Download How a Radio Telescope Works

How a Radio Telescope Works
Outline
Introduction
– purpose
– basic terms
Satellite TV System
– similar to radio astronomy
– system description
Radio Astronomy System
Overview
Antenna
Front end
– Key components
– Key design considerations
– Sample systems
Back end
Purposes
– Continuum
– Spectral line
– Pulsar
– VLBI
Key Techniques
– Switching
– Averaging
Examples of backends
Satellite TV System
Antenna—redirects energy to a
point
Feed—collects energy for LNA
LNA—”low noise amplifier”
amplifies signals
Cable—transposrts signal
Tuner—selects signal for
viewing
TV—displays signal
VCR—records displayed signal
for later viewing
Couch potato—creates a need,
supplies $$
Basic Radio Telescope
control computer
Front End
Preamplifies radio
waves and extracts
information (the
signal)
Back End
Further amplifies
signal until strong
enough to be
analyzed.
All components inside
dashed line are in the
control room.
Electronics
Control room
Parabolic dish reflector
concentrates radio waves to
antenna focal point where
electronic processing begins.
Systems Control
Positions telescope,
directs observing,
monitors receiving
systems.
Data Collection
Displays and monitors
individual data.
Records while observations are in progress.
Analysis
Computer
Does preliminary
analyses of
accumulated
data.
Computer display
Magnetic Tape
Stores raw data for later
and more thorough
analysis
Data recording and analysis equipment
How a Radio Telescope Works
• Basic Terms
– Time Domain
• Something plotted VS Time
– Usually voltage or power
– Frequency Domain
• Something plotted VS Frequency
Time Domain
Flashlight Battery Voltage VS Time
3.5
3
Voltage
2.5
2
1.5
1
0.5
0
1
2
3
4
5
6
7
Time (hours)
8
9
10
11
Frequency Domain
Key Componets
• Antenna
– The bigger the better
– Needs accurate surface
– Limits: mostly money
• Feed
– Collect ONLY signals from the antenna
• Amplifier
– Don’t want them to ADD noise.
– Usu. Cooled to -432〫F
Basic Front Ends
Direct Detection Type
G
Feed
RF Amp
B
Filter
To Backend for Detection
Input spectra
BPF 1 Response
BPF 2 Response
f
fR
fR-fL
BPF 2
BPF 1
mixer
Local oscillator
f
fL
fR
Front End
Feed
Backends
power
Y
pulsar
VLBI
Spectral line
continuum
X
frequency
Key Techniques
• Switching or Differencing
– Study the difference between two or more
data sets
• Averaging
– Noise is random
– Desired signal is not
– Averaging reduces noise by Root N
Pulsar Dispersion
VLBI
References
• M.L Meeks, Methods of Experimental Physics:
Astrophysics, Vol. 12, Academic Press (1976)
• J.R. Fisher, “Digital Continuum Receiver Users’
Manual,” NRAO Electronics Division Internal
Report No. 243 (1984).
• A. M. Shalloway et. al., “Autocorrelation
Receiver Model IV: operational Description,”
NRAO Electronics Division Internal Report No.
234 (1983).