Apply distortion when searching for astrometric reference objects

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Details

• Type: Story
• Status: Done
• Resolution: Done
• Fix Version/s: None
• Component/s:
• Labels:
None
• Story Points:
1
• Sprint:
DRP F16-3
• Team:
Data Release Production

Description

While investigating DM-6529 I found that LSST generally finds fewer reference objects than HSC when doing astrometry. For the CCDs on the edge of the focal plane the number of stars was typically very low causing frequent failures. I found that in the HSC code, there is a distortion being applied that shifts the exposure bounding box when getting objects from the reference catalog. This distortion is not being applied in the LSST code.

Activity

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Russell Owen added a comment -

Good catch. The LSST code is certainly attempting to correct for distortion. AstrometryTask has this:

 def _getExposureMetadata(self, exposure): ...  return pipeBase.Struct(  bbox=exposure.getBBox(),  wcs=getDistortedWcs(exposureInfo, log=self.log),... 

and that wcs is used for the search. However, getDistortedWcs uses the TAN PIX to PIX mapping to correct, and unfortunately that frame has the same center in both cases – it corrects local distortion, but not global distortion. If DM-2280 is fixed then that will fix this problem.

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Russell Owen added a comment - Good catch. The LSST code is certainly attempting to correct for distortion. AstrometryTask has this: def _getExposureMetadata(self, exposure): ... return pipeBase.Struct( bbox=exposure.getBBox(), wcs=getDistortedWcs(exposureInfo, log=self.log),... and that wcs is used for the search. However, getDistortedWcs uses the TAN PIX to PIX mapping to correct, and unfortunately that frame has the same center in both cases – it corrects local distortion, but not global distortion. If DM-2280 is fixed then that will fix this problem.
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John Swinbank added a comment -

Russell Owen – thank you for those details!

Sounds like we just need to run a check after DM-2280 lands to confirm that this is really fixed. Bob Armstrong, I'm putting your name on it to do that. Please shout if you object.

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John Swinbank added a comment - Russell Owen – thank you for those details! Sounds like we just need to run a check after DM-2280 lands to confirm that this is really fixed. Bob Armstrong , I'm putting your name on it to do that. Please shout if you object.
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Russell Owen added a comment -

DM-2280 is merged and Bob Armstrong has verified that it fixed this ticket. I think this ticket can be closed (using what status?).

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Russell Owen added a comment - DM-2280 is merged and Bob Armstrong has verified that it fixed this ticket. I think this ticket can be closed (using what status?).
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John Swinbank added a comment -

Let's just mark this as done, since Bob had to expend some effort to verify the fix.

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John Swinbank added a comment - Let's just mark this as done, since Bob had to expend some effort to verify the fix.

People

Assignee:
Bob Armstrong
Reporter:
Bob Armstrong
Watchers:
Bob Armstrong, John Swinbank, Russell Owen