Does Focal Ratio Affect Brightness?

[Splodger]

Does Focal Ratio affect brightness or is it all about field of view. Will an f/5 scope show the Andromeda galaxy more brightly in the eyepiece than an f/12 scope, or is it just about fitting Andromeda in there and seeing more of the sky? Also does the Focal Ratio of a scope have any effect on the amount or length of exposures taken in astrophotography? Thanks. 

[Peter Drew]

Visually no, aperture X is aperture X regardless of the focal length.  However with the same eyepiece a short focus X will give a brighter image than a long focus X as the latter will be providing a higher magnification.  At the same magnification the brightness from both X's will be the same.  I'll leave the photography issue to the imagers, can of worms and all that.    😆

[andrew s]

@Peter Drew I agree for extended objects but for stars I was led to believe a higher magnification darkened the background while leavig the "point source" stars largely unaffected.  

However, it's so long ago that I looked through a telescope I am relying on hear say.

Regards Andrew 

[Peter Drew]

@ andrew s.  Higher magnification does increase the contrast due to the darkened background making the stars look brighter, I think it would be a similar case for both X apertures assuming they were the same.    🙂

[Rusted]

The telescope itself matters. Modern coatings and optics can outshine old ones regardless of the number of optical components.
I have very recently compared my 85mm spotting scope with my trusty, Vixen M90mm f/11 achromatic refractor at the same magnifications.
Absolutely no contest on brightness, sharpness and field of view on terrestrial subjects. The Vixen was well behind at 30x, 40x and 60x.

I shall be grabbing my spotting scope, with a freshly baked solar foil filter, for scanning the sun for visible sunspots in future.
It is only 1/3 the length of the Vixen and needs no special set up. Its very shortness means it can be quickly plonked onto a tripod with video head.
The Vixen's meter-long tube has enough moment to make a decent old tripod and video head look downright flimsy. Boi-oi-oi-oi-ng.
Mounting long telescopes is a real pain in the wallet and often one's back from lifting them into place.
Ask me how I know?  [6" f/10 and 7" f/12 refractors plus a 10" f/8 Newt.]
The best view I have ever had of M42 was with my first, home-made mirror. An 8.75" at F3.8. With more flying gulls than a sunny seaside. [Coma!]

Now what was the question again?

 

[Waldemar]

For astrophotography the difference in f ratio is very important, especially for the exposure times.
For example a Celestron C11 at f/10 needs 10 minutes (600 seconds) for a certain DSO image.
The same C-11 with a HyperStar at f/2 will only need 600 : (10:2)² = 600:25 = 24 seconds for the same image,
but in this case the image size will be different as well. 
Note the square in the calculation, which is caused by the fact that the f-ratio is the quotient of focal length and aperture.

So the difference is huge and the possibilities of fast scopes are great. 

Edited April 29, 2020 by Waldemar

[jambouk]

Field of view is determined by focal LENGTH not focal RATIO.

For visual, get the largest aperture you can afford (certain potential exceptions for solar, lunar, and probably other targets and observation methods).

James

[Splodger]

On 29/04/2020 at 10:37, Peter Drew said:

Visually no, aperture X is aperture X regardless of the focal length.  However with the same eyepiece a short focus X will give a brighter image than a long focus X as the latter will be providing a higher magnification.  At the same magnification the brightness from both X's will be the same.  I'll leave the photography issue to the imagers, can of worms and all that.    😆

Perfect! Thank you 🙂