Astronomical Telescope - Practice Questions & MCQ

Astronomical Telescope

An astronomical telescope is an optical instrument which is used to see the magnified image of distant heavenly bodies like stars, planets, satellites and galaxies etc. An astronomical telescope works on the principle that when an object to be magnified is placed at a large distance from the objective lens of telescope, a virtual, inverted and magnified image of the object is formed at the least distance of distinct vision from the eye held close to the eye piece.

An astronomical telescope consists oftwo convex lenses : an objective lens O and an eye piece E. the focal length fo of the objective lens of astronomical telescope is large as compared to the focal length fe of the eye piece. And the aperture of objective lens O is large as compared to that of eye piece, so that it can receive more light from the distant object and form a bright image of the distant object. Both the objective lens and the eye piece are fitted at the free ends of two sliding tubes, at a suitable distance from each other.

The ray diagram to show the working of the astronomical telescope is shown in figure. A parallel beam of light from a heavenly body such as stars, planets or satellites fall on the objective lens of the telescope. The objective lens forms a real,inverted and diminished image A’B’ of the heavenly body. This image (A’B’) now acts as an object for the eye piece E, whose position is adjusted so that the image lies between the focus fe’ and the optical centre   of the eye piece. Now the eye piece forms a virtual, inverted and highly magnified image of object at infinity. When the final image of an object is formed at infinity, the telescope is said to be in ‘normal adjustment .

• . $f_{\text {objective }}>f_{\text {eyelens }}$ and $d_{\text {objective }}>d_{\text {eye lens }}$
  - The intermediate image is real, inverted and small.
  - The final image is virtual, inverted and highly magnified.
  - Magnification: $m_D=-\frac{f_0}{f_e}\left(1+\frac{f_e}{D}\right)$ and $m_{\infty}=-\frac{f_o}{f_e}$
  - Length: $L_D=f_0+u_e$ and $L_{\infty}=f_0+f_e$