ObsCore: o_ucd for uncalibrated data?

[Frederic V. Hessman]

Petr,

On 28 Jul 2015, at 17:10, Petr Skoda <skoda at sunstel.asu.cas.cz> wrote:

>>> phot.count represents the number of photons counted
>>> by the instrument, while what I'm talking about is
>>> really instrumental data numbers (ADUs) which are far
>>> from being the number of photons.

>> ADU’s are not fluxes (ADU/s/pix^2 might be) but simply uncalibrated counts, so I’d go with
>> 
>> 	phot.count;stat.uncalib
>> 
>> However, “phot” means “photometry”, not “photons”, and ADU’s are just a different unit of photometric counting, so
>> 
>> 	phot.count
>> 
>> (some number of photometric events) wouldn’t be so bad after all.  More metadata is usually better, but one has to stop somewhere….

> NO I think its a bad interpretation - I understand the UCD exactly as presented by Frederic
> 
> in http://www.ivoa.net/documents/REC/UCD/UCDlist-20070402.html
> 
> is said
> E | phot.count            | Flux expressed in counts
> 
> unlike
> 
> E | phot.flux                                        | Photon flux
> 
> what does this mean ?     number of photons per second ?
> 
> the term flux is well shaky as (in rigorous terms ) the flux is something per time but IMHO most people still understand the real meaning - BTW in VO lingua everything on vertical axis is FLUX (FluxAxis is synonymum for the measured variable axis evrywhere)

ACK! The whole purpose of the VO is to avoid “most people still understand the real meaning”!  You are perfectly right, of course, but this is a bug, not a feature.  Having phot.count but using it as phot.flux or vice versa is a terrible state of affairs, especially since the nominal problem is in the description, not the vocabulary.  I suggest

	- the description of phot.count be changed to “a photometric measurement expressed in photons, counts, or analogue-to-digital units (ADU)

	- the description of phot.flux be changed to “photometric flux in some units corresponding to energy, events, or photons per time and area (use phot.flux.density jto add per bandpass)

 
> But if the UCD vocabulary would allow phot.ADU it would me more easier to express all RAW data .....

in which case phot.ADU would be superfluous.  It might be good to have phot.count,rate….

BTW: sayting that “phot.fluence” means “fluence” isn’t very helpful.   Try what every young astronomer might do (google “physics definition of fluence") and you’ll find from serious websites

	- Fluence is the number of particles (particle fluence) or amount of energy (energy fluence) entering an imaginary sphere with a cross-sectional area of A cm^2

	-  optical energy per unit area

	- particle density or energy density,

	- radiant exposure or radiant fluence is the radiant energy received by a surface per unit area, or equivalently the irradiance of a surface integrated over time of irradiation

	- A measure of particle flux (that of a pulse of electromagnetic radiation).

and my favorite mangling of flux

	- Fluence is the number of particles that intersect a unit area. Units: 1/m² Flux is the rate at which something flows through a unit area. The units depend on what you're measuring.

so may I also suggest the description be

	phot.fluence		| energy or counts per area (time-integrated flux)

Where is “intensity” except in “spect.line.intensity"?  SI uses intensity and so can every well-educated astronomer.  How about

	phot.intensity			| photometric intensity in some units corresponding to energy, events, or photons per time, area, and solid angle (use phot.intensity.density for per bandpass)
	phot.intensity.density	| intensity per bandpass

Yes, I know no one uses “intensity density”, but it makes this compatible with the phot.flux usage.

Rick