Quote:
Originally Posted by bigjoe
FPL-53 is not just used to improve the chromatic aberration in an objective over that of FLP-51 , but really also to reduce the focal ratio requirements so a shorter F ratio can be used. F/8 Is quite ok and can get by with Fpl51 Fcd 1etc ...esp in triplets, as in Astrotech 115mm, a cracking scope! Its just FPL53 AND Flourite have more bragging rites.
bigjoe.
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That Astrotech 115 is probably the same scope as the TS115, but they are Triplets, and using FPL-51 & Lanthanum elements in the objective, at f7, certainly does produce nice results. Those scopes are price leaders as well. The design trend seems to be, for Imaging Triplet f7 and faster, is to go to FPL-53 for at least one element, or to FPL-51 and Lanthanum combos. As you say, life gets easier at f8, but the Skywatcher 152mm is a Doublet, so the devil is in that objective details.