Davoud:
...for eclipse photography purposes. I will be photographing the
eclipse with a "full-frame" DSLR. I have several 'scopes of varying
focal length and photographic fields of view and I want to ensure that
I do not crop the corona during totality.

Chris L Peterson:
I've seen images where the diameter of the corona exceeds 3°. It
depends on how tricky you're planning on getting with your processing.
Conventional single shot images tend to yield coronas that extend
perhaps a solar radius from the limb. But HDR imagery pushes this way
out. If you're planning the latter, I'd probably go with no more than
a 400-500 mm focal length. While that costs you some theoretical
resolution, most likely the seeing conditions will be what determines
resolution, and your pixel scale will be finer than the actual
resolution you can reach.

I'm planning on using a 400 mm FL refractor with a full frame camera
for my primary imaging instrument. I'll be running the camera from a
computer to shoot a rapid sequence of different exposures to try and
capture as much dynamic range as possible.

Thanks. That pretty much matches my perception and my plan. I had hoped
to use my Questar, partly for sentimental reasons: this is the 35th
anniversary year of my purchase from the factory. Indeed, it's at the
factory in New Hope, Pa. getting a complete rehab and drive upgrade @
$2950(!) Experience shows that is is perfectly suited for covering the
solar or lunar disc, but the Q cannot cover the long dimension of a 24
x 36mm frame, even with a .6 reducer (FL 840mm). It can cover a circle
of roughly 2 degrees.

Another advantage to the Questar is the reason I bought it:
portability. I have only to carry the Questar case and its Tristand,
plus camera(s). Nifty.

So that may lead to me taking my Tak FSQ-106, FL 530mm. Easily covers a
24 x 36 sensor with a field approximately 4 deg horiz x 2.5 deg vert.
If the diameter Moon/eclipsed Sun is taken to be about 31 minutes of
arc the image on the sensor will be 4.8mm, which is 13 percent of
sensor width and 20% of height. A little smaller than I would prefer,
but with a 30 megapixel camera, easily enlargeable for screen or print.

If I take the Tak or any 'scope other than the Questar to Tennessee I
have to carry my iOptron mount, not terribly bulky, but more than I had
hoped to have to take.

I have no idea why I'm thinking about this. I know that after driving
11+ hours I'm going to be clouded or rained out on eclipse day.

--
I agree with almost everything that you have said and almost everything that
you will say in your entire life.

usenet *at* davidillig dawt cawm