Astronomical Observing Techniques Lecture 7: Your Favorite Sta<on at 1420 MHz

Transcription

1 Astronomical Observing Techniques Lecture 7: Your Favorite Sta<on at 1420 MHz Christoph U. Keller

2 Overview 1. Introduc9on 2. Radio Emission 3. Observing 4. Antenna Technology 5. Receiver Technology 6. Back Ends 7. Calibra9ons

3 (c) National Radio Astronomy Observatory / Associated Universities, Inc. / National Science Foundation

4 The First Radio Astronomers Karl Guthe Jansky ( ) hkp://en.wikipedia.org/wiki/radio_telescope hkp://en.wikipedia.org/wiki/radio_astronomy Grote Reber ( ) Karl Jansky built (at Bell Telephone Laboratories) antenna to receive radio waves at 20.5 MHz (λ~14.6m) à turntable of 30m 6m à first detec9on of astronomical radio waves (à 1 Jy = W m 2 Hz 1 ) Grote Reber extended Jansky's work, conducted first radio sky survey. For nearly a decade he was the world's only radio astronomer.

5 Radio Astronomy Discoveries radio (synchrotron) emission of the Milky Way (1933) first discrete cosmic radio sources: supernova remnants and radio galaxies (1948) 21-cm line of atomic hydrogen (1951) Quasi Stellar Objects (1963) Cosmic Microwave Background (1965) Interstellar molecules ó star forma9on (1968) Pulsars (1968)

6 Radio Observa<ons through the Atmosphere Radio window from ~10 MHz (30m) to 1 THz (0.3mm) Low-frequency limit given by (reflec9ng) ionosphere High frequency limit given by molecular transi9ons of atmospheric H 2 O and N 2.

7 Radio: PhotonsàElectric Fields Directly measure electric fields of electro-magne9c waves Electric fields excite currents in antennae Currents can be amplified and split electrically.

8 Radio Emission Mechanisms Most important astronomical radio emission mechanisms 1. Synchrotron emission 2. Free-free emission (thermal Bremsstrahlung) 3. Thermal (blackbody) emission (also from dust grains) 4. Spectral lines Comparison of three emission components (for the starburst galaxy M82) Synchrotron radia9on dominates at low frequencies. Thermal dust emission dominates at high frequencies. Free-free emission

9 Synchrotron Emission Caused by highly rela9vis9c electrons, spiraling around galac9c magne9c field lines Polarized Con9nuous spectrum

10 Free-Free Emission Free-free emission produced by free electrons scakering off ions (e.g. in HII regions) without being captured: con9nuous spectrum Sun5GHz.jpg

11 Thermal Emission Rayleigh-Jeans tail of thermal emission from e.g. dust grains produces radio emission.

12 The HI 21cm ( MHz) Line Hendrik van de Hulst predicted in 1944 that neutral hydrogen could produce radia9on at ν = MHz due to two closely spaced energy levels in the ground state of the hydrogen atom. Hendrik van der Hulst ( ) First observed in 1951 by Ewen and Purcell at Harvard University, then by Dutch astronomers Muller and Oort. A$er 1952 the first maps of the neutral hydrogen in the Galaxy were made and revealed, for the the spiral structure of the Milky Way.

13 Map of the Milky Way Determine loca9on of hydrogen emission from rota9onal Doppler shis Contour plot of hydrogen concentra9on as seen from the top High concentra9ons in red

14 Astronomical Relevance of HI 21cm Line Main applica9ons: 1. Distribu9on of HI in galaxies 2. Big Bang cosmology: redshised HI line can be observed from 200 MHz to about 9 MHz: mapping redshised 21 cm provides the maker power spectrum aser recombina9on provides info on how the Universe was reionized (HI which has been ionized by stars or quasars will appear as holes) But the signals are intrinsically weak and plagued by radio interferences. Thilker, Braun & Walterbos (1998)

15 Famous Radio Telescopes (Single Dish) Parkes 64m Jodrell Bank 76m Arecibo, Puerto Rico, 305m Dwingeloo, 25m Effelsberg, 100m Greenbank, USA after structural collapse

