Question about F numbers

[randb]

So if f numbers dont make a difference to observers, and a low number only means lesser exposure time for photography (correct me if I'm wrong), then why dont they make all telescopes with low f numbers?

[Dave Mitsky]

For one thing, high quality optics are far more difficult, and therefore far more expensive, to manufacture when it comes to fast telescopes. (Check the prices of high-end mirrors such as Zambuto or high-end apochromatic refractors like the Astro-Physics Starfires sometime.) For another, the physics involved in some designs such as the various Cassegrains and ones based on the Cassegrain design (MCTs, SCTs, and so on) do not allow fast f/ratios. Fast telescopes are also much more demanding on eyepieces, especially wide-field designs.

Dave Mitsky

[aurora]

Other random comments about tradeoffs:

For a mirror, the lower the f number, the more glass that has to be removed to make it. And it becomes more difficult to get the mirror done with high quality, as Dave said.

For a Newtonian, the lower the f number the more crucial it is to get the collimation spot on. On the other hand, the tube or truss can be shorter, which makes it easier to mount on a GEM, or get to the eyepiece of a large Dob without using a tall ladder.

Extremely low f numbers make it difficult to get higher magnification, for those times when you need it.

[JohnW]

So if f numbers dont make a difference to observers...
Not exactly true. A low f number will give you a wider field and lower magnification for a given eyepiece. There is an upper limit to the the apparent field of view any eyepiece can provide, which is why you don't see eyepieces with focal lengths above 40-50mm. This means that a scope with a low f-number will let you see the whole of an extended object (the Veil Nebula, for instance), which would be impossible in a scope with a high f-number.

On the other hand, as aurora pointed out, it's harder to get high magnifications out of a fast scope. Tradeoffs, tradeoffs...

[randb]

But...why does a low f number mean lesser exposure time for deep space imaging? Light travels with such a high velocity that it shouldn't matter...rite?? (Dont laugh at me for my lame reasoning)..

Thanks for your help.

[Fr. Wayne]

In stellar photography focal ratio has no effect on the brightness of a star image, but on diffuse objects like comet tails or nebula it does. The shorter the focal length, the smaller the size of the diffused or extended object, thereby increasing its intensity on the film. (e.i. exposure time is proportional to the square of the focal ratio.)