Re: Image Orientation

[Stupendous Man <richmond@a188-l009.rit.edu>]

 [typographical errors noticed by Jure Skvarc have been fixed.  MWR 3/24/2001]

  I wrote:

>> Some programs will look at the header
>> and try to figure out which way is North, and which East,

  Tom asked:

> What key words do they then look at?  
> I would like to put them in the header.

  For a good description of the issues involved, see the 
World Coordinate Systems page

     http://tdc-www.harvard.edu/software/wcstools/wcstools.wcs.html

  You should use the following keywords:

     CTYPE1  = 'RA---TAN'     /  means that RA runs along axis 1
                              /    the 'TAN' part means that the image
                              /    is a projection of the celestial sphere
                              /    onto a plane tanget at the (RA, Dec) 
                              /    values we'll specify below.
     CRPIX1  =    1024        /  means that pixel 1024 along axis 1
                              /    will have the RA values specified by CRVAL.
                              /    You should probably use the middle of 
                              /    the chip.
     CRVAL1  =    12.3456     /  is the value of RA (in this example) 
                              /    at the pixel specified by CRPIX1.
                              /    Most people use decimal degrees.
     CDELT1  =     0.00208    /  means that advancing by one pixel along 
                              /    axis 1 increases the RA value by 0.00208
                              /    units.  For the Mark IV, each pixel
                              /    is 7.5 arcsec = 0.00208 degrees.

  One must also provide an analogous set of keywords for the other 
axis, CTYPE2 (which will be 'DEC---TAN'), CRPIX2, etc.

  The keys are

       1) to make sure that the proper axis is identified with RA
       2) to make sure that the signs of CDELT1 and CDELT2 are correct

  If the chip isn't oriented exactly North/South/East/West, one can
use the CROTA1, CROTA2 keywords to indicate a rotation angle between
the axes and the cardinal directions.  The rotation must be taken 
around the pixel specified by the CRPIX1, CRPIX2 keywords.

                                     Michael Richmond