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

[Barrett, Paul]

I am adding unit support to the Julia FITS package FITSFiles.jl. Table 4 of
the FITS specification (aka standard) lists several nonphysical units, such
as *pixel, voxel, photon, count, beam,* etc. The relationship and usage of
physical units is well known and understood, but this does not appear to be
the case for nonphysical units. For example, is a *photon* equivalent to a
*count* ? In other words, if I divide a *count* by a *photon*, is that
unitless? And what does *count^2* mean? Can anyone explain this or should I
just ignore it?

 - Paul

-- 
Paul Barrett, PhD
Department of Physics
The George Washington University
Washington, DC 20052
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