Coadd Artifact Removal via Warped Image Comparison (DM-8290)

Goals 

•  Remove artifacts from coadds by Identifying and masking/Interpolating artifacts in single epoch matchedWarps (CoaddTempExps). Replacement for sigma clipped coadds or "safe-clipped" coadds. 

Background and strategic fit

LDM-151 outlines algorithms for artifact detection at various stages of processing in both AP and DRP: snap combination, single epoch processing, and coaddition. During snap subtraction, we will remove cosmic rays and satellite trails (synonymous with “linear features” in  LDM-151).   During single epoch we will model optical ghosts (ISR using dithered star fields) and identify and remove satellite trails.  During coaddition, we will use the time series of warped images to identify artifacts. Most artifacts (see below) will only appear in one of a series of observations. Therefore a stack of warped, PSF-homogenized, background-matched images provide sufficient information to identify and mask pixels affected by artifacts and produce an artifact-free coadd. This epic will address artifact removal in the coaddition stage, by identifying them masking them in the single epoch psfMatchedWarps. 

Algorithms used by other multi-epoch imaging projects:

Dark Energy Survey “CosmoDM” (Desai et al. 2016),  removes artifacts from calexps by comparing to a catalog-generated model image (derived from measurement on a median-combined psf-matcched coadd). The algorithm consists of the following steps:

1. Make Psf-Matched (to Moffat Psf of median FWHM) Median Chi^2(i+z) Coadd

2. Run detection/measurement on Coadd.

3.  For each calexp:

   1. Produce a simulated image—via Skymaker (Bertin 2009)—of same projection/psf based on the catalogs built from measurement on the single epoch image. Exclude faintest sources.

   2.  Compare calexp to simulated calexp, by making a “deviation image:” sqrt(wieght)*(calexp - simulated_calexp)

   3.  Flag pixels with residuals above 5\sigma. (unflag pixels in vicinity of areas where coadd and coadd model image do not match).

   4. Pass through again and flag pixels adjacent to flagged pixels which deviate more than than 2.5\sigma, and again for all pixels adjacent to those which deviate more than 1.5\sigma.

SWarp ((Gruen et al. 2014)

Assumptions

Artifacts can be categorized as either an:

Imaged Artifact (Artifact larger than a PSF) vs. Non-Imaged Artifact  (Artifact smaller than a PSF. Artifact has not been imaged by the camera, and is produced by CCD electronics).

Single Epoch Artifact (appears in only one observation) vs Persistent Artifact (appears in many observations)

Concrete examples:

Single Epoch Artifacts:

• Imaged ( > PSF)
  
  • Satellite trails, meteors
  • Glints
  • Ghosts of bright stars: will be consistent for a given pointing a rotator angle (i.e persistent in snaps but not in Warps)
  • Astrophysical Transients (supernovae, asteroids). Though not an artifact, they are not part of the "static sky"
• Non-imaged (CCD) (< PSF)
  • Cosmic Rays
  • CCD defects (e.g. bad column) 

Persistant Artifacts:

• Imaged 
  • Diffraction Spikes

  • Astrophysical variable sources (variable stars, quasars)

• Non-imaged (CCD) (< PSF)
  

  • Cores of saturated stars 

  • Bleed trails 

Requirements

Questions

Below is a list of questions to be addressed as a result of this requirements document:

Not Doing