% @(#)fit_back.hlq	19.1 (ESO-IPG) 02/25/03 13:30:32 
\se
SECTION./BACKGROUND
\es\co
FIT/BACKGROUND                                                08-MAY-1990 RHW
\oc\su
FIT/BACKGROUND outframe = inframe(s) [deg,it] [clp1,clpn] [skew] [outbck]
FIT/BACKGROUND outframe = inframe(s) [coef] [outbck]
FIT/BACKGROUND inframe(s) [deg,it] [clp1,clpn] [skew] [outbck]
FIT/BACKGROUND inframe(s) [coef] [outbck]
	compute 2-dim. polynomial fit of the background
\us\pu
Purpose:     Compute coefficient of a 2-dim polynomial fit of the sky
             background. Optional create frames with the background and/or 
             the background subtracted input frame.
\up\sy
Syntax:      FIT/BACKGROUND outframe = inframe(s) [deg,it] [clp1,clpn] [skew] 
                            [outbck]\\
             FIT/BACKGROUND outframe = inframe(s) [coef] [outbck]\\
             FIT/BACKGROUND inframe(s) [deg,it] [clp1,clpn] [skew] [outbck]\\
             FIT/BACKGROUND inframe(s) [coef] [outbck]
\ys\pa
             outfram =    output frame, i.e. the background subtracted input 
                          frame; no default allowed.
\ap\pa
             inframe(s) = input frame and, if desired, the input mask and to 
                          be specified as {\tt inframe,inmask}. No default for
                          the input frame and no masking wanted.
\ap\pa
             outbck =     name of the output background frame. By  default 
                          no background frame is generated.
\ap\pa
             deg,it =     degree of the polynomial to fit and the maximum
                          number of iterations. Default values are 3,10.
\ap\pa
             clp1,clpn =  clipping parameter for thr first and clipping
                          parameter for the following iterations. Default
                          is 1.0,2.5.
\ap\pa
             up_skew =    upper limit for the acceptable skewness. Default
                          is 0.05.
\ap\no
Note:        The coefficients are calculated by an iterative least-squares
             fitting of the pixels in a rebinned input frame. If a
             predefined `masking frame' is given, only not masked pixels
             are taken into account.
\on\exs
Examples:
\ex
             FIT/BACKGROUND ccdbck = ccd 
             Compute the background in the frame ccd and store the background
             corrected frame in ccdback. Use all default values.
\xe\ex
             FIT/BACKGROUND ccd,ccdmsk 2,20 2,4 ? ccdbck
             Compute the background in the frame ccd and store the background
             corrected frame in ccdback. Use the frame ccdmsk to get the 
             pixels which should not be taken into account for the    
             background determination. The fit should be of the order of
             2 with 20 iterations at maximum. Clip values should be 2 for 
             the first and 4 for the following iterations. Use the default 
             skewness value.
\xe\sxe
