Basics of radio astronomy technology instrumentation:

Historical perspective

Sardinian Summer School in Astrophysics - 2nd cycle – Detecting Radio Waves (11-16 June 2012)

Marco BersanelliPhysics Department, University of Milano

Planck LFI Instrument Scientist & Deputy PI

Karl Jansky (1905-1950)1928 staff at Bell Telephone Lab, Holmdel, NJ. Investigate "short waves" (λ∼10-20 m !) for transatlantic radio telephone service Study sources of “static” as potential interference to radio voice transmissions

Radioastronomy: a technology-driven beginningGuglielmo Marconi (1874-1937)

First “wireless” telecommunication using radio waves (Nobel Prize 1909)

After several months he identified three types of static: 1. nearby thunderstorms, 2. distant thunderstorms, and…

“Jansky's merry-go-round”

Jansky built a 20.5 MHz (λ∼14.5m) antenna, mounted on a turntableRotation allowed to find approx direction of radio signals

3. …a “faint steady hiss of unknown origin”

Jansky spent 2yrs investigating the mysterious signal

It rose and fell once a day radiation from the Sun?

Jansky noted that the signal repeated not every 24h, but every 23h 56m Characteristic of “fixed stars”

“A faint steady hiss of unknown origin”

He eventually (1933) realized that radiation was coming from the Sagittarius region: Center of Milky Way!

Jansky proposed to Bell Labs to build a 30m dish antenna Bell Labs: static not a problem for transatlantic radio communication Observatories: could not afford take on new projects (times of great

depression)

Many scientists were fascinated by Jansky's discoverybut (almost…) no one followed up on it

Center of Milky Way!

Discovery publicized in the New York Times of May 5, 1933

“I can’t pinpoint the beginnings of my father’s interest in astronomy, whether or not it predated his discovery, but I know that it persisted afterward through my childhood. …

One probable effect of his informal pursuits in astronomy was his habit of dragging us all out of bed in the middle of the night to gaze at some celestial

Anne Moreau Jansky ParsonsDaughter of Karl Jansky(at a memorial symposium in 1983) This was always quite traumatic for David,

who always resisted any interference with his sleep, and would thrash around in his bed, objecting vociferously to my mother’s pleading. She’d call, “Karl, David doesn’t want to wake up!” My father would call back from out on the front lawn, “Well, you’ve just got to get that boy up—he may never get a chance to see this again in his entire life!” Finally, we’d all stand outside, with or without David, depending on who won the

dragging us all out of bed in the middle of the night to gaze at some celestial phenomenon. On such a night I would gradually awaken to the realization that Daddy was pacing back and forth in my bedroom, leaning far out of one after the other of the windows, which were the only ones upstairs with an uncluttered view of the sky. Then, at the right moment for viewing the “event,” he would get me up, and I would run outside with him, while my mother would try to get my brother out of bed.

without David, depending on who won the battle. Shivering in our pajamas, we’d watch an eclipse of the moon or a colorful display of the aurora borealis.

Another identification of his interest was a black box device he made and set up on the front stoop, which enabled us to see sunspots.

My father had a natural curiosity that was evident in our daily life…”

Grote Reber (1911-2002)Radio engineer, radio manufacturers in Chicago

Impressed by Jansky's discovery

1930s Reber applied at Bell Labs to work with Karl Jansky on cosmic radio waves No luck (time of great depression…) Reber decided to study radio astronomy on his own!

First two receivers (3300 and 900 MHz) failed to detect signals from outer space

In 1938 third receiver (160 MHz) confirmedJansky’s detection of radio emission from the Milky Way

In 1937 Reber built a radio telescope in his backyard Wheaton, Illinois (near Chicago)

Galactic synchrotron emission:( ) , 2.9T αν ν α−∝ ≈ −

Signal profile from Galactic center and SunPlus noise and sparks (from automotive engines!)

Reber surveyed the radio radiation from the sky

• Structure in the Milky Way, brightest signal from the center.• Other bright radio sources in Cygnus and Cassiopeia, were recognized for the first time.

Contour plots, in galactic coordinates, calibrated in Kelvin

G. Reber, 1949

1

12/2

3

−=

kThec

hB νν

ν

Blackbody radiation law

If the source has a black body spectrum, at a given ν, Bν is uniquely specified by TBν is uniquely specified by T

1h

xkT

ν≡ <<

2

22B

cT B

k νν=

At radio frequencies, for any physically meaningful T

Double lobed structure (NRAO, 1968): difficult optical identification in the early years!

At the end of the ’60s it was largely accepted that most radio sources in the sky were of extragalactic origin: Extragalactic radio sources

Radio sources: Cygnus A

Reber, 1948

Cygnus AVLA, 0.5 arcsec resolution

were of extragalactic origin: Extragalactic radio sources

Optical image

408MHz

Milky Way diffuse emission

Reber, 1948 Haslam, 1982160MHz

Planck (2011)

30-850GHz

Hyperfine transition of neutral Hydrogen

21cm line: a science-driven discovery

Aligned OppositeE E>

1940’s: theoretical prediction Neutral Hydrogen line (QM well developed)

Electron in fundamental state: “Hyperfine transition” Change in the electron spin

Easily supported even at the very low T of interstellar and intergalactic space!

