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LSSTDESC / imSim

GalSim based Rubin Observatory image simulation package
https://lsstdesc.org/imSim
BSD 3-Clause "New" or "Revised" License
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Apply DCR for faint objects #455

Open rmjarvis opened 5 months ago

rmjarvis commented 5 months ago

@esheldon discovered a bug in our rendering of very faint objects. We currently skip over the photon ops if the flux is < 100 photons. This is mostly fine, since we don't care if these objects have all the fine details of the sensor effects and such. However, if we are applying DCR as a photon op (as we typically do), this puts the faint objects in the wrong place. They don't have DCR applied to them.

This bit Erin when he used the output catalog to refit a more accurate WCS, including the DCR effect. He used all of the output objects' image and sky positions. But the majority of objects didn't have DCR applied, so their positions were systematically wrong, and the resulting WCS was pretty bad. Limiting the refit to flux > 100 fixed the problem, but we should find a way to get the faint objects in the right place to avoid this "feature" biting someone else in the future.

esheldon commented 5 months ago

Is there a need to skip photon ops for objects with less than 100 photons?

If they only have 100 photons, will they be fast in any case?