# Telescope optical path length vs focal length

• Stargazing
• balaonspace
In summary, the conversation discusses the design of a three mirror off axis telescope and the relationship between optical path length and focal length. The question is whether there is a relation between these two parameters for a system with 'n' elements/surfaces, and if a picture could help clarify the problem. The conversation also touches on the difference between optical and system focal length, and the potential for distortion or displacement of light.
balaonspace
Hi

I am trying to design a three mirror off axis telescope. The free parameters are the offset between the apertures and distance betwen the surfaces.

My question is for a system with 'n' elements/surfaces (n=3 in my case) is there a relation between optical path length and focal length of the system.

I don't want to go too descriptive about this problem. If it might help I can upload a pic in some webpage.

bala

the distance between well yes`. if you have one eye on a lens and the other open. Eye with more light will give more distance but less focus. some what a distortion ? or a dispacement of light'. i would like to see it

http://img828.imageshack.us/img828/5718/phyfor.png

In this image 'c' (which i interpret as the optical path distance) and 'f' the focal length can be constrained independently.
For example when i design a system with high focal length with optical path, and optimise the same in zemax, the system focal length reduces to optical focal length for a design with smallest 'rms' radius.
I cannot quiet grasp the difference between the two quantities here.
Thank you for reading.
I am not convinced that i explained my problem clearly. let me know if i need to clarify any part.

Last edited by a moderator:

## 1. What is the difference between telescope optical path length and focal length?

The telescope optical path length is the distance that light travels through the telescope's optical system, from the objective lens or mirror to the eyepiece or camera. The focal length, on the other hand, is the distance between the objective lens or mirror and the point where the light converges to form an image. In simpler terms, the optical path length refers to the physical distance that light travels, while the focal length refers to the distance at which the light is focused.

## 2. How does the telescope's optical path length affect its performance?

The optical path length of a telescope is an important factor in its performance. A longer optical path length allows for more precise and accurate focusing, resulting in sharper and clearer images. It also allows for a wider field of view, which is beneficial for observing larger objects in the night sky.

## 3. Can the focal length of a telescope be changed?

Yes, the focal length of a telescope can be changed by using different eyepieces. Eyepieces with shorter focal lengths will result in higher magnification, while eyepieces with longer focal lengths will provide a wider field of view. Some telescopes also have adjustable focal lengths, allowing for even more versatility in observation.

## 4. How does the optical path length affect the size of a telescope?

The optical path length does not necessarily affect the physical size of a telescope. However, telescopes with longer optical paths may require larger tubes or structures to accommodate the longer distance that light needs to travel. This is why some telescopes may appear larger than others, even if they have the same focal length.

## 5. What is the relationship between telescope optical path length and its maximum magnification?

The optical path length of a telescope does not directly determine its maximum magnification. Instead, it is the telescope's aperture (diameter of the objective lens or mirror) that determines its maximum magnification. However, a longer optical path length can help achieve higher magnification by allowing for more precise focusing and better image quality.

• Astronomy and Astrophysics
Replies
2
Views
2K
• Astronomy and Astrophysics
Replies
7
Views
8K
• Science and Math Textbooks
Replies
2
Views
757
• Astronomy and Astrophysics
Replies
10
Views
4K
• Optics
Replies
10
Views
2K
• Optics
Replies
9
Views
507
• Introductory Physics Homework Help
Replies
12
Views
3K
• Optics
Replies
11
Views
2K
• Introductory Physics Homework Help
Replies
2
Views
1K
• Introductory Physics Homework Help
Replies
1
Views
2K