Draft note on STC in the Registry

[Pierre Fernique]

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
> 60 Garden Street, MS 67   fax:  +1 617 495 7356
> Cambridge, MA 02138 arots at cfa.harvard.edu <mailto:arots at cfa.harvard.edu>
> USA http://hea-www.harvard.edu/~arots/ 
> <http://hea-www.harvard.edu/%7Earots/>
> --------------------------------------------------------------------------------------------------------------
>
>
> On Thu, Feb 1, 2018 at 5:27 PM, Pierre Fernique 
> <Pierre.Fernique at astro.unistra.fr 
> <mailto: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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