<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">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<div><br></div><div>rough-n-ready suggests</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- raw images are “phot.counts;stat.uncalib" with units “adu”</div><div><br></div><div>physically strict means that</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- 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)</div><div><br></div><div>Rick</div><div><br></div><div><div><div><div><div>On 10 Aug 2015, at 18:28, Louys Mireille <<a href="mailto:mireille.louys@unistra.fr">mireille.louys@unistra.fr</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
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<div class="moz-cite-prefix">Hi Rick , Hi all,<br>
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I also agree that 'phot' as the root element is defined for all
the photometric measurement in the UCD specification. <br>
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So in Alberto's usecase , "phot.count;stat.uncalib" is perfectly
valid. <br>
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 <i>unit='adu'</i>.<br>
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I take the point about "Fluence" : this is an addition following
the requirements addressed by the solar and planetary users. <br>
we will sort out a new definition as you suggest. More on this
very soon. <br>
Best regards , <br>
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Mireille Louys ,
CDS and Icube laboratory, Strasbourg University
IVOA Semantics WG Chair </pre>
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Le 28/07/2015 21:23, Frederic V. Hessman a écrit :<br>
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<pre wrap="">Petr,
On 28 Jul 2015, at 17:10, Petr Skoda <a class="moz-txt-link-rfc2396E" href="mailto:skoda@sunstel.asu.cas.cz"><skoda@sunstel.asu.cas.cz></a> wrote:
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<pre wrap="">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.
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<pre wrap="">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….
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<pre wrap="">NO I think its a bad interpretation - I understand the UCD exactly as presented by Frederic
in <a class="moz-txt-link-freetext" href="http://www.ivoa.net/documents/REC/UCD/UCDlist-20070402.html">http://www.ivoa.net/documents/REC/UCD/UCDlist-20070402.html</a>
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)
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<pre wrap="">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)
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<pre wrap="">But if the UCD vocabulary would allow phot.ADU it would me more easier to express all RAW data .....
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<pre wrap="">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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