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Calibration Access and Data Handbook

Procedure

The OM flatfield response is decomposed into two components, the small scale variation from subpixel to subpixel and the large scale variations with a scaling length above several CCD pixels. The overall flatfield is obtained by multiplying the two flatfield components. The flat field correction is applied to a science image by multiplication, thusthe inverse normalized sensitivity is returned by the CAL.
The large scale variations are mainly introduced by filter defects and contamination either on the filter or on the detector window. CAL_omLargeSenseVariation returns the flatfield component describing the large scale variations of the combination of detector plus specified filter. In case of the grism the zero and first order image are treated as different elements. Different extension of the CCF component LargeScaleSens are accessed for the various filter elements.

The coordinate system used in the CAL call is the PIXCOORD system. The image maps in the LrgScaleSens CCF components are sampled at a coarse resolution. The position (CRVAL1 and CRVAL2) and the address (CRPIX1 and CRPIX2) of the reference bin and the increment (CDELT1 and CDELT2) of the image array have to be interpreted when reading the extensions in the LrgScaleSens component. The position of the pixel (x,y) in an LrgScaleSens image extension corresponds to the position (CRVAL1+(x-CRPIX1)*CDELT1,CRVAL2+(y-CRPIX2)*CDELT2) in the PIXCOORD system.
The CAL has to interpolate the data to get from the coarser gridded map in the CCF file to a high resolution image required by the calling routine. Care must be taken at the edge of the FOV in the interpolation process. CAL_omLargeSenseVariation determines the large scale flat field response in the following way:

• determine filter setting
• read image array in Filter-FilterId extension of the LargeScaleSens CCF component
• read keywords AVERAGE, DAVERAGE and PIXERR and set them to the output variables ffavgDet(1:2) and ffavgPix respectively.
• extract area from image array as required by input parameters
  • interpret the positional keywords CRVALn, CRVALn and CDELTn
  • extract slightly enlarge area if required by interpolation
• interpolate extracted area
• extract output area from interpolated image