Quote:
Originally Posted by rogerg
phew .. my head is spinning from some of the intellectual replies to this  .. must be the end of the day.
my understanding is pretty simplistic but works for me ...
Same F-stop = same exposure length, regardless of aperture, focal length or colour of the OTA 
So if the 50" you're about to buy is F/6 and your 80mm is F/6, then exposure times would be the same to achieve the same image brightness.
If the larger scope you're looking at has a slower F-stop (F/10, F/12, rather than F/4, F/6 etc) then that larger scope will require longer exposures to get the same brightness of detail.
eg. If you go from an F/6 scope to an F/8 scope you'll go from 60 second to a longer exposure time (80 secs??) to achieve the same brightness of image.
Roger. |
This is essentially true but it is the image size that will change as you change the size of the scope.
An example to describe this is if you use a 200mm diameter f8 scope it will have a focal length of 1600mm. If you take a 1 min exposure you will get a certain brightness in your image. It image will be somewhere between 0.5 and 1 deg across depending on your camera.
If you swap to a 50mm camera lens at f4 (2 stops wider than f8) it will only take 15 sec exposure to achieve the same level of brightness in your image as the 200mm scope image.
The difference is the size of the field. It will be a 60deg wide image.
The point to this is example is that it isn't very good to compare an 80mm diameter fast scope with a short focal length with a bigger (ie >200mm) scope with a longer focal length but a slower f ratio.
The 80mm scope will give you a brighter image more rapidly but only because the image is much smaller.
It depends on what you want to image. Little objects like most pns and most galaxies can't be imaged with a decent size with a little 80mm refractor.
If you want to image widefield nebs than the little scope is good.
By the way. None of this applies to imaging stars as they are essentailly point sources and f ration makes little difference for them. Only light capture matters hence a big scope will image dimmer stars than a little scope.