Terrestrial Telescope - Practice Questions & MCQ

Terrestrial Telescope

A refracting telescope has inverting lenses or an eyepiece that presents an erect image. A telescope for use on earth rather than for making astronomical observations. Such telescopes contain an additional lens or prism system to produce an erect image.

 The erection of an image can be made by introducing a third lens between the objective and the eye-piece of the telescope. This modified telescope is known as the "Terrestrial Telescope" whose magnifying power is just equal to the magnification of an astronomical telescope but it just gives an erect image. 

The terrestrial telescope contains three lenses as compared to the astronomical telescope. It is also known as the spyglass. As an astronomical telescope forms an inverted image of the object so, the main difference between the astronomical and terrestrial telescope is the erection of the final image with respect to the object. The third lens of short focal length f is placed at 2f which forms an inverted image of the object. This image serves as the object for the eye piece. The lens placed in the centre of the telescope which actually erects the image is called the Erecting lens. The resolving power of the telescope can be given by the relations as follows:

$
\begin{gathered}
M=-\frac{f_o}{f_e} \times(-1)=\frac{f_o}{f_e} \\
L=f_o+f_e+4 f
\end{gathered}
$

Where,
$f_o=$ Focal length of the objective lens
$f_e=$ Focal length of the eye-piece lens
$f=$ Focal length of the lens placed between objective and eye-piece
- Magnification at $\mathrm{D}_{\text {, }}$

$
m_D=\frac{f_0}{f_e}\left(1+\frac{f_e}{D}\right)
$

- Magnification at infinity,

$
m_{\infty}=\frac{f_0}{f_e}
$