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Feb 17, 2006 Astro 100 Lecture 14 1

Lecture 14

Telescopes: Resolution and Seeing Instruments and Detectors

Ground-Based: visible, IR, radioSpace-based: suborbital, orbital

Observatories

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Field of ViewImportant reality: There is no telescope design that

is best for all kinds of astronomy. All good telescopes are specialized in some way. For instance:

� Field of View: The size of the image at the focus is proportional to the focal length.� Long focal length => high magnification but small field

of view. Good for small things (eg, Hubble Space Telescope)

� Short focal length => low magnification but large field of view. Good for faint diffuse things and large-scale surveys (eg, binoculars, "Schmidt" telescopes).

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Feb 17, 2006 Astro 100 Lecture 14 3

Spatial Resolution� Spatial resolution or resolving power. The

telescope image is always no better than� theoretical resolution: proportional to: (wavelength of

the light) / (aperture)eg: yellow light + 5-inch telescope: 1 arcsecond

� But, on the ground, this is made worse by motions in the atmosphere, called atmospheric "seeing":

� In Wisconsin, a few arcseconds; � At best sites about 0.5 arcsec.

� On ground, resolution of 4m telescope no better than 10-inch! Main advantage of Hubble: can attain its theoretical resolution of <0.1 arcsec.

Feb 17, 2006 Astro 100 Lecture 14 4

Instruments� What can you do with light? At each wavelength, can

measure� Intensity and/or Polarization

and how they vary with � Position and/or Time

� Spectrograph: variation of intensity with wavelength� Polarimeter: variation polarization with wavelength� High-speed photometer: variation of intensity with time� Imager: variation of intensity with position

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Feb 17, 2006 Astro 100 Lecture 14 5

Instrument Realities

� Resolution vs precision. � If your experiment requires separating very close

wavelengths, positions, or times, you need good "resolution".

� Since you are dividing the same amount of light into smaller pieces, you have less light in each "resolution element", giving lower precision.

� In practice, good resolution, high-precision measurements can only be made on bright objects.

Feb 17, 2006 Astro 100 Lecture 14 6

Detectors� Much of the progress in last decades is due to new

detectors:� Photographic film. Light causes chemical reaction,

made permanent and visible by "developing" after exposure.

� Phototube. Light strikes "photocathode", producing electron which is amplified and detected as electronic signal.

� Charge-Coupled Device ("CCD"). Light strikes silicon "chip", creates electrons trapped in tiny "pixels". Electronic signal "read out" sequentially from each pixel.

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Feb 17, 2006 Astro 100 Lecture 14 7

Ground-based Observatories -VisibleVisible-wavelength. Best features:� Remote: minimize light pollution� High altitude, Dry: minimize clouds, scattering� eg, Mauna Kea, Hawaii (13, 650 ft) But, remote sites can be very expensive to run and

inflexible. Small local sites can be better for trying new things and long-term monitoring projects.

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Ground-Based - Infrared/ Radio� Infrared

� Isolated wavelength "windows". Absorption by water vapor, carbon dioxide ("greenhouse gases").

� Worst problem: telescope and atmosphere glows in infrared. (solution: cool all or parts of 'scope)

� Radio� Because wavelength very large, theoretical resolution

very poor from the largest "single dish"� But: interferometry: frequency very low, so can

directly record signal from widely-separated dishes and combine to get effective resolution of telescope as big as separation. (up to size of earth: .001 arcsec!)

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Feb 17, 2006 Astro 100 Lecture 14 9

Airborne Observatories� Getting off the ground

� observe wavelengths absorbed by atmosphere� escape "seeing"� escape manmade and atmospheric glow

� Airplane� Good for infrared, since most absorption is from water

vapor in lower atmosphere. � Kuiper observatory: cooled 1m in C-141 jet (retired)� SOFIA: cooled 2.5m in 747 (1st light 2004) � Altitude < 50,000 ft. Few hours..

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Suborbital Observatories� Balloon

� Good for mid-ultraviolet wavelengths blocked out by ozone layer. Altitude < 140,000 ft (43 km). Few days

� Good for some X, γ ray (still some absorption)

� Suborbital ("Sounding") Rocket� Good for completely blocked

radiation (Far, extreme UV). � Altitude < 400 km. 8 minutes. � Good for developing new

instruments for these wavelengths. UW Wide-Field Imaging Survey Polarimeter (WISP)

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Feb 17, 2006 Astro 100 Lecture 14 11

Orbital Observatories� Low-Earth Orbit (eg, Hubble)

� 300-1000 km. � Years. � Short (30 min) "nighttime" can be disadvantage.

� High-Earth Orbit and Geostationary: � 40,000 km. Very long exposures.

� Interplanetary� eg Spitzer IR telescope: trailing earth Sun Lagrange Point� Decades. � Avoids earth-moon environment "geocorona". � Extremely long exposures.

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Hubble 2.4m Space Telescope

2 arcmin

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Feb 17, 2006 Astro 100 Lecture 14 13

Curtis 0.9m Schmidt

1 degree

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Seeing

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Feb 17, 2006 Astro 100 Lecture 14 15

Detector Comparison

Efficiency Wavelengths� Human eye <1% 400 - 700 nm� Film <5% 350 - 800 nm� Phototubes <20% <100 - 2500 nm� CCD <90% <100 - 1100 nm

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Light Pollution

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Mauna Kea

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Pine Bluff Observatory

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Atmospheric "Windows"

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Arecibo 1000'

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Very Large Array

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Airborne Infrared Observatories

KuiperObservatory SOFIA

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Scientific Balloons

Ascending140,000 ft

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Rocket Launch

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Feb 17, 2006 Astro 100 Lecture 14 25

Spitzer InfraRed Telescope