missing CD1_2 and CD2_1 in combined images

akooh wrote on Aug 29, 2010

Dear you,

I am using imcombine task (on IRAF V12.2.a) to combine a set of Hubble/ACS images. When I tried to extract corresponding X and Y positions in the resulting image for a set of objects with known RA and DEC (using rd2xy task), this error appeared:

ERROR: Image header parameter not found (CD1_2)

It seems that the CD1_2 and CD2_1 keywords are missing in the combined image. It should be noted that, these keywords are missing in the combined image only when CD1_2 and CD2_1 are zero in the input images.

Can you please help me with this problem? For your information, I have attached values assigned to imcombine parameters below:

Thanks very much,
Ali

input = "a.fits,b.fits"
output = "test.fits"
(headers = "")
(bpmasks = "")
(rejmasks = "")
(nrejmasks = "")
(expmasks = "")
(sigmas = "")
(logfile = "STDOUT")
(combine = "average")
(reject = "none")
(project = no)
(outtype = "real")
(outlimits = "")
(offsets = "wcs")
(masktype = "none")
(maskvalue = 0.)
(blank = 0.)
(scale = "none")
(zero = "none")
(weight = "none")
(statsec = "")
(expname = "")
(lthreshold = INDEF)
(hthreshold = INDEF)
(nlow = 1)
(nhigh = 1)
(nkeep = 1)
(mclip = yes)
(lsigma = 3.)
(hsigma = 3.)
(rdnoise = "0.")
(gain = "1.")
(snoise = "0.")
(sigscale = 0.1)
(pclip = -0.5)
(grow = 0.)
(mode = "ql")

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Mike Fitzpatrick wrote on Aug 29, 2010

The FITS convention is typically that if keywords like CD1_2/CD2_1 are zero then they may be ommitted, however the RD2XY task is looking for them explicitly since it pre-dates the MWCS interface. You can use HEDIT to add the keywords with zero values, or have a look at the WCSCTRAN task as another way to use the image WCS to generate x/y values.

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akooh wrote on Aug 29, 2010

Hello,

Thank you very much for your message and your attention. I modified the header using HEDIT and now it works.

Thanks again,
Ali

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Last post on Aug 29, 2010