TELESCOPES The main tools astronomers use are, of course, telescopes. However, there are many types, depending on the band of the Electromagnetic Spectrum being studied. The instruments at the back end: cameras, spectrometers, etc. are just as important as the “light buckets”.
But First, The Eye • Our natural way to collect and focus visible light • The pupil admits light to the lens, which along with the cornea, focuses light on the retina • The cones are retinal cells sensitive to color; rods are more numerous but only give us shades of gray
Refraction & Focusing • Light slows on entering glass, plastic, or cornea (n=1.3376) and lens: then it bends toward the normal • This focuses incoming rays from a point to a point and creates an inverted image (which our brain turns right-side-up)
Cameras Store Images • Our eye refreshes images ~30 times/s • Lets us see motion, but prevents long exposures or seeing faint objects • Film, or now, digital Charge Coupled Devices record for as long as the shutter is open
What Are Telescopes Good For? • Telescopes gather EM radiation and bring it to a focus. • MOST IMPORTANT: TO DETECT FAINT THINGS Observed BRIGHTNESS or INTENSITY is proportional to LUMINOSITY but declines inversely with the DISTANCE squared so bigger telescopes let you see to greater distances.
Better Sensitivity • The bigger the aperture (diameter) of the telescope, the more light it gathers in the same time: so it sees fainter objects. Bottom photo of Andromeda galaxy: twice the aperture.
What else do telescopes do? • MAKE IMAGES of EXTENDED THINGS, or SEPARATE IMAGES of NEARBY THINGS (resolution) • RECORD CHANGES OVER TIME • MAGNIFY images. While important for bird-watching on earth, and of some importance for planetary observations, higher magnification yields more jiggling and often worse resolution.
Focusing of Light • LENSES USE CURVED SURFACES TO TAKE A PARALLEL BEAM OF RADIATION FROM A VERY DISTANT SOURCE TO A (PRIME) FOCUS. • Such CONVERGING LENSES require CONVEX SURFACES: the bending is greater for larger differences from the normal (perpendicular to surface). More sharply curved lenses produce shorter focal lengths. • Images form as light from different parts of an object comes to a focus in different parts of the focal plane (inverted or upside-down).
Mirrors of Paraboloidal Shape Focus Light of All Colors to the Same Point and Invert Images Too Mirror Focus Inversion
Refractors vs. Reflectors Reflecting telescopes are superior because: • They only need perfect surfaces, not perfect solids. Therefore they are easier and cheaper to make -- MUCH cheaper for BIG telescopes! • No chromatic aberration in reflection. • Large lenses sag, and must be supported by their edges or else light is blocked.Big mirrors can be supported from behind w/o blocking light. • Refractors need to be much longer than reflectors of the same aperture--again more $$
Larger Mirrors Gather More Light • A telescope’s area is proportional to the square of its diameter, or aperture. Keck telescope: D = 10m has 2500 times the aperture of human eye (around 4 mm) or an area 6.3 million times bigger. Thus, in one second it can gather 6.3 million times the light, or see an object only one/six-millionth as bright. Or, the same object can be seen 2500 times further away! Light Bucket Applet Plus, telescopes can integrate (stare) for hours, while the human brain produces a new image about 30 times a second, wiping out the old one. Together, these mean that big telescopes can make images of objects billions of times fainter than we can see.
Larger Apertures Resolve Better • Here , the angle between the closest objects that can be seen separately is in arcsec, while wavelength, , and aperture, D, are in the same units (meters). • Example: a 3.5m telescope working in yellow light 500 nm = 5x10-7m, has a resolution angle, = 2.5x105 x (5x10-7m/3.5m) = 0.036 arcsec • Resolution on earth limited by SEEING --- spreading of an image via turbulence in the atmosphere: changes over < 0.1 s, and smears images out to > 0.5 arcsec. So little real improvement in resolution if D > 0.25 m. This is also why STARS TWINKLE and PLANETS DON'T (usually, as seen by the naked eye). • Angular Resolution Applet
Resolution and Turbulence Angular Resolution Car Lights Applet
Avoiding Resolution Limits • Go into Space: Hubble Space Telescope = 0.08” Next Generation (James Webb) Space Telescope ( = 0.02 arcsec ) • Speckle interferometry: take very short exposures -- works only with very bright stars ( = 0.002 arcsec ). (Also see regular interferometry, below.) • Active optics: quickly tip and tilt mirrors to make image crisper (motors + dome airflow + pistons) • Adaptive optics: measure blurring of a bright star (or laser spot) and very quickly adjust mirror shape to reduce it; then nearby images will also be crisper. ~ 0.1 arcsec can be achieved in IR and visible
Active Optics: 3.5 m NTT Star cluster R136: w/o & w/ active optics
Adaptive Optics: Starfire & Gemini NGC 6934, a star cluster without (1”, visible), and with (0.1”, IR) adaptive optics on 8-m Gemini North
Key Instruments (optical, UV, IR) • Typical research telescopes have several instruments which are attached to the secondary focus (Cassegrain and/or Coude). • INTENSITY (BRIGHTNESS) Phototube: linear, but only one or two objects at once. Photographic plates: non-linear, but compare many at once. Charged Coupled Device: CCD -- linear and get many at once. CCDs now dominate intensity measurements.
