Finally got all the pieces together: large aperture lens, adapter, mounting rings and plate, filter.
The goal: observe M8 and M20 the final time this year.
Rersults: Success! And failure.
This is is the Canon 300 2.8 FD L telephoto found used. It's probably older than me. It's a very heavy lens. Requires solid mounting. Shown here with 125mm ADM rings and Losmandy plate.
Features a rear 48mm filter holder. Normal astro 2" filters thread into it, but the filter cells are too wide to fit in the slot. The Canon filters made for these lenses have low-profile cells. The solution last night was to remove the filter from the cell. The glass simply press-fits into the Canon holder. Not too tight- don't want a broken or cracked filter. A filter under stress will probably distort the star images too.
The other solution is a four-inch H-alpha filter, but that ain't happening.
The waxing crescent moon was only a few degrees west of M8. The lens frames M8 and M20-21 in the same FOV. The filter is easily removed/replaced for different targets. I swept up the Milky Way to M24, M18, and the grand sights of M17 and M16.
After an hour or so the lens adapter fell apart. I switched to a different lens. Happy that the Canon lens works well for NV. Disappointed that our time was cut short.
For NV the lens exists somewhere between handheld observing and telescopic. It can be handheld for quick looks. It's very heavy. A small telescope is better in many respects (zenith viewing , focus lock for example )