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mscshutcor noao.obsutil



mscshutcor -- shutter correction from mosaic images of varying exposure


MSCSHUTCOR calculate the shutter correction for a mosaic camera 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.


mscshutcor images


List of overscan corrected mosaic exposures. These would usually be flat field exposures.
extnames = ""
List of extension names or patterns matching the full extension name. Each comma delimited segment is treated as a pattern so multiple patterns may be used.
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. Note that the section is in pixel coordinates and will be used on all the selected extensions.
center = "mode"
The statistical measure of central tendency that is used to estimate the data level of each extension. This parameter can have the values: mean , midpt , or mode . These are calculated using the same algorithm as the IMSTATISTICS task. When there is more than one extension the measured statistics over each extension are averaged.
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?


MSCSHUTCOR calculate the shutter correction for a mosaic camera given a sequence of overscan corrected exposures of varying durations. Typically these would be flat field exposures. For the selected extensions the exposure time specified in the EXPTIME keyword is extracted. A photometric measure, given by the center parameter, of the data over all the extensions is estimated. When there are multiple extensions in an exposure the statistics obtained separately in each extension are averaged. Note that this is valid even when the extensions have different gains provided all exposures have the same gains.

The shutter correction is the intercept divided by the slope from a plot of the exposure time versus photometirc exposure statistic. When the verbose parameter is set the statistics from each exposure and the fit values are output.

The first image extension in each exposure must contain the keywords OVERSCAN and EXPTIME otherwise an error will be given. A warning is given if the image contains the keyword FLATCOR.

Bad pixels should be eliminated to avoid affecting the statistics. This can be done with sigma clipping and/or an image section; though when there are multiple extensions the same image section is applied to each one. 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.

This task is a similar to the task obsutil.shutcor except that it handles multiextension mosaic files. However, this task will also work with simple single images and so may be used for both mosaic and non-mosaic data.


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

    cl> mscshutcor 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.


obsutil.shutcor, imstatistics

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