[fitsbits] Spectral FITS -- encoding extraction area

[William Pence]

That paper on FITS image compression was presented at ADASS IX, (1999):
      http://adass.org/adass/proceedings/adass99/P2-42/
The pixel mask image compression algorithm is supported by CFITSIO, and the 
implementation is described at:
      http://heasarc.gsfc.nasa.gov/docs/software/fitsio/compression.html

It is common practice in astronomical X-ray spectro-analysis to provide a 
weighted 2D image map (WMAP) to define the area on the detector or sky that 
was used to extract the spectrum.  An example of a ROSAT spectral FITS file 
is at:

ftp://legacy.gsfc.nasa.gov/rosat/wgacat/data/600000/rp600254a01/rp600254a01_in_94.pha.Z

in which the image map (uncompressed) is contained in the primary array, and 
the spectrum is stored in the following FITS binary table.

The details of the WMAP format are described in a document at
http://heasarc.gsfc.nasa.gov/docs/heasarc/ofwg/docs/spectra/ogip_96_001/ogip_96_001.html

Bill Pence


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.
>>
>>
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>>
> 


-- 
____________________________________________________________________
Dr. William Pence                          William.D.Pence at nasa.gov
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