3. Telescopes

Reflectors vs. Refractors

  • Large scientific telescopes are virtually always reflector telescopes

  • Lenses create Chromatic Aberration - light bends and focuses at the edges of the lenses (like a prism due to different wavelengths)

  • Glass can absorb some wavelengths of light while light passes through the lens

  • Lenses are very heavy

  • Both sides of the lenses have to be polished to a high precision

Chromatic Aberration

Lenses bend different wavelengths of light at different angles, similar to a prism, and focus light around the edges of the image.

Seeing and Diffraction Limits

Seeing

  • How much stuff gets smeared by the atmosphere

  • Telescopes take one image in many minutes

    • that much time is needed to collect enough light to see anything

  • imagine adding a bunch of turbulent images like this

  • it ends up just being a smear

Seeing is measured in arc seconds

  • 360 degrees = 1 circle

  • 60 arc minutes = 1 degree

  • 60 arc seconds = 1 arc minute

  • 1 arc second is 1 part in 1,296,000 of a circle (360×60×60)

Really good seeing = 0.3 arc seconds (only at Mauna Kea)

Really bad seeing = 3 arc second (here on a clear night)

The smaller the quantity of arc seconds the better the seeing

“Seeing” - how crisp or focused a space image is

Ranked worst to best image quality

  1. Sea-level in a humid environment

  2. Sea-level in an arid/desert environment

  3. On top of a high, dry mountain

  4. Empty (outer) space

Angular Diameters

  • The moon is 31 arc minutes across

  • Jupiter has an angular diameter of 48 arc seconds at its closest

Finding Jupiter’s Angular Diameter

Finding Alpha Centauri A’s Angular Diameter

Can we resolve Alpha Centauri A? Meaning, can we see it as a disk instead of a point?

No, not with our eyes (1 arc minute resolution)

Not with the most powerful ground-based telescopes either (0.3 arc second resolution)

Diffraction Limited Telescopes

  • Put a telescope in space, seeing is zero, right? Resolve everything

  • Now there is a problem with quantum mechanics

  • Thanks to QM, we cannot get perfect seeing, even in space

    • light is a wave, so it obeys wave mechanics, which smears even photons

Hubble vs. JWST Diffraction

Seeing Continents?

How large a space telescope do you need to see a continent (1000 km across) at 20 light-years distance?

1 LY = 9.46 × 10^12 km

Visible light (blue) = 4×10^-7 m

Adaptive Optics

  • Fire a laser into the upper atmosphere

  • observe how the laser beam is deformed by the turbulence in the atmosphere

  • Calculate exactly the reverse of the deformation from the atmosphere

  • bend and flex a mirror to exactly counteract the turbulence

  • repeat 20x a second