WD-AccessData-1.0-20140312

[Petr Skoda]

> data. While it isn't perfect and it isn't trivial, one can write a numeric 
> query of the ObsCore model such that most of the returned data is in a band 
> very much like R. Applications could help users formulate such queries.
Well I am very wondering who will write such an application and how will 
astronomers use such a tool. It would need connection with filter database 
and I would guess that no-one exactly knows what version of filter has 
particular survey (e.g. the Johnson (even from different photocathodes), 
Cousins or other derivatives very only approximate representation of the 
same range) There will be  a huge confussion in practical usage such a 
tool.

We try to do this in metadata for characterisation but it is 
nonsense anyway - the only sure query uses what is written in FITS as 'V' 
or 'U' and I know I am querying my survey. If I will query SDSS the only 
relevant query is in 'u' or 'g'  - other surveys are using similar filters 
but with slightly different ranges.

but fortunately the SIAP accepts any server specific parameter which is 
used for providing the "BAND" with letters - and by a hack in Aladin can 
be used .


> Or are you talking about creating a virtual R-band image out of a 3d cube? If 
> you are talking about a virtual image from a cube, the only kind of access 
> that is in scope for AccessData-1.0 is simple extraction is subsets. All 
> other kinds of operations are to be considered later in 1.x

I do not talk about VIRTUAL (e.g. computed ) subset of datacube.
I am afraid that everyone talking about datacubes thinks in terms of 
smooth values on each axis (probably some abstraction - in real 
everything is sampled)

But is a set of simultaneous or quasi-simultaneous (say 10sec exposures 
with fast rotating filter wheels) images in 20 filters a datacube or not ?
Anyway the AccessData should support even more simple structures 
(shouldn't it?) like stereo camera with 2 different filters (or 2 
satellites like BRITE). Such an "image" or datacube may be a 20-extension 
FITS. How do you access only the V-filter image ?


> PS-It is by design that the numeric BAND parameter is the same in SIA and 
> AccessData, because they both specify a value or interval on the spectral 
> axis. However, the two capabilities do different things now and will diverge 
> further (in allowed parameters) in subsequent versions.

I see the perfect symmetry in access and query part. In fact we already 
had a troubles using BAND both for selection of spectra and using it as 
getData (or newly datalink) for line cutout. It is more natural to cutout 
only the spectral range that is at least partly included in queried 
spectrum ;-)  - otherwise it is tricky to match in my cutout request the 
range I was selecting.

So for practical reasons too much flexibility (different range for query 
and different for operation ) maybe quite confusing .
Believe me - I was the first in IVOA who had put in operation  the 
spectral cutout in SSA. I had to misuse the BAND either as a selection or 
as a processing parameters (and thus needed two different serices) - but 
now after experiments with getData support (which is now replaced by 
datalink) I am starting to see the  practical bottlenecks and worse 
flexibility of the two-way negotiation with post-processing.

It would suffice to allow in SSA to have a parametr telling the BAND is 
used for cutout.

Now I am facing other troubles providing simple UBVRI images in SIAP.



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*  Petr Skoda                         Tel   : (323) 649201, l. 361      *
*  Stelarni oddeleni                          (323) 620361              *
*  Astronomicky ustav AVCR, v.v.i     Fax   : (323) 620250              *
*  251 65 Ondrejov                    e-mail: skoda at sunstel.asu.cas.cz  *
*  Ceska republika                                                      *
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