DASCH: Digital Access to a Sky Century @ Harvard
A New Look at the Temporal Universe

Status Update

October 18, 2022

The DASCH project has been a herculean effort spanning two decades. In order to ensure that the plate scanning is brought to completion, and that the resulting data are archived and as useful as possible, the project is undergoing an overhaul under the direction of the Harvard College Observatory. During this time, our servers will be down and access to DASCH data via existing interfaces will be limited. Please contact Purvang Patel, Executive Director at the Harvard College Observatory, if you have questions.

January 31, 2019

The Digital Access to a Sky Century @ Harvard (DASCH) project is pleased to release its sixth production data release (DR6), covering Galactic latitude b = +0 to +15 deg, joining results from DR1, which includes the 5 Development Fields from which the hardware and software pipelines for DASCH were developed; and DR2 through DR5. These represent ~36% of the Harvard plate data (1885 - 1992; full-sky excluding spectra and rejected plates). A brief overview of DASCH is given in Grindlay et al (2012) Opening the 100-Year Window for Time Domain Astronomy in arXiv:1211.1051 or IAU Symposium Vol. 285 p 29-34 . Additional project papers are listed in the DASCH publications web page. Digitized images (11 micron pixels) of ~195,000 plates and their fully reduced WCS solutions and SExtractor-based photometry of every resolved object are available from this Data Release from the ~230Tb of data on disk now available. Light curves (LCs) typically include 1500 points for an object with magnitude B ~12-13. LCs may be retrieved from input coordinates or SIMBAD object names for up to 10 at a time, and LC plots, data and images for each then downloaded individually.

We originally planed to release additional 15 degree increments in decreasing galactic latitude. In October, 2019, we revised plans to make DR7 through DR12 a single combined release of the Southern galactic hemisphere. The maps at the left illustrate this release strategy in galactic and equatorial coordinates: green indicates released areas and yellow indicates areas currently being scanned.

The following tables show the extent of released sky regions and the number of plates currently assigned to each region. Because many plates span multiple release regions, each plate is assigned to the release region which covers the most area on the plate. These numbers can not be final until we have completed both the logbook transcription process and full astrometric processing for all of the plates. We are currently releasing regions when we have scanned over 80% of plates currently estimated to have their plate centers in the new release region.

Release Fields
Region Galactic Longitude (l) degrees Galactic Latitude (b) degrees RA hoursRA degrees Dec degreesRadius degreesSquare degreesPlates
DR1 0 to 360 75.00 to 90.00 12h51m 192.86 27.13 15 1211 9075
DR2 0 to 360 60.00 to 75.00 1978 11962
DR3 0 to 360 45.00 to 60.00 3274 28368
DR4 0 to 360 30.00 to 45.00 4192 32861
DR5 0 to 360 15.00 to 30.00 4880 55426
DR6 0 to 360 0.00 to 15.00 5201 57499

The Development Fields are listed in the Table below and are centered on the coordinates given. Since adjacent fields have not yet been scanned, they are increasingly incomplete with increasing radius. The 3C273 field has been integrated into DR2 and DR3; the M44 field has been integrated into DR4 and DR5; and the Kepler field has been integrated into DR5 and DR6. Additional fields will be integrated into the Production scanning as they are covered in the data release plan described above. The counts of plates do not add up to the total plates in the release because many plates cover portions of multiple release regions.

Development Fields
Region Galactic Longitude (l) degrees Galactic Latitude (b) degrees RA hoursRA degrees Dec degreesRadius degreesSquare degreesPlates
M44 205.91 32.48 8h40m 130.09 19.67 5 78.5 14281
3C273 289.96 64.36 12h29m 187.27 2.05 5 78.5 11302
Baade's Window 1.03 -3.91 18h03m 270.88 -30.02 5 78.5 13488
Kepler Field 76.34 13.45 19h22m 290.73 44.50 8 201.1 23159
LMC 280.47 -32.89 5h23m 80.89 -69.76 5 78.5 12108

Release Photometry

An overview of plate processing and image magnitude calibration appears on the DASCH Photometry Results page. For each plate, the pipeline generates three sets of magnitude measurements, one for each of three calibration catalogs. The ATLAS All-Sky Stellar Reference Catalog uses Pan-STARRS DR1 supplemented with other catalogs to give the best photometric accuracy in SDSS g and r to 19th magnitude over the entire sky. The AAVSO Photometry All-Sky Survey (APASS) Release 8 Catalog uses Johnson B and V magnitudes which better match the unfiltered photographic blue of most of the plates. The GSC2.3.2 catalog provides the broadest magnitude coverage in Johnson B and V but with limited accuracy. A fourth catalog, the Kepler Input Catalog (KIC) using SDSS g and r, is no longer actively used in the DASCH project because of its limited sky coverage.

