A question about telescope

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In summary, the reason why the lens or mirror of a telescope often requires a long focal length is due to the practicalities of eyepiece construction, such as field of view, eye relief, and lens fabrication. Longer focal length also yields greater magnification, which is desired for viewing distant objects. This is easier to achieve with a longer focal length, as it only requires a modest curvature of the mirror's surface. Shorter focal lengths can be achieved, but they are more difficult and expensive to manufacture and may result in chromatic aberration. In contrast, cameras do not require magnification and instead focus on capturing the subject with a large aperture. The question can be answered by three main factors: higher magnification, easier manufacturing,
  • #1
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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.
 
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  • #2
Longer focal length typically yields greater magnification
 
  • #3
cadnr said:
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.
 
  • #4
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.
 
  • #5
cadnr said:
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?
 
  • #6
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
 
  • #7
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.
 
  • #8
russ_watters said:
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
 
  • #9
cadnr said:
...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

...
 
  • #10
cadnr said:
...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.
 
  • #11
pixel01 said:
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.
 
  • #12
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.
 

1. What is a telescope and how does it work?

A telescope is an optical instrument that is designed to gather and focus light from distant objects, making them appear larger and brighter. It works by using a combination of lenses or mirrors to collect and magnify light, allowing us to see objects that are too far away to be seen with the naked eye.

2. What are the different types of telescopes?

There are three main types of telescopes: refracting, reflecting, and catadioptric. Refracting telescopes use lenses to gather and focus light, reflecting telescopes use mirrors, and catadioptric telescopes use both lenses and mirrors to produce an image.

3. What is the difference between aperture and magnification?

Aperture refers to the diameter of the lens or mirror in a telescope, which determines how much light can be gathered and how sharp the image will be. Magnification, on the other hand, is the amount by which an object appears larger when viewed through a telescope. It is determined by the combination of the telescope's focal length and the eyepiece being used.

4. What can I see with a telescope?

With a telescope, you can see a wide variety of celestial objects such as planets, stars, galaxies, and nebulae. The type of telescope and its aperture will determine how much detail you can see, but even a small telescope can provide amazing views of the night sky.

5. How do I choose the right telescope for me?

Choosing the right telescope depends on your budget, observing goals, and level of experience. It's important to consider factors such as the type of telescope, its aperture, and portability. It's also helpful to read reviews and seek advice from experienced astronomers before making a purchase.

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