jfc@athena.mit.edu (John F Carr) (01/14/88)
In article <990@devvax.JPL.NASA.GOV> lwall@devvax.JPL.NASA.GOV (Larry Wall) writes: >> [I and others pointed out diffraction, etc, limit resolution >> to about 5 inches] >You guys are all assuming a big round mirror. Now, it's true that for looking >at faint stars you need a lot of mirror acreage, but there's plenty of light >bouncing off of Lebanon. You don't need a huge round mirror to get the >aperature you want--just build a frame that will stay rigid in microgravity >and hang several smaller mirrors on it with a common focus. What's the >resolution for a mirror with an effective aperature of, say 20 meters? How >many 1 meter mirrors would it take to get the interferometry to come out right? Another posting suggests a larger conventional mirror. The reason I used 2 meters in my article instead of (1) assuming a larger single mirror or (2) assuming an interferometer is that I think neither practical. I do not believe that the technology is available to do the right kind of interferometry at optical wavelengths with non-coherent sources. A more likely method would be to launch a 3 meter mirror in 3 1.5 meter pieces, then fit them together in orbit. I doubt that the required accuracy is possible now (a telescope is being built in Hawaii (??) which has 6 mirrors fitted together into one large mirror; I have heard that the technology to build it is quite recent and that it is not known if it will work). Unless the mirror array were solid, the gaps would cause diffraction and the result would be no better than a single mirror of the size of the individual components. I chose 2 meters because I am not aware that we can launch anything bigger. Does anyone know what the size of the Titan II(I) is? Does anyone know what the diffraction limit is for 3 mirrors, in an equilateral triangle [so no preferred direction], with a common focus ? --John Carr
tedrick@ernie.Berkeley.EDU (Tom Tedrick) (01/15/88)
Here is an extract from "The Puzzle Palace" (James Bamford) page 259: " ... the Code 467 satellite, better known as Big Bird ... first launched on June 15, 1971 ... built around an extraordinary, superhigh resolution camera capable of distinguishing objects eight inches across from a height of ninety miles ... "
bgarwood@cisunx.UUCP (Robert Garwood) (01/16/88)
In article <22587@ucbvax.BERKELEY.EDU> tedrick@ernie.Berkeley.EDU.UUCP (Tom Tedrick) writes: >Here is an extract from "The Puzzle Palace" (James Bamford) > >page 259: " ... the Code 467 satellite, better known as Big Bird >... first launched on June 15, 1971 ... built around an extraordinary, >superhigh resolution camera capable of distinguishing objects eight >inches across from a height of ninety miles ... " Lets see, 8 inches from 90 miles corresponds to an angular size of 0.3 arc seconds. This is only a factor of 3 or so smaller than the best seeing of ground based astronomical telescopes and a factor of 4 to 5 larger than the diffration limit of a 2-m telescope at optical wavelengths. Seems reasonable to me. Bob Garwood Dept of Physics and Astronomy Univ. of Pittsburgh