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Originally Posted by renormalised
Yeah, that's true....no CA, but the longer the FL the less CA you have. Around f12-18 it's pretty much non existent and in any case, you can get rid of any CA by post processing of the images.
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You can't barlow out CA in a refractor, so any CA present at it's native focal length will simply be magnified as you increase focal length through barlows and powermates. Since you are going to need long focal lengths for planetary imaging this is not a good thing. The amount of CA present is dependant just as much on aperture as focal length, so you can't accurately state any focal length range where CA is non existant in a refractor.
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However, the big advantages a short FL/f ratio refractor has over a reflector in any sort of imaging, planetary and DSO's is that it's far more portable, has far less loss of contrast hence much sharper and darker images (less skyglow and loss of light in optical path), far less need for collimation of the optics and they're far less affected by tube currents and cool down a lot quicker than a reflector. The only real advantage a reflector has, is its light gathering capacity and no CA.
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True, but I would add that it's not just light gathering advantage for the reflector, but also
resolution advantage with the larger aperture. Also contrast between a planet and skyglow is a non consideration with planetary imaging. As long as it is night time you can image a planet from the middle of a bright city and still get great results.
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The best scopes for planetary work are long focal length refractors, but an ED with a powermate might help.
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Simply not true. All of the worls best planetary imagers use reflectors or CATS. A quick browse of the images section of this forum will confirm this.
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The refractors are far more tolerant of not so good seeing than the reflectors are. Have a slightly humid night with temperature fluctuations and a slight breeze and any image you take with a reflector will look like it's been taken through a fishtank!!!. That's why long focal length refractors make much such good planetary scopes, but they're expensive. If you're interested in good, long focal length refractors, here's a site to goto... http://www.dgoptical.com/
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Tolerance to seeing is a function of aperture, not design of the scope. Typical atmospheric cells that affect seeing are around 6" - 12" in size, so going above this aperture range you will need to be more considerate of seeing conditions when imaging.