levy@cbnewsc.ATT.COM (Daniel R. Levy) (06/29/89)
The July, 1989 issue of Discover magazine has an article entitled "The Big Glass" by Terry Dunkle, a story about how a small team at Perkin-Elmer built the 94" mirror for the Hubble Space Telescope. It's an interesting read, but something about it bothers me. Dunkel's account sounded if P-E was having major reliability problems with its high voltage power supplies for the electron guns used in its vacuum vapor deposition equipment. According to the story, these supplies would often go overvoltage and destroy guns, and during the coating of the Big Mirror itself, a supply refused to work until the last second. I glean from the article that these power supplies were supposed to deliver about 16 kilowatts at 9 kilovolts. Now speaking as a humble EE-by-degree, the "reliability problems" sound incredible to me. Power engineers routinely design equipment for power distribution which reliably handles far, far greater powers and voltages than what the vapor deposition equipment needed. I mean who designed P-E's electrical equipment, a high school science fair contestant?!? It seems so incongruous considering all their other high-precision, high-tech wizardry. Was Dunkle, in the pursuit of an interesting story for the layman, reaching just a little too far into his hat for a _deus_ex_machina_? (I might also mention the part of the story where P-E ruined weeks worth of grinding with one smoothing that supposedly flattened the edges too much -- that sure would take one hell of a big underestimation of how much material the smoothing would remove, I'd think. Maybe it's not so incredible as the problems with the power supplies, but still a stupendous slipup if it DID occur....) Can anyone shed more light on the matter? -- Daniel R. Levy UNIX(R) mail: att!ttbcad!levy, att!cbnewsc!levy AT&T Bell Laboratories 5555 West Touhy Avenue Any opinions expressed in the message above are Skokie, Illinois 60077 mine, and not necessarily AT&T's.
ferguson@gorby.SRC.Honeywell.COM (Dennis Ferguson) (06/30/89)
In article <1479@cbnewsc.ATT.COM> levy@cbnewsc.ATT.COM (Daniel R. Levy) writes: >The July, 1989 issue of Discover magazine has an article entitled "The Big >Glass" by Terry Dunkle, a story about how a small team at Perkin-Elmer built >the 94" mirror for the Hubble Space Telescope. > (stuff deleted) > ...too far into his hat for a _deus_ex_machina_? (I might also >mention the part of the story where P-E ruined weeks worth of grinding with >one smoothing that supposedly flattened the edges too much -- that sure would >take one hell of a big underestimation of how much material the smoothing would >remove, I'd think. Maybe it's not so incredible as the problems with the power >supplies, but still a stupendous slipup if it DID occur....) I didn't read the article in question, but its easy to understand how PE could have wrecked a 94" mirror. Even your garden variety reflector telescope has to maintain a mirror flatness of 1/8 wavelength across its surface. To polish a mirror 94" in diameter and keep it flat to less than 1/8 wavelength is not exactly easy. Mirrors comparable to the 200" telescope at Palomar are prestressed these days so that they warp under gravity to achieve the levels of flatness required. To achieve extremely large reflective surfaces, astronmers have gone to multiple mirror telescopes (MMTs) where smaller separately gimbaled mirrors are aligned using lasers. The resulting composite mirror is lighter and easier to maintain. D.Ferguson
mink@cfa.HARVARD.EDU (Doug Mink, OIR) (07/01/89)
From article <24798@srcsip.UUCP>, by ferguson@gorby.SRC.Honeywell.COM (Dennis Ferguson): >> > (stuff deleted) > I didn't read the article in question, but its easy to understand how > PE could have wrecked a 94" mirror. Even your garden variety reflector > telescope has to maintain a mirror flatness of 1/8 wavelength across its > surface. To polish a mirror 94" in diameter and keep it flat to less than > 1/8 wavelength is not exactly easy. Mirrors comparable to the 200" > telescope at Palomar are prestressed these days so that they warp under > gravity to achieve the levels of flatness required. To achieve extremely > large reflective surfaces, astronmers have gone to multiple mirror telescopes > (MMTs) where smaller separately gimbaled mirrors are aligned using lasers. > The resulting composite mirror is lighter and easier to maintain. Actually astronomers are going back to the one big mirror concept for most of the next generation of telescopes. The MMT, testbed for multiple mirror concepts, ended up not using the planned laser alignment systems and is being replaced by a 6-1/2 meter mirror spin-cast by Roger Angel of the University of Arizona. Several 8-meter mirrors are planned using the same technique. The only multiple mirror telescopes currently being built are the 10-meter Keck and an array of telescopes whose size I don't remember at ESO. Doug Mink Internet: mink@cfa.harvard.edu SPAN: cfa::mink BITNET: mink@cfa Phone: (617)495-7408