How Do You Calculate the Focal Length of a Telescope's Objective?

[djerry]

Homework Statement

An optical telescope with a 12mm eyepiece makes the Moon appear to have an angular
diameter of 30 degrees. What is the focal length of the telescope's objective?

Homework Equations

magnification=focal length of objective/focal length of eyepiece

The Attempt at a Solution

[/B]
I'm not sure what I'm able to do given a diameter and angular diameter in order to get the magnification and focal length of the eyepiece as needed.

 

[collinsmark]

Homework Statement

An optical telescope with a 12mm eyepiece makes the Moon appear to have an angular
diameter of 30 degrees. What is the focal length of the telescope's objective?

Homework Equations

magnification=focal length of objective/focal length of eyepiece

The Attempt at a Solution

[/B]
I'm not sure what I'm able to do given a diameter and angular diameter in order to get the magnification and focal length of the eyepiece as needed.

There are a couple of different approaches from this point. But one way or another, you need to find the angular diameter of the Moon as seen with the naked eye.

And there are multiple ways to do that too.
(a) You could look up the answer, or
(b) you could calculate it yourself. If you calculate it yourself, you will still need to look up a couple of things: (i) the diameter of the moon (in units of length such as meters) and (ii) the distance between the Earth and the Moon (also in units of length). The ratio of the two is the fraction of the circle occupied by the moon along its path along the sky. Knowing that the total angular length of a complete circle is 360^o, how much of that circle is taken up by the moon's diameter, as measured in degrees? [Edit: what I describe here is technically an approximation, but it is valid if the diameter of the moon is << than the distance between the Earth and moon.]

 

[gneill]

Presumably the 12mm figure for the eyepiece is its focal length, no?

What's the naked eye diameter of the Moon in degrees?