DEFINE/MOS helps you to localize your objects and sky regions and by default works automatically. It averages (20) columns around the position (0 = center of frame) (in world-coordinates!). In the target frame the program will detect objects above the threshold ( , -0.04, see below) relative to the local background within the search window (5) and fits a gaussian to the spatial profile of any detected object. The threshold may be given in absolute (> 0.0) or relative (< 0.0) numbers. It may be advisable to do at least a rough sky subtraction ahead of this command to facilitate the detection of the objects. In this case you have to use an absolute threshold for the detection of the object spectra afterwards. One may also think about rebinning the object frame to constant wavelength steps because then the search could be done in the same wavelength region for all slitlets. The limits of the objects are defined at the position where the gaussian fit has reached the detection limit (0.001). A safety margin of 3 pixels is taken on both sides of each object where no sky is automatically defined (can be overridden manually later) and the remaining part of the slitlets is taken as sky region. The results are stored in .tbl (window) and can be displayed in the overlay channel display and/or the graphics window. If you are not satisfied with the results you can change the windows interactively. You may also choose the interactive mode from the very beginning with DEFINE/WIND. Then no automatic search is performed; instead you enter the the objects and sky regions for each slitlet by keyboard input. By default the sky region is defined as the complete slitlet.
The sky fit methods ( ) available for SKYFIT/MOS are a simple median along CCD columns within each slitlet (skymet=median) and a more appropriate polynomial fit along the columns (skymet=polynomial), respectively. These two methods use only rows marked as sky regions in the table .tbl (window) to fit the sky background. With skymet=nowindows, however, the table .tbl is ignored and the sky is determined as a simple median over the full slitlet. The limits of the slitlets are taken in this case from .tbl. This mode may be useful for a preliminary sky determination, before the object positions are known. If no sky regions are marked in some slitlet, the input frame is just copied to the sky frame for this slitlet. In this way, after sky subtraction, the slitlet contains only zeros thereby marking that the sky background is unknown for this slitlet. The keyword contains the order of the polynom fit or the width of the median filtering, respectively. If a polynom fit is performed the cosmics must be rejected. SKYFIT/MOS rejects (but not replaces) pixels that exceed a given limit before the fit is performed. Read out noise, gain and the detection limit (in units of ) must be given by keywords .