Astrometric Matcher of Euclid Targets: Why so few matches? What criteria is used?

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After many months of failure, I’ve finally managed to use the LSST DM stack w_2021_04 release to process a simulated image frame of a set of Euclid targets produced by an ImSim-like Rubin simulator developed within the Euclid Consortium. These simulations are rather idealised, e.g. they do not include distortions. We would like to understand what criteria goes into the astrometric matching, e.g. S/N cuts, etc that could result in such a low number of matches.

processCcd.calibrate.astrometry INFO: Matched and fit WCS in 3 iterations; found 62 matches with scatter = 0.065 ± 0.049 arcsec

Here is an image produced by the code.

While the resolution of that image is awful, I did manage to observe that there are a lot of red (reference) sources, and most of the green (detections) have yellow (matches) counterparts. That means the matching is working, and the log line you posted agrees: 62 matches with a scatter of tens of mas indicates the solution is solid (as opposed to 4 matches with a scatter of 1 mas, or 20 matches with a scatter of 3 arcsec).

I don’t think there’s a need to worry about the large number of references that don’t have a match: the purpose of the astrometric solution performed in processCcd is to get close enough to be able to associate objects (~ hundreds of mas); downstream processing (in particular, jointcal) will refine the astrometric solution using deeper detections.

As for the particular criteria for the astrometric matching, you should check the configuration (in particular, config.calibrate.detection and config.calibrate.astrometry). E.g., for HSC I see:

# Minimum allowed signal-to-noise ratio for sources used for matching (in the flux specified by sourceFluxType); <= 0 for no limit

# Threshold for footprints; exact meaning and units depend on thresholdType.

# Include threshold relative to thresholdValue

# Apply magnitude limit?

# Apply signal-to-noise limit?

Thanks for the prompt reply. I already confirmed that the matching was working. We were just curious to understand why the matches were so few. I will have a look at the configuration settings.

Hey Jennifer, as Paul says that’s an uncommon number of matches to have. The green points you see in that image are sources in the image that were selected to be of high signal to noise (40 in this case of Paul’s example config) and suitable or the astrometric fitting. If you look above the line you show a bit you should see output that tells you how many sources the pipeline is using to try to match and fit.