Compute the exposure time for a desired signal in a CCD image based on the elevel from a test image.

For a more detiled description see:
%  exposure_cal --help 

Usage: exposure_cal 50000.0 10000 30000 2.0 Y  
arg1 - desired peak signal level (adu)   
arg2 - detector bias level (adu) 
arg3 - observed level in test images  
arg4 - exptime (in sec) for test images  
arg5 - verbose/debug mode (Y/N)   

%  exposure_cal 50000.0 10000 30000 2.0 Y 

Desire peak level               = 50000.0  adu
Detector bias level             = 10000  adu
Observed signal in test image   = 30000  adu
exposure time of test image     = 2.0 sec

Observed level above bias =        20000.0 adu
flux rate from test image =   10000.000000 adu/sec
Desired level above bias =        40000.0 adu

Estimated exptime for desired peak level =       4.000000