Note that for sufficiently long focal ratios that the two lenses can be the very same radius as each other, just positive and negative. The f/12 versions made with such matching lenses will have fantastically tight spot diagrams across the visual spectrum. Married together they make an optical window since the opposite faces are planar and the glass types are also the same. They can be inspected against each other for fringes this way.
As the focal ratios get faster, the concave lens needs a longer radius than the convex lens. In exchange for my printing lens cells, and because Ed Jones is a heck of a guy anyway, Ed reworked my concave lens from Newport to be about 0.6" longer in radius to optimize it for my 8" f/10.8 primary.
A lot of these Chiefs could use commercial, off the shelf lenses of the same radii as each other and still be better than achromats on axis and at least comparable or sometimes still slightly better 0.25 degrees off axis. It all depends on your chosen tolerances and range of frequencies considered.
With a slight increase of the radius of the concave lens, they can do even better for any focal ratio, and much better for sub f/8 primaries.
I applaud Ed for working on an 8" f/7 version here with custom concave lenses (coated, even!) that are competitive for cost with COTS choices. 8" f/7 gets down to simple ergonomic designs that can use standard elliptical secondary mirrors since the focuser can reasonably be placed close to perpendicular to the optical axis of the primary. I expect the tiny amount of lateral color due to the different radii will be negligible. On axis, this appears to be a fantastic prescription.
Edited by jtsenghas, 14 August 2020 - 09:35 PM.