shef robotham litchfield hills amateur astronomy...
TRANSCRIPT
Shef Robotham
Litchfield Hills Amateur Astronomy Club
Brought to you courtesy of Storm Alfred
Radio Astronomy
Image courtesy of NRAO/AUI
Birth of the concept� 1666 Sir Isaac Newton demonstrated that light made up of colors
� He directed a beam of light through a prism to spread light into a ‘spectrum’
� Spectroscopy born� Detail analysis of light led to absorption and emission lines of
spectra� Each chemical element has a unique spectra
http://upload.wikimedia.org/wikipedia/commons/0/0b/Sodium_Spectra.jpg
Spectrum of Sodium
Spectrum of Potassium
Further Concept Developers� William Herschell 1800 discovered ‘calorific rays’
� Beyond red color� Produced heat� Later renamed ‘infrared radiation’
� Johann Ritter, 1801, wondered if ‘cooling rays’ could be found� Opposite end of visible spectrum from Herschell’s discovery� used ‘Silver Chloride’ paper and named his discovery ‘Oxidizing Rays”� Now named ‘ultra-violet’, above the color blue
Our expanding Understanding or
Confusion..
� Contributors to understanding of light’s spectra included [not limited to]:� Wollaston, 1802, Visible Solar absorption lines� Fraunhofer, 1814, detailed Solar Spectra studies� Niels Bohr, 1913, published Atomic model� Future analysis developed ‘Red Shifted’ light
• Maxwell, 1873, developed Electromagnetic Radiation Theory• possible at any wavelength• made up of perpendicular Electric and magnetic sinusoidal fields
Excerpts from How Astronomers make Sense of Starlight, Astronomy , December 2011
Heading towards Radio Astronomy
• Current ‘Understanding’ of light, consists of:• Huygens Wavelets
Light made up of ‘waves’Useful to understand optics
• Electromagnetic wave• Photon
Niels Bohr relationshipsQuantum Mechanics
• Radio Astronomy uses Electromagnetic radiation and Photon based theories
• Electromagnetic is RADIO!• Photon for specific atomic frequencies for radio
Spectroscopy
The Electromagnetic Spectrum
Credit: NASA/IPAC
First observations� Jansky, 1920, tasked by AT&T to find source of transcontinental
communication noise
� Observation Frequency 20.5 MHz
� Discovered thunder and solar ‘noise’
� ‘faint’ noise peaked at 23hr, 56min interval
� In direction of Sagittarius
� Later identified as the center of our galaxy, the MILKY WAY
� Unit of ‘Radio Brightness”
named after Jansky
Jansky and his Antenna
Jansky’s Antenna is on display at the National Radio Astronomy Observatory {NRAO}, Green Bank, WV
1Jy=10-26 W/(m2.Hz)
Watts per (meter2 in measuring BW)
Reber’s Equipment• Grote Reber, 1937, Amateur Astronomer compiled first sky map at 160 MHz
Built a 31.4ft dish in his backyard-No zoning laws!
• Reber’s Antenna on display at NRAO
Now Some Theory
Black Body Relationships
Note Peaks vs. Temperature
))1)/(/(1)(2/32()( −= kThvechvTv
B
Bv(T) Spectral Brightness as a function of Temperatureh = Planck’s Constantν ν ν ν = Frequency, Hzc = speed of Light, m/seck = Boltzmann Constant
A body’s apparent brightness will increase as it’s Temperature increases. Measuring the brightness at a frequency, the brightness at other frequencies can be predicted.
