Telescopes and the Atmosphere Notes

Telescopes and the Atmosphere

Uses a lens to focus light.
Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect).
- Can be corrected, but not eliminated, by a second lens out of different material.
Difficult and expensive to produce:
- All surfaces must be perfectly shaped.
- Glass must be flawless.
- Lens can only be supported at the edges.

They have no chromatic aberration.
Can be made bigger than refracting telescopes due to increased focal length.
A mirror can be made of a thin or honeycombed lightweight material with a thin aluminum coating.
The lens of a large refracting telescope will distort (sag) under its own weight.

Fundamentally, your eye and a telescope work in a similar manner.
- Light is bent by a lens (or mirror) to make an image.

Gather More Light – (bigger is better) making objects appear brighter.
See fine detail (called resolution).
Magnify
- magnification = (objective\ lens\ focal\ length\ /\ eyepiece\ lens\ focal\ length)

A larger objective lens provides a brighter (not bigger) image.
Light-gathering power:
- Improves detail
- Brightness proportional to the square of radius of mirror
- Collecting \ Area = π * (D/4)^2 = 0.8D^2
Since collecting area is proportional to the square of the diameter, a telescope with twice the diameter will have four times the collecting area.

Angular resolution refers to the ability to distinguish two objects that appear very close together in the sky.
In the absence of blurring effects from the atmosphere, the angular resolution of a telescope is determined by the wavelength of light and the diameter of the telescope.
A small angular resolution is good because it means you can separate objects that are very close together.
Angular resolution is determined by the formula:
- θ ≈ λ/D

Illustrates how increasing telescope size improves the resolution of an image, allowing for finer details to be observed.

What looks like a single star is actually two stars, as revealed by improved resolution.

Telescopes and detectors can be built to detect radiation from throughout the EM spectrum.
Because of the spectral resolution equation (θ ≈ λ/D), radio telescopes are often much larger than optical telescopes.
Examples:
- Very Large Array (VLA):
  - Maximum Separation 1 km (as shown), 13 km (at maximum).
- Arecibo Radio Telescope:
  - Diameter is 305 m (1000 ft).

Recall: Resolving power of a telescope depends on diameter D:
- αmin = 1.22 * λ/D
- This holds true even if not the entire surface is filled out.
Combine the signals from several smaller telescopes to simulate one big mirror → Interferometry.

The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter.
Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)!

Air refracts light just like glass or water, but to a lesser degree.
Cool air refracts light more than warm air.
Pockets of cool air in the atmosphere create moving lenses in the sky, shifting the light rays randomly.
This causes a twinkling effect, called scintillation.
A stable atmosphere causes less scintillation.
We say the seeing is good.
Seeing: the quality of observing conditions induced by turbulence in Earth's atmosphere.

Schematic of adaptive optics system:
- Feedback loop: next cycle corrects the (small) errors of the last cycle.
Deformable Mirror for real wavefronts
If there’s no close-by “real” star, create one with a laser.
- Use a laser beam to create artificial “star” at altitude of 100 km in atmosphere.
Keck Observatory
- Laser guide stars are operating at Lick, Keck, Gemini North, VLT Observatories
- Lick Observatory
Adaptive optics can help correct for this atmospheric distortion.
Earth-based image quality can compete with the Hubble Space Telescope in the visible.

Not all EM radiation can penetrate Earth’s atmosphere.
The Earth’s atmosphere absorbs most of the radiation incident on it from space.
This is a good thing for life – high energy photons would sterilize the planet!
This is not a good thing for astronomy, however!
Visible, radio, and some infrared wavelengths are not absorbed readily by the atmosphere.
- Optical and radio telescopes work well from the ground.
Gamma Rays, X-rays, and UV photons are absorbed.
- Observatories for these wavelengths must be kept above the Earth’s atmosphere!

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Cal Poly Pomona students participate in SETI research: Astrobiology and the Search for Extraterrestrial Intelligence
Research at the SETI Institute in Mountain View, California
Projects include:
- IR spectroscopy of ethane/water mixtures to study early solar system (Ashley Curry)
- Planetary geology and geomorphology of Mars (Amber Butcher)
- Mapping of meteor showers to study matter in the Solar System (Steffi Valkov)
Part of the Allen Telescope Array (ATA)
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