more on FITS and VO

[Doug Mink]

William Pence wrote:
> 
> The intent of this subtle change is wording is to allow the "image/fits"
> mimetype to be legally applied to FITS files in which the Primary
> array has NAXIS greater than zero, and has one or more NAXISn keywords
> equal to zero, and thus contains 0 pixels,  e.g.,
>   NAXIS  =   2
>   NAXIS1 = 100
>   NAXIS2 =   0
> 
> Take for example a web site or service that returns 2-dimensional FITS
> images to the browser, with mimetype 'image/fits'.   Some people have
> argued that there could be times when the web site would need to return
> a zero pixel length image, e.g., the user might only want the FITS
> header information (metadata) without the image itself, or a zero pixel
> image might be returned if the requested image position was not available
> for some reason (outside of the area covered by a survey).  For whatever
> reason, some people have argued, there *might* be occasions where the
> web service would need to return a zero pixel image, and in that case
> it would be more convenient, and less confusing to the end user, to assign
> 'image/fits' to the file, just like all the other images that it returns.

Not only might there be reasons for a web service to return a zero pixel image,
so the receiving browser realizes that there is a header, but no image, to
display, but such images also come in handy when working solely with header
information and information extracted from a very large image, such as an
objet catalog, which may follow as a table extension or be accessed in
another way.  In my own WCSTools software, I use BITPIX=0, keeping all of
the image dimensions.  FITS WCS uses WCSAXES to give the number of axes for
which a WCS is present; NAXIS could be zero and WCSAXES greater than zero
and various NAXISn > 0.

-Doug Mink