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The IPCS-II S-distortion correction is a rebinning operation performed by the DMS, and the parameters for the correction are determined by the DMS.
Set up the spectrograph and comparison lamp unit to give a narrow tungsten
spectrum with reasonably uniform illumination along the spectrum:--
Open the shutter carefully in OVERSCAN mode to check the count rate, and adjust the comparison filters, slit width and grating angle if necessary.
The s-distortion correction must be calculated at full (x8) resolution in
both X and Y directions, even if this resolution will not be used for the
observations. Run the ICL procedure:--
IPCSSDC
which will set up a full
resolution window 30 camera pixels wide centred on the spectrum. If, after
running this procedure the tungsten spectrum is not in the centre of the window
the value of the Y starting pixel of the CCD window must be changed, this can
be done with:--
SEND IPCS OBEY Y_CCD_WIN MOVE <ystart> 30
The existing s-distortion correction must be turned off by typing at the
DMS control window:--
0 ESD
At the ICL terminal take a fairly long exposure, it is important to have
a strong signal in the S-curve:--
IPCSOPEN
EXPOSENP IPCS 1000
After this exposure has finished, close the IPCS shutter with:--
IPCSCLOSE
then at the DMS control window type:--
CALC-SDC
This calculates the s-distortion correction, and takes about 80 seconds during
which time the DMS will not respond. After the correction has been calculated
it must be loaded and enabled on the DMS control window with:--
LOAD-SDC
1 ESD
S-distortion corrections can be stored on the DMS disk with:--
PUT-SDC-ARRAY
and recalled with:--
GET-SDC-ARRAY
After recalling an s-distortion correction it is necessary to load and enable it as described above. Unlike the first generation IPCS on the INT the s-distortion correction is a digital process, and no ``tweaking'' is necessary or possible.