Subject: CTYPE for pixel image

From: pence at tetra.gsfc.nasa.gov (William Pence)
Date: Wed, 3 Nov 93 11:19:44 EST

Does anyone have a recommendation for what values to assign to the WCS
FITS keywords when describing an image of the detector flat-field (or
similar image in the detector reference frame)?  In this case, there is
no projection onto the sky, and the pixel increments (CDELTn) are in
units of millimeters on the detector surface.

Would the following minimum set of keyword be acceptable?

NAXIS    =      2
NAXIS1   =    256
NAXIS2   =    256
...
...
CTYPE1   = 'PIXEL   '           / no projection is involved
CTYPE2   = 'PIXEL   '           / no projection is involved
CDELT1   =     .0027            / increment in mm/pixel on the detector
CDELT2   =     .0027            / increment in mm/pixel on the detector
CUNIT1   = 'mm      '           / physical units of the first image axis
CUNIT2   = 'mm      '           / physical units of the second image axis

The other WCS keywords (CRPIXn, CRVALn, CROTAn, CDiiijjj) would just have
default values, so there is no reason to explicitly list them
in the header.

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From: arots at xebec.gsfc.nasa.gov (Arnold Rots)
Date: Wed, 3 Nov 93 12:19:05 EST

I would submit that the flatfield image you are referring to is not
terribly different from a "real" image generated by the detector, at
least not where the coordinates are concerned: both contain a variable
(intensity, counts, flatfield correction) on the detector's pixel grid.
In the case of the real image, this is projected onto the sky, with the
projection determined by the telescope optics and the reference pixel
on the telescope's optical axis.
If you now decide to measure the flatfield coordinates in mm, you will
have to provide somewhere information on how to transform this
coordinate system to that of an image - that is, if you intend to
apply corrections to such an image.  It would be much simpler and less
error-prone if the flatfield were given in the same coordinate system
as the images, i.e., the projection of the detector pixels on the sky,
with the reference pixel along the optical axis, the position angle at
0, and defaults for the RA and Dec of the reference pixel.

  - Arnold Rots

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From: forveill at gag.observ-gr.fr (Thierry Forveille)
Date: Thu, 4 Nov 1993 14:19:41 +0100

It seems a strange convention to ascribe dummy angular coordinates
to a flat field. Which value should one one then choose for the RA
and Dec of the reference pixel? A flat field is one of the clear
cases where the only adequate unit along the axes is the pixel.
My choice for the header is however rather the following, since the linear 
scale on the detector is irrelevant.

NAXIS    = 2
NAXIS1   = 256
NAXIS2   = 256
..........................................
CTYPE1   = 'PIXEL   '           / no projection is involved
CTYPE2   = 'PIXEL   '           / no projection is involved
CDELT1   = 1                    
CRPIX1   = 1                    
CRVAL1   = 1
CDELT2   = 1                    
CRPIX2   = 1                    
CRVAL2   = 1
CUNIT1   = '        '           / physical units of the first image axis
CUNIT2   = '        '           / physical units of the second image axis