Astronomical Telescope

An instrument used for observing distinct images of heavenly bodies.

In the normal adjustment, the final image is formed at infinity. The magnifying power under situation is as follows.

O = objective

E = Eyepiece

O₁F₀ = Focal length of objective

O₂F_e = Focal length of eyepieces

A^lB^l = First image

Notice that in the ray diagram that the objective lens forms a real and inverted image, larger than the object. The eyepiece forms a second image but virtual and larger than the first.

Magnifying power is the ratio of the angle subtended at the eye by the final image to the angle subtended at the eye by the object directly, when both the final image and the object lie at infinite distance from the eye.

When the final image is at the distance of distinct vision (D)

In this case the final image is at the least distance of distinct vision.

Now v_e = -D; u = -u_e; f = +f_e

Negative sign implies the final image inverted.