On All Hallows' Eve, at night, Jupiter was in the southern sky, shining brightly. I tested the doublet's existing spacing, at 0.76mm, again, and with the new focusser; no change of which to report. So, I hurried back indoors, and swapped out the spacers with those at 0.52mm all round, half of that of the original spacing-ring.
With the 32mm Plossl, at 9.4x, the flaring on Jupiter was quite evident, but with the 20mm Erfle, at 15x, the flaring diminished somewhat.
By the time I got to the 12mm Konig, at 25x, the flaring vanished, not a trace; the same as I went up further in magnification.
I have to wonder as to the sweet-spot. Is it at 0.40mm, or 0.60mm?
An out-of focus Jupiter exhibited a perfectly circular ball of light, and well-defined round the edge even. When I used an erect-image diagonal to view Jupiter out-of-focus, and for that RACI -esque or -ish experience, I could see the diagonal's Amici-line even, there in the center, vertically.
During the testing, I'm began to think that the Celestron-kit erect-image diagonal is slightly better than the GSO, and that I had purchased separately...
That's not good. I have been somewhat curious as to why Agena Astro, of California, still has them in stock, whilst all the other GSO diagonals are sold out, and until next year. Incidentally, Agena Astro is the only, the sole vendor for GSO-branded products in the U.S.
Unfortunately, the blackening of the doublet, the completion, will have to wait, and until the Moon comes round again, then to test the 0.52mm spacing further. I have a good recollection of what I saw of the Moon at the 0.76mm spacing, and I must compare the two.