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The math of ground-based telescopes v. space-based telescopes v. interstellar probes

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  • #46
    Re: Sure

    Originally posted by pchang
    The problem is that working by hand in space is pretty tough. Last time I tried the spacesuit gloves, it was very hard for me to manipulate tools (compared to no gloves). Perhaps they have made advances in spacesuits since then.
    No, but they are making advances in habitat diameters. For instance, let's pretend that Bigelow Aerospace builds an inflatable habitat with suitable atmosphere (basically, a spacecraft dock). Piece together your mirror by hand. Depressurize the dock and launch.

    Inflatable habitats give you maybe 1.5x or 1.7x the diameter of regular habitats. So let's say that a space dock launched on the Ares V would have a max diameter of 12 meters x 1.7 = 20 meters.
    Last edited by DanS; February 19, 2007, 15:39.
    I came upon a barroom full of bad Salon pictures in which men with hats on the backs of their heads were wolfing food from a counter. It was the institution of the "free lunch" I had struck. You paid for a drink and got as much as you wanted to eat. For something less than a rupee a day a man can feed himself sumptuously in San Francisco, even though he be a bankrupt. Remember this if ever you are stranded in these parts. ~ Rudyard Kipling, 1891

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    • #47
      OK, so it looks like near-term, the max diameter for an optical telescope will be 8 or 9 meters, given that's what the Ares V will be, if they ever get the damn thing built...

      Encyclopedia Astronautica Index: 1
      http://www.astronautix.com/lvs/aresv.htm
      Encyclopedia Astronautica Index: 1


      What can you see with a diameter of 8 or 9 meters?

      You could probably make an oversized fairing. Might get you a couple extra meters.
      The Keck telescopes are at 10 m. And they should get a resolution of something on the order of centi-arcseconds in the visible spectrum...
      "Beware of the man who works hard to learn something, learns it, and finds himself no wiser than before. He is full of murderous resentment of people who are ignorant without having come by their ignorance the hard way. "
      -Bokonon

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      • #48
        Originally posted by DanS
        AFAIK, false color only gets you so far. The Earth is more beautiful than anything you would paint. The goal is to put something in orbit that would replace an interstellar probe. You want full visual impact.

        How big would a telescope need to be to replace an interstellar probe? Are we talking miles? Meters? AUs?
        For images of Earth or the planets, yes, the visual spectrum is preferable.
        But for images of stars/gas clouds/galaxies it is not obvious to me that the visual spectrum makes for better images. And though I don't use them myself, I have the impression that most of the Hubble wallpapers are of this kind.

        On the size of an interstellar probe, I read an article in Scientific American which said that the 100m OWL telescope would give the same quality of images of outer planet moons as a small probe send there.

        To rival an interstellar probe, I would guess you would need a pretty large Earth-based telescope. Light expands on the surface of a sphere, which is proportional to the square of the distance. The collecting area of a telescope is also proportional to the square of its diameter. So to take pictures as pretty as the images of Jupiter around a distant star, you need a telescope with N times the diameter, where N is the proportion between the distance to Jupiter and the distant star. A short calculation gives me N=50000 for Alpha Centauri at 4.3 LY, so you would only need a telescope with 50000 the diameter .

        On the other hand, making interstellar probes which has to function for tens of years cannot be easy either. There is no doubt that if you want at least some results in a reasonable timeframe then you start with relatively cheap Earth-based telescopes like OWL.
        http://www.hardware-wiki.com - A wiki about computers, with focus on Linux support.

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        • #49
          Re: You better believe it

          Originally posted by pchang
          Once you get past Jupiter, its too dark to use real color.
          I must admit that some of the recent interplanetary probe images (Saturn?) have been pretty amazing.
          I came upon a barroom full of bad Salon pictures in which men with hats on the backs of their heads were wolfing food from a counter. It was the institution of the "free lunch" I had struck. You paid for a drink and got as much as you wanted to eat. For something less than a rupee a day a man can feed himself sumptuously in San Francisco, even though he be a bankrupt. Remember this if ever you are stranded in these parts. ~ Rudyard Kipling, 1891

