Flat fielding is accomplished with option  AR  operating  on

            the general equation:


            The "bias" is simply a number representing an  average  intensity

            for  a  subset  of  pixels (say in a 50 by 50 area) taken with no

            light  entering  the  system  and  taken  immediately  after  the

            exposure   on  the  object  of  interest  (RAWFRAME  file).   The

            DARKFRAME file, which contains the spatial variations  about  the

            zero  level,  can  be  obtained by averaging several frames taken

            with no light entering the system.  Each  of  these  frames  must

            have  its  own  "bias"  subtracted  from it before the average is

            computed.  DARKFRAMES of long exposure may have too  many  cosmic

            ray  defects  or  may  be  impractical  to  obtain.   It  is then

            appropriate to average many short exposure dark frames and add on

            an  estimated dark current value.  The FLATFIELD file is obtained

            by exposing on a spatially uniform source of light.  Exposures on

            the  inside  of  telescope  domes generally are only uniform to a

            part in 100 and are not recommmended.  Exposures on the  twilight

            sky  should be long enough (at least 10 seconds) to avoid shutter

            vignetting effects.

                 Exposures on the night sky and also on the twilight sky will

            contain  star  images  which must be removed.  A median filter is

            not recommended for this operation since it does not  remove  the

            faint  outer  wings  of stars which overlap on more than half the

            exposures for a given pixel.  The most  accurate  way  to  remove

            stars is by modeling the light distribution with options PR or EA

            and subtracting the result with option FG.  Note that the working

            flat  field  file  should  be temporarily flat fielded by another

            approximate flat  field  for  the  modeling  operation  to  work.

            Sometimes,  after modeling and subtracting, the centers of bright

            stars will have unacceptable residuals.  The option OF  can  then

            be  used to perform a two-dimensional linear interpolation on the

            areas affected.  The areas can  be  selected  with  the  Grinnell

            cursor  box; either the entire area within the box can be written

            over or just the values which you have set to zero (with  options

            EP, EC, or ER).  F2 The box filters can clip out    points   >  a

            specified multiple of the   RMS and  replace  with  a  median  or

            mean.  BK   Fits  a  two-dimensional  surface to the map For IPCS

            spectra vignetting:

  RS    will scale rows with the averages stored from VP and FL.

  To preserve (roughly) counts when dividing by a flat field:

  NO    will normalize the flat fields to an average of 1.0

  For flat fields which are out of register (but not warped):

  XY    will shift by fractions of pixels in x and/or y.

  RO    will rotate thru any angle.

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  And for bad edges on the fields:

  CU    will trim them

  ME    will merge the trimmed pieces.