reliably by comparing the flux in the same pixel in different images, assuming that any differences well above the noise are positive deviations due to cosmic rays. If the images are shifted by an integral number of pixels, they can be realigned using a task such as imshift . As each CCD is oriented differently on the sky, this operation will need to be done on one group at a time, using the pixel shift appropriate for each CCD in turn. Another consequence of the undersampling of WFPC2 is that small pointing shifts will cause measurable differences even between images at the same nominal pointing.