Margaret Moerchen                                             2000 Mar 08
University of Texas

                      QSO MONITORING PROJECT
                   BASIC PHOTOMETRY DATA REDUCTION
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I. INTRODUCTION
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This is a concise description of the methods used to reduce and analyze
data images taken with the PFC on the 30" telescope at McDonald 
Observatory.   Through doing this, one can re-trace the steps involved
and reproduce the results. 
This work is done in IRAF (Image Reduction and Analysis Facility).


II. RAW IMAGE CALIBRATION
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A. Bias Correction

use zerocombine within ccdproc to combine all zero images
	-parameters are set by "epar ccdproc"; toggle
	zerocombine to 'yes' and enter the name of the list that contains
	the bias image names
	-set ccdtype = zero


B. Flat Correction

use flatcombine within ccdproc to combine all flatfield images
	-parameters are also set with "epar ccdproc"; toggle flatcombine
	to 'yes' (make sure zerocombine is set to 'no') and again enter
	flat file name list


III. MEASURING BRIGHTNESS
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Comparison Stars

(these routines are all found in digiphot/daophot/ in IRAF)

A. General Image Values

first, use imexam to determine values such as FWHM and background sky
counts for the image; open image with "imexam _filename_"
	
	-'r' in imexam will give a radial plot -- the 2nd to last number
	is the MOFFAT-fit FWHM, which is generally a good value to base other
	parameter values on
	
	-'h' in imexam will give a histogram of counts (useful for
	checking the sky counts if the field is crowded
	
	-'a' yields the same values as 'r' but gives no plot

B. General Parameters to Set

	DAOPARS -

	psfrad		5x FWHM or distance to nearest star if 5x FWHM
			starts to include other stars
	fitrad		0.85x FWHM or greater if there is no nearby star


	DATAPARS -

	FWHM		found with imexam
	readnoise	see observing log or header (make sure header does
			not just contain a dummy value)
	gain		" "


C. Daofind 

run daofind to obtain a list of candidate stars
	min good data value = 1000 counts (this really depends on the
						image)
	max good data value = 25000 counts (below threshold where CCD
						goes nonlinear)

D. Phot

run phot to do photometric calculations on daofind's list of stars
	inner radius = 2-3x FWHM to get far enough away from star
	outer radius = distance to nearest star
	(these are for sky measurements)
	aperture radius = ~4x FWHM (objective is to gather as much light
		as possible without including the nearest star's wings)

E. PSF

run psf to select comparison stars
	(a) with cursor over chosen star selects the star

	NB:

	-comparison stars chosen must be verified stars, observed more
	than once; watch for cosmic rays and radiation hits on PFC
	-comparison stars for the object must be consistently selected in
        the same sequence when running psf, e.g. *1, *2, *3, *4, qso

	
F. Specific Values for the QSO Monitoring Project at McDonald Observatory
Telescope = 0.8m (30")
Instrument= PFC
Detector  = LF1
   (a) plate scale: 1.355 arcsec/pixel 	
   (b) images are 700x700 pixels = 15.8 arcmin
   (c) nominal CCD readout noise = 3215 ADU
   (d) nominal CCD gain = 1.6 e- / ADU
   (e) nominal CCD bias level = 3215 ADU