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If you are looking at just one wavelength, e.g. sub angstrom H-alpha, a well corrected doublet will do the trick, and provided it has no geometric vices
e.g. spherical aberration or astigmatism, you'll get filigree detail
The problem with shifting from red through to blue narrow band wavelengths
with refractors is, the focal length often changes, and has a different barrel distortion to boot.
In short: makes image registration very tricky.
Not sayin' you can't get good results, but pay the penny ride.....
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