ccdgeometry: Discussion of CCD coordinate/geometry keywords
Package: ccdred
Description
The ccdred package maintains and updates certain geometry information about the images. This geometry is described by four image header parameters which may be present. These are defined below by the parameter names used in the package. Note that these names may be different in the image header using the image header translation feature of the package.
- DATASEC
- The section of the image containing the CCD data. If absent the entire image is assumed to be data. Only the pixels within the data section are modified during processing. Therefore, there may be additional calibration or observation information in the image. If after processing, the data section is the entire image it is not recorded in the image header.
- CCDSEC
- The section of the CCD to corresponding to the data section. This refers to the physical format, columns and lines, of the detector. This is the coordinate system used during processing to relate calibration data to the image data; i.e. image data pixels are calibrated by calibration pixels at the same CCD coordinates regardless of image pixel coordinates. This allows recording only parts of the CCD during data taking and calibrating with calibration frames covering some or all of the CCD. The CCD section is maintained during trimming operations. Note that changing the format of the images by image operators outside of the ccdred package will invalidate this coordinate system. The size of the CCD section must agree with that of the data section. If a CCD section is absent then it defaults to the data section such that the first pixel of the data section has CCD coordinate (1,1).
- BIASSEC
- The section of the image containing prescan or overscan bias information. It consists of a strip perpendicular to the readout axis. There may be both a prescan and overscan but the package currently only uses one. This parameter may be overridden during processing by the parameter ccdproc.biassec. Only the part of the bias section along the readout is used and the length of the bias region is determined by the trim section. If one wants to limit the region of the bias strip used in the fit then the sample parameter should be used.
- TRIMSEC
- The section of the image extracted during processing when the trim operation is selected (ccdproc.trim). If absent when the trim operation is selected it defaults to the data section; i.e. the processed image consists only of the data section. This parameter may be overridden during processing by the parameter ccdproc.trimsec. After trimming this parameter, if present, is removed from the image header. The CCD section, data section, and bias section parameters are also modified by trimming.
The geometry is as follows. When a CCD image is recorded it consists of a data section corresponding to part or all of the CCD detector. Regions outside of the data section may contain additional information which are not affected except by trimming. Most commonly this consists of prescan and overscan bias data. When recording only part of the full CCD detector the package maintains information about that part and correctly applies calibrations for that part of the detector. Also any trimming operation updates the CCD coordinate information. If the images include the data section, bias section, trim section, and ccd section the processing may be performed entirely automatically.
The sections are specified using the notation [c1:c2,l1:l2] where c1 and c2 are the first and last columns and l1 and l2 are the first and last lines. Currently c1 and l1 must be less than c2 and l2 respectively and no subsampling is allowed. This may be added later.