Radio Astronomy

1. Radio Astronomy • By looking at the radio part of the EM spectrum, we can get a different perspective on the nature of the universe. • the atmospheric window for radio covers a larger portion of wavelength (or bandwidth) than the optical • field started in 1931 when Karl Jansky detected a radio signal whose intensity peaked at a position in the sky that we now know is the center of the Galaxy • Radio telescopes are much different than optical telescopes • typically no tube, a large parabolic primary dish • a detector mounted at prime focus • detector tuned to one frequency (channel) per observation • diffraction-limited resolution is much worse than optical telescopes • primary dishes have to be large to get better resolution and to gather as many radio photons as possible • remember that intensity of the radio part of a blackbody curve is often many orders of magnitude less than the peak • dishes can be made out of material other than glass (usually metal mesh), so long as the irregularities are smaller than the wavelength of photons measured

2. Radio Astronomy (cont.) • Radio astronomy has some advantages over optical astronomy • can be done 24 hrs a day • not affected by weather too much • most strong radio emitting objects cannot be seen by optical telescopes • the Universe is fairly transparent to most radio photons • can measure hydrogen from a spectral line at 1420 MHz (21 cm) • this is the only way cold hydrogen can be measured in the Universe

3. Interferometry • To get around resolution limitations, the diffraction limit says we can do two things • choose to observe at shorter wavelengths • bad choice since the atmospheric window is more restrictive at lower wavelengths • make a bigger telescope • also hard to do with a single instrument because of expense • The answer is to use two or more simultaneous observations by telescopes separated by some distance • each observation must have a precise time history so that a computer can add the signals together • the principles of constructive + destructive interference eliminate noise and produce an observation with better resolution • observations will have the same resolution as if taken by a single telescope with an effective size of the distance between the two telescopes • radio astronomers routinely perform what is called Very Long Baseline Interferometry (VLBI) with two or more telescopes at different parts of the world • some optical astronomers are talking about putting a telescope in orbit around the Sun at the distance of Jupiter for the resolution needed to find other planets

4. Other Telescopes • Most objects in the Universe emit radiation over the entire EM spectrum, and most of that radiation occurs outside the optical window. • need to develop technologies to measure this radiation • Infrared telescopes and astronomy are currently the hottest field in astronomy • telescopes are very much like optical telescopes • CCD efficiency peaks naturally in IR (>80%) • observations are very sensitive to several conditions • atmosphere is fairly opaque to most IR, so a high mountain or balloon or satellite is required • need low humidity, because water absorbs IR • need supercooled detectors to eliminate radiation from the detector itself • some examples of IR telescopes • IRAS - infrared astronomical satellite (1983) • ISO - infrared space observatory (1995) • NICMOS - near infrared camera and multi-object spectrometer on HST (1997) • SIRTF - space infrared telescope facility (2002), built on the back of a 747

5. Other Telescopes (cont.) • Ultraviolet telescopes are necessary to study wavelengths just smalle than optical range • heavy atmospheric attenuation of UV photons dictates how observations are performed • can use CCDs optimized for what little UV radiation can be seen from Earth • only good for observations at wavelengths > 3200 Å • usually use telescopes mounted on rockets or satellites • very similar in construction to optical telescopes • some examples • IUE - international ultraviolet explorer (1978), the most successful satellite in history • EUVE - extreme ultraviolet explorer (1992), the first satellite to look at this part of the EM spectrum • GHRS - Goddard high resolution spectrometer on HST, removed when NICMOS was installed • X-ray and Gamma-Ray astronomy requires much different instrumentation • all observations require being above the ozone layer, which means high altitude balloon or satellite • low wavelength means that standard reflection telescopes won’t work • use a grazing incidence telescope to focus light for X-rays

6. Other Telescopes (cont.) • use scintillating crystals to measure gamma-rays • some examples • Uhuru, Copernicus, and EInstein satellites - first useful documentation of X-ray sky • ROSAT - currently providing observations • AXAF - US X-ray satellite delayed for past 10 years • Vela - US defense satellites that first detected a new astronomical object called gamma-ray bursts • HEAO-C - first gamma-ray telescope to detect 1.809 MeV emission from the decay of 26Al • GRO - gamma-ray observatory currently making groundbreaking observations