Capability extension for describing whole sky maps

[Pierre Fernique]

Le 20/05/2010 23:10, Tom McGlynn a écrit :
> It seems to me that there are three things being combined here:
>
>    All-sky data
>    Hierarchical imaging
>    HEALPix projection
>
> The first and last don't seem particularly special to me: many surveys
> have
> had all-sky coverage for a long time.  Nor does HEALPix seem
> particularly different.
> It's just another projection, albeit one that a lot of software does
> not yet
> support.  What strikes me as distinctive for the new Aladin service is
> the
> hierarchical imaging, which I don't think is well described in our
> current framework.

Tom, stop me if I'm wrong but neither HTM nor HEALPix are projections, but 
hierarchical sky division. The projection used for displaying the map is another 
thing.
If this sky division is specified in the registry (HTM + some parameters or 
HEALPix + some parameters, or...), anybody will be able to build a client for 
accessing any cells of this hierarchical sky resource. I do not describe more 
the framework only because it seems for me not required. If you have the ID of a 
cell (in HTM a triangle, in HEALpix a diamond), you know by construction the sky 
coordinates of their corners. It is enough for displaying  it in the client with 
the projection of your choice.

I will not try to convince VO people for using one unique sky division, but just 
have a way to describe the sky division used by a hierarchical sky server such 
as CDS one (Healpix), Microsoft one (HTM/TOAST) or Google one (?) (if they want 
to offer an free access).

Cheers
Pierre

>
> So I think that's the element I'd emphasize rather than HEALPix which
> I see as a detail
> in a particular implementation.  E.g., WWT proides a similar service
> using the TOAST
> projection and I believe GoogleSky does the same using a Cartesian
> projection (or maybe
> even Mercator).
>
>     Regards,
>     Tom
>
>
> Pierre Fernique wrote:
>>
>> Dear registry members,
>>
>> As I presented Tuesday 18 May during the App session at Victoria
>> interop, I was suggesting an extension of the "capability" section for
>> describing whole sky maps/surveys (for those who do not follow this
>> talk, here the pdf -
>> http://www.ivoa.net/internal/IVOA/InterOpMay2010Applications/AladindemonstrationInteropVictoria2010.pdf).
>>
>> Do you think that it is a good idea. And if yes, may you help me for
>> describing it properly in the schema of the registry.
>>
>> Here is my draft :
>>
>> <capability xsi:type= "SimpleAllSkyAccess">
>> <interface type="HEALPIXmap">
>> <accessURL use="base">http://..</accessURL>
>> <HEALPix>
>> <frame>Gal</frame>
>> <tileFormat>FITS</tileFormat>
>> <treeDepth>12</treeDepth>
>> </HEALPix>
>> </interface>
>> …
>> </capability>
>>
>> type : HEALPIXmap, HTMmap,... = sky tessellation method
>> frame : GAL, ECL, EQU = tessellation reference frame
>> titleFormat : FITS, PNG, JPG .... = image format for individual tile
>> treeDepth: number = hierarchical depth (in Healpix = Norder, or
>> log2(NSIDE))
>>
>> This kind of "SimpleAllSkyAccess" resource may be queried just by tile
>> ID as parameter. For Healpix it is a couple : Norder/npixNumber. For HTM
>> it will be the triangle ID...
>>
>> http://base.adresse..cgi?tile=ID
>>
>> With this very simple interface, any body should be able to build a
>> client for displaying this kind of resources.
>>
>> Thanks
>> Pierre Fernique
>>
>