diaObject time and coordinates

Support Data Products
#1

Hi!
I was looking at the latest alert packet schema and I didn’t understand something regarding the time (radecMjdTai) and coordinates (ra, dec) in the diaObject schema. Are these the parameters of the observation being reported (i.e. the same values of the diaSource) or a weighted average of the whole light curve?
I also ran a query in DP 0.2 to get extra info and I noticed that DiaObject.decl matches the coord_dec value of the respective DiaSources (“Fiducial ICRS Declination of centroid used for database indexing”) but DiaObject.radecTai has the value of the last detection (DiaSource.midPointTai).
Cheers,

Ignacio

#2

Hi @ireyes, could you provide a link to “the latest alert packet schema” that you’re looking at?

For the (ra, dec) and radecTai, there is a difference in how the future catalogs use these columns, and how they’re used for DP0.2. I appreciate that this is probably confusing! But I think I can shed a bit of light here.

The plans for how these table columns will be used in the future is currently defined by the Data Products Definitions Document (ls.st/dpdd). In Table 3, the schema for the DIAObject table are:

  • radec is the (ra, dec) position of the object at time radecTai
  • radecTai is the time at which the object was at a position radec

But for DP0.2, the columns are used a bit differently, and defined in the DP0.2 catalog schema. There, for the DIAObject table, ra and decl are the “Mean … of DIASources in the DIAObject”, and radecTai is not being used in DIAObject for DP0.2.

Let us know if that answers your question?

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#3

The AP team is looking into this: the specification in the DiaObject table is not precise enough. We will figure out a better description and update the APDB schema to reflect that. The DPDD does not reflect the current schema plans, it was the original aspirational document.

EDIT: Watch DM-47378 for updates on our sorting this out.

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#4

Hi!
@MelissaGraham I was looking at this alert packet schema alert_packet/python/lsst/alert/packet/schema/7/1 at main · lsst/alert_packet · GitHub
There’s a recursive definition between radec and radecTai, so it’s not clear to me what do they mean when you have many observations for a given object.

@parejkoj I will keep an eye on DM-47378 :+1:

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#5

Thanks @ireyes for asking the question – I was confused as well the first time I read the field! Keeping an eye on DM-47378 then :wink: