Crab et al., a warning Re: wcs.ps

[Lucio Chiappetti]

On Fri, 2 Apr 1999, Eric Greisen wrote:
> Lucio Chiappetti writes:
>  > On Thu, 1 Apr 1999, Eric Greisen wrote:
>  > > Arnold Rots writes:

>       Arnold's earlier reply that "we don't want to go there" about
> Crabs is well explained by your description of them.  I have no
> intention of putting them in the paper, until the community gets a
> much better definition.

  Ok for letting the Crab out

>  > One moment. Are we talking here of standardization of unit indication in
>  > generic FITS files (tables) or just of WCS ?
>  > And does not WCS apply "primarily" (or "originally") to IMAGES and
>  > "secondarily" (or "more recently") to SPECTRA ?
> 
>      WCS applies to any observation of a physical quantity.  The issue
> is most pressing when we attempt to associate coordinates with the
> voxels in an image, but images as well as single pixels can and do
> occur in tables and elsewhere.

  hmm ... tables within tables, or images within tables, look to me what
  you'd call "fancy" stuff.

  I do appreciate that an image can be other than a sky image (see one
  example further below), but most of the WCS image paper is for astrometry,
  or anyhow sky coordinate handling (and that's not particularly my field).
  
  Also a spectrum is in generally something quite defined (flux vs energy)
  although there can be peculiarities (and the WCS spectral papeer addresses
  many of them from optical astronomy, again things I'm not particularly
  familiar with) particularly for "raw" or "nearly raw" data.

  I could imagine the need of some standard for flux conversions of "reduced"
  spectra. I mainly think of the conversion of dN/dE vs E from/to Fnu vs nu,
  Flambda vs lambda, nuFnu (SED) etc.  with proper handling of error bars,
  and clear distinction of the case the single points in a spectrum are
  monochromatic or broad band fluxes (in which case proper conversion need
  to know the form of the underlying continuum !)


  But "any other thing" is just an "histogram" or "plot" of some y quantity
  vs some x quantity. Is it worthwhile to commit to a very hard enterprise
  of a fully general handler ?

  Or is it better to consider only the cases of images and spectra, and
  at most light curves and other timing analysis data structures (thinking of 
  x-axis which could be in time, frequency, or phase) ...

> I agree that Ohm is unlikely in astronomical data and so we are free
> to change its spelling, but Crabs and Angstroms are not unlikely.

Angstrom are extremely likely on the X (dispersion, or energy) axis of an UV
or optical spectrum. Crabs (a flux unit) are totally unlikely (probably
meaningless) as units of the axes of an image or a spectrum, and even if they
are flux units, quite unlikely also on the "Z axis" (BUNIT), or the Y axis of
a spectrum. They are likely to occur only in pure tables (catalog entries).


>      WCS are not plot labels.  They have that function, but they have
> a great deal more as well.

I've never told that WCS were plot labels, my reference to plot labels was to
the physical unit convention (i.e. the part of the Appendices which were not
immediately related to WCS). It was even farther from my mind to criticise or
diminish the WCS effort ! I do appreciate at least some usages of "original"
WCS for astrometry, and of course also the ones in the spectroscopy paper.

My doubts were on the insertion of the ENTIRE business of units into the WCS
approval process, particularly for what concerned "exotic" units, or anyhow
units outside the ra-dec-energy-flux (astrometry/spectroscopy) context.

After all the important thing in WCS are "internal transformation" (like sky
projections going beyond the original CRPIX CRVAl CDELT, or intrinsic usage of
"log(units)"   ... I tend myself to keep chi-square grid vs spectral index and
column density NH in form of images ... and of course NH has a log range ...).

But once there is a clear convention (-> WCS) to know that the value at pixel
coordinates i,j correspond a given log(NH),gamma, or instead to
log(pears),apples ... NH and gamma, or pears and apples, become plot labels.

----------------------------------------------------------------------------
Lucio Chiappetti - IFCTR/CNR - via Bassini 15 - I-20133 Milano (Italy)      
For more info : http://www.ifctr.mi.cnr.it/~lucio/personal.html             
----------------------------------------------------------------------------