Cosmic Ray Rejection in STIS CCD Images

We describe the method of noise model-based cosmic ray (CR) rejection in multiple, registered images, and discuss its implementation in the context of the STIS calibration pipeline. We focus on the method by which the various contributions to the uncertainty in the final, calibrated image should be combined. Unfortunately, the current design of the CR rejection module, calstis2, makes it difficult to apply the rejection threshold(s) correctly while also combining the uncertainties appropriately. We offer a solution to the problem, in the form of characterizing the uncertainties of the various calibration reference files with keywords in the science image header. We also characterize the properties of CRs in STIS CCD images, and describe the means by which the CR rejection parameters are optimized for pipeline processing. We conclude with a summary of recommendations for improving the calstis2 module and/or the corresponding off-line task, ocrreject.