[fitswcs] Question about the Mercator projection.

[William Thompson]

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
>>
>> _______________________________________________
>> fitswcs mailing list
>> fitswcs at listmgr.cv.nrao.edu
>> http://listmgr.cv.nrao.edu/mailman/listinfo/fitswcs
>>
> 

-- 
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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