• Light & EM spectrum: Visible \((400\text{–}800\,\mathrm{nm})\) only a small part. Space emits radio, IR, UV, X, \(\gamma\) too → different telescopes needed.
• Refracting telescope (lens): Objective collects light; eyepiece magnifies. Limits: making huge lenses is hard/heavy, long tubes, chromatic aberration.
• Reflecting telescope (mirror): Concave mirror collects light → no color error, easier to make large, even segmented. Types:
  • Newtonian: Primary concave mirror → flat secondary to side → eyepiece.
  • Cassegrain: Primary concave → convex secondary through central hole → eyepiece behind.
• India’s big optical: 3.6 m mirror at ARIES, Nainital (largest optical in Asia).
• Radio telescopes: Parabolic dishes focus radio waves onto receivers → computer makes images. GMRT: 30 dishes, each 45 m; array ~25 km aperture; studies solar wind, pulsars, supernovae, HI clouds, etc.
• Why telescopes in space? Atmosphere absorbs light and causes image shake; daylight/city lights/clouds hinder. Space telescopes avoid these → sharp, bright, stable images.
• Famous space telescopes: Hubble (optical; 94-inch mirror; ~589 km orbit); Chandra (X-ray; special grazing mirrors). Astrosat (India; UV & X-ray instruments on one platform).
• ISRO: Since 1969. INSAT/GSAT (telecom, TV, met), EDUSAT (education), IRS (resources & disaster management). Website: isro.gov.in

1) What is the visible light range?

About \(400\ \text{nm} \text{ to } 800\ \text{nm}\).

2) Which telescopes use lenses?

Refracting (optical) telescopes.

3) Which optical telescopes use mirrors?

Reflecting telescopes (Newtonian, Cassegrain).

4) What is chromatic aberration?

Color error in lens images due to dispersion.

5) Name the secondary mirror type in Cassegrain.

Small convex mirror.

6) Largest optical telescope in India (site)?

3.6 m ARIES, Nainital (Asia’s largest optical telescope).

7) What does GMRT stand for?

Giant Metrewave Radio Telescope.

8) Number and size of GMRT dishes?

30 dishes, each 45 m diameter.

9) Effective GMRT aperture span?

Array baseline ~25 km (acts like a 25 km dish).

10) Why place telescopes on mountains?

Thinner, steadier air; darker skies → better images.

11) Who first used a telescope for space?

Galileo Galilei (1609).

12) Name NASA’s optical space telescope launched in 1990.

Hubble Space Telescope.

13) Hubble mirror diameter (inches)?

94 inches.

14) Chandra studies which radiation?

X-rays from space.

15) Why X-ray telescopes can’t be ground-based?

Earth’s atmosphere absorbs X-rays.

16) What is Astrosat?

India’s multi-wavelength astronomy satellite (UV & X-ray telescopes).

17) What does INSAT/GSAT support?

Telecom, TV broadcasting, meteorology.

18) What is EDUSAT?

Education-focused satellite.

19) What is IRS used for?

Resource monitoring & disaster management.

20) Who discovered lens-based magnification (1608)?

Hans Lippershey.

1) Define refraction.

Bending of light when it changes medium (e.g., air→glass).

2) Define reflection in telescopes.

Mirrors redirect incident light/radio waves to a focus.

3) Why big objective is useful?

Collects more light → brighter images of faint sources.

4) One difficulty with very large lenses?

Heavy/warp easily; hard to make and support.

5) One advantage of mirrors over lenses?

No chromatic aberration; easier to scale/segment.

6) What does the eyepiece do?

Magnifies the focused image for the eye/detector.

7) Why city skies are poor for optical observing?

Light pollution & haze reduce contrast.

8) What shakes ground-based images?

Atmospheric turbulence (pressure/temperature variations).

9) Which waves pass through atmosphere to ground well?

Visible light and many radio wavelengths.

10) What is a parabolic dish?

Reflector surface focusing parallel waves to a single focus.

11) GMRT location?

Near Narayangaon, close to Pune, India.

12) What do radio receivers measure?

Weak radio signals at focus → sent to computer for imaging.

13) Hubble’s approximate orbital altitude?

About 589 km above Earth.

14) Chandra’s mirror specialty?

Grazing-incidence optics to reflect X-rays.

15) Who is Chandra named after?

Subrahmanyan Chandrasekhar (Indian-origin Nobel laureate).

16) Why combine smaller mirrors?

To emulate a large mirror’s area with easier fabrication.

17) Why do we prefer uninhabited mountain sites?

Dark, dry, stable air; less vibration and light pollution.

18) What is “space observation”?

Studying celestial objects using ground/space instruments.

19) One GMRT science target?

Pulsars / supernova remnants / interstellar H I.

20) Why are multiple telescopes necessary?

Universe emits across EM spectrum; each band needs suitable optics/detectors.

1) Compare refractor vs reflector.

Refractor: lenses; suffers color error; heavy at large sizes. Reflector: mirrors; no color error; scalable/segmented; compact focus paths.

2) Explain Newtonian telescope light path.

Concave primary focuses light; small flat secondary deflects beam sideways to eyepiece for viewing.

3) Explain Cassegrain telescope light path.

Concave primary reflects to convex secondary; light returns through primary’s central hole to rear eyepiece/instrument.

4) Why are large mirrors lighter than same-size lenses?

Mirrors can be thin menisci with back support; lenses need full thickness and exact bulk refractive quality.

5) State three problems for optical ground telescopes.

