1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Archived Reflecting telescope calculation

  1. Apr 7, 2013 #1
    1. The problem statement, all variables and given/known data

    A reflecting telescope has a radius of curvature of 3.0m for its objective mirror and a radius of curvature of -1.50m for its eyepiece mirror. If the distance between the two mirrors is 0.90m, how far in front of the eyepiece should you place the photographic film to record the image of a star?

    2. Relevant equations
    I could only find out the focal length of both objective mirror and eyepiece mirror.

    f=radius of curvature / 2 .....(1)
    1/f = 1/di + 1/do .....(2)

    3. The attempt at a solution

    For now, I got fo=1.5 and fe=-0.75
    as the object are from infinity , by using equation 2, di=fo=1.5
    Since the distance between the two mirrors is 0.90m , and 1.5-0.9=0.6
    0.6 is the object distance of the eyepiece.

    1/-0.75 = 1/di + 1/0.6
    di is -0.3333
    I haven't taught reflecting telescope in my class. I just find a similar solution in yahoo answer.
    But I feel the ans is totally wrong. Can anyone help me?
  2. jcsd
  3. Oct 29, 2016 #2


    User Avatar
    Science Advisor

    You can always "unfold" a system with mirrors, replacing the mirror with a lens of equal focal length (at least for the purposes of geometric optics). So your approach is basically correct, and I agree that the image distance for the first mirror is 1.5m. But the distance to the second mirror is only 0.9m - so the image (the object for the second mirror) is beyond it. So in the second calculation ##d_o=-0.6##m. Then$$\begin {eqnarray*}\frac 1 {d_i}&=&\frac 1 {0.6}-\frac 1 {0.75}\\
    &=&\frac 13\end {eqnarray*} $$and the answer you are looking for is 3m.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted