.help mkfringecor Feb88 noao.imred.ccdred
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NAME
mkfringecor -- Make fringe correction images from sky images
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USAGE
mkfringecor input output
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PARAMETERS
.ls input
List of input images for making fringe correction images.
.le
.ls output
List of output fringe correction images.  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.  If none is specified
then all types are used.
.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 background.
.le
.ls ccdproc (parameter set)
CCD processing parameters.
.le
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DESCRIPTION
The input blank sky images are automatically processed up through the
iillumination correction before computing the fringe correction images.
The fringe corrections are subset dependent.
The slowly varying background is determined and subtracted leaving only
the fringe pattern caused by the sky emission lines.  These fringe images
are then scaled and subtracted from the observations by \fBccdproc\fR.
The background is determined by heavily smoothing the image using a
moving "boxcar" average.  The effects of the objects and fringes in the
image is minimized by using a sigma clipping algorithm to detect and
exclude them from the average.  Note, however, that objects left in the
fringe image will affect the fringe corrected observations.  Any objects
in the sky image should be removed using \fBskyreplace\fR (not yet
available).

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.

To minimize the effects of the fringes and any objects in the blank sky
calibration images a sigma clipping algorithm is used to detect and
exclude features from the background.  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.
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EXAMPLES
1. The two examples below make an fringe correction image from a blank
sky image, "sky017".  In the first example a separate fringe
image is created and in the second the fringe image replaces the
sky image.

.nf
    cl> mkskycor sky017 Fringe
    cl> mkskycor sky017 frg017
.fi
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SEE ALSO
ccdproc
.endhelp
