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In the past we've used basic sigma-clipping to remove outliers (which leave faint wings of artifacts in the coadds), and "safe-clipped" coadds. Safe-clipped coadds difference each warp with the a sigma-clipped coadd, and mask the pixels with large deviations, and coadd again. Some drawbacks are The main drawback is that the comparison is done with direct rather than psfMatched Warps (seeing directWarps
rather than psfMatchedWarps
. Seeing variations can masquerade as artifacts) and it . It also doesn't handle broad but faint artifacts like faint ghosts. What else?, and pixels that have different artifacts in different observations.
Other projects use variations of the safe-clipped coadds.
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2) Need to use psfMatchedWarps
because extreme seeing variations can appear as artifacts: e.g. wings of a star on the worst seeing night is aberrant.
3) We want to mask both non-imaged artifacts mask and imaged artifacts, (Can’t make assumptions about what is behind an imaged artifact. its not necessarily sky.) as opposed to interpolating over the non-imaged ones. In normal single-epoch processing we interpolateover non-imaged artifacts like cosmic rays which are smaller than a PSF. However, a psfMatchedWarp
has been convolved with a kernel on order of the size of the PSF, thus broadening even non-imaged artifacts to the size of a PSF.
4) A masked footprint for an artifact, should be slightly larger than the identified outlier pixels (e.g. (Desai et al. 2016) checks surrounding pixels with lower and lower threshold sigmas). For an example why, recall outlines of clipped satellite trails in the sigma-clipped S13 Stripe 82 coadds.
5) Persistent artifacts. Need to clearly define desired behaviorTechnical definition of single-epoch artifact in time axis: Pixel is not in a detection in a robust coadd OR is detected in <10%** of observations. **Need to define in terms of probability of coincident artifacts and expected variability of real objects.
6) Technical definition of Persistent artifact in time axis: Pixel is in detection in a robust coadd OR is detected in >10%** of observations.
Requirements
# | TitleDescription | User Story | Importance | Notes |
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1 | Identify artifacts Per Patch |
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2 | Direct Remove single-epoch artifacts from direct and PSF-Matched Coadds are free of single-epoch artifacts | Hard decisions remain on persistent artifacts | ||
3 | Persistent Artifact BehaviorArtifacts | Four types of persistent "artifacts" have detection bit set in at least half
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4 | Mask single-epoch artifacts that coincidentally hit same pixels in multiple observations |
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Questions
Below is a list of questions to be addressed as a result of this requirements document:
Question | Outcome |
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LDM-151 says “we assume that everything we can detect morphologically will have already been detected.” Does this mean that we will not have to detect CRs in CoaddTempExps? | |
Where will these masks be stored? Currently, the only task which deals with the full stack of single epoch warps is AssembleCoaddTask. It reads in the CoaddTempExps/Warps and writes out a Coadd. It will need to write out more, specifically depth maps, which can also include the masks per visit. We need to think about the data structure for this. I'm imagining a data cube with mask-bit per visit, but I think Nate Lust's new Span Sets might have this capability too. | |
What are the failures of Safe-Clipped Coadds? | Per Bob Armstrong:
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What behavior do we want for the cores of saturated stars? |
Not Doing
- Images with guiding errors will not provide any useful information to the coadd and should be discarded.