[AstroPy] Need some help with wcs

Tue Jan 29 17:12:28 CST 2013

On 01/29/2013 03:49 PM, Christian Ambros wrote:
> Hi guys,
>
> I'm new to astropy but as I see the potential, I like to use it my Ph.d thesis.
> After I had fun with some of the tuts and their data I started out to use some of my own data and ran into some problems with wcs.
>
> After I opend up a fits with pyfits.open I tried to use wcs= pywcs.WCS(hdulist[0].header) to get the wcs data. I got the error mesg:    312                 # The header may have SIP or distortions, but no core
>      313                 # WCS.  That isn't an error -- we want a "default"

This isn't an error message -- it's merely a comment in the code around 
the point where the error occurred.  Can you post the full output from 
running the command?

>
> A typical header of the images I want to work with looks like this:
>   
> SIMPLE  =                    T / Fits standard
> BITPIX  =                  -32 / Bits per pixel
> NAXIS   =                    2 / Number of axes
> NAXIS1  =                 1023 / Axis length
> NAXIS2  =                 1023 / Axis length
> ORIGIN  = 'NOAO-IRAF FITS Image Kernel July 1999' / FITS file originator
> EXTEND  =                    F / File may contain extensions
> DATE    = '2000-08-24T17:53:06' / Date FITS file was generated
> IRAF-TLM= '18:52:47 (24/08/2000)' / Time of last modification
> TIMESYS = 'UTC     '           / P0S| time scale specification
> RADESYS = 'FK5     '           / P2S| WCS for this file
> EQUINOX =         2.000000E+03 / P0R| equinox
> OBSERVER= 'Reiprich / Englhauser' / P0R| observer(s)
> TELESCOP= 'Skinakas 1.3m'      / P0R| telescope
> INSTRUME= 'CH 360  '           / P0R| instrument name
> OBS_MODE= 'DIRECT MODE'        / RDF| observing mode
> FILTER  = 'R       '           / RDF| filter ID
> OBJECT  = 'R1825.3+30'         / P0R| name of object
> DATE-OBS= '2000-06-07T23:54:40' / P0R| time of obs start (UTC)
> EXPOSURE=         3.000000E+02 / JER| exposure time (s)
> DATE_CCD= '2000-06-08T00:00:20' / JER| end of CCD readout (UTC)
> AIRMASS1= '1.005   '           / JER| air mass at obs start
> AIRMASS2= '1.007   '           / JER| air mass at obs end
> BLANK   =                65535 / P0R| undefined integer values
> CTYPE1  = 'RA---TAN'           / P0R| (assumed) axis type for dim 1
> CTYPE2  = 'DEC--TAN'           / P0R| (assumed) axis type for dim 2
> CRPIX1  =                511.5 / P0R| 'pixel 1 from 0.5 to 1.5' convention
> CRPIX2  =         5.125000E+02 / P0R| 'pixel 1 from 0.5 to 1.5' convention
> CRVAL1  =         2.763750E+02 / P0R| (assumed) sky coord of 1st axis (deg)
> CRVAL2  =         3.045000E+01 / P0R| (assumed) sky coord of 2nd axis (deg)
> CDELT1  =    -1.3944444444E-04 / P0R| (assumed) X degrees per pixel
> CDELT2  =     1.3944444444E-04 / P0R| (assumed) Y degrees per pixel
> OBSGROUP= '07JUN00 '           / JER| group ID of observations
> OBSFNAME= 'DATA0134'           / JER| file ID of observation
> HISTORY   Changes by FIXQSRC:
> HISTORY     * add new keywords (everything except for SIMPLE,BITPIX,NAXIS.);
> HISTORY         filled by information from the observation log files
> HISTORY     * fix I2 bug of PMIS (negative pixel values)
> COMMENT   Keyword types:
> COMMENT     * P0R - reserved  in Paper 0
> COMMENT     * P0S - suggested in Paper 0
> COMMENT     * P2S - suggested in Paper 2
> COMMENT     * RDF - present in ROSAT RDF FITS files
> COMMENT     * JER - introduced by JER
> COMMENT   References:
> COMMENT     * Paper 0: NOST 100-2.0, FITS Definition, http://fits.gsfc.nasa.gov/
> COMMENT     * Paper 2: Paper II in http://fits.gsfc.nasa.gov/documents.html#WCS
> WCSDIM  =                    2
> CD1_1   =  -1.3944444444000E-4
> CD2_2   =  1.39444444440000E-4
> LTM1_1  =                   1.
> LTM2_2  =                   1.
> WAT0_001= 'system=image'
> WAT1_001= 'wtype=tan axtype=ra'
> WAT2_001= 'wtype=tan axtype=dec'
> ZEROCOR = 'Aug 21 19:55 Zero level correction image is Zerom'
> CCDSEC  = '[1:1024,1:1024]'
> CCDMEANT=            651373512
> CCDPROC = 'Aug 22  1:05 CCD processing done'
> FLATCOR = 'Aug 22  1:05 Flat field image is FlatRm with scale=40679.34'
> LTV1    =                  -1.
> NCOMBINE=                   11

Can you attach the original FITS file (or at least just the header?)  
E-mail clients tend to garble this stuff, and it's easiest for me to get 
to the bottom of what's going on with an actual file to test with.

>
> As it is written in the documantation the values i.e. CRPIX j, PC i j or CD i j, CDELT i,CTYPE i, CRVAL i, or CUNIT i have to be present. As you can see in the middle of the header they exist ok the WCSAXES value ist missing but there is WCSDIM instead.
> The error message says something about a 'default'. Do those values have to stand in a particular order at a particular position in the header?

There shouldn't be any dependency on ordering of the keywords.

> I ran verify() but with no negativ result.
>
> Is there, maybe, an example header to which I can refer to? If so I will write myself a tool to re-write the header of all of those 169 targets I have already in my databank. I would revise my header correcting program which update the header after camera output to get the 'right' header once and for all.

You can always create a WCS object without passing it a header to get a 
"default" header and write it out using to_header_string(). But first we 
should get to the bottom of what the error actually is -- it may be 
simpler than all that.

Cheers,
Mike

>
> Thanks in advance,
> Christian
> --
> "A little learning never caused anyone's head to explode!"
>
> "Ein wenig Lernen hat noch niemandens Kopf zum Explodieren gebracht!"
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