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 wont 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 26 Al –GRO - gamma-ray observatory currently making groundbreaking observations