Photometry

DASCH photometry currently consists of ~15 billion visible-band magnitudes of ~40 million sources, with thousands of measurements for most sources, to depths of 14th-16th magnitude. Typical lightcurve RMSes are about 0.15 mag. Extensive quality metadata are included.

Most plates were exposed without filters using blue-sensitive photographic emulsions. The resulting photometric measurements map well to the Johnson-Morgan B band, whose definition traces back to the very plates scanned by DASCH. Some plates used different emulsions or were exposed behind filters. The DASCH pipeline uses color corrections to homogenize their photometry.

The DASCH pipeline relies heavily on “reference catalogs” for both photometric calibration and assembling the lightcurve data. The pipeline uses multiple such catalogs, resulting in multiple photometric databases. Two such databases are recommended for use:

The apass calibration is based on the AAVSO Photometry All-Sky Survey (APASS), data release 8. Because APASS is based on the Johnson-Morgan B-band magnitude system, this “refcat” provides the most reliable data for long-term analysis. If in doubt, use apass.

The atlas calibration is based on the ATLAS All-Sky Stellar Reference Catalog (refcat2). DASCH’s use of ATLAS is based on its g-band magnitudes, which can result in discontinuities and false long-term trends in the lightcurves. However, ATLAS has better astrometry than APASS, which may be valuable in some use cases.

There are also historical photometric databases based on the following reference catalogs: gsc2.3.2, kepler, and gaia. They are not recommended for use.

Cutouts / Postage Stamps

Plate cutout images around any sky location can be obtained using the daschlab analysis toolkit.

In the Cannon data portal, positional searches have the option to download cutout images centered on targets of interest. Although queries in the Cannon portal are currently subject to DR6 data access restrictions on sources with negative galactic latitudes, such data are available upon request, or by using daschlab.

Calibrated Plate Mosaics

Full FITS images of entire plates, with WCS metadata including plate distortions, exist. Typical full-size mosaics are 750 MiB each; the largest are around 2.2 GiB. Downsampled mosaics, binned in 16×16 pixel blocks, are also available. These files can be downloaded by looking up individual plates in the StarGlass website, or by using the StarGlass API. For bulk access, reach out to the scientific contact named in the footer.

Plate Database

Information about the plates in the HCO Plate Stacks collection, both scanned and unscanned, is searchable. Plates can be searched using the StarGlass website, the StarGlass API, the daschlab toolkit, or the Cannon data portal.

Metadata for un-scanned plates may be inaccurate due to errors in the historical logbooks. Information about all plates, regardless of DR6 restrictions, is available.

Plate and Jacket Photos

Most plates were photographed prior to scanning. Photos covered both the paper jackets holding the plates, which sometimes have annotations of interest, as well as the plates themselves, which also sometimes have annotations. Prior to 2023, these annotations were erased before scanning, meaning that these photos are the only record of what was written on the plates. Individual photo files are typically 10 to 40 MiB in size.

Plate and jacket photos can be obtained using the StarGlass website, the StarGlass API, or the Cannon data portal. For bulk access, reach out to the scientific contact named in the footer.

Logbook photos

Along with the glass plates, the plate stacks collection includes written logbooks summarizing the corresponding observations. These logbooks have been photographed in their entirety. Citizen-scientist volunteers have transcribed limited forms of metadata from these photos, which have been incorporated into the plate database.

The entire logbook photo collection may be browsed using the Cannon data portal.

Raw Scan Data

Raw data and metadata associated with the plate scanning process are archived on magnetic tape and on Amazon AWS’s Glacier Deep Storage system. These data can be made available upon request, although we have not yet discovered any scientific applications for them that cannot be satisfied by the calibrated full-plate FITS mosaics. The archive of raw data is about half a petabyte in size.

Known Issues

As with any non-trivial survey, the DASCH data display various known issues. The Known Issues page summarizes them and provides links to more in-depth discussions.