How Does Portable--CGS4DR Select Calibration Frames?

The files of type BIAS, DARK, FLAT, CALIBR and STANDARD are generically termed calibration frames[2]In this document, I have tried to use the lowercase calibrate or calibration to refer to the technique or a generic frame and the uppercase CALIBR to refer to a specific wavelength calibration observation.. When such frames are required by the software to reduce another observation, a search is made on the index file of already reduced observations.

Normally, the user lets the software decide which calibration frames to use. In general, it is true to say that the nearest observation in time that matches the instrument configuration of the observed object is selected and such frames are memory mapped to prevent unnecessary processing thus speeding up the reduction considerably.

If a specific observation is chosen, the software will use that observation irrespective of any criteria mis-match. In verbose mode the software will report any mis-matched criteria to the user but otherwise the user may be unaware that the frame is unsuitable. Indeed, if a user wants to know why certain calibration frames are being rejected, setting verbose output to TRUE is the only way of obtaining such information.

  
Table: Calibration Frame Matching Criteria

There are two types of calibration frame checking within Portable--CGS4DR. Compulsory checks are hard-wired into the code and consist of basic integrity checks such as detector size and so forth. Optional checks are defined only for oversampled frames and are specified by the flat_match, calib_match and standard_match parameters in the task interface file. In both cases, these criteria are not configurable by the end user but are, for completeness, shown in table .

These matches are performed by evaluating and equating various FITS items pertinent to each observation. Note that the configuration index in the table is a time-stamp referring to when the grating was last moved and as such provides the strictest criteria.