Mirrors in the dark
by Dwayne Day
|As a wise sage once cautioned me: make sure to keep track of what you know is a fact and what you merely assume is a fact, and do not make the mistake of converting assumptions into facts merely because no contradictory evidence has emerged.|
However, sitting in a storage room in the visitor’s center for the MMT Observatory at the base of Mount Hopkins in Arizona lies part of the MOL legacy: six large optical mirrors constructed for the project and later used in an ambitious and technically challenging project known as the Multiple Mirror Telescope, or MMT. The MMT was built in the late 1970s and later disassembled in the 1990s, replaced by a new single large mirror which is now known only as the MMT Observatory, with the MMT no longer serving acronym duty.
Now to be precise, there is no confirmation that these mirrors are from the MOL program. As a wise sage once cautioned me: make sure to keep track of what you know is a fact and what you merely assume is a fact, and do not make the mistake of converting assumptions into facts merely because no contradictory evidence has emerged. We do not know that these mirrors came from the MOL program. What we do know is that: they are extremely lightweight and were therefore constructed for a spacecraft program; they are large; and they were left over from some canceled program. We know of no other canceled satellite reconnaissance programs from before 1975 that would have used these mirrors. MOL is therefore the best fit and this seems like a very solid assumption.
But MOL is a challenge to understand. The first year or so of the program’s history has been declassified by the USAF. This was a period during which MOL was intended as a military research spacecraft and the program was largely wandering around in search of a mission, and high-level support. At some point, probably in 1965, MOL transformed into an operational reconnaissance spacecraft. Its configuration changed and its mission became very dark. Virtually no information on the latter half of the MOL project has been declassified. As a result, the only clue to MOL’s mission and operations, other than the testimony of a few military astronauts trained to fly on it, are the former MMT mirrors.
Recently I had the opportunity to talk to somebody involved in the establishment of the Multiple Mirror Telescope. This person provided me with some information about the MOL mirrors and also suggested some further avenues for open-source research as well as interview subjects. Although he never asked for confidentiality, my goal was to use his information to gain access to better information, not for an end product. So his identity is unimportant, and the information presented here is tentative and not intended to be conclusive. None of this has been fact-checked. Yet.
According to the source, a famous telescope designer named Aden Meinel was instrumental in securing the mirrors for the MMT project. In the mid-1970s Meinel was Department Head of the Optical Science Center at the University of Arizona, and a colleague of Gerard Kuiper, the director of the University’s Lunar and Planetary Laboratory until his death in 1973.
In the early 1970s, Fred Whipple, the director of the Smithsonian Astrophysical Observatory, Kuiper, Meinel, and astrophysicist Frank Low discussed placing a telescope on top of Mount Hopkins, 55 kilometers south of the University of Arizona’s Tucson campus. At some point Meinel came up with the idea of a synthetic aperture telescope whereby multiple mirrors could combine their images to create the equivalent of a larger diameter mirror.
|Meinel never said where the mirrors came from “and we knew not to ask,” said my source.|
Meinel may have been prompted to this idea because he knew of a number of spare mirrors. In fact, he had one in his laboratory, which he used for testing optical equipment. The mirror was 70 inches (1.8 meters) in diameter. According to the source, Meinel could obtain six to seven of these mirrors for a telescope. Ultimately, he proposed a design using six mirrors, with an equivalent aperture of 186 inches (4.7 meters).
The mirrors were manufactured by Corning Glass of New York. Corning had manufactured a number of telescope mirrors over the years, most famously the 200-inch Palomar telescope in 1947, which was the Hubble of its day. The mirrors that Meinel acquired were made of borosilicate, a form of glass. Kodak also assisted in the assembly of the mirrors.
The mirror design was apparently both unique and ingenious. Each mirror weighed 1,250 pounds (565 kilograms). That may seem heavy, but it was actually only 13% of the weight of a normal ground-based telescope mirror of similar size. Put another way, the mirror was “eighty-seven percent light-weighted.” According to my source, this was incredibly impressive to those used to working with large ground-based telescopes.
It was this incredible light weight that clearly indicated that the mirrors were intended for space use. It was simply unnecessary to make a mirror that light for ground use. Meinel never said where the mirrors came from “and we knew not to ask,” said my source.
