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+.help mkskyflat Feb88 noao.imred.ccdred
+.ih
+NAME
+mkskyflat -- Make sky corrected flat field images
+.ih
+USAGE
+mkskyflat input output
+.ih
+PARAMETERS
+.ls input
+List of blank sky images to be used to create sky corrected flat field
+calibration images.
+.le
+.ls output
+List of output sky corrected flat field calibration images (called
+sky flats).  If none is specified or if the name is the same as the
+input image then the output image replaces the input image.
+.le
+.ls ccdtype = ""
+CCD image type to select from the input images.
+.le
+.ls xboxmin = 5, xboxmax = 0.25, yboxmin = 5, yboxmax = 0.25
+Minimum and maximum smoothing box size along the x and y axes.  The
+minimum box size is used at the edges and grows to the maximum size in
+the middle of the image.  This allows the smoothed image to better
+represent gradients at the edge of the image.  If a size is less then 1
+then it is interpreted as a fraction of the image size.  If a size is
+greater than or equal to 1 then it is the box size in pixels.  A size
+greater than the size of image selects a box equal to the size of the
+image.
+.le
+.ls clip = yes
+Clean the input images of objects?  If yes then a clipping algorithm is
+used to detect and exclude objects from the smoothing.
+.le
+.ls lowsigma = 2.5, highsigma = 2.5
+Sigma clipping thresholds above and below the smoothed iillumination.
+.le
+.ls ccdproc (pset)
+CCD processing parameter set.
+.le
+.ih
+DESCRIPTION
+A sky corrected flat field calibration image, called a sky flat, is a
+flat field that when applied to observations of the sky have no large
+scale gradients.  Flat field images are generally obtained by exposures
+to lamps either illuminating the telescope field or a surface in the dome
+at which the telescope is pointed.  Because the detector is not illuminated
+in the same way as an observation of the sky there may be large
+scale iillumination patterns introduced into the observations with such
+a flat field.  To correct this type of flat field a blank sky observation
+(which has been divided by the original flat field) is heavily smoothed
+to remove the noise leaving only the residual large scale iillumination
+pattern.  This iillumination pattern is divided into the original flat
+field to remove this residual.
+
+The advantage of creating a sky flat field is that when processing
+the observations no additional operations are required.  However,
+if the observations have already been processed with the original
+flat field then the residual iillumination pattern of blank sky
+calibration images may be created as an iillumination correction
+to be applied by \fBccdproc\fR.  Such a correction is created by the
+task \fBmkskycor\fR.  If a good blank sky image is not
+available then it may be desirable to remove the iillumination pattern
+of the flat field image using \fBmkillumflat\fR or \fBmkillumcor\fR
+provided the sky observations are truly uniformly illuminated.
+For more on flat fields and iillumination corrections see \fBflatfields\fR.
+
+The input, blank sky images are first processed, based on the
+\fBccdproc\fR parameters, if needed.  These parameters also determine
+the flat field image to be used in making the sky flat.  The residual
+iillumination pattern is determined by heavily smoothing the image using
+a moving "boxcar" average.  The effects of objects in the input image
+may be minimized by using a sigma clipping algorithm to detect and
+exclude the objects from the average.  The output image is ratio of the
+flat field image, for the same subset as the input image, to the
+residual iillumination pattern determined from the processed blank sky
+input image.  The iillumination pattern is normalized by its mean to
+preserve the mean level of the flat field image.
+
+The smoothing algorithm is a moving average over a two dimensional
+box.  The algorithm is unconvential in that the box size is not fixed.
+The box size is increased from the specified minimum at the edges to
+the maximum in the middle of the image.  This permits a better estimate
+of the background at the edges, while retaining the very large scale
+smoothing in the center of the image.  Note that the sophisticated
+tools of the \fBimages\fR package may be used for smoothing but this
+requires more of the user and, for the more sophisticated smoothing
+algorithms such as surface fitting, more processing time.
+
+Blank sky images may not be completely blank so a sigma clipping
+algorithm may be used to detect and exclude objects from the
+iillumination pattern.  This is done by computing the rms of the image
+lines relative to the smoothed background and excluding points
+exceeding the specified threshold factors times the rms.  This is done
+before each image line is added to the moving average, except for the
+first few lines where an iterative process is used.
+.ih
+EXAMPLES
+1. Two examples in which a new image is created and in which the
+input sky images are converted to sky flats are:
+
+.nf
+    cl> mkskyflat sky004 Skyflat
+    cl> mkskyflat sky* ""
+.fi
+.ih
+SEE ALSO
+ccdproc, flatfields, mkfringecor, mkillumcor, mkillumflat, mkskycor
+.endhelp