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Mosaicing of Frames

In particular in the IR range of the spectrum but also in the optical the user may want to combine several reduced science frames into a mosaic. To support this, the CCDRED context contains five mosaicing commands.

In order to align or match a number of images the user first has to run the command CREATE/MOSAIC. This command creates a master frame containing all input frames to be aligned and intensity matched as sub rasters. The order in which the sub rasters are put in the mosaic image is determined by a number of keywords all starting with `MO_'. E.g. the keyword MO_CORN determines the origin of the mosaic. Apart from the mosaic image, CREATE/MOSAIC also creates a MIDAS table containing all relevant information about the subrasters in the mosaic image and the ways it was created.

Using the mosaic frame and the database table, one can now do the alignment or alignment and matching of the sub raster. The alignment can be done by the command ALIGN/MOSAIC, the matching by two commands: MATCH/MOSAIC and FIT/MOSAIC. To be successful these commands need additional information were the images have to be glued together. The commands ALIGN/MOSAIC and MATCH/MOSAIC accept that information in three different formats: a fixed shift, a table with the x and y shifts for each subraster, or a table of common objects. The command FIT/MOSAIC only needs the database table that should contain the shifts per subraster.

The command SHIFT/MOSAIC is created in order to support the selection of common objects in adjacent subrasters. This command can be used to create a MIDAS table containing these objects and that can be used as input for the commands ALIGN/MOSAIC and MATCH/MOSAIC.

The keyword setting for the mosaicing can be inspected by the command SHOW/CCD MO. Changes can be done by the SET/CCD command. Below follows an overview of the mosaicing keywords and their default settings. The help files of the five commands give the full details.

Table 3.10: CCD keywords for mosaicing
Keyword Options Default Description
MO_SEC coord. [<,<:>,>] section for statistics
MO_SUBT yes|no no subtraction option?
MO_CORN ll|lr|ul|ur ll type of function to fit
MO_DIREC row|column row add subraster row/column wise
MO_RAST yes|no no add in raster pattern
MO_OVER integer 1,1 space between adj. frames
MO_INTER interpolant lin interpolation method
MO_MNPX integer 0 minimal number of pixels
MO_TRIM integer 1,1,1,1 trim columns and rows
MO_NUL real NULL(2) value for rejections

next up previous contents
Next: Miscellaneous Up: CCD Reductions Previous: Bad Pixel Correction
Petra Nass