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Palomar Hale Telescope Cassegrain f/16 to f/9 Focal Reducer lens profiles


Society of Photo-optical Instrumentation Engineers

Palomar Hale Telescope Cassegrain f/16 to f/9 Focal Reducer lens profiles

A little known (and little used) feature of the 200-inch Hale Telescope at Palomar Observatory is an opto-mechanical assembly that can be mounted up inside the Cassegrain focus tube that extends upward through the center perforation of the 5m primary mirror, carrying 18-inch diameter(!) lenses in hinged(!!) mounts, that are able to be swung into or out-of the beam via geared hand-cranks located at the bottom of the assembly. When swung into the optical path coming down from the telescope's secondary mirror, the Hale Telescope's ordinarily f/16 Cassegrain focus is converted into a wider-field-of-view f/9 focus. This figure shows the cross-sectional profile of these hinged 18-inch diameter focal-reducer lenses -- the Schott K7/F2 doublet lens is used alone for imaging in the blue/green/yellow parts of the spectrum, while for imaging work in the red and near-infrared, the additional negative meniscus lens (also made of Schott F2 glass) can be swung in behind the doublet for better color-correction at longer wavelengths.

(( In a past life -- starting back in mid-1991 -- I led an effort to refurbish this f/9 Cassegrain assembly and equip it with a CCD camera in order image the open cluster M67 as part of a coordinated global large-telescope campaign, led by astronomers Ron Gilliland and Tim Brown, to perform extremely accurate "ensemble photometry" [i.e., using a time-series of CCD images to constantly compare the brightness of each star in M67 to the mean [or ensemble] average brightness of all the other stars in the field] to search for evidence in these stars [1] of behavior analogous to the "solar oscillations" discovered some years earlier in the sun. Such solar oscillations enabled solar astronomers to probe the interior of the sun, opening up a new area of research called "helioseismology" [literally, solar seismology].

Just in case any of the ATM / optical design aficionados here on CN might be interested in the optical prescription for the Hale Telescope, including its f/9 Cassegrain focal reducer lenses shown above, I included the optical design data within a short S.P.I.E. Conference paper describing the refurbishment that can be found on-line here: https://authors.libr...264/1/163_1.pdf ))

Ref [1] https://www.research..._4_M_telescopes

This diagram is included in a CN post found here: https://www.cloudyni...mat/?p=11047691

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