The mask image can be used for fast extraction of orders from images taken at the same spectrograph configuration. This is done using MASKEXT. The SUbord controls which bits of the order are extracted into the output image. A SUb-order of 0 always extracts object and sky. SUb-orders of 1 and 2 can be used to extract object and sky separately. If the periscope is fitted, sub-order 2 corresponds to the first encountered part of the order and sub-order 1 corresponds to the second encountered part of the order (so if the tracked orders went from the bottom upwards the data values in the mask are monotonically decreasing as you go from bottom to top). If the periscope is not fitted, sub-order 1 always refers to object and sub-order 2 always refers to sky. This is confusing and will probably change!
$ MASKEXT
(IMage) name of image containing spectra [CONT] - OBJ
(MAsk) name of mask image [MASK] -
(MLow) Lowest order number to extract [68] -
(MHigh) Highest order number to extract [82] -
(SUBord) Sub-order to extract (0 => all) [2] - 0
(OUTput) Name of resulting set of spectra [ ] - OBJE