Imcombine using wcs: weird coordinate system error

rrg wrote on Jun 19, 2007

Hi all,
I have a stack of images (2048x2048) with reasonable WCS data in the headers, like:

CRVAL1  =      241.23971710972 /
CRVAL2  =      43.358557401931 /
CRPIX1  =     1025.00017818278 /
CRPIX2  =     1024.98884738801 /
CTYPE1  = 'RA---TAN'           /
CTYPE2  = 'DEC--TAN'           /
EQUINOX =                2000. /
WCSDIM  =                    2
CD1_1   =  -6.9046572469549E-5
CD1_2   =  -1.6518674845624E-6
CD2_1   =  -1.6910818686302E-6
CD2_2   =  6.90626502541518E-5
LTV1    =     2049.00003262489
LTV2    =                2049.
LTM1_1  =  -7.5497901264043E-8
LTM2_1  =   -0.999999999999996
LTM1_2  =   -0.999999999999994
LTM2_2  =  7.54979012640429E-8
WAT0_001= 'system=image'
WAT1_001= 'wtype=tan axtype=ra'
WAT2_001= 'wtype=tan axtype=dec'

When displayed in DS9, these look ok: N up, E left, with x increasing to the right and y increasing upwards.

I then try to imcombine a bunch of these images, using offsets=wcs. My full param set is

input   =      fit_//@testlist  List of images to combine
output  =             testadd4  List of output images
(headers=                     ) List of header files (optional)
(bpmasks=                     ) List of bad pixel masks (optional)
(rejmask=                     ) List of rejection masks (optional)
(nrejmas=                     ) List of number rejected masks (optional)
(expmask=                     ) List of exposure masks (optional)
(sigmas =                     ) List of sigma images (optional)
(logfile=               STDOUT) Log file

(combine=               median) Type of combine operation
(reject =            avsigclip) Type of rejection
(project=                   no) Project highest dimension of input images?
(outtype=                 real) Output image pixel datatype
(outlimi=                     ) Output limits (x1 x2 y1 y2 ...)
(offsets=                  wcs) Input image offsets
(masktyp=                 none) Mask type
(maskval=                    0) Mask value
(blank  =                   0.) Value if there are no pixels

(scale  =                 none) Image scaling
(zero   =                 none) Image zero point offset
(weight =                 none) Image weights
(statsec=                     ) Image section for computing statistics
(expname=                     ) Image header exposure time keyword

(lthresh=                INDEF) Lower threshold
(hthresh=                INDEF) Upper threshold
(nlow   =                    1) minmax: Number of low pixels to reject
(nhigh  =                    1) minmax: Number of high pixels to reject
(nkeep  =                    1) Minimum to keep (pos) or maximum to reject (neg)
(mclip  =                  yes) Use median in sigma clipping algorithms?
(lsigma =                  2.5) Lower sigma clipping factor
(hsigma =                  2.5) Upper sigma clipping factor
(rdnoise=                   0.) ccdclip: CCD readout noise (electrons)
(gain   =                   1.) ccdclip: CCD gain (electrons/DN)
(snoise =                   0.) ccdclip: Sensitivity noise (fraction)
(sigscal=                  0.1) Tolerance for sigma clipping scaling corrections
(pclip  =                 -0.5) pclip: Percentile clipping parameter
(grow   =                   0.) Radius (pixels) for neighbor rejection
(mode   =                   ql)

The resulting image has a WCS problem. x and y are correct, but N goes diagonally up/right, and E goes diagonally down/left !!!! N and E are 180 degrees from each other. The header now says

CRVAL1  =      241.23971710972 /
CRVAL2  =      43.358557401931 /
CRPIX1  =     1188.00017818278 /
CRPIX2  =     3111.99993175194 /
CTYPE1  = 'RA---TAN'           /
CTYPE2  = 'DEC--TAN'           /
EQUINOX =                2000. /
WCSDIM  =                    2
CD1_1   =  -6.9046572469549E-5
CD1_2   =  6.90465776824204E-5
CD2_1   =  -1.6910818686302E-6
CD2_2   =  1.69108199630373E-6
LTM1_1  =                   1.
LTM2_2  =                   1.
WAT0_001= 'system=image'
WAT1_001= 'wtype=tan axtype=ra'
WAT2_001= 'wtype=tan axtype=dec'

The CRVALs are ok. The CD matrix is garbled. The CRPIX are wacky. So, I took a shot:
I set CRPIX2=CRPIX2-2048
I set CD2_2 to what was in CD1_2
I set CD1_2 to what was in CD2_2 * -1

and that works! A check against SDSS shows the coordinates are correct (or very close). It looks like a bug to me...

Is this a known bug in imcombine? I am using IRAFNET PC-IRAF Revision 2.13-BETA2 on a MacIntel running OS 10.4.9

Thanks
Roy

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rrg wrote on Jun 19, 2007

The images that were used here were rotated (using IRAF rotate) and flipped (using imcopy[-*,*]) to get the orientation right. Following a colleague's suggestion (thanks Dale Kocevski!) I found that removing the LTV and LTM header keywords before running imcombine resulted in a correct final image.

This sort of solves my problem in an ugly way, but hopefully it points to the source of the error (if there is one) in imcombine.

Roy

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Last post on Jun 19, 2007