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