DASCH “DRnext”

This page documents the data currently available through the DASCH data access services. These services are not currently versioned by data release, so the results of any queries or downloads will change as scanning proceeds and the data processing pipeline is updated. We refer to this “rolling data release” as “DRnext”.

Because the DASCH pipeline is always evolving, the available data may contain undocumented errors or limitations. Use them cautiously, and do not hesitate to reach out to the scientific contact if you have any questions or concerns.

Likewise, the material here is, inevitably, provisional. If you find any inaccuracies, please alert the scientific contact. For documentation that has been vetted but may be out-of-date, see the most recent snapshot data release: Data Release 6 (January 31, 2019).

Noteworthy changes in the data will be logged on the news page.

If you just want to get your hands on some data:

To learn more about where the data came from:

Spatial Coverage

The publicly-available DASCH data currently match DR6: anything with a galactic latitude b > 0 is available.

Plates covering the whole sky have been scanned and processed, and the plan is to make them publicly available as soon as possible. For preview access, reach out to the scientific contact named in the footer.

Temporal Coverage

To first order, DASCH data span the entire history of astronomical photography: from the invention of the technique (1880’s) to the triumph of the CCD (1990’s). Data across this entire range are available.

Collection Coverage

Approximately 420,000 plates in the HCO Plate Stacks collection have been digitized. This represents about 96% of all of the plates that are believed to be viable for photometric analysis. The remaining plates are large-format “A-series” plates, including Henrietta Swan Leavitt’s Cepheid observations.

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.

Photometry has been computed for sources covering the entire sky, but is only publicly available in self-service format for sources covered by DR6 (b > 0). For preview access to other sources, reach out to the scientific contact named in the footer.

The DASCH pipeline relies heavily on “reference catalogs” for photometric calibration. 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 B-band magnitude system whose definition traces back to the very plates scanned by DASCH, 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 photmetric databases based on the following reference catalogs: gsc2.3.2, kepler, and gaia. They are not recommended for use.

Data Products

DASCH creates the following data products, sorted approximately from “high level” to “low level”:

Photometry / Lightcurves: You can search by source to plot lightcurves and download data tables. Extensive quality metadata are included. Although data for sources with negative galactic latitudes are currently subject to DR6 data access restrictions, they are available upon request.

Cutouts / postage stamps: Positional searches have the option to download cutout images centered on targets of interest. Although data for sources with negative galactic latitudes are currently subject to DR6 data access restrictions, they are available upon request.

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. Although these files are currently subject to temporary data access restrictions and not available for public self-service download, they are available upon request.

Plate database: Information about the plates in the HCO Plate Stacks collection, both scanned and unscanned, is searchable. 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. These files used to have restricted access, but are now available for self-service download.

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, and the entire logbook photo collection may be browsed. Citizen-scientist volunteers have transcribed limited forms of metadata from these photos, which have been incorporated into the plate database.

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.

If you make use of DASCH data in scholarly work, you should properly cite and acknowledge DASCH.

Known Issues

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