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Abstract In the second paper in this series, we improve on our previous demonstration of the ability of a commercially available graphic arts scanner and cost-effective analysis tools to produce scientifically useful scans of astronomical photographic plates. We describe a method using freely available tools to extract magnitude measurements from the star images on sky-survey plates, such as are stored in observatory archives around the world. We detail the use of this method on one plate in particular, Plate 8 in E. E. Barnard’s A Photographic Atlas of Selected Regions of the Milky Way , examine the effects of our scanning method on our magnitude measurements, discuss the difficulties encountered when measuring the magnitudes of stars in crowded fields, and present a case study of red supergiant stars appearing within the field. Our work results in a catalog of more than 66,000 measurements of stellar positions and magnitudes in the central 6.°8 × 6.°8 field of view. 

ABSTRACT We present a sample of 706, z < 1.5 active galactic nuclei (AGNs) selected from optical photometric variability in three of the Dark Energy Survey (DES) deep fields (E2, C3, and X3) over an area of 4.64 deg2. We construct light curves using difference imaging aperture photometry for resolved sources and non-difference imaging PSF photometry for unresolved sources, respectively, and characterize the variability significance. Our DES light curves have a mean cadence of 7 d, a 6-yr baseline, and a single-epoch imaging depth of up to g ∼ 24.5. Using spectral energy distribution (SED) fitting, we find 26 out of total 706 variable galaxies are consistent with dwarf galaxies with a reliable stellar mass estimate ($$M_{\ast }\lt 10^{9.5}\, {\rm M}_\odot$$; median photometric redshift of 0.9). We were able to constrain rapid characteristic variability time-scales (∼ weeks) using the DES light curves in 15 dwarf AGN candidates (a subset of our variable AGN candidates) at a median photometric redshift of 0.4. This rapid variability is consistent with their low black hole (BH) masses. We confirm the low-mass AGN nature of one source with a high S/N optical spectrum. We publish our catalogue, optical light curves, and supplementary data, such as X-ray properties and optical spectra, when available. We measure a variable AGN fraction versus stellar mass and compare to results from a forward model. This work demonstrates the feasibility of optical variability to identify AGNs with lower BH masses in deep fields, which may be more ‘pristine’ analogues of supermassive BH seeds.