[fitswcs] Question about the Mercator projection.

[William Thompson]

I'm running into an issue with the Mercator projection that makes me ask whether 
  I understand the way that cylindrical projections work.  I have an image with 
the following FITS header (relevant keywords only):

SIMPLE  =                    T / file does conform to FITS standard 
BITPIX  =                  -32 / number of bits per data pixel 
NAXIS   =                    2 / number of data axes 
NAXIS1  =                  500 / length of data axis 1 
NAXIS2  =                  272 / length of data axis 2 
CTYPE1  = 'HGLN-MER' 
CTYPE2  = 'HGLT-MER' 
CRPIX1  = 250.500000 
CRPIX2  = 136.500000 
CRVAL1  = 59.889179 
CRVAL2  = 21.644938 
CDELT1  = 0.030000 
CDELT2  = 0.030000 
CUNIT1  = 'degree' 
CUNIT2  = 'degree' 
END

In the WCS software that I wrote, the resulting lines of longitude and latitude 
are slightly curved, because the reference pixel is not on the equator. 
However, if I open this same file in ds9, the lines of longitude and latitude 
are straight across.  Am I missing something?

Bill Thompson

-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov