.help shutcor Nov01 noao.obsutil
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NAME
shutcor -- shutter correction from images of varying exposure
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SYNOPSIS
SHUTCOR calculate the shutter correction for a detector given a
sequence of overscan corrected images of varying durations.  Typically
these would be flat field exposures.  The shutter correction is the
intercept on a plot of exposure duration versus exposure level.
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USAGE
shutcor images
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PARAMETERS
.ls images
List of overscan corrected images.  These would usually be flat
field exposures.
.le
.ls section = ""
The selected image section for the statistics.  This should be chosen
to exclude bad columns or rows, cosmic rays, and other non-linear
features.
.le
.ls center = "mode"
The statistical measure of central tendency that is used to estimate
the data level of each image.  This can have the values:  \fBmean\fR,
\fBmidpt\fR, or \fBmode\fR.  These are calculated using the same
algorithm as the IMSTATISTICS task.
.le
.ls nclip = 3
Number of sigma clipping iterations.  If the value is zero then no clipping
is performed.
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.ls lsigma = 4, usigma = 4
Lower and upper sigma clipping factors used with the mean value and
standard deviation to eliminate cosmic rays.
Since \fBfindgain\fR is sensitive to the statistics of the data the
clipping factors should be symmetric (the same both above and below the
mean).
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.ls exposure = "exptime"
Keyword giving the exposure time.
.le
.ls verbose = yes
Verbose output?
.le
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DESCRIPTION
SHUTCOR calculate the shutter correction for a detector given a
sequence of overscan corrected images of varying durations.  Typically
these would be flat field exposures.  The shutter correction is the
intercept on a plot of exposure duration versus exposure level.

The images must contain the keyword OVERSCAN otherwise and error will
be given.

Bad pixels should be eliminated to avoid affecting the statistics.
This can be done with sigma clipping and/or an image section.
The sigma clipping should not significantly affect the assumed gaussian
distribution while eliminating outlyers due to cosmic rays and
unmasked bad pixels.  This means that clipping factors should be
symmetric.
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EXAMPLES
A sequence of flat fields with varying exposure times are taken and
processed to subtract the overscan.

.nf
    cl> shutcor flat*

    Shutter correction = 0.538 +/- 0.043 seconds

    Information about the mode versus exptime fit:

	   intercept        slope     (and errors)
	    5.347105      9.933618
	   0.4288701    0.01519613
	
	chi sqr:  0.2681   ftest: 419428.   correlation:      1.
	 nr pts:      4.   std dev res: 0.422769
	
	x(data)     y(calc)     y(data)     sigy(data)
	     3.      35.148     34.6725          0.
	    12.     124.551     125.015          0.
	    27.     273.555     273.778          0.
	    48.     482.161     481.949          0.
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SEE ALSO
imstatistics
.endhelp
