ObsCore: o_ucd for uncalibrated data?
mireille.louys at unistra.fr
Mon Aug 10 18:28:45 CEST 2015
Hi Rick , Hi all,
I also agree that 'phot' as the root element is defined for all the
photometric measurement in the UCD specification.
So in Alberto's usecase , "phot.count;stat.uncalib" is perfectly valid.
UCD are not dedicated to fully identify the nature +format+unit of a
quantity , so we should keep the Unit information separate, as this is
done in the VOTable specification for PARAM of FIELD, where the unit
attribute can be used as /unit='adu'/.
I take the point about "Fluence" : this is an addition following the
requirements addressed by the solar and planetary users.
we will sort out a new definition as you suggest. More on this very soon.
Best regards ,
Mireille Louys ,
CDS and Icube laboratory, Strasbourg University
IVOA Semantics WG Chair
Le 28/07/2015 21:23, Frederic V. Hessman a écrit :
> 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
>>> However, “phot” means “photometry”, not “photons”, and ADU’s are just a different unit of photometric counting, so
>>> (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
>> 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.
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