Quote:
Originally Posted by raymo
Something strange; should have been MUCH larger in the 127, not slightly,
[using the same eyepiece].
The CA is bound to be obvious through the 127, which at f9.4 is very short on focal length for an achromat, but the manufacturers try to keep the scopes manageable. A traditional f/15 version would be about 1.8 metres long, and
somewhat heavier, necessitating a MUCH higher and sturdier tripod. Depending upon the quality of the lens, an f/15 would show very little if any CA. Very long focal length achromats can rival apochromats, but are hugely
inconvenient to use, anything over about 125-150mm usually being a
permanent installation. An easy way to get a more equal comparison between the two, lens quality wise, would be to make two aperture stops for the 127
out of matt black stiff paper, one 100mm aperture [f/12], and one 80mm aperture[f/15]. The CA should be noticeably less at f/12, and very slight at f/15. No point in using the scope at 80mm, but you might find the loss of aperture an acceptable exchange for less CA at 100mm.
raymo
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That's very interesting and something I'll look into doing.
When l get another mount I will have them side by side and then the fun will really begin.