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NSEA · Elementary Astronomy

Telescopes and Instruments

Refracting/reflecting telescopes, magnification, resolving power and modern observatories.

In the official syllabus: IAPT — Syllabus for National Standard Examination and Indian National Olympiad in Physics, Chemistry, Biology,…, Astronomy syllabus, Part 3: Elementary Astronomical notions (read 29 Sept 2026).

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3 practice questions on Telescopes and Instruments for NSEA, with answers

Shishya's practice questions, written with AI. Each answer was checked by an automated second pass, not by a person.

  1. 1.A telescope has a primary mirror diameter of 2.4 meters and operates at a wavelength of 600 nm. What is the approximate angular resolution (in arcseconds) of this telescope according to the Rayleigh criterion?

    • (A)0.03 arcsec
    • (B)0.06 arcsec
    • (C)0.12 arcsec
    • (D)0.25 arcsec
    Show the answer and solution

    Answer: (B) 0.06 arcsec

    Solution: Using θ = 1.22λ/D, where λ = 600×10⁻⁹ m and D = 2.4 m. θ = 1.22 × 600×10⁻⁹/2.4 ≈ 3.05×10⁻⁷ radians ≈ 0.063 arcsec.

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  2. 2.A Newtonian reflector telescope has a primary mirror of diameter 20 cm and focal length 100 cm. What is its f-number (focal ratio)?

    • (A)f/2
    • (B)f/5
    • (C)f/10
    • (D)f/20
    Show the answer and solution

    Answer: (B) f/5

    Solution: The f-number is the ratio of focal length to aperture diameter: f-number = f/D = 100 cm / 20 cm = 5, expressed as f/5.

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  3. 3.A refracting telescope has an objective lens of focal length 120 cm and an eyepiece of focal length 2.5 cm. What is the magnifying power of this telescope in normal adjustment?

    • (A)36
    • (B)48
    • (C)60
    • (D)72
    Show the answer and solution

    Answer: (B) 48

    Solution: The magnifying power of a telescope in normal adjustment is M = fo/fe = 120/2.5 = 48.

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