BIGGER IS BETTER WHEN IT COMES TO TELESCOPES

LARGER MIRRORS RESOLVE BETTER

theta = 0.25 (lambda/D)

Here theta, the angle between the closest objects that can be seen
separately is in arcsec, lambda is in micrometers (um) and D is in meters.

Example: a 3.5m telescope working in yellow light (500 nm = 0.5 um) has a
resolution angle, theta = 0.25x(0.5/3.5) = 0.036 arcsec

Resolution is limited by SEEING --- the spreading of an image
via turbulence in the atmosphere, which typically changes over < 0.1 s,
and smears images out to > 0.5 arcsec.
This implies little real improvement in resolution if D > 0.25 m.

This is also why STARS TWINKLE and PLANETS DON'T (usually).

This limit on resolution can be gotten around by:

Going into Space: Hubble Space Telescope ( theta = 0.08 arcsec );
Next Generation (Webb) Space Telescope ( theta = 0.02 arcsec )

Speckle interferometry: take very short exposures -- this works
only with very bright stars (theta = 0.002 arcsec ). (Also see regular interferometry, below.)

Adaptive optics: measure blurring of a bright star and very quickly
coarsely adjust mirror shape to reduce it; then nearby images will also
be crisper (theta = 0.1 arcsec can be achieved).

Active optics: use a bright natural star OR a laser guide star
to measure seeing and rapidly correct details of the mirror shape (theta < 0.1 arcsec can be achieved).

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KEY INSTRUMENTS (optical, IR, UV)

Typical research telescopes have several instruments which
are attached to the secondary focus (Cassegrain and/or Coude).

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INTENSITY (BRIGHTNESS)

Phototube: linear response (current proportional to light intensity),
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 use millions of pixels.
  • In each one the collected charge is proportional to the number of photons falling on it.

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    IMAGES

    Photographic plates -- use multiple filters 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.

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    SPECTROMETERS

    Most telescopes spend most of their time spreading the light out into all
    frequencies: SPECTROSCOPY gives FAR MORE DETAILED INFORMATION
    than does IMAGING.

  • Temperatures
  • Composition and abundances
  • Pressures
  • Velocities (Doppler shift)
  • Rotation
  • Magnetic fields

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    POLARIMETERS

    Special materials can rotate different linear polarizations by
    different amounts and allow weak polarizations to be detected.

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    RADIO TELESCOPES

    INTERFEROMETRY OVERCOMES POOR RESOLUTION

    Single dish radio telescopes can't resolve better than
    approx 20 arcsec --- or North Georgia as seen from the Moon.

  • On the other hand, radio telescopes work night and DAY and can penetrate clouds.
  • Only thunderstorms or very heavy precipitation or very strong winds stop them.

    Combining and interfering signals can produce much better resolution;
    the EFFECTIVE APERTURE becomes the MAXIMUM SEPARATION
    (BASELINE) between the telescopes.

    Examples:

  • Very Large Array: 27 telescopes, up to 27 km baselines, so
    theta = 0.1 arcsec (at 1 cm): GSU from the Moon.
  • Very Long Baseline Array: 10 telescopes, 6000 km baseline so
    theta = 0.0004 arcsec (at 1 cm) --- you from the Moon.
  • VLBI from Space: HALCA -- up to 21,000 km baselines so
    theta = 0.0001 arcsec (at 1 cm) --- your head from the Moon.

    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.

    THE CHARA ARRAY is the largest optical interferometer:

  • a GSU project, headed by Prof. McAlister, the longest baseline is 350 m
  • light from 6 one-meter aperture telescopes can be combined
    to give resolutions of about 0.0004 arc sec (like VLBA)
    Can measure the sizes of stars, the separations between nearby stars
    and make images of nearby giant stars.
  • First results showed that Regulus, a nearby luminous star, is
    highly distorted in shape because of fast rotation and
    that it is hotter near its poles than near its equator.

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    X-RAY and gamma-RAY TELESCOPES

    MUST BE PLACED IN SPACE -- essentially all these high energy photons
    are blocked by the Earth's atmosphere.

    X-RAY TELESCOPES For X-rays, only small angle reflection is possible, so complex designs are
    needed if decent angular resolution is possible. Stacked pieces of paraboloidal
    and hyperboloidal mirrors around a cylinder work well.

    Much more difficult to perform X-ray spectroscopy than in the visible,
    but it is now possible.

    Important X-ray telescopes include

  • Uhuru, Einstein, ROSAT, ASCA (previous),
  • Chandra -- arcsec spatial resolution, and
  • XMM-Newton -- good energy (spectral) resolution (current)

    These X-ray telescopes have told us a lot about the SUN, SUPERNOVAE,
    ACTIVE GALACTIC NUCLEI, RADIO JETS, and
    the INTERSTELLAR AND INTERGALACTIC MEDIUM
    but not much about the PLANETS. So more is postponed until Astr 1020.

    GAMMA-RAY TELESCOPES

    For gamma-rays, no focusing is possible -- they are too powerful.
    Collimators restrict entry into solid-state detectors, yielding positions
    only accurate to several arc minutes.

    Important gamma-ray telescopes include:
    CGRO (Compton Gamma Ray Observatory, now deceased), Beppo-SAX, HETE, Swift.

  • They are detecting incredibly powerful Gamma Ray Bursts, most from very distant dying stars.