The Development of Telescopes

Question 1

Pre-telescope Astronomy

Answer 1

could only observe position, brightness and colour

Question 2

Astronomical Coordinates

Answer 2

Celestial coordinate system
- Defined by Earth’s equator and axis
- Celestial equator is the projection of the Earth’s
equator onto the sky

Other coordinate systems
- Elliptical coordinate system, with sun-Earth
orbital plane as equator
- Galactic coordinate system

Question 3

Refractive vs Reflective Telescopes

Answer 3

Refractors

• Lens absorbs some of the light, harder to see dim objects
• Light is broken into colors when it bends, and focus at different points
• Lens can be very heavy and telescope very long

Reflectors

• Less absorption, no color aberration
• Incoming parallel light reflected from a perfect sphere focus at different points
• Eyepiece near the front of the telescope = hard to operate when straight up = add mirror to reflect light back to bottom

Question 4

Development of Photon Detectors

Answer 4

eyes

photographic plates

photomultiplier tube

charge-coupled devices

Question 5

Bigger Telescopes

Answer 5

Better sensitivity from
larger collecting area

Better angular resolution

Question 6

Adaptive Optics

Answer 6

Turbulence in atmosphere deforms the wave front of the light

• Makes the star twinkle
• Places a limit on how sharp the images can be

Adaptive Optics
- use nearby guide star or laser to measure the atmospheric turbulence
- deform the surface of the (secondary) mirror in real-time to compensate the
deformed wavefront of the light

Question 7

Radio Telescope: The First Few

Answer 7

“Wire” Antenna: Only sufficient for long wavelengths with large numbers of small, relatively isotropic elements

Horn Antenna: Can be used for strong source, measuring absolute flux density

Paraboloidal reflector: Commonly used for wide frequency range

Question 8

Single-Dish Radio Telescopes

Answer 8

64 m Parkes Telescope

300 m Arecibo Radio Telescope

500 m FAST Radio Telescope

Question 9

Interferometers – Aperture Synthesis

Answer 9

Correlate light from different dishes

Measure time delay to
determine source direction

More collecting area, longer baseline

complex electronics, high precision timing and intensive computer processing