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

[Lucio Chiappetti]

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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