Known Issue: Undetected Plate Defects

Synopsis: DASCH images contain all sorts of effects that do not correspond to astronomical objects: dust, scratches, dents, annotations, and so on. While the DASCH processing pipeline contains many checks to detect and remove defects that might masquerade as astronomical objects, some will inevitably make their way into the lightcurve data.


Although a major goal of the DASCH scanner is to provide plate images that are as pristine as possible, virtually every plate image in the DASCH collection contains various non-astronomical artifacts. Not all of these are undesired: for instance, the handwritten annotations on various Harvard plates represent a unique historical record of the women astronomical computers and their work.

The DASCH pipeline contains various filters that attempt to detect and remove artifacts from the photometry data. But in a dataset as large as DASCH, some artifacts will inevitably make their way into the final data products. If you have a science result that hinges on a single detection, you should carefully vet the detection to ensure that it is astronomical.

Things to check include:

  1. Validate the overall astrometry of the plate in question to confirm there is no gross positioning error.
  2. Consider also whether a seeming sudden appearance may be due to a change in emulsion rather than intrinsic.
  3. Examine the plate image around the source in question for issues.
    1. If the source is blended or affected by a scratch, annotation, etc., proceed with extreme caution.
    2. The apparent brightness of the source should seem appropriate for its pipeline magnitude and significance above the local limiting magnitude.
    3. The overall shape and profile of the source should be consistent with that of nearby sources.
    4. The position of the source should agree well with expectations.

It is possible in exceptional circumstances that a physical plate defect, such as a small dent, may pass the above tests. Physical examination of the plate in question with side-illumination can reveal these. Contact the Plate Stacks Curator if you wish to arrange for such an assessment.

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