s_region problem

[Walter Landry]

Markus Demleitner <msdemlei at ari.uni-heidelberg.de> wrote:
> Felix,
> 
> On Wed, Nov 29, 2017 at 10:27:32AM +0100, Felix Stoehr wrote:
>> > Question 2: Assuming some tool support (i.e., "STC-S-to-MOC
>> > converter", and MOC support in their backen databases), how would
>> > current STC-S providers feel about migrating to MOCs on a time frame
>> > of a few years?
>> > 
> [...]
>> ALMA can certainly compute MOCs and has actually been planning to do so.
>> My personal impression is that however the MOCs serve a different
>> purpose than our STC-S footprints. MOCs: fast x-match, high-order
>> display in AladinLite. STC-S footprints: exact outline and precise
>> display in AladinLite.
> 
> I don't think precision is a major issue here -- at level 29, the
> typcial extent of a healpix cell is of the order 0.5 mas.  Even in
> the Gaia age, that, I believe, should go a long way.
> 
> It is true, though, that MOCs hide some physics; it is, I suspect,
> virtually impossible to reconstruct that a certain coverage, for
> instance, has been produced as a union of n distinct circles, and...

As an aside, even n distinct circles do not exactly reproduce
footprints.  The projections of telescope observations on the sky do
not follow great or small circles.  That is why WCS is so complicated.
We ran into this when we started using great circle polygons in the
database for our datasests.

So there will always be a place for exact footprints with full WCS
using high order polynomials.

Cheers,
Walter Landry