Reducing an Object, Sky or Arc

To reduce an OBJECT, SKY or ARC, Portable--CGS4DR reads the raw data from store in $idir/ and FITS header information from store in $odir/, applies a bad pixel mask, optionally subtracts a reduced BIAS frame, linearises the data (if required), optionally subtracts a reduced DARK, optionally divides by a flat field and wavelength calibrates the data (if required), and writes the reduced data parameters into the index file and stores the reduced OBJECT, SKY or ARC in $rodir/ as shown in figure .

Note that if the bad pixel mask is changed between the calibration frame and the astronomical data, the resultant quality array will contain the logical AND of the two masks until the calibration data is re-reduced with the new mask and the RED4 task is re-initialised to force it to memory map the new calibration frame. This has caused some confusion in the past.

  
Figure: Reducing an OBJECT, SKY or ARC