Mosaicing blocks

Each DPOSS plate has traditionally been stored as 529 blocks, each of which is a 1024x1024 image. Each individual block is just 20'x20'. Freqently one needs a larger part of the sky. To mosaic an entire plate, use:

idl_mosaic.pl xYYY

Here, x is either j, f or n for the three filters and YYY is the plate number.

The plate blocks are downloaded from HPSS (until the fast connection is down, plate blocks are assumed to be under the directory xYYY), vignetting corrected and sewed together. The header of the resulting file, xYYY.fits, is altered to include WCS information.

The sewing itself is done as follows:
take all 13 blocks from a row,
write first row (1024 pixels) of each block
write second row (1024 pixels) of each block
...
...
write last row (1024 pixels) of each block
go to the next row ...

The header is obtained from skicat (xYYY_Header), keywords
CNPIX1 1
CNPIX2 1
PLTSCALE 1.007
are added using the ftools task fmodhead. Finally, the equinox is made 2000.0 and the header included in the fits file.

Last Outdated: 14 Nov 2000