K07.0≈k

hT

ν≈21cm erg/K1038.1

erg1041.916

18

−

−

××≈

erg1041.9 18−×=∆E

Aligned OppositeE E>

Hz1042.1

cm/s1039

10

××==

νλ c cm 21≅

s erg1063.6

erg1041.927

18

−

−

××=∆=

h

Eν MHz 4.1420=

First discovery: Edwin Purcell et al.25 March 1951 (Nobel prize 1953)

Horn antenna used for first detection of the 21 cm emission.

1940’s “Race” to the detection of 21cm line

Neutral Hydrogen in the universe

- Dust is transparent at radio waves- Doppler measurements- Zeeman effect

“21 cm universe”:

Today: Excitingperspectives use 21cm line to study earlyuniverse

Enormous impact in ourstudy of the universe (ISM, Galaxy structure, Galactic dynamics and magnetic fields, rotation curves, … )

- Zeeman effect

Atomic H in Our Galaxy: GBT et al

Pioneer 10 (&11) Launch 3 March1972 towards Jupiter, and beyond Solar System

Length units: 21cmTime units: 1/(1400 GHz)

= 7.14 x 10-10 sec

HI Hiperfine transition

Location and time of launch

Position of 15 pulsars (in units of 21 cm)

Decrease of the period of those pulsars(in units of 7.14 x 10-10 sec)

Altitud

e fo

r which r

adiation

is

atte

nuat

ed b

y a

fact

or 1

/2

Fra

ction

of a

tmos

her

e fo

r which r

adiation

is

atte

nuat

ed b

y a

fact

or 2

λ > 15m

λ < 5 mmHigh atmospheric

λ< 5 mm mostly absorbed by molecular bandsλ>15 m, absorbed or reflected by the ionosphere

Radio Astronomy:the 2nd window on the Universe:

Altitud

e fo

r which r

adiation

is

atte

nuat

ed b

y a

fact

or 1

/2

Fra

ction

of a

tmos

her

e fo

r which r

adiation

is

atte

nuat

ed b

y a

fact

or 2

λ > 15mIonosphere becomes reflective

atmospheric molecular absorption lines

Large, ground based radio telescopes

Effelsberg Radio TelescopeMax Planck Institute for Radio Astronomy in Bonn

Green Bank TelescopeNational Radio Astronomy Observatory (NSF)

100 meter class radio telescopes

Angular resolution(rad) 1.22

D

λθ∆ ≅

• Angular Resolution– at 21 cm (1.4 GHz) ~10' – at 3.5 mm (86 GHz) ~10“

• Pointing Accuracy: ~5" • Surface accuracy: ~0.5-1mm

Typical performance:

Pranu Sanguni (CA)Primary 64 m

Versatile, several focal solutions

Frequency range: 0.3--40GHz (100 GHz)

Radioastronomy, Geodynamics e Space Science

SRT (Sardinia Radio Telescope)INAF, MIUR, ASI, Regione Sardegna

Space Science

Radio receivers: Total power

Signal from the sky must be amplified by a factor ~ 107 (70 dB) before being detected by a diode

Tsky

TNoise,A

Filter selects frequency band and shields against RFI

TNoise,R

Diode produces DC voltage proportional to the square of the RF signal (quare-law detector)

After detection, a DC amplifiers boosts signal (typically) by factor 100-1000

VOUT = α α α α (TSky + TNoise,A + TNoise,R)

Output signal is the sum of the sky signal plus the power introduced by the instrument

a = Calibration constant

GBT 20GHz System

2

min sysdet

( )1( ) T

RT

G fT f k T

n G

δν τ

∆ = + ∆ ⋅ < >

Sensitivity

White noise Stability Push detector sensitivity to fundamental limits Cool to cryogenic temperatures

(HEMTs: factor ~10 from 300K to 20K) Make large arrays of detectors

Differential receivers(Dicke, pseudo-correlation, …)

Internal or sky reference Make large arrays of detectors

Pol X

A

B

Pol Y

C

D

Planck-LFI receiver

A.Penzias & R.Wilson

1978: Nobel Prize

Discovery of the Cosmic Microwave Background

1964-65, Bell Telephon Labs, NJ

• Low-noise receiver

• Accurate cryogenic calibrator

• Low sidelobes antenna

Main chatacteristics:

Bell Labs Horn Reflector AntennaDesigned for telecomunications

Bell Labs Horn Reflector AntennaDesigned for telecomunicationsParabolic

reflector

Incoming radiation

Piramidal horn

Rotary joint

Control cab Receiver

Penzias & Wilson’s Experiment set upObjective: Absolute measurement of galactic emissionCalibration: observations Cas A

