Mosaicing is the most complex part of infrared imaging data reduction.
Several crude levels of mosaicing are provided in the redimage
task. Discussion of full interactive mosaicing is deferred to
§7. To mosaic a dataset using the redimage
task, set the mosaic flag, enter the mosaic output filename in
the mosfile parameter, and define the mosaicing type using the
mostype parameter.
Selecting mostype=blind causes the object images in the dataset to be combined at their nominal offsets from the base position, as specified in the DO file used to acquire the data and as recorded in the image header entries `offra' and `offdec'. This type of mosaic is generally useful for a first look, or for minimally overlapped mosaics where blind offsetting is all that can be achieved.
Selecting mostype=manual causes the object images to be display at their nominal offsets so that the user can mark the location of a suitable reference point with the image display cursor. The reference point should be located within each of the images in the dataset, but need not correspond to a particular object. No automatic centroiding is performed on the marked position, so this option is most suitable for noisy images where the centering determination is subjective. The xoffset and yoffset parameters specify the position of the reference object relative to the base position of the mosaic.
Selecting mostype=auto is useful if there is a moderately
unresolved reference object in the mosaic, and this source is in each
object image of the dataset. This will often be the case for dithered
observations of a single object. When mostype=auto is selected,
the nominal offsets are corrected by centroiding on the reference
object in each frame using the IRAF proto.imcntr task. The
redimage parameter cboxsize defines the size of the
centroiding box used. This option produces excellent results for
suitable datasets with a moderately unresolved reference object. Some
care should be exercised in deciding whether centroiding has been
successful. This can usually be gauged from the appearance of
off-center stars in the mosaic. The xoffset and yoffset
parameters specify the position of the reference object relative to
the base position of the mosaic.
Observations obtained with the radio.do pattern, described above, require special treatement to estimate the nbL image offsets from offsets determined from interspersed Kn images of the same object. This is achieved by selecting mostype=radio.
These options provide a convenient way of assessing mosaiced data at the telescope and of gauging the result of the data reduction steps performed so far, before committing significant effort to the more involved full mosaicing.