NAME

shutcor -- shutter correction from images of varying exposure

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.

USAGE

shutcor images

PARAMETERS

images

List of overscan corrected images. These would usually be flat field exposures.

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.

center = "mode"

The statistical measure of central tendency that is used to estimate the data level of each image. This can have the values: mean , midpt , or mode . These are calculated using the same algorithm as the IMSTATISTICS task.

nclip = 3

Number of sigma clipping iterations. If the value is zero then no clipping is performed.

lsigma = 4, usigma = 4

Lower and upper sigma clipping factors used with the mean value and standard deviation to eliminate cosmic rays. Since findgain is sensitive to the statistics of the data the clipping factors should be symmetric (the same both above and below the mean).

exposure = "exptime"

Keyword giving the exposure time.

verbose = yes

Verbose output?

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.

EXAMPLES

A sequence of flat fields with varying exposure times are taken and processed to subtract the overscan.

    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.

SEE ALSO

imstatistics