Synopsis: Some DASCH-provided lightcurves are missing measurements that should be present.
Details
The DASCH lightcurves are built up as scanning proceeds. For each scanned plate that is successfully photometrically calibrated, its detected sources are matched to each of the “reference catalogs” and the associated measurements are added to a database that organizes the measurement by the catalog source ID.
For various reasons, this matching process can fail, resulting in magnitudes that should be present in the lightcurve database, but are missing. This is unfortunately not a rare phenomenon.
Due to the way that DASCH lightcurves are constructed, if a photometric point for a source on a plate is missing in this way, the associated lightcurve data product will include an upper limit (limiting magnitude) that may be much fainter than the actual source.
Therefore, if you have a science application that depends on non-detections of one or more targets, you should visually examine relevant plate image cutouts to check for missed measurements. In general, your eye is a more-than-adequate source detector. However, the HCO plates contain various artifacts, dust specks, chips, etc., that may masquerade as your target of interest. If you are searching for a faint or transient target that is undetected the vast majority of the time, and you see something at its position, the odds are good that you are seeing some kind of defect rather than an actual source detection.
Plates with detections that are missing from the lightcurve database can be reprocessed in an effort to restore the missing data. Such reprocessing can and should be performed by the DASCH team, who can use the full DASCH photometric pipeline to obtain the best possible calibrations and measurements.
See also the known issue of source splitting, which has a similar effect but is easier to mitigate.