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Anom-R2

Anom-R2: Cosmic-ray rate is within specifications

Description:
The rate of cosmic rays [per chip area per time] found in science images should be within the specifications. There is basically two types of things that mimic cosmic rays in the images: actual cosmic rays (at a rate of approx. 5 events cm^{-2} minute^{-1} when pointing to the zenith) and radioactivity from the camera itself (rate unknown, but the requirements say it should be below the actual cosmic ray rate). See also http://www.noao.edu/meetings/decam/media/DECam_Technical_specifications.pdf

Prerequisites:
Since DECam has ~1000 cm^2, 10 science images with DES exposure times should be enough to get sufficiently good statistics. We can safely assume that the cosmic ray rate is practically constant in time, so these need not follow a specific cadence. There is no need for astrometry or flat-fielding, but we need a valid bad pixel mask. The frames should be taken as close to the zenith as possible, since the expected rate follows a power-law dependence on cos theta, where theta is the zenith angle. http://snap.lbl.gov/ccdweb/groom.pdf

Procedures:
Take 10 dark frames with DES exposure times in all filters. After masking bad pixels, run SExtractor to get a catalog of what should only be cosmic rays. The shutter will not influence the rate much unless in very strange cases. Also run a piece of code that identifies CRs in sky images on images from the MiniSurvey.

Codes required:
  • SExtractor
  • cosmicfits [used at USM to identify CRs]

Verdict:
If event rate exceeds 10 cm^{-2} minute^{-1}, the requirement fails.

Consequences:
If requirement fails, radioactivity of camera components and other material in the dome should be re-examined. Also more detailed tests (more exposures so we can find accurate differences in rates between different chips) should follow in this case.