Magnitude Counts Per Year

The leftmost plot show that the released data contains over 15,395,000 ATLAS refcat2 calibrated magnitude estimates and 14,119,000,000 GSC2.3.2 calibrated magnitude estimates spanning the years 1885 to 1989. The data comes from 47 telescopes which may be divided into 33 wide field patrol telescopes with scales greater than 350 arcsec/mm and objective diameters of 1 to 3 inches; and 14 narrow field telescopes with objective diameters of 4 to 24 inches. The main patrol programs ran from about 1900 to the 1950's. A smaller patrol program using six Damon telescopes with 1.65" diameter objectives ran from 1970 to 1990.
The middle plot on the left shows that the APASS calibrated dataset contains over 13,194,000,000 magnitude estimates with limited coverage deeper than 15th magnitude.
The release contains 50,307,000 lightcurves calibrated with the ATLAS refcat2; 38,329,000 lightcurves calibrated with the GSC2.3.2 catalog; and 34,559,000 lightcurves calibrated with the APASS DR8 catalog. The third plot shows the median number of points for any given lightcurve as function of magnitude. This third plot is dominated by the patrol plate measurements.

Photometry Accuracy

The Atlas refcat2 dataset may be divided into 10,097,000,000 images matched to the catalog and 2,522,000,000 unmatched images; the GSC2.3.2 dataset may be divided into 9,532,000,000 images matched to the catalog and 2,052,000,000 unmatched images; the APASS DR8 dataset may be divided into 9,062,000,000 matched images and 1,630,000,000 unmatched images; and the Kepler Input Calibration dataset contains 262,066,000 matched images and 325,630,000 unmatched images. The unmatched images are mostly plate defects, astrometry matching errors, multiple exposure matching errors, and asteroids. The plots on the left use better quality matched images where the quality is described by a set of flags (see AFLAGS). There must be at least 10 good quality points in a lightcurve to allow calculation of a reasonable lightcurve RMS. The leftmost plot assumes that the median lightcurve RMS removes variable stars. The detailed RMS histograms for each magnitude range appear in the plots on the right for each calibration catalog.

Release Fields Coverage Plots vs. Limiting Magnitude

The first plot on the left shows the deepest limiting magnitude per plate for the GSC2.3.2 and APASS DR8 calibrations. The second plot shows that the wide field patrol plates have a limiting magnitude of approximately 12 before circa 1935 and 14 after that date. Stars deeper than 16th magnitude have limited coverage from non-patrol plates after 1935.

All limiting magnitude coverage plots below have a resolution of one degree. The red angular borders around the coverage area are artifacts of the plot shading algorithm. For each field, the GSC2.3.2 calibration catalog results appear on top, and the APASS DR8 catalog results appear on bottom. From left to right, the limiting magnitudes of the plots are 10, 12, 14, and 16.

DR6: b = 0 deg to 90deg

M44 Cluster Field

This field is the first scanned because of the availability of accurate photometric catalogs. This field is now part of DR4 and DR5.

3C273 Quasar Field

This field is the second scanned to investigate the usefulness of DASCH photometry for the study of quasars. This field is now part of DR2 and DR3.

Baade's Window Field

This field is the third scanned to study algorithms for nova searches and to test DASCH astrometry and photometry in crowded fields.

Kepler Field

This field is the fourth scanned to take advantage of the superior accuracy of the Kepler Input Catalog over the GSC2.3.2 catalog for the calibration of DASCH plates. This field is now part of DR5 and DR6. Kepler Input Catalog calibrations are available for this field and its immediate surroundings above b = 0 degrees as shown below.

Large Magellanic Cloud Field

Because of the historic discoveries made by Henrietta Leavitt, the LMC field provides the deepest magnitude coverage and highest plate density for the Harvard plates scanned to date. Improved photometry (for crowded fields) will be done for both the LMC and Baade's Window when these fields are re-processed for Production scanning.


The DASCH project at Harvard is grateful for partial support from NSF grants AST-0407380, AST-0909073, and AST-1313370; which should be acknowledged in all papers making use of DASCH data.

We acknowledge the one-time gift of the Cornel and Cynthia K. Sarosdy Fund for DASCH, and thank Grzegorz Pojmanski of the ASAS project for providing some of the source code on which the DASCH web-interface is based.

The ongoing AAVSO Photometric All-Sky Survey (APASS) has improved DASCH photometric calibration and is funded by the Robert Martin Ayers Sciences Fund.