Life in the Universe

Radio AstronomyASTR 3010

Lecture 25

Intro to Radio Astronomy

Concepts - Amplifiers - Mixers (down-conversion) - Principles of Radar - Radio Astronomy basics: System temperature, Receiver temperature Brightness temperature, The beam (q = l / D) [ its usually BIG] Interferometry (c.f. the Very Large Array VLA) Aperture synthesis 2History of Radio Astronomy (the second window on the Universe)

1929 - Karl Jansky (Bell Telephone Labs) 1030s - Grote Reber 1940s - WWII, radar - 21 cm (Jan Oort etc.) 1950s - Early single dish & interferometry - `radio stars, first map of Milky Way - Cambridge surveys (3C etc) 1960s - quasars, pulsars, CMB, radar, VLBI aperture synthesis, molecules, masers (cm) 1970s - CO, molecular clouds, astro-chemistry (mm) 1980s /90s CMB anisotropy, (sub-mm)

3NRAO/AUI/NSF4

4Astronomers use telescopes to study objects at all wavelengths. Here you see examples of objects at multiple wavelengths: Saturn, and the Sun Taken from http://www.ipac.caltech.edu/Outreach/Multiwave/multiwave.html

Look at the cool aurora in the UV image of Saturn. Look at how different the radio image is. This is a false color image where red shows the most radio waves, blue the least. The rings of Saturn do not emit radio waves, but the planet does.

The images of the sun show a similarity between the visible light image and the radio image.

NRAO/AUI/NSF5Optical and Radio can be done from the ground!

5Nice when you need to fix your telescope!NRAO/AUI/NSF6Radio Telescope

Optical TelescopeNowadays, there are more similarities between optical and radio telescopes than ever before.6Radio telescopes also have parabolic mirrors, but theyre not shiny enough to reflect visible light well. The receiver is at the focal point. We cant see radio waves and so must put an electronic detector at the focal point.

OutlineA Simple Heterodyne Receiver Systemmixers and amplificationObserving in the Radioresolutionbrightness temperatureRadio InterferometryAperture synthesis7Df = 1850 Hzf transFreflect = f trans + / - DfMixing: Adding waves together8Mixerssignal inLOlocal oscillatorw1w2signal outw1+w2 andw1-w2A mixer takes two inputs: the signal and a local oscillator (LO).The mixer outputs the sum and difference frequencies.In radio astronomy, we usually filter out the high frequency (sum) component.9MixersfrequencysignalLOoriginalsignalmixedsignal0 Hz10MixersfrequencysignalLOoriginalsignalmixedsignalThe negative frequencies in the difference appear the same as a positive frequency.To avoid this, we can use Single Sideband Mixers (SSBs) which eliminate the negative frequency components.0 Hz11

W-band (94 GHz,4 mm) amplifier

12Local oscillatorDownconverted signalFrequency Single sideband mixer:f = 10 GHzF + Df = 10 GHz + 1850 Hz 1850 Hz Df = fIFBand-pass of amplifier: Intermediate frequency = IF13A Simple Heterodyne Receiverlow noiseamplifierfilterreceiver hornLOtunablefiltersignal @ 1420 MHz1570 MHz1420 MHztunableLO~150 MHzAnalog-to-DigitalConverterComputer++outputs a power spectrum150 MHz14AmplificationWhy is having a low noise first amp so important?the noise in the first amp gets amplified by all subsequent ampsyou want to amplify the signal before subsequent electronics add noiseAmplification is in units of deciBells (dB)logarithmic scale3 dB = x25 dB = x310 dB = x1020 dB = x10030 dB = x1000

15Observing in the Radio IWe get frequency and phase information, but not position on the sky2D detectorA CCD is also a 2D detector (we get x & y position)16Observing in the Radio II:Typical Beamsize (Resolution)i.e. The BURAO 21 cm horn (D ~ 1 m)

17Observing in the Radio IIi.e. The NRAO GBT (D ~ 100 m)

at 21cm = 1.420 GHzat 0.3 cm = 100 GHz

18Observing in the Radio IIi.e. The Arecibo Telescope (D ~ 300 m)

at 21cm = 1.420 GHzat 0.3 cm = 100 GHz

19

Observing in the Radio III:Brightness TemperatureFlux: erg s-1 sr-1 cm-2 Hz-1 (1023 Jy)Bu(T): erg s-1 sr-1 cm-2 Hz-1 (1023 Jy)We can use temperature as a proxy for flux (Jy)

Conveniently, most radio signals have hu/kT