How Is Telescope Magnification Calculated?

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Discussion Overview

The discussion revolves around the calculation of telescope magnification, focusing on the relationship between the focal lengths of the objective lens and the eyepiece. Participants explore methods to determine the effective focal length of a multi-lens eyepiece and the implications for magnification values.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Exploratory

Main Points Raised

  • One participant inquires about calculating the magnification of a telescope with a 500mm focal length objective lens and a four-lens eyepiece, each with a 500mm focal length.
  • Another participant suggests that magnification can be calculated using the formula F/f, where F is the focal length of the objective lens and f is the focal length of the eyepiece.
  • A participant questions how to find the effective focal length of the eyepiece, given that it consists of multiple lenses touching each other.
  • One participant proposes a method to measure the focal length of the eyepiece by projecting sunlight onto paper and adjusting the distance until the image is smallest, noting that this method may yield a focal length less than 100mm.
  • Several participants confirm that the distance from the lens to the paper when the image is smallest corresponds to the focal length.
  • Another participant calculates their telescope's magnification as 7x with a 500mm focal length objective and a 10mm focal length eyepiece, and later confirms a magnification of 50x with the same objective and a different eyepiece.
  • A participant reminds others that magnification should be considered alongside the resolution of the telescope.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the calculation of magnification and the determination of effective focal lengths, with no consensus on the method for calculating the focal length of the multi-lens eyepiece.

Contextual Notes

There are limitations regarding the assumptions made about the focal lengths of the eyepiece lenses and the applicability of the proposed measurement method, particularly for concave lenses.

josue2503
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How to get the magnification of my telescope if my objective lens (double convex) has a focal length of 500mm, and an eyepiece w/ 4 lens (double convex) that is close together (touching each other) and has a focal length of 500mm each lens?
 
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The magnification equals F/f where F is the focal point of object lens, f is the focal point of eyepiece. You can calculate the f of eyepiece or even measure it (approx.).
 
how to find the focal length of my eyepiece w/ 4 lens touching each other having 500mm focal length each lens??
 
josue2503 said:
how to find the focal length of my eyepiece w/ 4 lens touching each other having 500mm focal length each lens??

A typos? the focal point of each eyepiece lens is 500mm, isn't it?
You can measure roughly the focal point of the whole eyepiece by putting it into the sun and get the image of the sun on a piece of paper. Adjusting the distance between the lenses and the paper so that the sun's image is smallest (the paper may burn). That is the eyepiece's focal point. Probably it is less than 100mm.

Edit: this method is not used for concave lens
 
Last edited:
thanks...so..when the sun rays converges to the paper, the distance between the paper and the lens is the focal length?
 
josue2503 said:
thanks...so..when the sun rays converges to the paper, the distance between the paper and the lens is the focal length?

Exactly, that is what the theory says.
 
thanks very much so my telescope magnification is 7x and if i used an objective having 500mm focal length and an eyepiece having 10mm focal length.. my telescope magnification is 50x, right?
 
josue2503 said:
thanks very much so my telescope magnification is 7x and if i used an objective having 500mm focal length and an eyepiece having 10mm focal length.. my telescope magnification is 50x, right?

Yes. But remember the magnifition must go along with the resolution of the telescope.
 

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