Max Planck, 1901, developed a relationship that was the foundation of Quantum Mechanics
Observational Frequency
Brightness
Predicted Brightness
Black Body Equation and Simplification
1 10 100 1 .103
1 .104
1 .105
1 .106
1 .107
1 .1020
1 .1019
1 .1018
1 .1017
1 .1016
1 .1015
1 .1014
1 .1013
1 .1012
1 .1011
1 .1010
1 .109
1 .108
1 .107
1 .106
100K500K1000K10000K
Black Body Radiation
Frequency, GHz
Wat
ts/H
z/m
^2
1.896072 107−×
1 1020−⋅
P T1 ν g( )P T2 ν g( )P T3 ν g( )P T4 ν g( )
1.999998 106×1 10
0× ν g1 10 100 1 .10
31 .10
41 .10
51 .10
61 .10
71 .10
20
1 .1019
1 .1018
1 .1017
1 .1016
1 .1015
1 .1014
1 .1013
1 .1012
1 .1011
1 .1010
1 .109
1 .108
1 .107
1 .106
100K Full100K R-J500K Full500K R-J
Black Body Radiation
Frequency, GHz
Wat
ts/H
z/m
^2
6.14471 107−×
1 1020−⋅
P T1 ν g( )P RJT1 ν g( )P T2 ν g( )P RJT2 ν g( )
1.999998 106×1 10
0× ν g
P rj f t,( )2 f
2⋅ A⋅
c2
k b⋅ t⋅ BW⋅:=
If hf << kbT then:
Rayleigh-Jeans Reduction
P f t,( )2 h⋅ f
3⋅
c2
BW A⋅
e
h f⋅
k b t⋅1−
⋅:=
Full Black Body Equation
Equations in MathCAD format
Valid ‘Area’
Wien’s Displacement Law
� Using Black Body Thermal relationships� Knowing:
� Observational Wavelength� Measured Temperature
� The targets peak temperature can be calculated
� Used in calculating the absolute ‘brightness’ of the Star
� The absolute brightness can be used to determine the Class of Star and its Mass
http://en.wikipedia.org/wiki/Wien%27s_displacement_law
K nm108978.2 6xTpeak =λ
Was found that product of Peak Wavelength and Temperature was a Constant
http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html
Nyquist Noise Temperature Theorem
Using en = [ 4 k T R BW ] 1/2 the maximum available noise power delivered to a noiseless load from a resistor at a Temperature is given by:
P = Wattsk = Boltzmann ConstantT = Temperature in Deg KBW= Bandwidth of measurement, Hz
P = k T BW
-174 dBm, 290 Deg K, 1 Hz BW
in = en/2RPn=in^2*R
Quick Calculation:Noise Power ‘Floor’ [ dBm ], Room Temperature, 1 Hz Bandwidth
Using the theory…
So….. What does all this mean?
� The Nyquist Noise equation will predict the Noise Power from a Reference Resistor of Temperature and BW� Measuring the resulting ‘noise power’ from the Reference at the
Radio Telescope’s input will calibrate total processing gain
� ‘Dicke Switch’ Configuration looks at reference load then star
� Knowing the telescope’s gain, Frequency of observation and Bandwidth, the object’s Temperature can be calculated using Nyquist Noise Equation
� A Star’s Temperature relates to its Brightness� defines the Class of Star
� Used to determine Star’s Mass
� where it is in its lifetime, H R Diagram
� A Star’s Peak Brightness can be calculated using Planck’s relationship
Continuum Observations� Telescope in “Total Power” Configuration
� Measures the total Signal Power at the Antenna, Continuously� Over a Bandwidth, or block of Frequencies� ‘Signal’ EXTREMELY small� Appears as ‘white noise’� Telescope Pre-amp liquid Helium cooled, 3 Degrees Kelvin
� The telescope electronics drift will hide the signal� Electronics contained in a temperature controlled enclosure� Dicke Switch Used
� Alternatively switches the telescope’s input between the Antenna and a known temperature reference load
� The object’s Temperature can be measured� Doppler effects not detected� Spectral Content not detected
Continuum Telescope with Dicke Switch
Antenna
50 OhmReference
"Dicke"Switch
LowNoise
Pre-Amp
HelicalFilters
Post BPFAmp
DBM
UserSelected
Band PassFilters
AsRequired
GainDetector
Typical Gain ~130 dBComputerControlledSynthesizer
Serial Data
Serial Clock
Device Select
Block Diagram
16 bitA/D
Serial Data
Serial Clock
Device Select
Reference Temperature
Pre-Amp Source
ReferenceTemperature
Ch 1
Ch 2
Head End
Dual LowPass Filter
"Warm and Cumfy"
Inside Continuum Observations� If Continuum Signal can be broken down into smaller
bandwidths� Spectral Analysis of the Continuum Bandwidth
� Target elements can be resolved
� Using Spectroscopy
� Target receding velocity detected
� Observing the Doppler Shifted spectrum
� Estimate of Target age
� Total Power Configuration changed from 1 large bandwidth to multiple smaller bandwidths
� BW necessary to resolve spectral lines
� Typically 100KHz or less
� Spectral broadening due to Doppler effects
Spectroscopy Observations� Telescope observes element’s radio lines or spectrum
� Signal even smaller� Electronics has multiple channels of very small bandwidth
� Necessary to resolve spectrum ‘bins’
� Object’s element make-up can be determined� Objects receding velocity can be determined
� Using known element spectral content� Velocity measured using the Doppler shift of the spectrum
Spectral observations take more time, Signal integrated for longer times
Radio Astronomy Challenges
� Radio wavelengths much longer than optical
� To see the effect of wavelengths:
� Define ‘Observing Gain’ as (aperture area/wavelength)
� Define human Eye as 1
� Fully night adapted 7 mm diameter… 0.007 meters
� Green light 535 nm… 535 x10^-9 …. 0.000,000,535 meters
� Human Eye Gain (HEG)
� Typical Amateur Instrument 8 Inch
� 8” = 0.203 m : 843 Gain over Human Eye
� 200 Inch Mt Palomar Gain
� 200 “ = 5.08 m: 526,663 Gain over Human Eye
� Reber’s 31.4 ft Dish @ 160MHz 0.537 over Human Eye
93.71910535/2)2/007(. =−= xHEG π
Further Challenges
� Human Eye has 1000 million pixels
� We ‘see’ a complete image immediately
� A Radio Astronomy “Image” made of pixels
� EACH pixel takes TIME !!
