[fitsbits] Re: Reading floating point FITS files

[Mark Calabretta]

On Fri 2003/11/14 17:32:49 +1100, Mark Calabretta wrote
in a message to: Steve Allen <sla at ucolick.org>
and copied to: FITSbits <fitsbits at nrao.edu>

>>Yes.  I suspect that most original flux data have been eradicated.
>>In an astronomical image of this sort I can imagine wanting to
>>compare relative fluxes and change the mapping in ways that
>>this JPEG does not allow.

Well you could always twiddle the R, G, and B balance on your monitor!

>It's not hard to think of with ways to handle this (e.g. using the
>PVi_ma keywords on an 'RGB' axis for scaling).  However, the
>constituent images can always be stored separately, even in the same
>FITS file.  The aim is simply to store a colour image in a standard
>way, something which FITS currently cannot do!

Alternatively, store the three images in a data cube with primary WCS
defined by 'WAVE-TAB' (or 'FREQ-TAB', etc.).  This WCS would record the
wavelength (or frequency, etc.) of the radio, optical, and X-ray images
(in the Crab Nebula example).  Pixels values, (BUNIT, BSCALE, BZERO)
would be stored in any desired units (e.g. flux density or even
magnitudes).  Naturally, these units apply to all three planes.

So far we have an "ordinary" data cube.  Now add colour via a secondary
WCS; the 'RGB' coordinate type would have six PVi_ma; an offset and
scale (default values 0.0 and 1.0) for each of the three planes (R, G,
and B).  Voila!

(Getting metaphysical for a moment, it would appear that this sort of
WCS stretches the envelope in applying to pixel values, not just pixel
coordinates.  However, it is essentially the same as the 'COMPLEX' and
'STOKES' "conventional" types, provided that you interpret the 'R' in
'RGB' as meaning "a red colourspace coordinate where the range defined
by PVi_ma maps to 0.0 to 1.0 in a standard colourspace".  This is a
general feature of the conventional coordinate types; they differ
fundamentally in qualifying the value of a pixel, rather than simply
locating it in some coordinate space.  This relates to the fact that
such axes have to be treated specially, principally because they are
not interpolable.)

Mark Calabretta
ATNF