Using Prematched stars and the USNO-A2.0 Catalog

If an image has features which make it hard to match specific stars, it is useful to match up a few image stars (5 is a reasonable number) to catalogued positions by hand. imwcs, as of version 2.8.7, accepts a file of x,y,RA,Dec matched positions and fits an initial world coordinate system to them. That WCS may be either written to the image header or used as an initial guess to fit a more complete list of images star positions and a deep catalog, such as the US Naval Observatory A2.0 Catalog, to get a better WCS.

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This FITS image, courtesy of Andy Szentgyorgyi of the Harvard-Smithsonian Center for Astrophysics, contains the planetary nebula NGC 246, the coordinates of whose features we want to find. The header keywords


NAXIS   =                    2 / Number of axes
NAXIS1  =                  495 / Axis length
NAXIS2  =                  519 / Axis length
RA      = '00:44:32.90'        /MEAN RA
DEC     = '-12:08:44'          /MEAN DEC
EPOCH   = 1950.0               /MEAN EPOCH
SECPIX  =                 0.63

provide a nominal WCS which is used at the left by imua2 to plot the catalog over the image.

Because the nebula could confuse the automatic star-finding program and the reference catalogs, a prematched set of x, y, right ascension, and declination coordinates for 5 stars in the field were placed in the file ngc246.stars:

72.04   143.88  00 47 10.984    -11 53 18.92
114.91  185.80  00 47 09.153    -11 52 52.70
370.15  400.11  00 46 58.055    -11 50 36.60
325.49  191.93  00 47 00.064    -11 52 49.89
418.23   30.94  00 46 56.044    -11 54 32.13

These selected stars are plotted in the image at the right. These matches are used by imwcs to fit a WCS using header and command-line information for an initial value.

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To quickly find the 100 brightest stars in the image, Emmanuel Bertin's SExtractor program was used through a shell script, imsex, which sets up the parameter files it needs and prepares the image, if necessary:


$ imsex ngc246.fits
ngc246.fits -> ngc246.fits
Measuring from: "NGC246_NeV" / 495 x 519 / 32 bits FLOATING POINT data
(M+D) Background: 0.0070038  RMS: 0.00744432 / Threshold: 0.0111665  
Objects: detected 115      / sextracted 55      
> All done

$ imwua2 -ew -d ngc246.sex -u ngc246.stars ngc246.fits

The verbose mode gives more information about how the fit is going:

$ imwua2 -ewv -d ngc246.sex -u ngc246.stars ngc246.fits

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The final FITS image contains standard FITS world coordinate system keywords and a logging keyword showing what version of imwcs fit the WCS:


CRPIX1  =             247.5000      / Optical axis X coordinate
CRPIX2  =             259.5000      / Optical axis Y coordinate
WCSRFCAT= 'ua2     '                / Reference catalog
WCSIMCAT= 'NGC246_NeV_F.sex'        / Catalog of image coordinates of stars
WCSMATCH=                    9      / Number of stars matched and fit
WCSNREF =                   25      / Number of reference stars in the image
RA      = '00:47:03.422'            / Right ascension of optical axis
DEC     = '-11:52:05.70'            / Declination of optical axis
WEQUINOX=                 2000      / Old Equinox
EPOCH   =                 2000      / Epoch (really equinox) of WCS
RADECSYS= 'FK5     '                / Coordinate system of WCS
CTYPE1  = 'RA---TAN'                / X-axis WCS projection
CTYPE2  = 'DEC--TAN'                / Y-axis WCS projection
CRVAL1  =         11.764280414      / Right ascension of optical axis
CRVAL2  =        -11.868264512      / Declination of optical axis
CDELT1  =         -0.000176282      / Degrees of RA per pixel
CDELT2  =          0.000176973      / Degrees of Dec per pixel
CROTA1  =                0.215      / WCS rotation angle
CROTA2  =                0.215      / WCS rotation angle
WCSTOL  =              10.0000      / WCS matching tolerance in pixels
EQUINOX =                 2000      / WCS equinox
SECPIX1 =               0.6346      / Arcseconds per pixel in right ascension
SECPIX2 =               0.6371      / Arcseconds per pixel in declination
WCSSEP  =                0.839      / Mean arcsec offset from catalog to image position 
IMWCS   = '2.8.7, 11 January 2001, Doug Mink, SAO'

Last updated 11 January 2001 by Doug Mink dmink@cfa.harvard.edu