� Each Radio detector integrates [adds up] the signal
� Telescope ‘Resolution’, Radians..
Degrees=1.02 λ/Aperture
� At Radio, wavelength longer and aperture smaller when compared to Optical
� Radio Telescope front-end needs to be cooled to reduce Thermal Noise, Nyquist Theorem
Telescope Resolutions
Excel Spread Sheet ‘Snap-Shot’ showing Telescope ‘Gain’ over Human Eye, HEG, and ‘Image Resolution.
Interferometry
� Developed to increase Angular Telescope Resolution
� Multiple antennas combining signals
� Preserving instantaneous phase
� Phase causes addition and cancelation of signals
� Telescope Resolution Increased, Sensitivity slightly increased
� More capture ‘Area’
� Large ‘Synthetic Aperture’
Very Large Array, New Mexico
Interferometer Development
10− 5− 0 5 100.94
0.96
0.98
11
0.941
Ant.gain θ( )
1010− θ
Single Antenna
W ave Front
BaseLine,B
e2=sin( t)
sin
e1=sin( t+dt))
Interferometer
The use of two, or more, antennasIncreases the resolution greatly. TheLonger the Baseline is, the greater theResolution.
The Trigonometry
� Φ caused by the Earth’s Rotation or antenna steering
� e1 and e
2phase must be
preserved� The output is the two, or
more, signals from each antenna Interfering with the other
But is a distanced=v*t, using the
speed of light
d=c*t
Interferometer
The Signals:
W av e F r ont
e2=sin( t)
sin
e1=sin( t+dt))
X2
Eout( )
Edet( )
0 5 109−× 1 10
8−× 1.5 108−× 2 10
8−×1−
0.5−
0
0.5
11
1−
e.1 t.ime( )e.2 t.ime( )
2 108−×0 t.ime
Phase Shift caused by Angle Φ
60− 40− 20− 0 20 40 601−
0.5−
0
0.5
11
0.914−
E.out θ( )
E.det θ( )
4545− θ
Signal Processing
60− 40− 20− 0 20 40 600
0.2
0.4
0.6
0.8
11
1.232 106−×
E.det θ( )
4545− θ
Resulting Fringes
Interferometer Gains
60− 40− 20− 0 20 40 600
0.2
0.4
0.6
0.8
11
1.232 106−×
E.det θ( )
Ant.gain θ( )
.707
4545− θ
• Interferometer increases Resolution• Increase in Telescope Complexity• Increase in Post Signal Processing• Baseline can be extended to earth
and beyond
Single Antenna
Interferometer
-3dB
Graphs from MathCAD using a baselineof 10 meters and a Frequency of 100MHz
Conclusion
� Big-Bang remnants discovered using Radio Telescope
� Radio Images expand understanding of Optical targets
� Radio Astronomy extends our ‘seeing’ above and below the visible spectrum
� Allows observations obstructed by visible dust
� Radio itself extends to deep infra-red and X-Ray
� Allows verification of relativistic theories
� Encompasses optics, microwave, radio and digital technologies
References
� Society of Amateur Radio Astronomers, http://www.radio-astronomy.org
� Astronomy by Dinah Moche
� An Introduction to Radio Astronomy by Burke and Smith
� Tools of Radio Astronomy by Rohlfs and Wilson
� Radio Astronomy by John D Kraus
� Radiotelescopes by Christiansen and Hogbom
� High Sensitivity Radio Astronomy by Jackson and Davis
� Interferometry and Synthesis in Radio Astronomy by Thompson, Moran and Swenson