[fitswcs] Question about the Mercator projection.

[William Joye]

All

There is a problem with DS9. In particular, I needed to distinguish between an spherical coordinate system as defined by CTYPE and a straight linear WCS. (you not believe some of the values I see in the CTYPE keyword)

In the past DS9 was using RADESYS to try to determine if the WCS was a celestial system. However, RADESYS which is only valid for equatorial systems and will not be present for systems such as Heliographic coordinates.

I'll have new beta available shortly which will correctly distinguish between a valid spherical system from a straight linear system.

Regards

bill joye

On Dec 10, 2010, at 11:40 AM, William Thompson wrote:

> The starlink website matches the output of my own software, which makes me feel relieved.  However, I'm still having problems with ds9.  I've attached GIF images of the output from both.
> 
> Bill Thompson
> 
> 
> David Berry wrote:
>> Hi Bill,
>>           As I understand it DS9 uses the AST library for plotting
>> its coordinate grids, and AST does produce curved lines, so I'm
>> confused. Maybe Bill Joye can comment. If you go to
>> http://starlink.jach.hawaii.edu/cgi-bin/ast/fits-plotter
>> and paste your FITS headers into the "Headers" box,  then enter
>> "grid=1" into the "plot settings" box (to get grid lines rather than
>> just tick marks) and then press submit, you will see what AST
>> produces.
>> David
>> On 9 December 2010 22:32, William Thompson <William.T.Thompson at nasa.gov> wrote:
>>> 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
>>> 
>>> _______________________________________________
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>>> 
> 
> -- 
> William Thompson
> NASA Goddard Space Flight Center
> Code 671
> Greenbelt, MD  20771
> USA
> 
> 301-286-2040
> William.T.Thompson at nasa.gov
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