Draft note on STC in the Registry

[Arnold Rots]

In principle, each catalog should provide, in its description,
the part of the sky that is covered in selecting its records.
It may take some work to dig it out and turn it into a MOC,
but it is not impossible and should be pretty straightforward
- unless the authors are deficient in their description.

Providing a coverage description of the records may be
nice, but is scientifically much less interesting than the
the coverage of the data used to construct the catalog;
and the reason is simply that non-detections are generally
as significant as detections when one is looking for
specific locations.

Cheers,

  - Arnold

-------------------------------------------------------------------------------------------------------------
Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------------------------------------------


On Mon, Feb 5, 2018 at 2:27 AM, Pierre Fernique <
Pierre.Fernique at astro.unistra.fr> wrote:

> Thanks Arnold to clarify, and sorry to miss your previous explanation.
>
> 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) ?
>
> 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.
>
> 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 ?
>
> Cheers
> Pierre
>
>
> Le 02/02/2018 à 20:23, Arnold Rots a écrit :
>
> I have mentioned this before.
> A common positional query to catalogs basically asks three questions:
> 1. Is this location covered by the catalog?
> 2. If so, is there an entry associated with this location?
> 3. If so, what is that entry?
>
> My understanding is that the MOCs associated with (most) catalogs
> provide the coverage of the *records *in the catalog. As a consequence,
> they can only give a definite response if the answer to the second
> question is yes.
> For the Chandra Source Catalog our coverage represents the union
> of the fields of view of all the observations used to create the catalog.
> This means that if a location is included in the coverage, but does
> not correspond to a source, the user knows that there is a non-
> detection at that location which, in many cases is as significant as
> knowing there is a source.
> If on the other hand, the coverage is solely based on the catalog's
> records, a NO-answer is ambiguous; it may be:
> *no, we had a non- *
> *detection*; or: *we don't know since we didn't look there*.
> I think that is a serious shortcoming. Non-detections are significant,
> particularly in the context of SEDs and the time domain.
>
> Cheers,
>
>   - Arnold
>
>
> ------------------------------------------------------------
> -------------------------------------------------
> Arnold H. Rots                                          Chandra X-ray
> Science Center
> Smithsonian Astrophysical Observatory                   tel:  +1 617 496
> 7701 <(617)%20496-7701>
> 60 Garden Street
> <https://maps.google.com/?q=60+Garden+Street&entry=gmail&source=g>, MS
> 67                                      fax:  +1 617 495 7356
> <(617)%20495-7356>
> Cambridge, MA 02138
> arots at cfa.harvard.edu
> USA
> http://hea-www.harvard.edu/~arots/
> ------------------------------------------------------------
> --------------------------------------------------
>
>
> On Thu, Feb 1, 2018 at 5:27 PM, Pierre Fernique <Pierre.Fernique at astro.
> unistra.fr> wrote:
>
>>
>> Le 30/01/2018 à 20:13, Arnold Rots a écrit :
>>
>>> Another issue is that MOCs generated
>>> for catalogs generally reflect the distribution of the catalog's
>>> records, not the true coverage of the catalog.
>>>
>>
>> Arnold can you detail this point ? I'm not sure that I caught your
>> argument.
>> Thanks
>> Pierre
>>
>>
>
>
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