The Plate Collection of the
Nantucket Maria Mitchell Association
 

Digitized Copies Now Available

The Maria Mitchell Observatory's collection of over 8,000 plates taken over the last century has been digitized and put onto CD-ROM. We are making the plates available for use by our colleagues.

The Plate Catalog

An Excel-readable version of the plate catalog can be downloaded here, and a version is available from the Wide Field Plate Database maintained by the Bulgarian Academy of Sciences.

To obtain more information or request specific plates please contact Dr. Vladimir Strelnitski, Director of the Maria Mitchell Observatory.

  

Coverage

Time:
1913 to 1995

Sky:
Major areas are in Sagittarius, Scutum, and Cygnus where the coverage is quite rich.

Plates

Plate Characteristics:
8 x 10 inches or 13deg (RA) x 16 deg (DEC)

Scale: 240 arcsec/mm

Limiting Magnitude:
Varies with exoposure and emulsion. With 103a-O exposed for 30 minutes -> about B=14

Emulsion:
Common emulsions for the time were used. From 1949 on Kodak 103a-O and IIa-O were the usual choice.

 

Scans

Low Resolution scan: Sufficient for most purposes
Full plate coverage - 840 ppi
Size: 65 Mbytes

High Resolution scans:
2 per plate East and West - 1 inch overlap in center
2,500 ppi - 10 microns per pixel
Size: about 550 MBytes per plate

Format: TIFF

The Telescope

The historic 7.5-inch Alvan Clarke telescope seen on the left dates from 1913 and carries a Cooke triplet lens in a mount made by Alvan Clarke and Sons.

The lens was remounted in 1941by J.W Fecker of Pittsburgh and the effective focal length was measured to be 33.75 inches.

Following this the lens was further adjusted in 1983-4 by Paul Valleli with the Amateur Telescope Makers of Boston. James Baker of Harvard-Smithsonian did a ray trace of the lens after the composition of the elements was determined.

More telescope pictures and an offer.

Scanning Procedure

In March 2001, the MMO started and in October 2002 finished a complete digitization of its photographic plate archive.

The project was funded by a grant from the William R. Kenan Jr/ Fund for Engineering and Science.

A commercially available scanner, AgfaScan T5000, customized by the MMO, was used for this project.

Three image files (about 600 MB in total) were burned on a CD for each 8x10 inch plate - the overview scan 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.

 

  

Photometric Accuracy

In order to check whether digitization with this scanner entails any serious losses of information, we carried out photometric measurements of several scanned plates containing the open cluster M25. This study, performed by the MMO/REU student Kristina Barkume, showed that the simplest eye photometry of standard stars from this cluster is equally precise for estimates on the plates directly and on their digitized images (BAAS, vol. 33, n.4, p. 1322, 2001, abstract 10.13).

Previously, it was demonstrated in another MMO/REU project (BAAS, vol.30, n4, p. 1266, 2000, abstract 11.11) that the eye photometry of stars on photographic plates is not less precise than other methods, such as microphotometry or numerical reduction of digitized images. We conclude that digitization with the AgfaScan T5000 provides a copy of the original plate without appreciable losses of photometric
information for stars.

This material is based upon work supported by the National Science Foundation under Grants No. AST-9820555 and 0097694. Any opinions, findings, and conclusions or ecommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
       
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Page maintained by Peter Boyce  

email: pboyce@aas.org