          Comment

          • #50
            Thue: So 5,000 km diameter? Hmmm...
            I came upon a barroom full of bad Salon pictures in which men with hats on the backs of their heads were wolfing food from a counter. It was the institution of the "free lunch" I had struck. You paid for a drink and got as much as you wanted to eat. For something less than a rupee a day a man can feed himself sumptuously in San Francisco, even though he be a bankrupt. Remember this if ever you are stranded in these parts. ~ Rudyard Kipling, 1891

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            • #51
              To get equivalent images to what a 10cm telescope can take of Jupiter, yes. (If my calculation is correct...). Give or take a factor 10.
              http://www.hardware-wiki.com - A wiki about computers, with focus on Linux support.

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              • #52
                ITYM, equivalent images to what a 100 meter telescope (i.e., 100 meters times 50,000) can take of Jupiter?
                I came upon a barroom full of bad Salon pictures in which men with hats on the backs of their heads were wolfing food from a counter. It was the institution of the "free lunch" I had struck. You paid for a drink and got as much as you wanted to eat. For something less than a rupee a day a man can feed himself sumptuously in San Francisco, even though he be a bankrupt. Remember this if ever you are stranded in these parts. ~ Rudyard Kipling, 1891

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                • #53
                  Ah yes, 100m, not 10cm.

                  When dealing with numbers this size it is easy to lose a factor 1000...
                  http://www.hardware-wiki.com - A wiki about computers, with focus on Linux support.

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                  • #54
                    Re: That's what

                    Originally posted by pchang
                    false color is for
                    QFT.
                    It's all false color nowadays, in a way, even what's in the visual spectrum.
                    For one thing, it's numerical, and they will often enhance one thing or the other because otherwise things are too bland.

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                    • #55
                      Originally posted by DanS
                      AFAIK, false color only gets you so far. The Earth is more beautiful than anything you would paint. The goal is to put something in orbit that would replace an interstellar probe. You want full visual impact.

                      How big would a telescope need to be to replace an interstellar probe? Are we talking miles? Meters? AUs?
                      False color doesn't mean you have to "paint".
                      You can get beautiful pictures in false color.

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                      • #56
                        Re: the answer is too depressing

                        Originally posted by pchang
                        It's why I don't work at places like JPL anymore. I went for the money instead and am much happier.
                        What did you do there and what do you do now?

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                        • #57
                          My life story

                          I got out of grad school at MIT (aeronautics and astronautics) and started working as a government contractor. I never worked directly for NASA/JPL, but I was loaned out to them for several projects dealing with space sensors and modifying interstellar probes. I got disenchanted with working on government projects and went totally commercial. I am now a sales engineer. I sell expensive network/application appliances to large companies.
                          “It is no use trying to 'see through' first principles. If you see through everything, then everything is transparent. But a wholly transparent world is an invisible world. To 'see through' all things is the same as not to see.”

                          ― C.S. Lewis, The Abolition of Man​

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                          • #58
                            Re: My life story

                            Originally posted by pchang
                            I got out of grad school at MIT (aeronautics and astronautics) and started working as a government contractor. I never worked directly for NASA/JPL, but I was loaned out to them for several projects dealing with space sensors and modifying interstellar probes. I got disenchanted with working on government projects and went totally commercial. I am now a sales engineer. I sell expensive network/application appliances to large companies.
                            So do you get sent by an employer to institutions to basically give a spiel for their products/services and rely on juicy commissions to make it all so worthwhile?

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                            • #59
                              If he's happier, it's worthwhile.
                              No, I did not steal that from somebody on Something Awful.

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                              • #60
                                Re: Re: My life story

                                Originally posted by Geronimo


                                So do you get sent by an employer to institutions to basically give a spiel for their products/services and rely on juicy commissions to make it all so worthwhile?
                                I actually have to make the equipment work in the customer's environment. However, I get to rely on a high salary as well as juicy commissions to make it all worthwhile.
                                “It is no use trying to 'see through' first principles. If you see through everything, then everything is transparent. But a wholly transparent world is an invisible world. To 'see through' all things is the same as not to see.”

                                ― C.S. Lewis, The Abolition of Man​

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