Quote:
Originally Posted by Terry B
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....
|
Of course Terry, but Duncan specifically said he had an understanding of the FOV changes, but was asking about exposure differences. Good to clarify the whole situatio I suppose though
