Quote:
Originally Posted by multiweb
My understanding is that you need at least 3 lenses (apo) to correct for chromatic aberrations. Isn't your ED a doublet?
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It is possible to make an apochromatic scope with a doublet but not a fast one (the Telescopes, Eyepieces, Astrographs book has an example of a f/15 air-spaced doublet apochromat.) I expect that the ED80 is not a true apo.
Quote:
Originally Posted by Amaranthus
Every scope needs regular refocusing if the temperature is changing, although this is mitigated to an extent by carbon fibre bodies.
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Rolando from Astro-Physics claims that a metal body works better in a refractor because dimensional change in the scope body tends to counteract the change in focal length of the lens with variation in temperature. I haven't looked into this in any depth but I'm prepared to take his word for it
Sorry for hijacking your thread, Lee. I do see effects like this sometimes but it's normally a symmetric halo where stars in one or more colours are bigger but much the same shape.
Cheers,
Rick.