1. Radio Telescopes

2. Jansky’s Telescope Karl Jansky built a radio antenna in 1931. –Polarized array –Study lightning noise Detected noise that shifted 4 minutes each day. –Direction of Sagitarrius –Consistent with galactic source

3. Reber Telescope In 1937, Grote Reber built a 32- foot-diameter parabolic dish antenna in his backyard in Wheaton, Illinois to seek cosmic radio emissions.

4. Reber’s Map In the spring of 1939, he was able to detect cosmic radio emissions with his equipment. In 1941, he made the first survey of the sky at radio wavelengths (160MHz).

5. Radio Spectrum The radio spectrum is divided into three bands.

6. Atmospheric Window The atmosphere is transparent from 50 m to 0.5 mm –Long wavelengths reflected by ionosphere –Short wavelength absorbed by O 2, H 2 O; less at altitude

7. Radio Emission Radio sources are measured in Janskys (Jy). –1 Jy = 10 -26 W m -2 Hz -1 –Differential flux F Thermal emission follows the Rayleigh – Jeans law. Synchrotron radiation comes from magnetic fields.

8. Antenna For MHz radio waves a telescope is a half-wave dipole antenna. –Consists of two conductors –Short separation –Quarter wavelength each Cables at the center connect to the receiver. Jocelyn Bell and 81.5 MHz radio telescope (1967)

9. Radio Horns Horn antennas are used to collect waves at GHz and higher. –Factor of 8 in bandwidth –May be dielectric filled –Waveguides to detector VLBA telescope ~1 meter ~2cm

10. Sensors Radio sensors are matched to the desired wavelengths. High frequency sensors are usually SIS. –Superconductor- insulator- superconductor –NiO – AlO –Photon assisted tunneling VLBA telescope

11. Noise Small radio signals can be lost in noise. –Minimum detectable brightness B min –Integration time t –Frequency bandwidth  –Receiver-based constant K Noise temperature based on multiamplifier stages with gain G n.

12. Cryogenics One way to lower noise is to operate the electronics at low temperature. –Reduces thermal noise Cooling to less than 20K is accomplished with high pressure helium gas. –Compressor and pump –Thermally isolated cryostat Helium pump Cryostat

13. Components Horn Purpose of the horn is to collect the radiation directed to it from the antenna. Amplifier Increases the amplitude of the signal Mixer Used to change the frequency to a more easily used frequency

14. Heterodyne The simplest receiver is a heterodyne receiver similar to a consumer radio. –Preamp gain 10-1000 A local oscillator mixes with the input signal. –Beat or intermediate frequency Further amplification may be by factors of 10 6 to 10 9. Heterodyne receivers have high system temperatures. –10 K at radio –10 M at millimeter Phase sensitive techniques reduce noise.

15. Receivers Store data Radio Preamp Mixer Convert to desired form to record/analyze Millimeter and Submillimeter Store data Convert to desired form to record/analyze Horn

16. Radio Receiver Receivers are inside the dome Aricibo

17. Millimeter Wave Receiver Horns Amplifiers ~0.5 m

18. Millimeter Wave Oscillator Local oscillator

19. Millimeter Wave Electronics Intermediate frequency (IF) plate

20. Submillimeter receiver CSO, Mauna Kea

21. Submillimeter receiver 230 GHz mixer block CSO, Mauna Kea

22. Antenna Dish A radio telescope is often noted for the large dish. This is for the same optics as an optical reflecting telescope. –Rayleigh criterion applies –Beam width at first nulls –Dipole length x ReceiverSteering gear Antenna mirror

23. Gain The maximum gain for a radio telescope depends on the effective area. –Typically 1.6 for half-wave dipole The effective area compares the output power to the incoming flux. –Must be correctly polarized

24. Surface Errors The surface error of a dish must be controlled. –Less than 1/20 wavelength –Losses to less than 30% 1/20 =0.15mm Radio VLBA (3mm-3m) Mauna Kea

25. Very Smooth The surface error requirements are much stricter for sub- millimeter telescopes than for radio telescopes. 1/20 =0.05mm Millimeter 1/20 =0.015mm Sub-millimeter NRAO 12 meter (1mm- 3mm) Kitt Peak JCMT (0.3mm-2.0mm) Mauna Kea

26. Holographic Test This image was taken during the SMT reflector's holographic testing showing that the deviations of the reflector. Deviations are nearing the targeted 15 microns (about the thickness of a human hair).