Since it's less work I'm going to copy and paste what I said in Miscellany.
It's funny you brought this up since I was just thinking about these spacer rings.
I'm not sure but I think they might be steel. It looked like the one in my Crescent was a bit rusty. It also appeared to be uneven in thickness, like it had a tiny uneven lip on it's edge from being stamped out. The one in my '63 60/700 9TE is metal and was not round as in bent. The 60/1000mm Penncrest has two plastic (nylon?), ones. One between the lenses and one that the flint rests on. I also have a 900mm Carton lens that the previous owner put in a Sears cell with a nylon spacer. since Carton didn't use rings, it makes me think it would perform better if it had three foil spacers. Anyway, I'm begining to think between the two types, the plastic ones might be better.
Has anyone ever measured a metal or plastic ring spacer's thickness? I'm sure they are much thicker than the typical foil ones. They seem to space the lenses further apart so that the Newton rings cannot be seen. I once replaced the ring in the Cresent with three foil spacers and it seemed to perform even worse, so switch it back.
Maybe this topic could use it's own thread?
I think the biggest problem with a ring spacer is, without precise manufacturing, it's likely to have a inconsistant thickness and I doubt that Towa made them that precise. My guess is they were used for mass prodution since it would be much faster to assemble than appling three foil spacers. Afterall, what other decent telescope maker even used them on a Fraunhofer lens?
I've had a few turkeys so agree, foil spacers don't necessarily equal good optics. On the other hand there seems to be a ton of members who extol the performance of their Towa made (and a few other makers), ringed spaced scopes, but without a DPAC, or side by side evaluation, or a look for your/myself, the rest of us will never know how good or not they really are.
It might be a good or fun stand alone topic to ask and press them on the subject. Though it might get heated.
My 60/1000mm Penncrest (Towa with two plastic ring spacers described in Post 1) seems to have fairly good optics. I don't have a DPAC so I won't really know how good it is, or how it rates until I put it up against some my other long refractors.
Edited by Kasmos, 24 July 2020 - 05:22 PM.