AST 443: Submm & Radio Astronomy November 18, 2003Atmospheric TransmissionLocations of mm-wave telescopesSome DefinitionsSimple Radio Telescope: a wireAmplitude ModulationAeffSlide 8Slide 9Antenna TemperatureBrightness TemperatureReal AntennaThe BeamAperture EfficiencyAn exampleSlide 16Detecting a sourceDetectability, cont.Telescope designObservations with a single-dish telescopeSingle-dish telescope observations, contThe RadiometerIntermediate FrequencySpectroscopyApplicationsAST 443: Submm & Radio AstronomyNovember 18, 2003Atmospheric Transmission•Visible, radio, & mm radiation reach the ground•Mm (1-3 mm) and submm (< 1 mm) radiation are susceptible to absorption by water vapor in the atmosphere•Thus, mm & submm telescopes are typically located in the desert, or on high mountains.Locations of mm-wave telescopes• Owens Valley, California• Elevation ~ 5000 ft• Operational September – May of every year• Mauna Kea, Hawaii• Elevation ~ 14,000 ft• Operational 365 days per yearSome Definitions•Unit of intensity for radio and mm astronomy is the Jansky, •1 Jy = 10-26 W m-2 Hz-1•Most detectable sources have 10-3 – 106 Jy•Most radio sources generate thermal radiation•Or synchrotron radiation(0.2 ≤  ≤ 1.2)222TckF vF(h / kT << 1)Simple Radio Telescope: a wire•I.e., like car radio•A radio wave has amplitude, frequency, phase, and polarization•The car antenna detects radiation that is parallel to it•For unpolarized radio with total incident power, P [W], the power detected by a matched antenna is Pm = ½ PAmplitude ModulationA radio station emits a carrier wave where A = amplitude,  . A is modulated in proportion to the “message” signal m(t). Thus, where m(t) ≤ 1 andNote that the AM signal has frequency components at c, c + m, and c – m. I.e.,)cos( tASc)cos()](1[ ttmASc)sin()( ttmm))cos(())cos(()cos()cos()]sin(1[21ttAtASttASmcmcccmAeffSuppose a flux density S [W m-2 Hz-1] is incident on the antenna. The power, P, produced by the antenna is,Aeff may be comparable tobut is dependent on the direction of the radiation. I.e., for our wire example, SAPeff2124DAgeomLow AeffHigh AeffAntennaGiven this,where, if P is the beam pattern or power pattern of the telescope,),(effeffAA ),(),(maxPAAeffP(is measured by pointing at a bright sourceSimilar structure to AiryfunctionThus, if the radio telescope is used to examine a source with brightness B() [W m-2 Hz-1 sr-1], the power measured is,dPABdABPmeffm),(),(),(),(max44Antenna TemperatureBy comparison, the power generated by random thermal motion in a resistor is,where Ta is the resistor temperature. Thus, antenna temperature is defined as,I.e., it is the temperature of a resistor that generates the same output power per Hz as the radio telescope.akTP dABkTeffma),(),(4Brightness TemperatureIn the Rayleigh-Jeans limit, h / kT << 1, the flux density is given by,Thus, the telescope will measure,And thus,The brightness temperature, TB, is the temperature of a blackbody (BB) that radiates the same brightness as the sources (regardless if the source is a BB or not)2),(2BkTB 221),(BmkTBB dATTeffBa),(),(142Real AntennaThe antenna “beam” solid angle on the sky is4),( dPADirectivity, which is a measure of how big the beam is on the sky is,AyDirectivit4The Beam= aAperture Efficiency9.08.0 geomeffAAAAn exampleSuppose we’re using the Kitt Peak 12m diameter telescope to observe an unresolved source which is emitting a signal at 105 GHz. The telescope has a telescope efficiency of  ~ 0.64. The solid angle subtended by the telescope is,The flux to brightness temperature ratio at that frequency is,sr1062.622.14481052 telGHzDRJy/K4.22222 kcTBBBecause the telescope measures an antenna temperature, it is useful to know the antenna temperature to flux ratio,In general,Jy/K35BATBTBJy/K32262telADTBDetecting a sourceAs is the case with optical/NIR astronomy, one must integrate on a source long enough such that the signal from the source is readily apparent over the noise. For radio astronomy, the uncertainty T is given by,Where  is the number of measurement of length , and Ts is the noise, or system, temperature. It has many components,sTT nrcSTTTT ScienceSourceAtmosphere,Side lobes,Losses in antenna structureReceiver noiseDetectability, cont.sTT Time = t1Time = 3t1Telescope design• Alt-azimuth mounting• Main Reflector• Sub-Reflector• Waveguide FeedObservations with a single-dish telescope•Observations of faint mm sources are done in a similar manner as NIR sources. I.e., the noise contributions from the sky and the instrumentation are large.•Beam switching: Nutating the subreflector (see last viewgraph) is a very efficient way to observe faint sources. The sub-reflector switches at a rate of ~ 1.25 Hz from a beam containing the source+sky to a beam containing just the sky. Subtracting these two gives you the source (+ noise).Single-dish telescope observations, cont•Position switching: This is only useful for brighter sources. The telescope is moved from source to sky at a much slower rate (every 30 – 60 seconds).•Frequency switching: instead of moving the telescope, the frequency are “shifted” back & forth by some minute frequency (~15 MHz).•Note that calibrators are routinely put in the beam to recalibrate the telescope.The Radiometer• The signal is amplified and then is “mixed” with a local oscillator of frequency LO. The resultant signals have frequency components at 0 + LO and 0 – LO.Intermediate Frequency• Unwanted frequencies are filtered out, and only signals within the band centered on 0 + LO and 0 – LO are converted and admitted by the filter.• This conversion to an intermediate frequency, or IF, is done because it is easier to manipulate lower frequency signals than higher onesSpectroscopyFilterbanksAutocorrelatorApplicationsNeutral HydrogenRadio ContinuumStar-forming