The resulting mask image can be used for fast extraction of orders from (in the case of the CCD `irot90-ed' and `irevy-d') raw images taken at the same spectrograph configuration as it. This is done using the `maskext' program. `maskext' needs to be told the range of order numbers that you want to extract and this determines the Y size of the extracted file (referred to as a collapsed échellogram) and its Y units.
The `sub-order' controls which bits of the order are extracted into the output image. A sub-order of 0 always extracts object and sky and 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!
ICL> maskext image=arc mask=mask mlow=68 mhigh=82 subord=0 output=arce
FIGARO A general data reduction system