Using a long achromat telescope as a general long tele lens for 135 film (coverage, apparitions etc.)

[reddesert]

Diaphragm?

The thread began by talking about telescopes, and telescopes traditionally don't have adjustable apertures. If you want to stop down a telescope, you can do this by putting a pupil mask over the objective. (At large or professional telescopes, one puts the pupil mask in a reimaged pupil, but on a small amateur telescope this is usually not possible nor necessary.) There are sometimes good reasons to do pupil masking, but they rarely have to do with exposure control. For a long slow lens like many refractors, you want decent image quality wide-open, and it's so slow that stopping down would be painful anyway, so just control the exposure with shutter speed.

 

[xkaes]

The thread began by talking about telescopes, and telescopes traditionally don't have adjustable apertures.

Kinda, sorta. The OP mentions:

"ED and apochromatic lenses are usually not much longer (if at all) than for example the terrific 400mm Rokkor I already own. But of cours you can use a Barlow or tele extender, but that brings us back to the question of coverage."

That 400mm has an aperture, even though it's not a "telescope". My Tokina 800mm f8 with a doubler in a 1600mm f16 -- with an aperture -- and the results are comparable to my Honeywell Lumitar 1250mm f10.5 (Celestron C5) telescope -- without an iris.

And both the Tokina and the Lumitar cover 24x36mm.

 

[moggi1964]

I used to use William Optics and also APM telescopes as long lenses but always sought out apo rather than achro. Better colour correction and less CA.
Used 80mm, 105mm and a80mm if I recall. I was first introduced to the idea by a couple of guys at an astronomy store in the USA back in 2007who were taking shots of an eagle on an apo. I still have their images and have attached them here for you to see (Copyright Dave and Grant at Highpoint Scientific NJ)