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  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  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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