Atmospheric absorption, turbulence (image blur), light pollution/daylight/clouds.

6) How does placing telescopes in space help?

No atmosphere → no seeing blur/absorption; stable pointing; continuous access (no day/night limitation for some bands).

7) What is the role of a radio receiver at dish focus?

Amplifies and converts weak radio signals into data for imaging/spectra.

8) How does GMRT act like a 25 km dish?

Interferometry: many 45 m antennas spread over 25 km combine signals → synthesized aperture ≈ 25 km.

9) Why can’t X-ray telescopes be on ground?

X-rays are absorbed by the atmosphere; only space-borne platforms can detect them.

10) Mention two achievements of Hubble.

Deep field galaxy imaging; precise cosmic distance scale & accelerating universe evidence (with other data).

11) What is Astrosat’s uniqueness?

Multiple telescopes (UV & X-ray) on one Indian satellite enabling simultaneous multi-band studies.

12) How do INSAT/GSAT help daily life?

Enable telecom, TV broadcast, weather data → connectivity and safety nationwide.

13) Why does image “dance” in big telescopes on ground?

Atmospheric turbulence refracts light randomly, shifting focus—called “seeing”.

14) What is a parabolic focus advantage?

Parallel rays converge to a single point → strong signal concentration for detectors.

15) Give two reasons refractors are long.

Large focal lengths for magnification; objective–eyepiece at opposite ends increases tube length.

16) Why do reflectors avoid color fringing?

Reflection does not disperse wavelengths as refraction does.

17) How do computers make radio images?

Digitize signals, calibrate phases/amplitudes, Fourier-synthesize sky brightness (aperture synthesis).

18) State two reasons to put observatories at uninhabited high sites.

Low light pollution; dry/stable air improves transparency and seeing.

19) Why multiple space telescopes with different bands?

Each band reveals distinct physics (hot gas in X-ray, star birth in IR, etc.).

20) Name two Indian space applications beyond astronomy.

Resource mapping (IRS) and education broadcasting (EDUSAT).

1) Fill in the blanks with the proper words.

a) The wavelength of visible light is between 400 nm and 800 nm.
b) GMRT is used for radio waves.
c) A certain X-ray telescope is named after scientist Chandrasekhar (Chandra).
d) The first scientist to use a telescope for space observation was Galileo.
e) The biggest optical telescope in India is situated at ARIES, Nainital.

2) Form pairs – match the groups.

(i) X-rays → (d) Chandra
(ii) Optical Telescope → (c) Hubble
(iii) Indian radio telescope → (a) GMRT
(iv) Launching artificial satellites → (b) ISRO

3) What are the difficulties in using ground-based optical telescopes? How are they overcome?

Difficulties: (1) Atmospheric absorption reduces intensity; (2) Turbulence blurs/shifts images; (3) Daylight, city lights, clouds limit use.
Remedies: Place telescopes at high, dry, dark, uninhabited mountain sites; use larger mirrors; for complete avoidance, put telescopes in space (e.g., Hubble).

4) Which type of telescopes can be made using a concave mirror, convex mirror, plane mirror and a lens? Draw diagrams of these telescopes.

Newtonian Reflector: Concave primary + small plane secondary + eyepiece (lens).
Cassegrain Reflector: Concave primary + small convex secondary + rear eyepiece (lens).
Refractor: Objective lens + eyepiece lens (no mirrors).
[Use textbook figures 18.1, 18.2, 18.3 for labeled sketches in your notebook.]

5) Study the figure and answer:

a) The figure shows a Newtonian reflecting telescope.
b) Main parts: Concave primary mirror, small plane secondary mirror, eyepiece, tube.
c) It uses a concave (curved) primary mirror.
d) Another curved-mirror telescope is the Cassegrain reflector.
e) Working: Parallel light from space reflects off the concave primary toward focus; before focus, a small plane mirror deflects the converging beam sideways into the eyepiece which magnifies the image.

6) Answer the following questions.

a) Construction of Galileo’s telescope: A simple refractor—large objective lens in front collects light; a smaller eyepiece at the other end magnifies the image. Tube length equals sum of focal lengths (in the basic design).

b) Construction of a radio telescope: A large parabolic dish reflects parallel radio waves to its focus, where a radio receiver/feed picks up signals. Signals are amplified and sent to a computer for processing into images/spectra. Arrays (like GMRT) combine many dishes for higher resolution.

c) Why optical telescopes on uninhabited mountains? High altitude has thinner, steadier, drier air ⇒ less turbulence/absorption; remote sites reduce light pollution—all yielding sharper, deeper images.

d) Why an X-ray telescope cannot be based on Earth? Earth’s atmosphere absorbs X-rays; detectors must be above the atmosphere in space (satellites like Chandra, Astrosat).

Type	Approx. Wavelength	Ground telescope?	Typical instrument
Radio	\(>20\ \text{cm}\) down to mm	Yes	Parabolic dishes (e.g., GMRT)
Infrared	\(0.8\ \mu m \text{ to } 0.3\ \text{mm}\)	Partly (dry high sites)	IR telescopes
Visible	\(400\text{–}800\ \text{nm}\)	Yes	Optical refractors/reflectors
Ultraviolet	\(300\ \text{pm}\text{–}400\ \text{nm}\)	No (mostly)	Space telescopes
X-ray	\(3\ \text{pm}\text{–}300\ \text{pm}\)	No	Space telescopes (Chandra)
Gamma	\(<3\ \text{pm}\)	No	Space instruments