[fitsbits] The meaning and use of nonphysical FITS units {External} {External} {External}

[Barrett, Paul]

Yes, I understand all of this. Note that *cts/s* should technically be *Hz*.

That is what I am asking: does dividing two dimensionless units have any
meaning? As noted above, there is an alternate and preferred way of
specifying such units that has meaning and proper mathematical behaviour.

The FITS specification lists these units, but does not describe their
usage. As a result, it leaves their interpretation up to the software
developer, the creator of the FITS file, and the user, who may all have
different interpretations. A common interpretation would be preferable.

 -- Paul

On Fri, Jun 6, 2025 at 10:16 AM Lucio Chiappetti via fitsbits <
fitsbits at listmgr.nrao.edu> wrote:

> On Fri, 6 Jun 2025, Barrett, Paul via fitsbits wrote:
>
> > the FITS specification (aka standard) lists several nonphysical units,
> > such as *pixel, voxel, photon, count, beam,* etc.
>
> I'm not talking for any oast or present FITS committee but only for myself
> (and common sense ?)
>
> More than "nonphysical" I'd call them "dimensionless" from the point of
> view of dimensional analysis
> https://en.wikipedia.org/wiki/Dimensional_analysis?useskin=vector
>
> > For example, is a *photon* equivalent to a *count* ?
>
> I guess it depends on the context. E.g. in n X-ray astronomy it is common
> to refer to a "count spectrum"  as cts/s or cts/s/PHA channel or cts/s/bin
> (where channel or bins are also dimensionless), which is what the detector
> measures. And to a "photon spectrum" dN/dE as photon/cm2/s/keV is what
> the celestial source emits. The relation between the two, used in
> spectral fitting, is given by the convolution with the response matrix
> (RMF and ARF)
>
> > In other words, if I divide a *count* by a *photon*, is that unitless?
>
> I would say "dimensionless", you divide two dimensionless quantities and
> obtain a dimensionless quantity. If that has any meaning.
>
> > And what does *count^2* mean?
>
> Never encountered, and seems to have no meaning for me
>
> --
> Lucio Chiappetti - INAF/IASF - via Corti 12 - I-20133 Milano (Italy)
> For more info : http://www.iasf-milano.inaf.it/~lucio/personal.html
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