Focal length short and long. Which to choose?

[Tyrion101]

I'm building a telescope and one of the more important choices I'm making is what focal length to make the mirror, my understanding is the longer the focal length the closer you see far away stuff, but the field of view diminishes a bunch. I've seen both long tube dobsonians and short tube ones. (This is what I'm building) I'm wondering what I'd see from one vs the other? (Assuming both are the same diameter)

 

[Drakkith]

The magnification and field of view of a telescope depend on the focal lengths of both the mirror (or lens) and the eyepiece. You divide the focal length of the telescope by the focal length of the eyepiece to get your magnification. So a 1000 mm mirror with a 20 mm eyepiece gives you the same magnification as a 500 mm mirror with a 10 mm eyepiece.

In addition to worrying about the magnification, you must also consider the focal ratio (focal length divided by aperture diameter). A mirror with a "fast" focal ratio, such as 250 mm aperture 1000 mm focal length (focal ratio of f/4), is harder to construct and collimate than the same diameter mirror with a "slow" focal ratio, such as a 2000 mm focal length (f/8).

Honestly in all really depends on what you use the telescope primarily for. If you're using it for planetary viewing, then a longer focal length is usually desired, as you can use your larger focal length eyepieces, which have better eye relief and wider fields of view than shorter focal length eyepieces. I'd also consider the physical size of the tube. A long focal length mirror requires a longer tube and is much more unwieldy and harder to transport than a shorter tube. This is especially true for dobsonians, as the length of the tube is almost the same as the focal length of the mirror.

 

[Tyrion101]

Thanks for the advice, I like to look at it all but primarily DSO hunt, the design I'm thinking of is not quite a tube but close enough. I've been thinking shorter since this is going to be big and I don't want a ladder to stand on just to see stuff, and possibly move the thing to look at more than one target. Does the focal length also change the level of detail or is that more a job of the mirror?

 

[Drakkith]

The resolving power of a telescope is primarily a function of aperture diameter (assuming flawless optics). A mirror with a 1000 mm focal length can have exactly the same resolving power as one with a 2000 mm focal length if both are equal in diameter.

It sounds like you're building a pretty big scope. What's the diameter of your primary mirror going to be?

 

[Tyrion101]

The hope is 16" but I'll settle for 14"

 

[Drakkith]

Hmmm. At that size you're probably going to want an f/4 or f/5 focal ratio just to keep the size of the scope down. The biggest dobs I've seen are typically even F/3 because they become unmanageable at larger sizes.

Have you been to the forums at http://www.cloudynights.com/? You should be able to find a wealth of information there, as they are dedicated to astronomy.

 

[HallsofIvy]

What are you planning to observe? A short focal length, "rich field", telescope will give you a larger field of view (brightness of object depending on the diameter of primary lens or mirror). That's what you need for observing stars. But magnification is "focal length of primary lens or mirror divided by focal length of eye-piece" so if you are observing the moon or planets, with a short focal length primary to get the same magnification you would need very short length eyepieces that are very difficult to focus. (When you are observing stars magnification is not relevant because no matter how you "magnify" a star, it still is just a point of light.)

 

[Tyrion101]

I've used the site for research in the past but never joined I don't think. I'll look into it. Part of this project is just to learn about telescopes in general. Thanks for your help.