30-inch mirror

Title

30-inch mirror

Subject

Astronomy
Optical instrument
Telescope

Description

Wood, metal, glass

Frustrated by the selenium cell photometer's lack of sensivity, Joel Stebbins purchased a large 30-inch reflector in 1912. The original telescope was purchased from C.W. Draper, $1500, #573, July 26 1912. It has previously been owned by Elmer Gates who used the telescope as a burning glass. According to Robert Baker: “The original telescope was made by the Brashear Company after the specifications of an individual whose plan was apparently not astronomical. According to one account he purposed using it as a burning glass for manufacturing diamonds. Another story is that he expected to observe the flight of souls. However this may be, the instrument was finally on the market at a very small price and my predecessor, Professor Joel Stebbins, recommended its purchase by the University. The telescope was quite unusual as its original purpose appears to have been. The 30-inch mirror had a focal length of only 20 inches and such necessarily imperfect definition that it was not the easiest matter to distinguish between a star and the moon.” A 1914 article in the Urbana Daily Courier states that the original mirror was by Brashear and was used in Washington DC as a huge burning glass, used to melt platinum.

Once in place, Stebbins and his assistant Elmer Dershem used the 30-inch telescope as a test bed for the selenium photometer as late as 1919. The photoelectric cell photometer being used in the main Observatory was proving to be very sensitive so the work on the selenium cell was eventually ended by the time Stebbins left Illinois in 1922.

Since the original Brashear mirror had proven unsatisfactory, a new mirror was ordered around 1925. The new primary mirror was 29 ½ inches in diameter, 4-inches thick with a 4-inch hole in the center. The missing secondary mirror was 7-inches in diameter. Both were made of borosilicate crown glass figured by John E. Mellish, an amateur telescope maker turned profession located outside of Wilmette Illinois around 1925. As an amateur he discovered six comets and is credited with being the first to see craters on Mars. The new focal length of the primary was 75-inches and the focal ratio of the Cassegrain system was 12.

The original mount from 1915 was an equatorial with a fork mounting. It was later adapted to the new 30-inch telescope in 1925 and to the Ross camera in 1939. The wooden dome was constructed in 1915 with a double slit.

With the new optics and longer focal length, the telescope mount needed to be modified. Six brass rods supported the secondary mirror in its new position. Focus was achieved by racking the secondary along the optic axis of the primary. Slow motion controls and clamps were added as were long counter-balance weight rods. The clock drive was also modified to handle the extra load. The 30-inch observatory's new site was on the south side of Florida Avenue, just east of Wright Street, about a quarter mile south of the Observatory. The site had darker skies courtesy of a cemetery to the north and crop fields to the south. The telescope was operational by December 1927. In 1938 Baker replaced the 30-inch mirror with a 4-inch Ross Fecker camera.

Currently on the Mellish 30-inch mirror remains. The mirror has lost the silver coating and has a 7-inch crack but test have shown it still retains it’s figure.

Creator

John Mellish

Source

Baker, Robert H. "The 30-inch Reflecting Telescope and Photoelectric Photometer of the University of Illinois." Popular Astronomy. 122:86-91.

"University buys big telescope" (August 5 1914) Urbana Daily Courier, page 3.

Publisher

Astronomy Department, University of Illinois, Urbana, Illinois

Date

1927

Contributor

Michael Svec

Rights

Copyright MIchael Svec

Format

image/jpg

Language

English

Type

physical object

Coverage

University of Illinois Observatory, Urbana, Illinois

Files

30-inch Mellish mirror
The 30-inch solar furnance in Washington DC about 1910.
The 30-inch with equipment for the selenium cell photometer circa 1915.
The rebuilt 30-inch reflector with photoelectric photometer attached. Robert Baker and an assistant used this telescope to help open the Chicago Worlds Fair in May 1933.

Collection

Tags

Citation

John Mellish, “30-inch mirror,” University of Illinois Observatory Collection, accessed August 16, 2017, http://uiobservatory.omeka.net/items/show/57.