F Ratio Question

[gareththegeek]

If I had two scopes with different f ratios but the same focal length e.g.

100/1000 (f/10)

200/1000 (f/5)

And I use the same 20mm eye piece in each then I should achieve 50x magnification in each case. Aside from the benefit of wider aperture with the f/5 and being less demanding of eye pieces in the f/10 will there be any other difference?

Will the field of view be affected?

[brianb]

Same field of view, image in the larger scope will be 4x as bright (but may appear less as the eye's response to brightness is logarithmic rather than linear). The corresponds to 1.5 magnitudes more light grasp i.e. if you can just see mag. 12 stars in the 100mm you should be able to see mag. 13.5 in the 200mm.

[NGC 1502]

Post #2 is a nice summary of the differences between the 2 scopes. The visibility of fainter stars makes objects like open & globular clusters & DSOs in general, much more enjoyable to view. Regards, Ed.

[John]

The 100mm F/10 scope with that eyepiece would result in a 2mm exit pupil while it would be 4mm with the 200mm F/5 scope.

[gareththegeek]

Ah ha! Bigger exit pupil, that makes sense. I was having trouble understanding the advantage of 'fast' f ratio...so you would see the same area of sky but it would be 'blown up' to make a larger image...

Thanks John / everyone !

[FraserClarke]

Ah ha! Bigger exit pupil, that makes sense. I was having trouble understanding the advantage of 'fast' f ratio...so you would see the same area of sky but it would be 'blown up' to make a larger image...

Well the image would be the same size (both as on the sky, and as perceived by your eye) in both scopes -- that is controlled entirely by the focal lengths.

But, because the exit pupil is twice the size (four times the area), you're illuminating four times as many photo-receptors in your eye, so you can detect objects four times as faint (in theory at least, the eye is probably a bit more complex in reality)