TATM ~ 2.3 KTGR < 0.1 K

TLOSS ~ 0.9 K

TGAL < 0.1 K

TREF

Technique: ∆∆∆∆T = TSKY - TREF

TREF ~ 5.0 K

Penzias & Wilson’s absolute calibratorReference calibrator cooled at Liquid Helium Temperature

Waveguide interfaceTREF ~ 5.0 K(& adjustable attenuator)

Expected T from antenna (no source):TSKY = 0.1 + 0.9 + 2.3 + 0.1= 3.4 K

Expected T from reference load:TREF = 5.0 K

Expectation: TREF > Tsky

T low T high

Signal from antenna(loss + atm + ground + ?)

Signal from LHe calibrator

TSKY > TREF

K 5.3? ≅• Ground radiation?• Atmosphere (factor 2?)• RFI (Near New York)?• Galaxy?• Excess antenna losses? • Excess antenna losses? • Seasonal changes?• 1962 nuclear explosions?• Pigeons (!?)

Bell Labs Antenna beam pattern

Physical temperature of ground

∫∫ Ω≡Ωπ

φθ4

),( dPnA

2),( HPBWnB

Main

dP θφθ ≈Ω≡Ω ∫∫Ω

1≤ΩΩ≡

A

Bε

K 300≈≈ Bphys TT

Bell labs antenna:

0.05 KMeasuredT ≈

TAtm

Atmosphere Z

TAtm (Z)CMB

)]([)( 0 ZTTZS Atm+= α

1)sec(

)0()(

−−=Z

SZSTAtm

a ba = b cos(Z)

Atmospheric emission measurements with the Bell Labs 20 foot antenna

2.2 K < Tatm < 2.4 K2.2 K < Tatm < 2.4 K

K 5.3? ≅

1964: Princeton group (Dicke, Peebles, Roll & Wilkinson) predictprimordial thermal radiation and plan experiment to detect it

“Perhaps all discoveries in science have some elements of a good mistery story – This one does!”

(D.Wilkinson , 1983)

3.5 KCMBT ≅ Penzias & Wilson (1965): discoveryDicke et al (1965): interpretation

1948: Gamow, Halpher & Herman predict thermal radiation at T ~ 5-7 K to explain big bang nucleosynthesis

George Gamow

1959: Low-noise radiometry at Bell Lab Measurements show “unusually high system noise temperatures”

Predicted: 18.9 K

Measured: 22.2 K

∆∆∆∆T ~ 3.3 K

1941: Adams, Dunham & McKellar detect unexplained eccess (T = 2.3K) of excitation radiation at mm wavelengths from observations of interstellar CN lines

R. Hanbury Brown, University of SydneyConclusion of Workshop on 50th anniversary of Karl Jansky’s discovery

“It seems to me that the moral of the story that we listened to yesterday about Jansky … [is] that science cannot dispense with an appeal to experience … This is a very serious thing I’m talking about, because it’s one of the great philosophical illusions of history that everything can be produced from a simple series of laws and that our knowledge of the universe can be reduced to this series of laws. It’s one of the great lessons of the science of the last 300 laws. It’s one of the great lessons of the science of the last 300 years that you cannot dispense with an appeal to experience.

Most of the inventions on which our civilization depends now would not have been supported by a committee of review which gives out money, the sort of committee that I have sat on and probably many of you have as well.”

Serendipitous Discoveries in Radio Astronomy: Proceedings of a Workshop Held at the National Radio Astronomy Observatory, Green Bank, West Virginia on May 4, 5, 6, 1983, ed. K. Kellerman and B. Sheets (Green Bank, WV: National Radio Astronomy Observatory, 1983)

John C. Mather George F. Smoot

“For their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation”

COBE-FIRAS COBE-DMR

Cosmic Background Explorer

510T

T

ρρ

−∆ ∆≈ ≈

WMAP full-sky map

WMAP, 2007

PLANCKLooking back to the dawn of time

• Sensitive receivers

• Accurate calibration

• Low sidelobes

Main requirements

1 µK/deg

0.1%90dB

10%50dB

0.1 K/deg

Planck P&W

• Angular resolution

• sky coverage

• Spectral coverage

10°

1%

30-850GHz

0.1°

100%

4GHz

• Low sidelobes 90dB 50dB

PLANCKLooking back to the dawn of time

Planck Telescope1.5x1.9m off-axis

GregorianT = 50 K

LFI Radiometers 27-77 GHz, T = 20 K

HFI Bolometers100-850 GHz, T = 0.1 K

«Those who have reached the stage of no longer being able tomarvel at anything simply show that they have lost the art of

reasoning and reflection.»Max Planck

«Those who have reached the stage of no longer being able tomarvel at anything simply show that they have lost the art of

reasoning and reflection.»Max Planck