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<div class="moz-cite-prefix">Thanks Arnold to clarify, and sorry to
miss your previous explanation.<br>
<br>
In fact, more than discussing the method used to describe an area
on the sphere (by STC regions or MOC or whatever), it is the
meaning of this area that you highligth. And, if I correctly
follow you, the question is : do we want to characterize the
observed regions ? or the union of the effectives detected sources
(catalog case) or pixels (image case) ?<br>
<br>
Now, in the first case - for catalogs with ponctual sources -
how to build this coverage ? I do not know how to do with STC
regions (no source area => empty union - default radius ?
agregation algorithm with minimal distance ? ). At the opposite,
MOC provides something well defined : the union of all MOC cells
having at least one source. The accuracy of the result is
dependent of the MOC resolution.<br>
<br>
In any case, observed regions or effective detected elements -
each option has its own use cases. Now which one (or both) do we
want to have in the registry ?<br>
<br>
Cheers<br>
Pierre<br>
<br>
Le 02/02/2018 à 20:23, Arnold Rots a écrit :<br>
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<blockquote type="cite"
cite="mid:CAJXToE9jWL0E18s0RPODTpP96h04WK0+QoCcKKtKHSb8Gx2vgQ@mail.gmail.com">
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<div>I have mentioned this
before.<br>
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A common positional query to
catalogs basically asks three
questions:<br>
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1. Is this location covered by the
catalog?<br>
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2. If so, is there an entry
associated with this location?<br>
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3. If so, what is that entry?<br>
<br>
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My understanding is that the MOCs
associated with (most) catalogs<br>
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provide the coverage of the <b>records </b>in
the catalog. As a consequence,<br>
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<div>they can only give a definite response
if the answer to the second<br>
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<div>question is yes.<br>
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For the Chandra Source Catalog our coverage
represents the union<br>
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of the fields of view of all the observations
used to create the catalog.<br>
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This means that if a location is included in the
coverage, but does<br>
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not correspond to a source, the user knows that
there is a non-<br>
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detection at that location which, in many cases is
as significant as<br>
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knowing there is a source.<br>
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If on the other hand, the coverage is solely based on
the catalog's<br>
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records, a NO-answer is ambiguous; it may be: <i>no, we
had a non-<br>
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<i>detection</i>; or: <i>we don't know since we didn't look
there</i>.<br>
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I think that is a serious shortcoming. Non-detections are
significant,<br>
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<div>particularly in the context of SEDs and the time domain.<br>
<br>
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<div>Cheers,<br>
<br>
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<div> - Arnold<br>
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<div class="gmail_extra"><br clear="all">
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<div class="gmail_signature" data-smartmail="gmail_signature">
<div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>
Arnold H. Rots
Chandra X-ray Science Center<br>
Smithsonian Astrophysical Observatory
tel: +1 617 496 7701<br>
60 Garden Street, MS 67
fax: +1 617 495 7356<br>
Cambridge, MA 02138
<a href="mailto:arots@cfa.harvard.edu"
target="_blank" moz-do-not-send="true">arots@cfa.harvard.edu</a><br>
USA <a
href="http://hea-www.harvard.edu/%7Earots/"
target="_blank" moz-do-not-send="true">http://hea-www.harvard.edu/~arots/</a><br>
--------------------------------------------------------------------------------------------------------------<br>
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<div class="gmail_quote">On Thu, Feb 1, 2018 at 5:27 PM, Pierre
Fernique <span dir="ltr"><<a
href="mailto:Pierre.Fernique@astro.unistra.fr"
target="_blank" moz-do-not-send="true">Pierre.Fernique@astro.unistra.fr</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex"><span
class=""><br>
Le 30/01/2018 à 20:13, Arnold Rots a écrit :<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
Another issue is that MOCs generated<br>
for catalogs generally reflect the distribution of the
catalog's<br>
records, not the true coverage of the catalog.<br>
</blockquote>
<br>
</span>
Arnold can you detail this point ? I'm not sure that I
caught your argument.<br>
Thanks<span class="HOEnZb"><font color="#888888"><br>
Pierre<br>
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