Program:       FLAT

Purpose:       Apply a flat field correction to a map by fitting a
               2-D polynomial to the background.

Category:      MODELS, CALCULATION, FITTING, CALCULATION

File:          flat.c

Author:        M. Vogelaar

Keywords:
               
   INSET=      Give set, subsets:
               Maximum number of subsets is 2048.   
               Dimension of the subsets must be 2.               

   BOX=        Area which needs to be flat fielded           [whole map]
   
   OUTSET=     Output Set/Subset(s)  

   ORDER=      Order of polynomial to be fitted to the map           [1]
               highest allowed order is 21


** RANGE=      Give range of levels to include in fit:      [all levels]
               Input consists of two values. The first value may 
               be greater than the second. See the description 
               for the correct use of this keyword.



Description:   This program can be used for example to extract a 
               background by fitting a polynomial to it.
               The program uses a set (INSET=) which usually
               is blotted first. The subsets must be two dimensional.
               The area on which FLAT works is given in BOX=
               It writes the results to OUTSET= in the same box.
               Beware if OUTSET= already exists. It only overwrites
               data in the given box. A polynomial plane will be fitted 
               to the data using the least squares method. The order
               of this polynomial is given in ORDER= This number cannot
               exceed 21.
               Values outside the range defined in RANGE= are treated 
               as blanks, i.e. they are not included in the fit and are 
               interpolated after a plane is fitted to the data.
               Examples for the use of the RANGE= keyword:

               RANGE=2, 5
               (IF 2<value<5 THEN include in fit ELSE value==>blank)

               RANGE=5, 2
               (IF 2<value<5 THEN value==>blank ELSE include in fit)
                             
               At the RANGE= keyword, the values -INF and INF can 
               be input also. These values represent the minimum
               and maximum values of the current system.
               
               RANGE=5, INF
               (IF value>5 THEN include ELSE value==>blank)
               
               The fitted background can be subtracted from the 
               original set with the program COMBIN.


Example:

Notes:         

Updates:       Oct 14,  1991: MV, Document created.