The fact that Meinel had one of these mirrors in his lab, and was able to gain access to a bunch of them, indicates that he clearly had ties to the world of satellite reconnaissance. What is unknown is if he was involved in any way with the manufacture of these mirrors in the first place. A quick perusal of Meinel’s publication list indicates that he had some esoteric interests, from the atmosphere of Venus to solar power to large optics design. Working on spy satellites would not have been out of character for him.
Besides the lightweighting, the most interesting aspect of these mirrors is that they were flat. They had no curvature and only reflected light, but did not focus it. In order to be used in a telescope, they had to be curved. So Corning heated each mirror and put it over a graphite mold until it slumped into a f2.7 curvature focal ratio.
Mirror construction started in 1974. The mirror supports were started in 1975. In 1976 the structure for supporting all six mirrors was underway and workers dynamited the top off of Mount Hopkins to prepare it for the rotating building to house the new telescope. Each of the mirrors was supported with an air bladder system. According to my source, the mirrors had no good support structure of their own, but it would have been relatively easy to pad them on their back to support them for spaceflight. He also added that it is a little known fact that, for telescopes, the support structure often costs as much as the mirror itself.
The Multiple Mirror Telescope was finished by the late 1970s and soon entered service. It was a relatively inexpensive way to get a large diameter mirror, although designers faced a number of challenges in combining the images from each of the mirrors.
The most interesting and confusing aspect of this story is the fact that these mirrors were all flat and had to be curved for use. What this implies is that the mirrors were reflecting mirrors, not concentrating mirrors. That has implications for the design of the MOL.
There are two competing theories about how the MOL was constructed. One is that it was designed like a periscope: it would fly along in its orbit horizontal to the Earth (with the Gemini pointing forward) and light from below would enter the MOL cylinder from the side, strike a reflecting mirror, and then travel down to a curved primary mirror that would focus it onto a secondary mirror which would then send it on to a camera, or an eyepiece (or more likely both). This design was used for the KH-6 LANYARD reconnaissance satellite and the more successful KH-7 and KH-8 GAMBIT reconnaissance satellites. The fact that the MMT mirrors were flat strongly implies that they were intended for this kind of system.
|The fact that the mirrors were flat implies that they were reflecting mirrors, not concentrating mirrors. That has implications for the design of the MOL.|
The competing theory is that MOL was constructed like the Hubble telescope with a Gemini spacecraft at one end. The Gemini pointed up, and the telescope pointed down toward the Earth. Light entered the telescope barrel directly and then was concentrated by a curved primary mirror onto a secondary mirror that sent it on to a camera or eyepiece. A source that I trust who used to work for the intelligence community once told me that he saw an illustration of the MOL system in this configuration.
The benefit of the straight tube design is that it eliminates the image-reflecting mirror entirely, thus saving weight and complexity (and possibly reducing light loss). This is the way that a satellite designer would make such a system if given a choice. After all, it is the system Hubble uses. The KH-6, -7, and -8 used an image-reflecting mirror because they were forced into it due to the need to physically return film to Earth: the film was stored in a reentry capsule at the nose of the spacecraft and the Agena upper stage that powered and stabilized the spacecraft had to be at the bottom. The image-reflecting mirror was the only way to bend light into the camera between them.
Because MOL apparently did not need an image-reflecting mirror, and because my reliable source said that none was included in the design he saw, I assumed that MOL used the Hubble configuration. This information about the optically flat mirrors for the MMT now causes me to doubt that conclusion.
One additional data point might help, or might confuse the analysis. The MMT mirrors shown in pictures of the observatory had holes in their centers to allow light reflected from the secondary mirrors to travel through the primary mirrors and back to the instruments. Were these holes in the original optically flat mirrors, or were they cut into the optical flats for the MMT project? They would have been unnecessary for MOL, unless they served some other purpose.
MOL was designed for a 30-day mission. But no artwork of the planned MOL in orbit depicts it with solar panels (although there is other concept artwork of different MOL configurations with solar panels). Even today spacecraft do not use fuel cells or batteries for an entire month. One possibility is that artwork depicting MOL in operational configuration, with deployed solar panels, may have revealed its mission and so the artists were told not to draw them. But right now that is only speculation.
According to my source, several publications such as Sky & Telescope and Physics Today published contemporary articles on the MMT’s construction, and there was apparently a conference proceedings produced soon after the construction of the observatory. I do not have these, but they may shed some additional light on the subject. But for now MOL still retains its mysteries.