I wasn't very precise in my first comment.

My adapters are from 4 manufacturers/distributors, including Vivitar.
These are definitely all 5x.
Bought my first one some 30 years ago, so they've been around a long
time.

My statement on 80-210 & 300mm mirror lenses was a useability comment,
not an optical judgement. My primary interest for these telescopes is
for a light, small, high-quality, telescope for wildlife & maritime
stuff, 'tho I used to be a pretty serious aircraft anorak...

I've tried the adapters with many lenses. It's helpful to have zoom --
makes target-finding quicker. Short lenses below 75mm don't work as
well. You don't need automatic, autofocus or anything that adds costs
or complexity. Least moving parts the better -- telescope image is
limited by the quality of your eye, not film resolution (unless you're
trying to be very
clever, & use this adapter to produce images to be photographed )

Little use with my 120-600 Vivitar unless on a rock-solid tripod base
(which I do have as a permanent fixture, as I overlook an interesting
waterway).

Problem with really long lenses is that the adapter makes them much
longer! Firld of view becomes a degree or so, & locating what you are
trying to see can be tiresome without a finder -- some wildlife is very
small -- & fast ..... Hand-holding a 3000mm lens is an art form
destroyed for me by 60 years of beer drinking.

Some camera accessory folk do (did?) sell detachable tripod collars,
usually some form of clip-on collar that surrounds the lens barrel.
Must confess that on my adapter set of ancient cast-off lenses I've
used crude rigs including jubilee clips 'thru to aralding a suitable
nut straight onto the lens barrel. Easy to make somthing up, either
permanently or temporarily. Often see old lenses on eBay for a few
pounds with tripod mounts.

The best optical quality lens I've used with an adapter for everyday
usage in the field is a '60's 200mm f3.5 prime pre-set by Asahi Pentax.
Mine has tripod mount, light in weight & image, wonderful quality --
but mint, & has sentimental family connections, so not used in the
hurly-burly of cliff walking etc. Must find a battered one .....

Adapters get called different things. No universal name. Quite often on
eBay -- I've bought 3 there, between $30 & $50 each. (All new, or as
new).. Nothing to go wrong with them, so a safe buy.