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Hi
While Hitchhiker post is spot on when imaging a point source such as a star/deep space object it does not relate to planetary imaging.
Planetary imaging is an art in itself, Planets are extended objects, and the Dawes or Rayleigh criterion does not apply here as these limits refers to point sources of equal brightness on a black background. For example, a 254mm aperture telescope has a dawes limit of 0.45" arc seconds. The dawes limit is really of little use the Planetary observer, as it applies to stellar images. Planetary detail behaves quite differently, and the resolution that can be achieved is directly related to the contrast of the objects we are looking at. A great example that can be used from modern images is Saturn's very fine Encke division in ring A. The narrow gap has an actual width of just 325km - which converts to an apparent angular width at the ring ansae of just 0.05" arc seconds - well below the Dawes criterion of even at 50cm telescope. In `fact, the division can be recorded in a 20cm telescope under excellent seeing, exceeding the Dawes limit by a factor of 11 times!. (Quote from D Peach)
From my experience in planetary imaging, magnification is important, remembering that magnification is a function of focal length and the focal length will determines image scale. So the reason many planetary imagers use such high f/ratio e.g. f28 to f50 is to get a large enough image to fill as much of the CCD chip as possible. This way there are more pixel use to record finer detail.
Also for imaging planets there is no problem with critical sampling,
except the rotational speed of the planet itself. For example if you are to image Jupiter you will need to keep the video capture time to 90sec as the image will begun to blur after this lenght of time due to the rotaional speed of Jupiter.
You can also under sample if the video capture time is to short e.g there is not enough frames in the avi file to produce a decent image.
Final I should add an image of Mar. The original file was 10000 frames from that 2000 where used to produce the final image. I used a 10" LX200 @ f50 plus a Lumenera LU075C camera @ 37fps.
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