On Fri, Mar 18, 2011 at 12:29 PM, Hal Murray <[email protected]> wrote: > >> From what I can learn about "seeing" the atmosphere instability is too >> great to allow making measurements optically in the 1 ms area. > > There has been some interesting amateur work done on taking lots of > short-exposure pictures and sifting through them to find the good ones. (I > can't find a good URL right now.)
Video is used for imaging planets. They are bright because like earth they are lit by the sun. But measurement is different from imaging. For this job we don't need pictures. The method using by FASST is called "drift scan". The images of the sky is allowed to move over a large CCD sensor. The telescope does not track the sky so the stars images drift. CCD sensors are read out one row at a time then the entire images is shifted down and the next ros is read. It a drift scan camers the charge shifting in the CCD is synchronized to the motion of the image. The result is a very. long image. Think of it as like a flat bad scanner ony the image moves not the sensor. This allows for some longer exposures and also covers a lot of the sky. One BIG advantage of drift scan is that the telescope is bolted down solid and never moves, so uncertainty in pointing is greatly reduced. The FASST camera continously a large part of the sky as it drifts overheadand they match that data to a catalog of stars and then know the rate the sky is drifting and thenthe rate the Earth turns I worked on a drift scan camera project a few years back. We were able to generate the wave forms to control the CCD in software. The Earth does not really turn all that fast I had a prtottype camera mounted on my Garage roof for a long time, It used a chaep telephoto lens (135mm f/2.8) as the telescopes and we were getting surprizing accuracy for a "junk box" optical system. An example of one of the prototypes is the white tripple lens unit here http://www.tass-survey.org/. Software ran under a rel-time version of Linux with timming by NTP but later re-calibratd by the stars themselves. Our system not not nearly as good as FASST, not even close but used many of the same techniques -- ===== Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
