Compute a test FITS image. Use a simple circle with a radial gradient.

%  tim_circle.sh --help  

usage: tim_circle.sh 50.0 60.0  30.0 100.0 50.0 test.fits [-v] [-h]  
arg1 - Xo of circle 
arg2 - Yo of circle 
arg3 - Ro, radius of circle 
arg4 - I value at R=0 
arg5 - I value at R=Ro 
arg6 - name of output test image
Additional options: 
 -v = print verbose comments and run in debug mode 
 -h = just show usage message  

You can read some old descriptions HERE.