I've played around with this. Somewhere, I'd seen that the magnitude of the sun is about 13 times brigher than the moon. (Try this link http://en.wikipedia.org/wiki/Apparent_magnitude)
That means the sun is about 500,000 times brighter. That worked out to about a ND 5.0 filter (2^19=524,000, remember that every 0.3 is one stop of exposure - 19*0.3=5.7. I suppose a 6.0 ND would have been closer...).
Using the sunny 16 rule and the moon needing expsure similar to daylight exposure given by sunny 16, you can expose the disk of the sun with 1/film speed at f/16 with a 5.7 ND filter. So I tried this, with a 2.0 and a 3.0 Wratten ND filters. Since I only had 5.0 ND of filtration, I had to stop down about 2 stops (5.7-5.0=0.7, 0.7/0.3=2.3 stops).
My test shots showed this was pretty close - maybe a little more exposure on the sun would have been good. I was shooting near sunrise so it was probably a little dimmer due to the light going through more atmosphere.
Since I was not using telephoto lenses, just a 240 and 150 mm, I did not have to worry about heat on the shutter or film plane. I focused on the landscape as normal, did an exposure of the scene before the sun had risen, and then did multiple exposures for the disk of the sun with the filter. It worked pretty nicely, I just needed to work out my timing interval between multiple exposures to make a nicer looking shot. (This was all on a normal day, not for a solar eclipse or anything.)
The Wratten does transmit some IR, but for normal lens lengths, don't worry about focusing with the sun in the field of view. Just don't use your loupe on the sun to focus it...
As far as hooking large format to a telescope, I built a platform that bolts to the top of my Meade LX-200 8 inch Catadioptic telescope and I've used both my Linhof Technika on it as well as a Fuji 617 camera. I never really got enough exposure time to make a really cool photo, but it shows a lot of promise. It was quite a load on the telescope, but you can get balancing weights to counterbalance with.
Also, look into Barn Door Mounts for doing exposures that are in the 10 minute ranges. They can be built big enough to mount a 4x5 - or even an 8x10 on them. I built one that uses a stepper motor to run the thing so you can do shots without having to touch the mount.
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