A question from my ignorance here.
I have a very inexpensive 70mm F4.3 refractor that I've been tinkering with to see if I can turn it into a decent finder.
With a 5mm EP in it, it was apparent that the Achro doublet was improperly collimated. Airy pattern was a fan, not rings.
On disassembly the the objective was two elements separated by a thin (~ 1mm) plastic ring which was poorly made with rough edges. As far as I could tell by eye, the two adjacent surfaces the lenses were a match so, just to see, I put the two lenses in contact (they appeared to fit exactly), reassembled and tried it out.
The collimation was now near perfect - nice Airy rings at 60x.
I was thinking about trying to make a spacer out of a loop of nylon fishing line, but then I wondered if I needed to bother.
By putting the spacer ring on top of the second element, the clamping ring held everything in place without rattling. I could also bond the two elements together with Canadian balsam or similar.
So is there a good optical reason to fiddle with making a spacer if direct contact appears to work? (at least as indicated by Airy pattern).