FreeLists / az-observing / [AZ-Observing] Re: The Orion 120mm ED Refractor

["Paul Lind" <pulind@xxxxxxxxx>]

Folks,

I want to apologize for misquoting Roger Ceragioli on doublets and the 
strict APO criteria.  Here's one last comment before I'm run out of town. 
The main point is that it's possible to design a high quality f/7.5 ED 
doublet that will run circles around one using traditional glass from 20 
years ago.  To prove this to myself  I designed two lenses using ZEMAX. 
The first is an "Orion 120mm f/7.5" using FPL53, and the second is a 120mm 
f/7.5 achromat using BK7 and SF2.  The FPL53 design has a strehl ratio above 
0.8 for most of the visible spectrum, no coma, a decent field, and lots of 
good features.  I can't bring the BK7 design anywhere near that. There's too 
much data to show here, but if you'd like to see the ZEMAX files (.ZMX), 
email me at pulind@xxxxxxxxxx  I could also do screen captures of the plots. 
Finally, as Jack Jones says, terms like APO and semi-APO are confusing, and 
apparently Orion is misusing the term APO, even if it's a fine telescope. 
Also, I completely agree with Dwight Bogan and would choose Takahashi over 
Orion.
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