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On Orbit Measurement of SXT CCD Electronic Gain

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Date submitted: 06-Jun-1996

	On Orbit Measurement of SXT CCD Electronic gain

Special observations of the Al.1 pinhole image were made on
19-Apr-96 to check the SXT CCD gain (electrons/DN).  The
panels show:

Upper left, top: Average of the 30-sec exposures of the pinhole,
		128 x 64 Full-res PFI.
Upper left, bottom: Variance image from the same exposures.  The 
		south coronal hole is at the upper right.  The weak
		X-ray flux (about 1 % of the optical) produces high
		variance because of pointing drift, solar evolution,
		and the large number of photoelectrons per x-ray photon.
		Only the bottom 11 rows are used for the gain
		measurement to avoid the X-ray flux.

Upper right: Distributions of the intensity residuals after subtracting
	the average image for 3.7 sec (+) and 30 sec (x) exposures.
	The distributions are Gaussian and the fits, plotted as lines,
	were used to measure the variance.  A low order polynomial
	surface is used to account for the slow variation in the
	pinhole image shape/brightness when each image is matched
	to the average.

Lower left: The raw variance is corrected for systematic effects,
	including the bias in the A/D converter: twice as many 
	pixels have even data numbers than have odd.

Lower right: A linear fit of variance against intensity has
	a slope whose inverse is the gain.

Values for camera parameters are:

		    Canonical		Measured
	Bias		13		14.9 +/- 0.1 DN
	Gain		100		87.9 +/- 1.4 e-/DN
	Read noise	20		46   +/- 19  e-

Submitted by Barry LaBonte
June 6, 1996

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