I score between 3 and 4, color calibrated monitor.
Speaking of fast achros, I have a barride optics 6" f/5 and I needed the prescription for it for modeling barlows and field flattener for it, so I went through the hassle of making a spherometer with ball feet, calibrated it the best I could with Jo Blocks, etc., measured the glass volume and density by immersion method...
With help of Mike I. Jones, who pointed me out to CDGM glass catalog, without any doubt, the glasses Barride used for this are H-K9L and C-F4.
Knowing the exact Back focal length for best polychromatic focus, I nailed the prescription accurately enough.
Now, the thing with that f/5.9 doublet is that it is obviously corrected for d line (588nm).
My Barride is corrected for e (546nm). The difference is smaller spherochromatism for f-line and above, which is producing smaller purple halo.
here is the plot for LSA (for the same lines and same 76mm aperture radius)
and here is the chromatic focal shift
On bright stars there is a small indigo halo and much larger, dimmer, purple halo.
When stopped down to 100-110mm, it is very easily bearable on the Moon at 150x mag. I'll get an EP to bump the mag up to 200, it feels like it can bare that much, and stop there with that scope.
Anyhow, about that purple halo business, correcting a fast achro for e line, it seems like a much more usable thing to do than correcting it for d line and let 430nm go too wild.
Edited by raal, 22 December 2015 - 07:53 PM.