# A Cassegrai

Given:

Distance between the objective mirror and the secondary mirror, d = 20 mm

Radius of curvature of the objective mirror, R1 = 220 mm

Therefore, focal length of the objective mirror, f1 = 0.5 × R1 = 110 mm

Radius of curvature of the secondary mirror, R2 = 140 mm

Therefore, focal length of the secondary mirror, f2 = 0.5 × R2 = 70 mm

The image of an object placed at infinity, formed by the objective mirror, acts as a virtual object for the secondary mirror.

Let the virtual object distance for the secondary mirror be u,

u = f1 – d …(1)

u = 110 – 20

u = 90 mm

By applying the mirror formula for the secondary mirror, we can calculate image distance (v) as:

…(2)

By putting the values in equation (2), we get,

v = 315 mm

Hence, the final image will be formed 315 mm away from the secondary mirror.

NOTE: Cassegrainian telescopes are one of the most widely used telescope of current times. It is a combination of a parabolic primary mirror and secondary hyperbolic mirror.

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