Nobel Prize in Physics 2009 • Willard S. Boyle and George E. Smith who were at Bell Labs in 1969 share half the prize for the invention of the Charge Coupled Device sensor: CCDs were used first in spy satellites, then by astronomers and today in digital cameras. • The other half went to Charles K. Kao, who while working in England in 1966 demonstrated pure enough glass would allow fiber optic cables to work; hence the internet. • All are Americans, though Kao is also British and Boyle also Canadian
Making Images • Photographic plates, use multiplefilters and combine for color images. • CCD -- resolution now about as good and linearity far better; data is DIGITAL and can be processed more easily to get more precise results. Star cluster R136 in the Large Magellanic Cloud, from: ground, original HST, processed HST, corrected HST
Spectrometers & Polarimeters • Most telescopes spend most of their time spreading the light out into all frequencies: SPECTROSCOPY gives FAR MORE DETAILED INFORMATION than IMAGING. Temperatures Composition and abundances Pressures Velocities (Doppler shift) Rotation Magnetic fields • POLARIMTERS: Special materials can rotate different linear polarizations by different amounts and allow weak polarizations to be detected.
Spectrograph & Spectra • Diffraction gratings, not prisms, actually used
Timing Studies • Along with IMAGING and SPECTROSCOPY measuring changes with time is a very important use of telescopes: in brightness (light curve -- here Mira, P = 331 days);positions (astrometry); spectra (binary stars and planets around stars)
RADIO TELESCOPES • INTERFEROMETRY OVERCOMES POOR RESOLUTION Single dish radio telescopes can't resolve better than 20 arcsec --- Georgia as seen from the Moon. • Combining and interfering signals can produce much better resolution; the EFFECTIVE APERTURE becomes the MAXIMUM SEPARATION (BASELINE) between the telescopes.Very Large Array: 27 telescopes, up to 30 km baselines, so = 0.1” (at 1 cm): GSU from the Moon. Very Long Baseline Array: 10 telescopes, 6000 km baseline so = 0.0004” (at 1 cm) --- you from Moon. VLBI from Space: HALCA -- up to 21,000 km baselines so = 0.0001 arcsec (at 1 cm).
Principle of Interferometry • Constructive and destructive interference depend on the exact direction of incoming waves. As earth rotates, different path lengths are sampled, so images can be built up. • Resolution equals that of telescope w/ aperture = separation
Optical Interferometry • The same techniques can now be used in the optical band, where it is much more difficult to combine and interfere the signals. (A little easier in infrared.) • CHARA ARRAY is the largest optical interferometer: a GSU project, headed by Prof. Hal McAlister, its longest baseline is 330 m Light from 6 one-meter aperture telescopes can be combined to give resolutions of about 0.0004 arc sec (like VLBA) Measure: sizes of stars (directly), separations between nearby stars, and even make images of nearby giant stars!
First Results of the CHARA Array Regulus, a nearby hot star had its size, shape and temperature(s) accurately measured by CHARA. Its fast rotation makes it much fatter at its equator and it is also hotter at the poles, so it’s brighter there. Compared to Sun.
SHORT WAVELENGTH ASTRONOMY • UV, X-ray and Gamma-ray astronomies are newer: must be done in space, above the blocking atmosphere, so started in the 1960s • Hubble Space Telescope (HST) works in the UV, along with IUE, FUSE, EUVE and others -- doing both imaging and spectroscopy. • X-ray missions require grazing incidence mirrors as X-rays can’t be focused; current missions: Chandra and XMM-Newton (past UHURU, Exosat, ASCA, Einstein) • Gamma-rays can’t be focused: collimators and special detectors are used: CGRO; now HETE, SWIFT, FERMI • IR astronomy also better done from space: Spitzer is currently up there, but mountains and planes work too.
Chandra X-ray Observatory “false color” image of the supernova remnant Cas A 1” resolution!
Compton Gamma-Ray Observatory CGRO launched from shuttle Atlantis; Image of the blazar 3C 279
Spitzer Space Telescope 4, 8, 24 m+ false color composite, of M81