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Originally Posted by Kal
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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Quote:
Originally Posted by renormalised
CA is entirely due to the degree to which the lenses of the scope bring the various wavelengths of light to focus at the focal point. It has nothing to do with aperture and everything to do with how the lenses are figured and the refractive indices of the elements of the lens assembly. The reason why they use fluorite is that it is optically more transparent than normal Schott glass and has a low RI which means there is little or no subsequent deviation of the light path through the lenses and hence little or no misalignment in the focusing the various wavelengths of light...they are brought into focus as close to the desired focal point as you can get.
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As I stated above, CA in a refractor
is dependant on aperture as well as focal length. If all things are equal, and you have the same design and the same glasses used, and you went from a 4" F8 refractor to an 8" F8 design, everything will scale, including the spot sizes at the focal plane. To achieve the same amount of correction you therefore need to double the focal length, so a 4" F8 refractor will have the same amount of color correction as an 8" F16 refractor. You can see why refractors are not popular at larger apertures!
The reason why fluorite is used is used has nothing to do with it's transperancy compared to schott glass. For starters, schott is just a company, and they make all kinds of glass, and not just for telescopes. The reason why telescope makers use fluorite and PFL-53 glass is because of their dispersion value off the abbe line. To get better colour correction you need to use at least one glass that deviates from the abbe dispersion line, and fluorite makes a good match to the readily available and cheap BK7.