ObsCore: o_ucd for uncalibrated data?

[Frederic V. Hessman]

There is a remaining question about whether the metadata should be rough-n-ready or physically strict - since we’re apparently in the process of defining best-use, this may be a good point to ask ourselves what we want: for instance

rough-n-ready suggests
	- raw images are “phot.counts;stat.uncalib" with units “adu”

physically strict means that
	- raw images are “phot.fluence;stat.uncalib" with units “adu.pix-2” (integral over exposure time, bandwidth, and square-pixel area of uncalibrated monochromatic flux spectrum)

Rick

On 10 Aug 2015, at 18:28, Louys Mireille <mireille.louys at unistra.fr> wrote:

> 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 :
>> 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
>> 

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