Harvard Plate Collection Metcalf Telescope

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

DASCH Data Release 4 (DR4)

March 17, 2015

The Digital Access to a Sky Century @ Harvard (DASCH) project is pleased to release its fourth production data release (DR4), covering Galactic latitude b = +30 to +45 deg, joining results from DR1, which includes the 5 Development Fields from which the hardware and software pipelines for DASCH were developed; DR2; and DR3. These represent ~12% of the Harvard plate data (1885 - 1992; full-sky). 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 ~55,000 plates and their fully reduced WCS solutions and SExtractor-based photometry of every resolved object are available from this Data Release from the ~81Tb of data on disk now available. Light curves (LCs) typically include ~1000 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.

The DR4 release includes a complete recalibration of all images North of +20 degrees declination with the newly available release 8 of the APASS photometric database. This release from AAVSO fills in many of the coverage gaps in earlier DASCH releases, resulting in an 8% increase in lightcurves with at least two good points.

We plan to release an additional 15deg increment in galactic latitude (b = 30 to 15deg) as DR5. We recently revised plans for subsequent releases to continue in 15 degree increments through the plane of the Milky Way to the South Galactic Pole, completing the project by 2017. This revision avoids an originally planned skip from galactic latitude 30 to the South Galactic Pole to work back towards the plane of the Milky Way. 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 15 702 17086
DR2 0 to 360 60.00 to 75.00 2061 9081
DR3 0 to 360 45.00 to 60.00 3278 19634
DR4 0 to 360 45.00 to 60.00 4272 22275

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; and most of the M44 field has been integrated into the DR4 release. 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 4662
3C273 289.96 64.36 12h29m 187.27 2.05 5 78.5 4825
Baade's Window 1.03 -3.91 18h03m 270.88 -30.02 5 78.5 3593
Kepler Field 76.34 13.45 19h22m 290.73 44.50 8 201.1 5767
LMC 280.47 -32.89 5h23m 80.89 -69.76 5 78.5 7541

Release Photometry

An overview of plate processing and image magnitude calibation 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 GSC2.3.2 calibration catalog provides the broadest magnitude coverage but with limited accuracy. The Kepler Input Catalog provides more accurate magnitude and color measurements, but for a limited region of the sky. Finally, the APASS DR8 catalog provides good photometry accuracy, but is limited in magnitude range and is not yet complete in all regions of the sky.

Magnitude Counts Per Year

The leftmost plot show that the released data contains over 2,500,000,000 GSC2.3.2 calibrated magnitude estimates spanning the years 1886 to 1989. The data comes from 30 telescopes which may be divided into 18 wide field patrol telescopes with scales greater than 350 arcsec/mm and objective diameters of 1 to 3 inches; and 12 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 2,300,000,000 magnitude estimates with limited coverage deeper than 15th magnitude.
The release contains 2,833,000 lightcurves calibrated with the GSC2.3.2 catalog and 2,075,000 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 GSC2.3.2 dataset may be divided into 2,400,000,000 images matched to the catalog and 673,000,000 unmatched images; the APASS DR8 dataset may be divided into 1,800,000,000 matched images and 581,000,000 unmatched images; and the Kepler Input Calibration dataset contains 88,000,000 matched images and 17,600,000 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 calibations. 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 primarily during the 1940's.

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.

DR1: b = 75deg to 90deg

This field is centered on the North Galactic Pole and has a radius of 15 degrees.

DR2: b = 60deg to 75deg

This field is centered on the North Galactic Pole and extends from galactic latitude +60 to +75.

DR3: b = 45deg to 60deg

This field is centered on the North Galactic Pole and extends from galactic latitude +45 to +60.

DR4: b = 30deg to 45deg

This field is centered on the North Galactic Pole and extends from galactic latitude +30 to +45.

M44 Cluster Field

This field is the first scanned because of the availability of accurate photometric catalogs. Most of this field is now part of DR4 except for a segment centered on b = 206deg, l = 31deg (J2000 RA 127deg and declination 17deg).

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. Kepler Input Catalog calibrations are available for this field only and are shown in the middle set of graphs.

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.