I now see my previous post was foolish, as the 90FL f/4 is NOT the same design as the 107 ED f/3.9 despite having the same number of elements (6).
From the descriptions, the 107 is a Petzval design. That's a whole different thing, and in my opinion, Petzval is the best way to go in an imaging refractor.
One nice thing is a "true" Petzval design (all elements forward of the focuser) eliminates the need for precise spacing of the camera. If you get the sensor at the focal point, you are at the correct spacing, not the other way around as it is for the 90FL, where the last 4 elements are a reducer-corrector, aft of the focuser, requiring a specific distance from the last element to the sensor, with maybe 0.5mm of tolerance before things go bad.
I also think Petzval's have better color correction and perhaps a larger flat field.
What I don't know is if achieving this large flat field and good color correction was at the expense of a larger point spread, i.e, larger spot sizes (though I am guessing/hoping, still diffraction limited, at least near the center of the field).
I started designing my own Petzval 4-element 100mm focal length, f/4 lens, using OSLO, but cannot get good results with off-the-shelf lenses, even if I start with a N-SSK5/LAFFN7 achromat. I think a good Petzval is quite a feat, but more and more are showing up. Even Meade has a 71mm f/5 Petzval refractor now. Is this a fad or the future for imaging? I don't know, but I really like my William Optics 71mm f/4.9 Petzval. It is by far the easiest scope to image with that I have ever owned. Had I gotten this when I started 4 years ago, (instead of a 6-inch f/4 Newt) I may have not purchased anything but a Petzval after that.
Edited by StevenBellavia, 11 January 2019 - 08:44 AM.