helplog.13: mosaic astrometry, WCS solutions for multiple amps/CCD

Frank Valdes wrote on Jan 21, 2005

NUMBER:	13
KEYWORDS:	mosaic astrometry, WCS solutions for multiple amps/CCD
DATE:	Fri Jan 21 11:51:46 MST 2005
FROM:	valdes

Q:	I am trying to derive a WCS for a new mosaic camera.
	I have used CCMAP to derive a fair WCS using ten bright stars on
	one amplifier of one ccd.  To simplify the situation, I have cut
	three of the CCD's out of the mosaic, so I have only two amplifiers
	(and two extensions) to work with.  My first question deals with
	how to treat both amps at once.  CCMAP seems to want only one
	extension at a time, or a frame that has been put into a simple
	fits format.  I can create the simple format by copying the two
	extensions into one simple fits file (but I lose the header info
	from extension [0]), but I don't think this is what I should be
	doing.  I assume I derive a WCS for the mosiac and then make a
	simple fits file which is corrected and shifted to a point common
	with the other dithered images of the same object.  Am I right?
	How do I treat the two amps (extensions) with CCMAP?

A:	Your are right that you should derive a single WCS for a CCD since
	multiple amplifiers are geometrically linked.  This is what is done
	with the CTIO mosaic that has 2 amplifiers per CCD and I do a
	single WCS solution for each CCD which are then divided across the
	amplifiers.  The way this is done is to derive a database solution
	for the merged single CCD using MSCTPEAK.  In the database it
	should say pixsystem is physical.

	Then the other thing is setting up the mapping between physical
	coordinates (think of this as CCD coordinates) and the pixels in
	the amplifier images correctly.  This is done with the LTV1
	keyword.  Normally you would have no LTV or LTM keywords in the raw
	data but it might depend on the presence of any over/pre scans on
	the left side of the image.

	LTV1 should be the offset from the first pixel of the real data,
	i.e. the merged CCD image and the first pixel in the amplifier
	image.  An example is best here.  Suppose you have two amplifier
	image extensions called im1 and im2.  In im1 there is nothing on
	the left.  This means pixel (1,1) is the first CCD pixel
	corresponding to (1,1) in the image you used for MSCTPEAK.  Then
	the offset in x is 0 and you can either set LTV1=0 or leave it out
	because the default for a missing LTV keyword is 0.  Now the first
	amplifer reads out the first 1024 pixels of the CCD.  The second
	amplifier reads out the second 1024 pixels.  Again, if there is no
	dummy data on the left of the image for im2 then the first image
	pixel maps to pixel (1025,1) in the merged CCD image.  Then the
	offset is LTV1=1024.

	Once you have this setup to define this "physical" or CCD
	coordinate system then MSCSETWCS will apply a database solution
	appropriately.  This only modifies the WCS keywords and will set
	CRVAL1/CRVAL2 to some real point on the sky for the exposure based
	on a keyword in the header.  That is, the one solution for the CCD
	will be set appropriately for each of the two amplifiers.  Note
	that what this really means is that the keywords will be the same
	except that the CRPIX1 keywords will differ by some amount.  As a
	reminder CRPIX1 is the pixel relative to the image that corresponds
	to the "tangent point" where the CRVAL1 keyword gives the RA (in
	degrees).  As noted in the astrometry write up the celestial point
	on the sky needs to be the same for all the pieces of a mosaic and
	then the CRPIX keywords define the different distances of a WCS to
	that point on the sky in terms of the origin of the particular
	image.

	To illustrate what I do for the CTIO mosaic I make 8 solutions for
	the 8 CCDs.  These are labeled im1, im3, im5, etc in the database.
	I then copy the first 8 to the end of the file and change the
	labels to im2, im4, etc.  There is nothing else changed in terms of
	the WCS.

	Once you have the database file then running MSCSETWCS to populate
	the WCS in the mosaic data will preserve the headers and only
	change the WCS keywords.

	References:

	http://iraf.noao.edu/projects/ccdmosaic/astrometry/astrom.html
	http://iraf.noao.edu/projects/ccdmosaic/generic/generic.html

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Last post on Jan 21, 2005