Dear all,
I am extracting AGN lightcurves from DP1 data, and I want to experiment with both PSF and aperture magnitudes. As far as I understand aperture mags are stored in the Source tables, but it seems the ObjectID is missing, so I would need to cross-match by position every single source in every visit to build the light curve. Is this a DP1 limitation and will it be fixed in DP2?
Hi @paolillo, thanks for your question. You are correct that the aperture fluxes are in the Source table and that does not have ObjectId. This will also be true for the DP2 and you can browse the schema for the DP2: Data Preview 2.
The ForcedSource table has an ObjectId since the forced photometry is done at the coordinates of Objects (which have an ObjectId) detected in the Deep Coadd images, and the forced photometry is PSF photometry done on visit images. The Source table has 5 sigma detections from the visit images, and this is not related to detection of Objects in the Deep Coadds. So you can think of it as the Source table goes with visit images and Sources, and the ForcedSource table goes with Objects.
It is generally recommended to use the ForcedSource and ForcedSourceOnDia for light curves since the measurements are associated by ObjectId or DiaObjectId. But if you are interested in using other flux measurements, you can do so by cross-matching.
If you have not already used it, our tutorial for the Source Table shows you how to retrieve all the sources that overlap a particular coordinate, and how to get visit information.
Thanks @gfonsecaalvarez, I understand your explanation although originally in the description of the Source table the objected was listed as well. Now the solution of doing a cross match means matching every visit with the Object table which seems challenging given the number of sources.
An alternative would be to have the apertures already measured in the ForcedSource table, but this is not present at this stage. Maybe we should consider it because PSF photometry would be useful for, e.g., AGN lightcurves.