DASCH Scanning is Complete — What’s Next?
2024 March 28
Today’s the day! After two decades of work, the DASCH scanning “prime mission” is now complete. Today we celebrated the scanning of the final DASCH-able plates in the HCO collection with a small event and a champagne toast.
This is the end of a chapter, but far from the end of the story. Just how much work is left to do is a question whose answer depends on how broad of a perspective you want to take.
When it comes to making the existing DASCH data available and useful to the astronomical research community, there’s still plenty to be done. In the near future, that work will mostly take the form of the next data release, DR7, currently under development as DRnext. As my recent posts have perhaps conveyed, this is going to be a lot of effort — there’s basically an unbounded amount of work that could go into writing documentation, fleshing out software like daschlab, and other support activities — let alone actual improvement of the underlying data products. But DR7 also needs to come out in a finite amount of time. I can already tell that it’s going to be difficult to draw the line at which the polishing has to stop and the thing simply has to get out the damn door.
Regardless of where exactly that line gets drawn, there’s absolutely no chance that DR7 will completely plumb the depths of the DASCH data. You could spend the rest of your life improving and expanding the analysis of a dataset as large and heterogeneous as DASCH. Whether we’ll see Data Releases 8, 9, etc. off into the future is, as ever, going to depend on money. If you ask me, a dataset as rich and unique as DASCH certainly deserves lots of funding to analyze and enhance it — and that’s not even considering its cultural and historical implications — but lots of projects deserve funding.
It’s also important to underline that “DASCH-able plates” represents only a portion of the Plate Stacks holdings. About 430,000 plates have been scanned for DASCH, compared to an estimated 550,000–600,000 plates in the entire collection. This gap exists because DASCH explicitly focused on plates suitable for astronomical photometric and time-domain analysis. Plates that did not fit this definition include solar, lunar, and eclipse observations, and above all, the spectrum plates, containing the hundreds of thousands of stellar spectra studied by Annie Jump Cannon. These plates might not be useful for time-domain photometry, but that is not at all to imply that they're not valuable in their own right. For instance, it’s believed that the spectrum plates could provide unique insight into climate change by measuring the gradual changes in the absorption lines that Earth’s atmosphere imprinted on the data.
But wait, there’s more! The Plate Stacks holdings also include film, written materials, and other artifacts. We can consider DASCH to be one tentpole among many in the broader effort to digitize the holdings of the Plate Stacks. It was certainly the tallest tentpole, but others remain.
Heading in a different direction, we can look beyond the Plate Stacks collection. The DASCH scanner is still going strong, and I think it’s fair to say that in many ways it remains the highest-quality plate scanner in the world. It’s very tempting to consider digitizing other plate collections. This presents logistical challenges: the scanner is essentially immovable, but large collections of glass plates are difficult and expensive to transport as well. It’s also true that the scanner is a one-of-a-kind piece of hardware, analogous to a telescope instrument. Many of its components are virtually irreplaceable — and getting quite old. If you wanted me to promise that the system will keep working for the next decade, you’d have to give me a fairly significant sum of money to invest in documenting the existing setup, assessing potential failure modes, and developing recovery plans.
Granting that baseline risk, the Plate Stacks are already in possession of a few external plate collections whose digitization with the DASCH scanner we hope to demonstrate. We have copies of POSS-I, the ESO Southern Sky Survey (which appears to be surprisingly undocumented, but see Schuster in Messenger, 1980), the SERC Equatorial Red survey (SERC-ER) and associated AAO Second-Epoch Survey (AAO-SES, aka AAO-R), and the Palomar “Quick V” (Pal-QV, Lasker+ 1990) survey done in support of the Hubble Guide Star Catalog project. Together these form a substantial fraction of the plates that went into building the Digitized Sky Survey. I’d love to compare DASCH results to the DSS data! This could dovetail beautifully with my longstanding desire to upgrade the “DSS Terapixel” map that’s the default optical basemap for WorldWide Telescope, which is generally great but has a few longstanding issues (most notably for scientists, a few-arcsecond global astrometric offset). What’s kind of amazing is that, as far as I’ve been able to figure, digitizations of these photographic surveys still, in the year 2024, represent the best way to obtain a deep, high-resolution optical map of every last nanosteradian of the sky. Modern surveys go awfully deep and awfully wide, but I’m not aware of a set of surveys that can be homogeneously combined across the entire sphere.
To try to make concrete the sophistication of the DASCH system — not just the scanner, but the people and processes surrounding it — I’ll note that we could probably digitize all of POSS-I in a couple of good weeks.
Finally, a call to action: these are things that I would like to see happen, but in truth probably none of them are going to occur (except DR7) without community support and collaboration. If any of these topics are at all interesting to you, please let me know! I would genuinely love to help other groups launch their own projects based on the DASCH data, the scanner, or other elements of the Plate Stacks collection.