Nice post - thanks. I noticed my old Tak FS152 was stunning visually even compared to high triplets.
Also Yuri used FL51 in the now discontinued TEC160ED.
Greg.
Quote:
Originally Posted by wulfgar
That's precisely the issue. In theory an apochromat can be made from ordinary glass, but the curvatures of the element figures becomes something wicked. This in the past limited practical apochromats to very long focal lengths.
Generation one style ED enabled practical shorter designs in both true apo triplets and "semi-apo" doublets, but F7.5 is very short and more the province of the later generation ED's.
Chromatic aberration isn't the only issue with a refractor and these short focal lengths that are popular for imaging face increasing other problems that the old F15 standard for achromats never had to deal with.
But the introduction a third element risks the loss of more contrast due to scattered light which is perhaps part of the reason that "semi-apo" doublets remain popular.
I tried my old Vixen 102 triplet fluorite against a friends later doublet version. Racked slightly in and out of focus the triplet remains dead white while the doublet shows color. Is it decided then? No, even though the doublet showed some color, the doublet had better contrast in other ways with a blacker sky background. I can assume the extra element also means more scattered light. You gain some to lose some, everything comes with a price. I assume that greater curvatures on the elements of fast telescopes, will scatter some more light as well.
It looks to me like these cheaper 5" triplets are trying to cash in on the reputation of high end imaging products like the Tak's.
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