Gather a DSS image and the corresponding catalog for a section of sky. The image can be displayed with ds9 and the catalog overplotted with ds9 regions. Origianlly this routine provided access to multiple catalogs (USNO,2MASS,SDSS) but now USNO is used almot exclusively.

% skygrab --help  
% skygrab 08:13:15.10 +45:59:29.0 5.0 Y USNO Y  

Usage: skygrab 08:13:15.10 +45:59:29.0 5.0 Y USNO Y
arg1 - Ra J2000 in sexigecimal
arg2 - Dec J2000 in sexigecimal
arg3 - Search Radius in arcmin
arg4 - view with ds9 (Y/N) 
arg5 - USNO to use ONLY usno 
arg6 - run in debug/verbose mode 
NOTE: See --help message for description of backdoor files. 

A number of tasks are performed in the skygrab script. Some brief explanations, along with the links to more detailed explanations of each script are given here now. To get each data resource (DSS, 2MASS, etc...) skygrab must call tools that need positions (in various formats) and field sizes in varrious units. The first order of business is to retrieve a DSS image that matches the position and size of the CCD image we are calibrating. The dssg script is an OTW code takes the input RA,DEC from the image header, and prepares information needed for the actually bash script that uses a wget call to retrieve the DSS images. This latter script is called dssget.

The next set of tasks (2 python, one OTW) are used to process the coordinates and image size. In the order they are used in skygrab we have:

1. psex.py takes the Ra,Dec in sexigecimal format and converts them to degree and hour units in floating point form.
2. RadCon.py reads the field radius (image size) in units specified by the users, and then predicts the size in degrees, arcminutes, and arcseconds.
3. c2r is an OTW task that reads the field center position and size and computes the Ra and Dec min,max boundaries (presently needed for the SDSS query).

At this stage we have the forms of information we need to construct star catalogs.