Reading Atlas Images

The atlas images are in a binary heap format that is not easily readable by normal humans; think soft encryption.

We have created a standalone code that serves to both read them and as a template library for inclusion in other codes.The code is available as: readAtlasImages-v5_4_11.tar.gz.

Compiling

1. % make clean

2. % make

If you are on a big-endian machine, remove -DSDSS_LITTLE_ENDIAN from CFLAGS in the Makefile.

Using

% read_atlas_image -h
Usage: read_atlas_image [options] input-file row output-file
Your options are:
       -?      This message
       -b #    Set background level to #
       -c #    Use color # (0..ncolor-1; default 0)
       -h      This message
       -i      Print an ID string and exit
       -v      Turn up verbosity (repeat flag for more chatter)

If one wanted to read the r-band atlas image of an object with id=432 in run 752, rerun 20, camcol 3, field 177, one would say:

% read_atlas_image -c 2 /data/dp3.b/data/752/20/objcs/3/fpAtlas-000752-3-0177.fit 432  myAtlasImage.fits

where one should know the code that filters u,g,r,i,z are 0,1,2,3,4 respectively. The background level is an artifical offset added to all pixels. The SDSS convention is 1000..

Developer Comments

I don't expect that many users will actually want to use the read_atlas_image executable (although it is perfectly functional). The main use of the product will probably be to link into custom built executables that need to process atlas image data. I believe that the code should be easily reused for this purpose.

If you look at the code you'll see that it actually manipulates a type called an ATLAS_IMAGE. This contains a field called a master_mask that contains inter alia the bounding box of the atlas image ([rc]{min,max}) in the r band, and offsets to that band (d{row,col}).

The standalone programmes read_mask (reads fpM files) and read_PSF (reads psField files) are similar; build instructions are identical.