DRnext: Known Issues

As with any survey, the DASCH data display various known issues. This page summarizes them, with links below to individual pages that discuss the each item in more depth.

There are surely other gremlins lurking in the DASCH data that have not yet been discovered. If you are working with DASCH data and notice anything suspicious, please reach out to the scientific contact to discuss.

  • False long-term trends in ATLAS photometry: DASCH’s photometric calibration against the ATLAS-refcat2 catalog uses g-band magnitudes. The g filter profile is not a perfect match to the plates’ chromatic response, which can lead to false long-term trends or features in lightcurves. The APASS calibration should be used for analyses relying on precise long-term photometry.
  • Missing lightcurve points: some DASCH lightcurves are missing detections that should, by rights, be present. If you have a science application that depends on non-detections of one or more targets, you should visually examine relevant plate images to check for missed measurements.
  • Source splitting: the DASCH catalogs may contain multiple entries corresponding to a single astronomical source, causing lightcurve measurements to be split quasi-randomly across the different catalog entries. It is often safe and appropriate to merge the detections associated with multiple close catalog entries into a single lightcurve.
  • Undetected plate defects: although the DASCH pipeline contains many checks aimed at detecting and rejecting plate defects that masquerade as astronomical sources, some will slip through. Science results hinging on a single detection should include detailed vetting of the underlying data, potentially up to the level of physical examination of the plate in question.
  • Undetected blends: plates with low spatial resolution may blend multiple sources, causing apparent brightenings that are not real. These couple into the long-term lightcurves because different observing programs operated over different time periods.
  • Incorrect astrometric solutions: the automated astrometric solutions for some plates are simply wrong. This problem is most likely to affect the “meteor” and “patrol” plates that cover very large areas on the sky.
  • Unspecified emulsions: Different plates used for DASCH photometry used emulsions with different color sensitivities. The DASCH data do not make it easy to tell which measurements come from which emulsions, leading to apparent "variability" that is actually a color effect.