16 ALMA: Atacama Large Millimeter Array

17 Antennae: The Hertz Dipole Antennae required to focus power into feed Feed is device that efficiently transfers power in the electromagne9c wave to the receiver Proper9es of antennae (beam pakerns, efficiencies, ) are the same for transmission and recep9on. Hertz dipole: total power radiated from Hertz dipole of length Δl carrying an oscilla9ng current I at a wavelength λ is: P = 2c IΔl 3 2λ 2

18 Radia<on from Hertz Dipole Radia9on is linearly polarized Electric field lines along direc9on of dipole Radia9on pakern has donut shape, defined by zone where phases match sufficiently well to combine construc9vely Along the direc9on of the dipole, the field is zero Best efficiency: size of dipole = 1/2 λ

19 Radio Beams, PSFs and Lobes Similar to op9cal telescopes, angular resolu9on given by D θ = k λ Radio beams show just like the Airy pakerns of op9cal PSFs pakerns of lobes at various angles, direc9ons where the radiated signal strength reaches a maximum, separated by nulls, angles at which the radiated signal strength falls to zero. where k ~1.

20 Main Beam and Sidelobes Highest field strength in main lobe, other lobes are called sidelobes (unwanted radia9on in undesired direc9ons) Side lobes may pick up interfering signals, and increase the noise level in the receiver. The side lobe in the opposite direc9on (180 ) from the main lobe is called the back lobe.

21 Ω A = Main Beam Efficiency The beam solid angle Ω A in steradians of an antenna is given by: 4π 2π π ( ϑ, ϕ) dω P ( ϑ, ϕ) sinϑdϑdϕ Pn = 0 0 With the normalized power pakern Hence, P n = 1 for Ω A for an ideal antenna. Main beam solid angle Ω MB is: Ω MB = P n main lobe ( ϑ ϕ), dω And the main beam efficiency η B is: n 1 P ( ϑ, ϕ) P( ϑ, ϕ) P n = (This is the frac9on of the power concentrated in the main beam.) η B = Ω Ω MB A max

22 Coherent (Heterodyne) Receivers Problems with detec9ng and amplifying signals (electromagne9c waves): 1. The signals are usually very weak 2. The frequencies are too high for standard electronics λ = 1 µm ó ν = 300 THz ó Δt = s λ = 100 µm ó ν = 3 THz ó Δt = s λ = 1 cm ó ν = 30 GHz ó Δt = 33 ps Solu9on: Mixing (mul9plica9on) of the source signal with a reference wave (provided by a local oscillator):

23 encodes signal over a wide wavelength range à ideal for spectroscopy typically, power(ω LO )» power(ω S ) à amplifica9on by oscillator signal down-conversion to frequencies where low-noise electronics exist Principle of Frequency Mixing 1. Signal S 1 is mixed with local oscilla9ng field S 2 2. The mix produces a down-converted difference, intermediate, or beat frequency at ω S1 ω S2 (and ω S1 + ω S2 ).

24 Mixer Technology Example of a mixer: Source signal à Local oscillator signal à Mixing element Problem: good & fast tradi9onal photo-conductors do not exist for ν < 7.5 THz è SchoKky diodes SIS junc9ons Hot electron bolometers

25 Mixer Output linear device (a) yields no output power at any frequency. non-linear device (b,c) can convert power from the original frequencies to the beat frequency even if the mixer has an odd func9on of voltage around the origin (b) the conversion efficiency is zero. but if biased above zero (A) the average change in current is larger for posi9ve than for nega9ve voltage peaks. If I ~ V 2 (as in a diode) then output ~ (field strength) 2 ~ power, which is exactly what we want to measure!

26 Basic Heterodyne System local oscillator signal wave diplexer mixer Back End specifies the devices following the IF amplifiers. Many different back ends have been designed for specialized purposes such as con9nuum, spectral or polariza9on measurements.

27 Polarimeters Antennas with fixed-dipole feeders or horn feeders receive only the frac9on that is polarized in the plane of the orienta9on of the feeder. Rota9on can be measured by rota9ng the feeder about the antenna s beam axis or by two orthogonally polarized antenna feeders. Heterodyne dual polariza9on receiver = two iden9cal systems, connected to the same local oscillator, and sensi9ve to only one of the two orthogonal polariza9ons. Can provide values of all polariza9on parameters simultaneously.

