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Old 19-03-2009, 06:03 PM
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gregbradley
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Join Date: Feb 2006
Location: Sydney
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There are 2 possible causes;

1. your scope is not APO and no doublet is by the way.
2. your lens is out of collimation even though it is a refractor.

Doublets fall into semi-APO category which is not to say they can't be good. But that much violet fringing is unusual. There are plenty of ED80 images that are quite sharp.

The Tak FS series is one of the higher quality doublets made and they give blue halos on bright stars.

What it means is green is in focus and blue and violet are way out of focus. APO means all 3 are in focus within a tight tolerance. Usually this means a triplet lens with the middle element made of exotic glass (like FPL53 which is expensive and are hard to make, hence the high cost).

Overzealous marketing from telescope sellers have hijacked the definition of APO and degraded it to a lesser meaning. The original standard is quite high and unfortunately only a very few scopes would qualify and even more unfortunately they are very expensive. Tak TOA, Astrophysics, TEC, some Stellarvue, TMB/APM triplets, some William Optics (most are doublets) are the bulk of them.

Having one element of FPL51 or 53 is only meaningful if the mating elements glass is also known. Better results in doublets come from a higher quality mating element. Roland Christen says its the mating elements that determine colour aberration.

The other possibility is your lens is out of collimation. If the 2 elements
are not aligned properly then they will also cause that effect.

Try a free download of CCD inspector and see what it says about your collimation after it analyses that image.

Either way your scope is not performing well enough and you can reasonably expect better performance from a doublet. You should consider sending it back. If enough people did that it would give the false marketers a black eye.

Greg.
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