Cosmic rays will obliterate ~20 pixels per second per CCD. It is imperative that two or more images be obtained at each pointing position, if these artifacts are to be removed from the data. The default action by the Phase II proposal processing software is to split exposures longer than 600s into two near equal parts, so as to allow removal of the cosmic ray tracks. The CR-SPLIT and CR-TOLERANCE optional parameters on the Phase II proposal allow observers to adjust this behavior. CR-SPLIT can be set to either DEF (default), NO, or a numeric value (0.0 to 1.0) giving the fraction of the total exposure allotted to the first sub-exposure of the pair. CR-TOLERANCE indicates the spread allowed in dividing the exposure, as a fraction of the total exposure time. For example, the default CR-TOLERANCE=0.2 allows the first sub-exposure to range from 0.3 to 0.7 of the total exposure. Setting CR-TOLERANCE=0 will force equal-length sub-exposures.
The required degree of cosmic-ray avoidance will depend on the science goals of the proposal; observations of a single small target will usually suffer much less impact from cosmic rays than programs needing very "clean" data over a large area. Table 7.3 gives very rough recommendations for the number of sub-exposures for a given total exposure time. Note that splitting into many sub-exposures introduces additional overhead time and will increase the noise for "read noise" limited exposures (usually exposures in UV or narrow band filters), and hence one should not use more sub-exposures than are truly required by the science goals.
Table 7.3: Recommended Exposure Splittings.