DASCH Data Release 2 Now Available
The second data release (DR2) for the Digital Access to a Sky Century @ Harvard (DASCH) is now available from the project website. DASCH provide images and fully-processed astrometry (~0.5–3 arcsec, depending on plate series) and photometry (RMS ~0.1–0.15 mag) from the digitized Harvard plates for the full century (1885–1992) of coverage. Lightcurves (~500–2,000 points, typically) and magnitudes are readily available for each resolved object, with limiting magnitudes typically B = 14–17 depending on plate series and exposure times.
DR2 covers galactic latitude b = 75°–60° as well as improved data processing and access to DR1 (b = 90°–75°) and the five initial Development Fields listed on the website. Each DR covers the full range of galactic longitude (l = 0°–360°) in the given galactic latitude band; see the website for the DR1 to DR12 sequence planned. DR1 and DR2 together cover 2,686 deg² from 27,193 plates, or only about 6% of the estimated total. Data from an additional 17,896 digitized plates of the five Development Fields are also part of the ~90 Tb of data now online.
Improved data-access capabilities are now provided: lightcurve data and images for up to 10 objects at a time can be downloaded either individually (in several formats) or as a single tar file. The number of objects per download may be increased if server loads allow, but our intent is to enable rapid (~10 sec) access to any given (or few) objects of interest. Improved astrometry is now available, with recent inclusion of UCAC4 proper motions for the given plate epoch for both full plate calibration objects (from the APASS or GSC2.3.2 or KIC catalogs) as well as the object of interest. This results in fewer nearby “DASCH objects,” which are either new variables or (more likely) are not properly matched to a catalog object, and are usually due to either PSF issues or [previously] astrometry or proper-motion errors. Plate defects are still not totally filtered out but can be eliminated in lightcurve plots by clicking on “Hide: plate defect." The APASS calibration for DASCH provides the best photometry (and color corrections, given its B, V, g, r, i filters) and will soon (we hope) be upgraded to DR8 that will improve sky coverage as well as multiple measures of each star.
We project the DR3 (b = 60°–45°) release for April 2014. With increasing sky coverage per Δb = 15° release as we approach galactic latitude b = 15°, the number of plates will increase over DR3–DR5, and then again over the southern galactic cap (DR6–DR10) and finally the galactic plane (DR11–DR12). Continued NSF support in our new grant will now enable more scanning and plate-processing staff to be hired to achieve the original production processing goal of ~400 plates/day and enable the full-sky DASCH database to be online by 2017.
DASCH is grateful for the support provided by NSF, now with grant AST-1313370. Coauthors on this report are Alison Doane, Edward Los, George Miller, Robert Simcoe, and David Sliski, all at the Harvard-Smithsonian Center for Astrophysics.