TELESCOPES AND INSTRUMENTS

What are telescopes good for?

All telescopes gather EM radiation and bring it to a focus.

WHY? MOSTLY TO DETECT FAINTER THINGS

Observed BRIGHTNESS or INTENSITY declines inversely
with the square of the DISTANCE:

I = L / (4 pi d^2)

so bigger telescopes let you see to greater distances.

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RESOLVE IMAGES of EXTENDED THINGS, or
SEPARATE IMAGES of NEARBY THINGS

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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.

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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.

More sharply curved lenses produce shorter focal lengths.

Problem: DIFFERENT COLORS are DISPERSED and come to
SLIGHTLY DIFFERENT FOCI --- resulting in CHROMATIC ABERRATION

MIRRORS OF PARABOLOIDAL SHAPE CAN ALSO BRING EM RADIATION TO A FOCUS

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REFRACTORS vs. REFLECTORS

Why reflecting telescopes (those using mirrors) are superior:

  • They only need perfect surfaces, not perfect solids.
  • Therefore they are easier and cheaper to make.
  • No chromatic aberration in reflection.
  • Large lenses tend to sag, and can only be supported by their edges;
    otherwise light is blocked. Big mirrors can be supported from
    behind w/o blocking light.

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    MAIN TYPES OF REFLECTING TELESCOPES:
    depend on where the secondary focus is placed.

  • NEWTONIAN (still good for amateur work)
  • CASSEGRAIN (typical for research telescopes)
  • COUDE (when large spectrographs are needed)

    FIGURES OF ALL OF THESE WERE SHOWN IN CLASS
    AND SHOULD BE UNDERSTOOD; See Figs. 5.6 (and 5.5) in the text.

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    FOR TELESCOPES, BIGGER IS BETTER

    LARGER MIRRORS GATHER MORE LIGHT:

    A = (pi/4) D^2

    The Keck telescope with D = 10m has 2500 times the aperture of the
    human eye (around 4 mm) or an area 6.3 million times as great.

  • Thus, in one second it can gather 6.3 million times the light,
    or see an object only one/six-millionth as bright.
  • Equivalently, the same object can be seen 2500 times further away!
  • Plus, telescopes can integrate for hours, while the
    human brain produces a new image about 30 times a second, wiping out the old one.
    Therefore, big telescopes can make images of objects billions of times fainter than we can see.

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