On the other hand, with some star testing over many nights over a few months, and with attempts at masking and adjusting, my opinion is that the quality of these lenses is modest, probably not as good as quarter wave.
I don't have a flat here-so I really worked on learning star testing. (I was tutored in autocollimation testing by David G and Dick Parker at Delmarva and folks at Stellafane, and I've got a setup in the US).
Here, I was using the Suiter book, and I’ve been reading the book by Cooke, “On the Adjustment and Testing of Telescope Objectives.” 1899. Good book, recommended by David Grotski. The main star testing chart is good, I spent time transferring the key directly to the chart. Then pages 29, Spherical Aberration, to 31 zonal aberration, to 33, perfect figure, to 34, The Star Image. This book of free for download-just Google it.
An example of some of my notes on the 6inch f/10.
"At focus pretty nice (as much as I could discern through turbulence). Star, and one clear ring, some flaring as I move my eye. It is cleaner with the #58A green filter. Not very much extraneous junk in the image (at least as I could make out in the turbulence).
I then tried the #11 yellow-green filter.
-bright outer ring, fuzzy on the outer edge
-pronounced dark first ring
-now and then when it is still, nice concentric rings, maybe five inside first dark ring
-views fuzzier without the filter, but I still see the same
-bright ring, then purple color over the rings inside
-this side looks a bit cleaner, sharper
-less fuzzy outside the outer ring
Later in the evening Sirius came into viewing range of the scope as mounted.
Sirius, At focus:
-Pretty dancy, can see a good ring around the center star, but dancy flaring
-What I saw on the other star, just brighter
-hairy outer ring
-strong first dark ring
-purple now only strongly on the central 1/3 of the image
-rings are clearly defined
-hairy outer, strong first disk
-pretty even brightness, outside to the center
1. nice concentric rings both sides
2. number of rings are the same both sides, with equal focuser travel
3. outer ring is a bit fuzzier on the inside, maybe a bit cleaner on the outside
4. the bright outer ring is the same both sides, first dark ring is the same, then even illumination to the center, both sides.
Therefor, as interpretation:
1. -there is no stig
-lens elements are aligned well in the cell
2. I have a decent comparison set up.
3. So, outer ring is a bit hairier on one side, and a bit cleaner on the other. Maybe this is a slight turned down edge? Green filters 58 and 11, help this by diminishing the effect.
4. illumination of outer ring, first dark ring, and 5 or 6 rings to center are pretty even both sides, meaning so significant SA, and no zones (so far as I can see on this evening)
Based on this data, I have the lens well mounted in the cell, and the tube works well for it to produce a decent image.
The overall quality is still unclear, but the possible range is, I believe, from OK to good. This is based on the star testing so far, and the fact that I can push powers on the moon to well above 200X on nights when the seeing is modest. A two times barlow, at well over 400X, was not super sharp on these evenings, though. This is a quality lens, but maybe not a high quality lens. This lens provides some very nice views of the moon.