[fitsbits] Spectral FITS -- encoding extraction area

[Tom Jarrett]

Thanks to all for these very helpful suggestions and pointers.
We now have much food for thought. I personally like the idea
of using a separate image (or "mask") to describe the spatial
complexities of the spectral 1-D or 2-D data.  There may be a way
to combine the virtues of both WCS-space and Detector-space
information that is encoded in an pixel mask image.  The Infrared
Science Archive (IRSA), e.g., may be able to create a tool that
reads the image WCS, interprets the masking (detector) code,
and creates an appropriate polygon or HTM tesselation for
the encode sky coverage.  In some sense, a pixel mask passes
on the hard work to the next tool down the line.  But at least
all of the information is there.

Can someone point me to an example FITS image where something
like this has been done -- namely:

1. fits binary table of 1-d or 2-d spectra
2. 2-d array consisting of spatial "mask" (integer code)
3. optionally, as Jaffe suggests, a spatial plane consisting of a weighting mask

or something comparable.  I'd like to see just how it is done
with FITS (e.g., can you mix within one file a binary table and
a 2-D image array?  not sure I've seen this before ...)

Thanks again folks,
-tom


Doug Tody wrote:
> The "pixel mask" facility used in IRAF and some other projects does exactly
> this.  Bill (Pence) - do you know what happened to that paper we did on
> compression in FITS some years back, which included a discussion of pixel
> masks?  Pixel masks are good for outlining irregular areas and can be
> expressed in a compressed from in a binary table.
> 
> The purpose of the region stuff in VO is mainly to outline regions on the
> sky in world coordinates.  This gets compiled into more efficient formats
> such as HTM or pixel mask for use by software.  This approach is better
> for working in "world coordinate" space; the pixel mask approach may be
> preferable for describing masks such as extraction masks, exposure masks,
> etc.  at the level of an instrument or detector.     Doug
> 
> 
> On Thu, 17 Feb 2005, Walter Jaffe wrote:
> 
>> For encoding irregular extraction areas, I recommand NOT using
>> a list of polygon vertices, but rather an image, which is much
>> more general and intuitive.  So in one FITS file you could
>> include both your spectra and a "finding chart" image, say of
>> integer values=0,1,2,3.  Pixels where chart values=1 were included
>> in spectrum 1, etc.  A more complicated variation is where pixels are
>> included in spectra with various unequal weights.  Then I would
>> use a 3-d image with the weights for each spectra in one plane of
>> the image.
>>
>>
>> _______________________________________________