28 Mul<channel Spectrometer The IF input signal is divided among the bandpass filters ( filter bank ) and the output of each is processed by a detector/integrator stage. The outputs of these stages are switched sequen9ally to the computer where the spectrum can be displayed. Such mul9-channel spectrometers can have up to 512 parallel channels.

29 Acousto-Op<cal Spectrometer (AOS) AOS converts frequencies to ultrasonic waves that disperse a monochroma9c light beam onto an array of visible light detectors. from Wikipedia The acous9c wave can be created in a crystal ( Bragg-cell ) and modulates the refrac9ve index à induces a phase gra9ng. The angular dispersion is a measure of the IF-spectrum.

30 Autocorrela<on Spectrometer Reminder: f ~ f ~ f ~ f ( x) ( s) = FT{ f ( x) } ( s) 2 2 ( s) = f ( x) f ( x) Function Fourier transform of f(x) Spectral density or power spectrum of f(x) Wiener - Khinchine (autocorrelation) theorem k + ( x) = f ( u) f ( u + x) Autocorrelation 1. Given: time dependent IF signal f(x) 2. Want: Power spectrum I(v) = f(s) 2 3. f(s) 2 could be computed via FT{f(x)} du 4. Better and faster: compute autocorrelation function of f(x) 5. à Digitize and delay x(t) n-times and compute autocorrelation

31 True Brightness Temperature Rayleigh-Jeans approxima9on: B RJ ( T ) ν = 2ν c, 2 brightness and effec9ve temperature are strictly propor9onal Can use brightness temperature to describe source intensity: T B 2 c = 2 2kν B RJ Note: usually only fulfilled if source fills beam (very extended sources) If the source is a real black body hv << kt, then T B is independent of v If the emission is non-bb (e.g., synchrotron, free-free, ) T B will depend on v but the brightness temperature is s9ll being used 2 = kt 2 λ 2k B RJ

32 Main Beam Brightness Temperature Rela9on between flux density S v and intensity I v : For discrete sources, the source extent is important and we need to combine the above equa9on with the previous one to: or simplified for a source with a Gaussian shape: Generally, for an antenna beam size θ beam the observed source size is: which relates the true brightness temperature with the main beam brightness temperature: S Sν = Jy ν = Ω B I ν 2kν c ( θ ϕ) 2 Sν = T 2 B T MB T B, cosθ dω ΔΩ θ arcsec θ = θ + θ 2 observed ( 2 2 ) 2 θ + θ = T θ source 2 source beam 2 2 beam λ mm B 2 source

33 Noise Temperature The power spectral density (PSD) entering the receiver is given by P = kt ν and is also called the antenna temperature. A receiver shall increase the input power level. The amplifica9on involves a noise factor F, defined via the S/N as: F = S S input output output For coherent receivers this noise factor is expressed as noise temperature: / / N N input ( F 1) K T R = 290

34 Receiver Calibra<on Noise temperature (receiver temperature) can be measured by comparing the signals of two ar9ficial sources with effec9ve temperatures T 1 and T 2. The total power is given by: where P α = Gkδν 1 ( T + T ) and P = ( T T ) = α α R 1 2 R + depends on gain G and bandwidth δν. 2 Now we can define a Y-factor as: Y = P P 2 1 With the Y-factor and solving for the receiver temperature T R we get: T R T2 YT = Y 1 1

35 Receiver Stability and Dicke Switching Source signals are weak à gain must be high à small gain instabili9es can dominate the thermal receiver noise. à Compare source signal with a stable reference signal by beam switching or Dicke switching (1946). It also compensates for atmospheric changes. Disadvantage: 50% of total 9me is spent to look at flux reference. Robert Henry Dicke ( )

36 Spectral Line Observa<ons Three common observing modes to detect weak spectral lines: 1) Posi9on Switching and Wobbler Switching: The signal on source is compared with a measurement of a nearby sky posi9on. Obviously, there should be no line radia9on coming from the sky posi9on. 2) On the Fly Mapping (extension of method (1)): spectral line data is taken at a rate of perhaps one spectrum or more per second while the telescope slews (scans) con9nuously across the source field. The background/con9nuum emission is reconstructed from the en9re data set. 3) Frequency Switching: For most sources, the spectral line radia9on is restricted to a narrow band. Changing the frequency of the receiver on a short 9me by ~10Δν produces a comparison signal with the line well shised. The line is measured all of the 9me, so this is an efficient observing mode.