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There are two stages to the mapping between the mask focal plane coordiante system and the detector coordinates. The first is the linear mapping of pixels at the detector to mm at the mask, and the second is to account for the small optical distortion introduced into the detector coordinates by the LDSS-2 optics.
Although generally small, field distortion is can be used for use with PREBOX and with mask manufacture from direct images. It is not used in the extraction routines which have algorithms for following distortions empirically.
The important routines are:
If the /STORE qualifier is present then it is assumed that the fit defines the mapping of the mask coordinates onto the detector and resets the values of the relevant internal parameters accordingly. This allows the subsequent use of the TRANSFORM command.
This routine can be used to define the distortion of a matrix mask image (which has been analysed by, say, MAP), compared to the expected image (see below). The fit is currently stored in an external transformation file. It can be subsequently used by the routine DISTORT.
The transformation is defined by the internal variables ROTATION, XPIX, YPIX, FRED, XCENTRE and YCENTRE.
If these are available then they will be used by DISTORT in preference to the raw spline coefficients, since it saves on CPU time.