RESEARCH – ASTRONOMY

Astronomical Plates Collection

What Are Astronomical Plates?

Astronomical Plates are essentially an early form of photography that used glass plates to capture images of the night sky. The application of photography to astronomical research, in the last quarter of the 19th century, revolutionized the science of the sky. The possibility to record permanently the images of thousands of stars and other celestial objects on a photographic plate looked like a miracle to astronomers who, until then, could only record their visual impressions. Much of our knowledge of the universe comes from measuring changes in objects over time as recorded on photographic plates. For a full century, photographic recording of the sky provided the basic information from which astronomers have developed the present picture of the universe.

Why Are Astronomical Plates Important Today?

During the last two decades photographic plates have given way to the even more accurate electronic camera. However, many changes in the heavens take place on time-scales that are too long to be covered at all adequately by modern observing techniques, no matter how sophisticated the equipment. Innumerable questions can only be solved or even tackled by recourse to archival material that spans many decades. When objects which look like normal stars explode (as novae or supernovae), or are discovered to be a source of x-rays, or radio radiation, we try to learn more about them by studying their past behavior on photographic plates.

The MMO Astronomical Plate Collection

Fortunately, the century of photographic astronomy has bequeathed us a legacy in the form of collections of scientific photographic plates which now reside in observatories around the world, including an important collection at the Maria Mitchell Observatory. 


The MMO plate collection, consisting of approximately 8,000 photographic glass plates, was produced by the efforts of several professional astronomers and hundreds of students from 1913 to 1995. While not as large, it is one of the richest and most uniform for a few selected areas of the sky in the constellations of Cygnus, Sagittarius, and Scutum that are especially rich in variable stars.


In 2001-2002, the Maria Mitchell Observatory digitized all the 8000+ photographic plates obtained with its 7.5-inch Cooke/Clark refractor during those eight decades of research (see Schaefer, B.E., Sky & Telescope, vol. 103, No.3, p. 42). The digitized copies were first put on CD-ROM and later copied to a hard-disk storage device. Their CD or DVD copies are available for the scientific community on request that should be addressed to Dr. Regina Jorgenson at rjorgenson@mariamitchell.org


Most of the plates are 8 x 10 inches, which, with the scale 240 arcsec/mm, corresponds to the field of view 13 deg (RA) x 16 deg (DEC). As mentioned, the richest sky coverage is for Sagittarius, Scutum, and Cygnus.


The limiting (B) magnitude varies with exposure and emulsion and is, typically, between 14 and 16 mag.

Catalog of Digitized Astronomical Plates from the MMO

The downloadable catalog file is here. To import the catalog, save the file on your computer as a plain text file. Then open it with Excel as a coma-delimited file and you will have the catalog available for manipulation. The resulting Excel file should be about 700KB in size. The columns of the catalog give: (1) Plate number, (2) Plate’s center Right Ascension (hours), (3) Right Ascension (minutes), (3) Declination (degrees), (4) Exposure time (minutes), (5) Calendar date in Y_M_D format, (6) Julian Date, (7) Emulsion type, (8) Notes. The work on the catalog is still in progress. The current version may have some gaps, which are being filled when detected. We will appreciate any comments/suggestions.

Plates and Their Scanned Copies

A commercially available scanner, AgfaScan T5000, customized by the MMO, was used for scanning. Three TIFF image files (about 600 MB in total) were burned on a CD for each plate – the overview scan (65 Mbytes) of the whole plate with resolution of 840 dpi, and two high-resolution (2,500 dpi) scans of the western and eastern halves of the plate (with some overlap). The higher resolution corresponds to about 10 microns on the plate.


The photometric accuracy of the plate copies was investigated in several MMO REU student projects (Bull. Am. Astron. Soc., vol. 33, 2001, abstract 10.13; vol. 36, 2004, abstract 153.03; ASP Conference Series (San Francisco: Astronomical Society of the Pacific), Vol. 338, 2005, p.359), with the conclusion that the scanning typically introduced less than 0.05m additional uncertainty for stars not too close to the plate limit.

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