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Yerkes Plate Digitization

A new process highlights an innovative thanks to get old glass plates online… and turned up a possible extra-galactic discovery over a century old.
You never know what new discoveries could be hiding in old astronomical observations. for nearly 100 years starting within the late 19th century, emulsion-coated dry glass plate photography was the quality of choice employed by large astronomical observatories and surveys for documenting and imaging the sky. These large enormous glass plate collections are still out there round the world, filed away in observatory libraries and university archives. Now, a replacement project shows how we’d bring the stories told on these old plates back to light.
More than an estimated 2.4 million glass plates are out there in collections in North America alone. These were taken starting within the 1890s right up until the 1970s, when CCD (Charged Couple Device) detectors began to come online for astronomy. Of these, only an estimated 400,000 plates are digitized to research quality, most notably by the DASCH (the Digital Access to the Sky Century at Harvard) and therefore the international APPLAUSE (The Archives of Photographic Plates for Astronomical USE) projects.
Astrophysics, and therefore the Kavali Institute for Cosmological Astrophysics wondered if there could be a neater thanks to bring these old plates into the fashionable digital era.
“The process of plate scanning is much easier,” Will Cerny (University of Chicago) told Universe Today. Once the plate got selected, we just make sure that the surface is clean in order that dust particles don’t get mistaken for stars within the final image. Then, we set our scanner to the very best quality we will and produce a picture file. In effect, we are considering the scanner to be a scientific instrument: for every small piece of data on the plate, we get a digital rendition of the quantity of sunshine transmitted through the photograph. From there, we upload the resulting file to an internet site which maps the celestial coordinates onto the image, which also creates an enter a uniform format for astronomical analysis.”
The team turned to a close-by source, the Yerkes Observatory. For the study, the Yerkes Plate Digitization Team wanted a plate ideal for calibrating both stellar brightness and therefore the sky background, covering a swath of sky located far away from the galactic plane. The team also wanted plates taken under excellent sky conditions, with long exposures depicting an honest sort of galactic in extra-galactic objects so as to measure limiting magnitude. Located on the shores of Lake Geneva in southern Wisconsin and built by American astronomer and telescope maker George Ellery Hale in 1897, Yerkes Observatory also houses a set of 150,000-200,000 glass plates. Though Yerkes is home to the good 40” telescope—the largest operational refractor within the world—most of the plates within the collection were taken using the 24-inch Ritchey reflector at Yerkes starting in 1901 or at the McDonald Observatory in western Texas.

The utilization of glass plates over a period of time for astrophotography was often tedious and cumbersome. Often, astronomers had to custom-shape the plates to suit specific cameras by hand using diamond cutters. What then followed was often a chilly dark night at the eyepiece following a guide star, while the required exposures were made. These resulting plates, however, function a chronicle of the sky spanning nearly a century.

Interpreting the magnitude scale on the scans and calibrating the plates for factors like glow, surface brightness and saturation (artifacts often introduced by the photographic and scanning process) yield a limiting magnitude of +19, and therefore the scanning process obtained a precision of higher than a tenth of a magnitude in brightness. For context, an outsized backyard telescope can typically see right down to about magnitude +14 on a transparent night with good seeing, and modern ground all-sky surveys like PanSTARRS-1 have a limiting degree of brightness of about 10,000 times fainter, at around magnitude +24.
“The simplicity of the method makes it possible to digitize an outsized number of plates during a relatively bit of your time,” says Cerny. “It also has the advantage of not requiring a custom scanner, making it accessible to groups without the time to style or purchase one. Custom scanners are prohibitively expensive. If our methods are often generalized, then plate collections from multiple observatories might be rendered available to be used in research project.”
The team has selected about 50 plates in the end that met the criteria for the study. The team used a commercially available Epson Expression 12000XL graphic arts scanner, greatly speeding up and streamlining the method. Files were initially scanned as positive .TIFF files (with black stars on a white background) then saved as FITS files, a format familiar to several modern astrophotographers. The targeted scan area resulted during a field of view 1.5 degrees wide, about 3 times the diameter of a full-of-the-moon. One among all the plates scanned by the team (Ry60) taken in 1903 centered on the +10th magnitude galaxy NGC 7331 located 45 million light-years distant within the constellation Pegasus also turned up a surprise visitor: a guest ‘star’ or possible supernova, not visible in SDSS (Sloan Digital Sky Survey) comparison images. If confirmed, this is able to be the fourth known supernova observed during this galaxy.

Cerny said that his team had actually scanned variety of plates before deciding on this particular plate (Ry60) for our paper… however, we had absolutely no idea initially that this plate was hiding this candidate supernova! “We were browsing the image of the galaxy on the plate as a part of our analysis, which involved comparing the plate with a contemporary image of an equivalent field of sky. At one point, we blinked (rapidly alternated) between the 2 pictures, and noticed what seemed to be a star present on the plate image.” The team also eliminated other potential false positives—such as an asteroid, dust fleck or a galactic classical nova—before measuring the object’s brightness, according to a foreign supernova.

What good are old glass plate images of the sky? Many other recent studies have turned to the record documenting the sky back over a century ago. When astronomers noticed an anomalous dimming seen in Tabby’s Star KIC 8462852, they checked out old glass plates of an equivalent region to point out that the strange star is really fading over longer time scales. Another study checked out the nearby white dwarf star named Van Maanen’s Star and demonstrated that astronomers had potentially documented evidence for exoplanets waaaay back in 1917… had they known to seem for it.

In addition to watching the variability of stars over long periods of your time, old plates open up the likelihood of watching stellar astrometry or the position and movement of stars via proper motion over a century-plus long baseline. The team used measurements from the European Space Agency’s Gaia mission for comparison within the study to demonstrate this very technique. Gaia released its DR2 (Data Release 2) catalog with over 1.6 billion stellar position measurements in 2018, and only recently went public with EDR3 (Early Data Release 3) on December 3, 2020, with the complete release set for late 2021.

In the end, the team and therefore the study demonstrated a low-cost but effective technique to simply scan astronomical glass plates for research level quality, using off-the-shelf commercially available equipment. The team also has long-term plans to form Yerkes plate scans and logbooks available online to the general public via the University of Chicago Library website.

It’s definitely well worth the effort to preserve those glass plate images of yore. Who knows what other astronomical discoveries are waiting to ascertain the sunshine of day.