Why Do Telescopes Have Long Focal Lengths?

[pixel01]

Hi all,

Do you know why the lense (or mirror) of telescope often requires that the focal lengh be rather long? What is different between a backyard telescope and a normal camera?
Thanx.

 

[cadnr]

Longer focal length typically yields greater magnification

 

[pixel01]

Longer focal length typically yields greater magnification

I know that, by replacing the eyepiece, they can adjust the manification of a telescope. So longer focal length is not only parameter necessary to yields higher manification.
There may be other reasons.

 

[cadnr]

Magnification = telescope focal length / eyepiece focal length

There are many practicalities surrounding eyepiece construction; field of view, eye relief, exit pupil size, lens fabrication etc. that dictate a minimum feasible eyepiece focal length. Thus, the only reasonable thing to do if you want to increase magnification is to increase the telescope's focal length.

 

[pixel01]

Magnification = telescope focal length / eyepiece focal length

There are many practicalities surrounding eyepiece construction; field of view, eye relief, exit pupil size, lens fabrication etc. that dictate a minimum feasible eyepiece focal length. Thus, the only reasonable thing to do if you want to increase magnification is to increase the telescope's focal length.

That's right, it's not easy to make eyepiece with FL of about 1cm or less. But do you still think there's something related to aberration of the lense?

 

[chroot]

The reason is actually VERY simple:

A long focal length requires only a fairly modest curvature of the mirror's surface, which is easy to manufacture. A very short focal length mirror has a much more pronounced curvature which is harder to manufacture and harder to thermally stabilize.

There's a huge market for short focal-length refractors, for example -- they are many people's "dream telescopes," but they are very expensive.

- Warren

 

[russ_watters]

It's also tough to make a short focal length scope without chromatic aberration.

And btw, the eyepiece thing is only partially relevant. Cameras don't use eyepieces. Having too much glass between you and your subject tends to degrade the image, so the best way to view is by placing your detector at prime focus (magnifying-glass style). For viewing with your eyes, that isn't really feasible (focus needs to be precise), so an eyepiece is needed.

 

[cadnr]

It's also tough to make a short focal length scope without chromatic aberration.

And btw, the eyepiece thing is only partially relevant. Cameras don't use eyepieces. Having too much glass between you and your subject tends to degrade the image, so the best way to view is by placing your detector at prime focus (magnifying-glass style). For viewing with your eyes, that isn't really feasible (focus needs to be precise), so an eyepiece is needed.

...in the case of no eyepiece a longer focal length still leads to greater magnification

 

[pixel01]

...in the case of no eyepiece a longer focal length still leads to greater magnification

In the case of camera, we do not need magnification. Longer FL will lead to bigger camera though.
After all, the question why in telescope the lense has long FL, can be answered by:
1/ Higher magnification
2/ Easier manufacturing
3/ Lower aberration

...

 

[russ_watters]

...in the case of no eyepiece a longer focal length still leads to greater magnification

Yes, sorry if that wasn't clear. There are also artificial ways of increasing (or decreasing) the focal length to change the magnification that don't work like eyepieces. The result of putting in a barlow lens is a telescope that behaves like one of a longer focal length.

 

[russ_watters]

In the case of camera, we do not need magnification. Longer FL will lead to bigger camera though.

Not sure if there is a wording issue between us*, but how can you not need magnification on a camera? I have a decent camera with a 10x zoom lens, which means it provides 10x magnification using a variable focal length lens. The whole reason why camera lenses get so long is so they can provide higher magnification. The corresponding aperature is large because of the desire for a low focal ratio.

*From what I understand, some photogaphers prefer to talk in terms of field of view instead of magnification, but they are fundamentally the same thing. For astrophotography, you calculate fov using the focal length and the size of your detector chip. Until digital cameras came around, the size of the detector was a piece of 35mm film, so focal length and magnification or fov were always directly comparable across different brands of cameras.

After all, the question why in telescope the lense has long FL, can be answered by:
1/ Higher magnification
2/ Easier manufacturing
3/ Lower aberration

Yes.

 

[Chronos]

Telescopes are bulky. Ease of use is roughly proportional to the square of the focal length - which explains why cat's are so popular, despite their relatively high price. A scope you can set up and break down in a few minutes . . . priceless.