From: jim bell <[email protected]> To: Troy Benjegerdes <[email protected]>
> [I didn't get a bounce off of CP the first time] >Izvestia'.) An approximation I once heard is that a lens or mirror of about >4.5 inch in diameter can resolve an >>angle of one arc-second. A mirror of >the size of the Hubble Space Telescope (which I assume approximates >>that of the typical spy satellite today) is about 20x larger, so the resolution should be 20x better, or 1/20 arc->>second. That's 1/(57 degrees per radian)(3600arcseconds per degree)(20) = 1/4,100,000 radian. From an >>altitude of 500 kilometers, that's about 1/8 of a meter, or 120 millimeter. Maybe that's a pixel-pair, but it's far >>too large to resolve the text on a newspaper. >>> The best prospect to improve on this resolution would be to use a >>> 'multiple-mirror-telescope' technology. > >Light-gathering capability isn't >>> important in this application; high resolution is. Making a spy-telescope >>> out of a >>few different mirrors, held precisely many meters apart, could >>> conceivable achieve resolutions substantially >>greater than this. >> Jim Bell >Such a mirror array would at some point reflect enough light at odd angles to be visible with the >naked eye. >I find it more likely that multiple-mirror-telescope tech would be implemented >with a swarm of small >satellites and extremely precise location tracking and a lot of signal >processing later on. I sure find that difficult to imagine! Particularly because the assemblage would presumably be flying at about 500 kilometers altitude, and would therefore be buffeted by extremely-small-but-significant orbital winds. In addition, the amount of information that would have to be interchanged (phase and amplitude, in TWO dimensions!) of an entire field of view would be phenomenal. What I suspect the US military would really like to see is a spy satellite at geosync altitude (22,000 miles) with an apparent aperture of perhaps 150 meters, so that it has roughly the same resolution on the ground as existing fast-orbital spy satellites. (orbital period circa 90 minutes or so). Jim Bell Curiously, I just saw this article: (http://news.yahoo.com/us-military-developing-foldable-space-telescope-video-images-110058353.html) The United States military's advanced research arm is working on a foldable space telescope that could image Earth in high resolution at a relatively low cost. The Defense Advanced Research Projects Agency (DARPA) says the telescope design — known as theMembrane Optical Imager for Real-Time Exploitation, or MOIRE — would be of great use in geosynchronous Earth orbit, the spot 22,000 miles (35,000 kilometers) up where most telecommunications satellites reside. "Membrane optics could enable us to fit much larger, higher-resolution telescopes in smaller and lighter packages," Lt. Col. Larry Gunn, MOIRE program manager, said in a statement. [Giant Space Telescopes of the Future (Infographic)] "In that respect, we’re ‘breaking the glass ceiling’ that traditional materials impose on optics design," Gunn added. "We’re hoping our research could also help greatly reduce overall costs and enable more timely deployment using smaller, less expensive launch vehicles." MOIRE is now in Phase 2 of development since work began in 2010. When this phase is completed, a 16-foot (5 meters) prototype of the telescope's mirror should be completed for ground testing. No space missions have been set for MOIRE yet, DARPA officials said. There are both advantages and disadvantages to the MOIRE design. The membrane is not as efficient as the usual glass, but it is lighter — which allows prime contractor Ball Aerospace & Technologies Corp. to make larger lenses to increase the telescope's efficiency. DARPA estimates that a membrane system should weigh 86 percent less than a more traditional system of the same resolution and mass.w gallery Most telescopes either reflect light (using mirrors) or refract it (using lenses), but MOIRE's behaves differently. Each membrane will instead diffract light using a piece of equipment known as a Fresnel lens. "It is etched with circular concentric grooves like microscopically thin tree rings, with the grooves hundreds of microns across at the center down to only 4 microns at the outside edge," DARPA officials said in a statement. "The diffractive pattern focuses light on a sensor that the satellite translates into an image." If the design ever reaches orbit, DARPA envisions the membrane stretching to 66 feet (20 meters) in diameter — about eight times the diameter of the Hubble Space Telescope and more than three times bigger than the mirror for NASA's huge James Webb Space Telescope, which is scheduled to launch in 2018. The membranes would ride to space as "petals" packed into a tight package about 20 feet (6 m) wide, small enough to fit on a rocket. These petals would then unfurl in orbit, and provide an estimated resolution of 3.3 feet (1 m). Follow Elizabeth Howell Elizabeth Howell, or Space.com @Spacedotcom. We're also on Facebook and Google+. Originally published on Space.com. ===============
