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Quantum Efficiency Correction (a.k.a. `flat-fielding')

Although the spatial variation of the quantum efficiency across the HRI is nominally within $\pm$10% for most of the detector, occasionally it may be desirable to flat-field a map with preflight calibration measurements.

1.

Determine the roll angle of the satellite for the observation.

        cl> imhead rh*.qp | match ROLL_NOM

NOTE: The roll angle for US processed REV 0 data is given with different signs in the header compared to that of the printout (which is correct). Therefore the above method should be used only for RDF data.

2.

Retrieve the quantum efficiency map from anonymous ftp at SAO. This is a 512$\times$512 pixel array with 8 $^{\prime\prime}$ resolution.

	> ftp sao-ftp.harvard.edu
	> cd /pub/rosat/calfiles
	> ascii
	> get README
	> binary
	> get roshri_qe8.fits
	> quit

3.

Convert the QE map fits file to an Iraf image file using the task `dataio.rfits'

4.

Rotate the QE map to the rotation of the data.

        > images 
        > rotate

NOTE: use the following parameters

rotation angle=	(switch the sign of the ROLL_NOM)
	xin=252.5
	yin=252.5
	xout=INDEF
	yout=INDEF
	ncols=512
	nlines=512
	interpo=poly3
	boundary=constant
	constant=0.0

5.

Store the new gain map image using ``blink" in SaoImage. Under the item ``scale", click on ``blink" with the mouse button you would like the image stored under.

6.

Display the image file created from rarc2pros (rh*_im1.imh).

        > display rh*_im1.imh

7.

Flip between both images to determine the QE value at the source position by hitting the mouse button with the stored image. Note that the ``scale" item in Saoimage must be selected.

8.

Pixel intensities on both maps can be determined by the iraf task ``listpix".

	In Iraf:

	>images
	>listpix "rh*.imh[4050:4070,4070:4090] wcs=physical"
		
		-where the coordinates are the image section of interest.

9.

The source intensity can be relatively corrected to the field center. The quantum efficiency correction (QEC) is the inverse of the intensity determined from the QE map at the source position. The source intensity can be multiplied by the QEC factor to give the on-axis value. As well as point source corrections, this QEC factor can also be used for 'flat fielding' extended sources.

N.B. The source intensity correction given in the SASS output is the QEC factor.

The wobble means that a general area of the detector/QE map will contribute to any give source; hence the above method is approximate.