[fitsbits] [mhvk at astro.utoronto.ca: Question about FITS format for logarithmic units]

[Michael Droettboom]

We should definitely get involved in the VOUnit standardization effort on
these things, as the current draft hints at, but doesn't really
sufficiently address these issues.

Mike


On Thu, Dec 12, 2013 at 9:22 AM, Marten van Kerkwijk <mhvk at astro.utoronto.ca
> wrote:

> Hi Peter,
>
> >> (1) How would I indicate a dimensionless but logarithmic quantity such
> >>     as dex? If I understood the standard correctly, log(surface gravity)
> >>     might have the unit "log(cm/s2)", but how about a dimensionless one
> >>     (like metallicity).  Would it be "log()", or, by analogy with
> >>     magnitude, just "log"?
> >
> > If something has no unit that the unit could be viewer to be unity.
> > Hence I would say use log(1). Leaving the argument empty would otherwise
> > probably break parsers.
>
> Thanks!  definitely seems much more logical than the log() or log('') I
> suggested, especially since the standard explicitly defines it as the
> function having taken an argument and divided it by the unit in
> parenthesis.  (And it also matches nicely with our internal astropy use,
> where dimensionless unscaled can be represented by Unit(1).)
>
> My colleage Michael Droettboom looked at wcslib in a bit more detail and
> it looks like that package assumes that prefactors for the log()
> function are not allowed, and hence that it is not possible to pass that
> package a unit of dB(mW) or mag(AB).
>
> From the discussion -- which I do find interesting! -- it seems that (as
> yet) the FITS standard similarly does not define these cases.  Like we
> already do for units with non-factor-of-ten scale, it may be best for
> now to simply warn the user that this cannot be stored in a way that one
> can count on other packages understanding, and give a hint on what would
> be needed to make it more easily supported.
>
> I do think eventually one would like to be able to handle such units,
> however ugly they are (they are just too commonly used...).  In the
> recommendations on units by NIST [1], two possibilities are mentioned,
> the default one perhaps more alike to Tim's, where the unit is "dB" (in
> that example) and the description "L_P (re 20 μPa)" (where "re" stands
> for reference level, which I must say I rather dislike) and a "condensed
> version" that is more similar to what I had in mind, as that unit
> carries all the required information in one place: "dB (20 μPa)"
>
> Thanks again,
>
> Marten
>
>
> [1] http://physics.nist.gov/Pubs/SP811/sec08.html#8.7
>



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