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I noticed a strange thing last night while observing the moon, apart from the fact that it was freezing-gave for some good viewing though!

If I took off the middle section of the lens cap on my achromat refractor I didn't seem to get ANY CA whatsoever, but as soon as I took off the rest of it, CA appears!! :)

Sorry if I'm a bit behind on this one, but observing the moon without CA was much better (still would like a WO Meg though ;-))

Richard.

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Indeed. What you have done is, in effect, increase the focal ratio of your telescope, making it 'slower'. You are only using the centre part of the lens so you've reduced its aperture but the focal length remains the same. In refracters CA rises as the focal ratio diminishes which is why fast apos are so expensive and why very slow achromats give superb freedom from CA. The more the lens has to 'bend' the incident light the more the different wavelengths separate and the harder it is for the optician to gather them all together again at the focal plane! Also a 'fast' system has the light coming in from a wide angle so moving the EP or chip only a tiny amount moves it way out of focus. In a slow system the light converges from a narrower angle so is more tolerant of being slightly out of focus.

Olly

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You have now had a taste of relatively CA-free viewing - is there any going back now ?!!

Your resolution will have dropped with the reduction in aperture as well though so there is no "free lunch" :)

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Olly has it about right ... just to add that cheap eyepieces (and some not so cheap ones) also contribute chromatic aberration when used with a scope design which is too "fast" for them ...

... but don't forget, when stopping down an objective, that resolution is dependent on aperture, and light grasp is proportional to the square of the aperture.

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