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Old 08-12-2017, 06:28 AM
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Atmos (Colin)
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Join Date: Aug 2011
Location: Melbourne
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Quote:
Originally Posted by Merlin66 View Post
Colin,
For a good achromatic the spot diagrams for red/green/ blue will all show tight focused results - when the chromatic aberration curve is taken into account.
I can cleanly focus target stars from CaK (393nm) through to Ha (656nm)with the achromat in the spectroscope. Yes, there has to be re-focusing between the wavelengths, but when this is done the image and result is very sharp.
(Spherical aberrations could be an issue......)
That works well when you’re looking at 0.5-1 angstrom passband but when shooting with a green filter you are actually going from almost red to almost blue. It gets worse when you are going from effectively IR to green with the red filter and green to UV with the blue.
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