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

[Lucio Chiappetti]

On Thu, 1 Apr 1999, Eric Greisen wrote:

> Arnold Rots writes:
>  > I noticed that your list was missing the unit that is dear to many HEA
>  > types' hearts: the "Crab".  

>      if the Crab is as well defined as the Solar units, we could add
> it.  What is the definition?

About the definition of "1 Crab" I guess it could be good to agree a standard
definition. I believe everybody has one's own variant ...

In principle it should be a "flux" unit (ANY flux, from erg/cm2/s to ph/cm2/s
or even cts/s for a particular mission instrument) which results from the
comparison of the integrated flux (in a "given" energy band, see below) of
a Crab-like spectrum, i.e. the spectrum of the synchrotron component in the
Crab nebula   dN/dE = 9.8 * E**(-2.1) * absorption(NH=3.10E21).

At least that's what I use. Little variations in norm, spectral index and NH
are possible according to your favourite reference.

However what is not well defined is the band in which the integral is taken.
In principle you can integrate the above spectrum in ANY band, and compare the
flux of your source in the same band to the Crab-like spectrum.

In practice the definition was born in 2-10 (or 2-6) keV X-ray astronomy, and
it would be of less use and ease in other bands ... of course a, say, 1 mCrab
source in 2-10 keV would'nt be a 1 mCrab in 100-300 keV if the spectrum is
steeper or flatter than 2.1 !

                           BUT .. BUT ... BUT ...

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 ?

I would expect it would be very difficult that the "spatial" axes of an image,
or the "dispersion" axis of a spectrum, would be measured in Crab units (or
in Ohm, if that matters).
So aren't we discussing about "the sex of angels" ?

                            AND MORE BUT'S ...

Even if we would be discussing standardization of units in generic tables, I
reiterate my provocation, is it really worth while to go in extreme detail ?
For what are essentially "plot labels" ?
I'd expect humans (astronomers) be able to read a column heading and know what
it is (and convert to their favourite customary units) anyhow, while software
will just copy and use labels as they are, as "semantic-less" strings.
Do we really expect to have an all-encompassing universal routine library
which will convert any unit to any unit ?

I heard a saying "build a system than any fool can use, and only a fool will
want to use it". But it could also be reversed "build a standard that is too
clever, and nobody will be clever enough to want to use it".

I'm thinking of a recent discussion about a largish database private to a
smallish consortium (less than 10 institutes), of course to me it was obvious
to say FITS had to be used for interchange and not ASCII ... but it was
pointed to me that the programming competences of the "mean quadratic
astronomer" were quite below that mark ...

And if he/she will have difficulty in using age-old consolidated good old
plain FITS, is it likely it will adhere, or even check, a "marginal"
convention ?

Even when major agencies (funded also by my taxes) involved in major missions
produce and distribute tools which measure fluxes in eV/mm2/s (!) instead of
the more usual erg/cm2/s ?  :-) [sorry, I could not resist]

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Lucio Chiappetti - IFCTR/CNR - via Bassini 15 - I-20133 Milano (Italy)      
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