Mosaicing

Mosaicing is the name applied to the process by which images of astronomical targets are combined in such a way that the positions of the objects are matched up. This can also lead to a final image larger than the input images if you are, for example, mapping out the IR emission in an extended target. Different reduction packages have several different methods for doing this, but all basically rely on the user specifying the positions of the objects to be matched up from one image to another, and/or specifying the relative shifts between each image that has to be combined.

A series of commands in MIDAS are used to perform mosaicing, all included in the CCDRED context. CREATE/MOSAIC is used first to create a master frame including all the subframes. Alignment of the frames is done using ALIGN/MOSAIC, and background levels are matched using MATCH/MOSAIC and FIT/MOSAIC. Objects to be used in matching the subimages together are selected using the SHIFT/MOSAIC command. Overall parameters for the mosaicing routines can be examined using SHOW/CCD MO and set using SET/CCD MO. More extensive documentation on the mosaicing routines is supplied in the chapter on CCD reductions, Chapter 3.