how does a smaller focal ratio allow shorter exposure times?

Nicely explained Dave.
I have a clearer picture of how that works now...

Maybe you can help me answer this...
What are the main differences between these two scopes?


5'', f/15, Focal Lenght 1900mm, MCT

6'' f/15 Focal Lenght 762mm, Newt

Would they both give the same views visually?
From what you say above, the f/5 newt would be a 'faster' scope.
Better for astrophotography?

Cheers.

Frank,
The F15 MAK adds up, a focal length of 1900 gives you a focal ratio of F15.2, but whos going to argu over 0.2 :lol:
The reflector on the other hand doesnt add up, if its F15, it has to be the same focal length (approx) as the MAK, where is 762 coming from, is the scope a F6 ??
This makes these scopes radically different. The views through the reflector with a given eyepiece would yield a lower magnification, hence a wider field of view, this is good for widefield nebula/galaxy images.

The MAK would give a higher field of view making it better for the planets/moon and smaller galaxies and nebula, especially planetary nebula and most star clusters.
Then again the MAK is an expensive piece of equipment.

There is a simple, (but whapping), multiplication error there!
6 X 15 can't be less than 5 X 15 !

:oops:
sorry that should have read:
6'' f/5 Focal Lenght 762mm, Newt