<div dir="ltr"><div><div><div><div><div><div><div><div><div><div><div><div><div><div><div><div><div>I have mentioned this before.<br></div>A common positional query to catalogs basically asks three questions:<br></div>1. Is this location covered by the catalog?<br></div>2. If so, is there an entry associated with this location?<br></div>3. If so, what is that entry?<br><br></div>My understanding is that the MOCs associated with (most) catalogs<br></div>provide the coverage of the <b>records </b>in the catalog. As a consequence,<br></div><div>they can only give a definite response if the answer to the second<br></div><div>question is yes.<br></div>For the Chandra Source Catalog our coverage represents the union<br></div>of the fields of view of all the observations used to create the catalog.<br></div>This means that if a location is included in the coverage, but does<br></div>not correspond to a source, the user knows that there is a non-<br></div>detection at that location which, in many cases is as significant as<br></div>knowing there is a source.<br></div>If on the other hand, the coverage is solely based on the catalog's<br></div>records, a NO-answer is ambiguous; it may be: <i>no, we had a non-<br></i></div><i>detection</i>; or: <i>we don't know since we didn't look there</i>.<br></div>I think that is a serious shortcoming. Non-detections are significant,<br></div><div>particularly in the context of SEDs and the time domain.<br><br></div><div>Cheers,<br><br></div><div> - Arnold<br><br></div></div><div class="gmail_extra"><br clear="all"><div><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">arots@cfa.harvard.edu</a><br>USA <a href="http://hea-www.harvard.edu/~arots/" target="_blank">http://hea-www.harvard.edu/~arots/</a><br>--------------------------------------------------------------------------------------------------------------<br><br></div></div></div>
<br><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">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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