Fwd: Light curves in the IVOA

[Roy Williams]

-------- Original Message --------
Subject: Re: Light curves in the IVOA
Date: Wed, 16 Jun 2010 07:52:08 -0700
From: Roy Williams <roy at cacr.caltech.edu>
To: Raul Gutierrez Sanchez <raul at cab.inta-csic.es>
CC: Doug Tody <dtody at aoc.nrao.edu>, Rob Seaman <seaman at noao.edu>

Raul
I would like to start launching and interoperating light curve services, 
and what you have done is a most of the work that is needed, so I am 
very happy about your real live services!

Perhaps we can think about writing a real document about what a light
curve service should look like? That way we can ensure that they real
interoperate. I have a few small questions and concerns about what you
have implemented, and I wonder if I could ask if there could be slight
modifications to your services? Please find attached the two files
(lightcurve-menu.xml and light-curve-instance.xml), where the first is a 
listing of links, and the second is a light curve.

(1) I see in both of the attached files that you have GROUP containing
GROUP. This recursion in VOTable is something that is difficult for many 
parsers and code-binders, and I fought against it when it was introduced 
into the VOTable specification. Would it be possible to just delete 
GROUPS altogether, since the utype of the PARAMs already reveals its 
position in the hierarchy. The GROUPs seem like a lot of extra
complication that adds no information!
        <GROUP utype="sed:Segment.Frame">
         <GROUP utype="sed:Segment.Frame.Sky">
            <PARAM utype="sed:Segment.Frame.Sky.Type" ……./>

(2) I see that time is represented by barycentric MJD as in the types
and values below. I am not clear on the meaning of your MJD -- it is
1300 days from which reference point? But I do not see much of the STC
technology that has been crystallizing in the IVOA with the efforts of
Demleitner (*). Do you think it would be possible to modify your service 
to report something more like that?

sed:Segment.Frame.Sky.Equinox value="2000.0"
sed:Segment.Frame.Time.Type value="MJD"
sed:Segment.Frame.Time.RefPos value="BARYCENTRIC"
sed:Segment.Coverage.Location.Time.Value value="1305.88"
sed:Segment.Coverage.Region.Time.Start value="1305.87"
sed:Segment.Coverage.Region.Time.End" value="1305.89"

(*) http://www.ivoa.net/Documents/latest/VOTableSTC.html


(3) I see that some of your data values have "99.0" for the flux value,
which obviously means missing data. But is there any way in the SSAP or
VOTable specification to say in the metadata "whenever you see 99.0 it
means missing data".

Thank you
Roy Williams

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