L3 - Introduction to Radio Astronomy & Radio Physics 2

Question 1

What are the main issues for a astronomical observations in the radio regime?

Answer 1

Noise
Angular Resolution
Phase Stability
Mirror Shape

Question 2

Describe the noise issue when executing astronomical observations in the radio regime.

Answer 2

Problem No. 1: Noise

• The receiver produces thermal noise.
• The antenna receives thermal noise from the sidelobes (ground clutter, spill-over).
• The sky has some thermal emission (Earth atmosphere).

How to beat noise:
• Use a big antenna.
• Integration: average signal over long time interval.
• Switch periodically between the object and a stable comparison source:
- Terminating resistor (thermal noise).
- Empty sky (beam switching, moving secondary mirror).

Question 3

Describe the angular resolution issue when executing astronomical observations in the radio regime.

Answer 3

Problem No. 2: Angular resolution

Diffraction limit: to distinguish two point objects with an instrument of aperture diameter, D, at wavelength, λ, they must be separated by an angle larger than sin(θ)> λ/D

By combining the outputs of several radio telescopes placed some distance, B, (baseline) gives the same angular resolution of an instrument of that size. The longer the baseline, the finer are the structures an interferometer can detect: sin ∆θ = λ ∆φ/B

A multi-antenna interferometer has several baselines of different length and direction. From the fringe pattern one can reconstruct the image (Fourier transform). As the Earth rotates during observation time, the projected baselines change, and thus provide more information

Question 4

Describe the phase stability issue when executing astronomical observations in the radio regime.

Answer 4

Problem No. 3: Phase stability

The receivers of an interferometer must preserve the phase of the signal.

All local oscillators must be phase-locked to each other, and preferably to a stable master oscillator (atomic clock).

Question 5

Describe the mirror shape issue when executing astronomical observations in the radio regime.

Answer 5

Problem No.4: mirror shape

The shape of a parabolic reflector must be as accurate as λ /10. But surface need not be solid metal surface (e.g. ‘chicken wire’ in GMRT), if any holes are smaller than λ /10.