Quote:
Originally Posted by jimmyh1555
What's the difference between a big Newt or a small refractor? I find the smaller one is easier to set up, and doesn't vibrate so much in wind. Do you have to take longer exposures with a small refractor? An 8 inch Newt has 7 times the light-gathering power of a 3 inch refractor - does that mean you have to have 7 times the exposure with the smaller scope  |
With that 7 times, you also have to factor in focal length. The big Newt gives you more focal length and a fast F ratio. Focal ratio is what it's all about. Focal ratio = focal length / diameter of the objective. Your typical ED80 has 80mm objective with 600mm focal length. So 600 / 80 = F7.5. A typical 8" F5 is 1000mm FL / 200 (diameter) = F5. So the big fat Newt has both a longer focal length AND a faster F ratio.
If we even it out and make the focal length of the ED80 1000mm, the same as the 8" F5 Newt, the ED80 would have a focal ratio of F12.5. Which is very slow photographically.
Bottom line, DSLR's like fast scopes, up to about F7. You can get away with slower focal ratios but you have to acquire enormous amounts of capture time to make up for it.
On the other hand, yes a little ED80 is so much easier to handle with vibration, tracking and wind etc. It's much more forgiving in that respect.