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Limits of Observations
Part A (Worlds & Light)
➤ Week-2: Observational Tools & The Architecture of the Solar System
➤ Topic-5: Limits of Observations & Radio Astronomy
1. Observation Limits
1. Observation Limits
In theory you can reach 0.02" with a 5 meter telescope.
In reality you cannot do better than 1":
Earth's turbulent atmosphere which blurs the image even before the light reaches our instrument.
Blurring: The light from the objects are refracted slightly in the atmosphere.
➤ the image dances around on the detector (or on our retina)
➤ creating twinkling of stars.
So;
on a good night
at the best observing site
the best angular resolution is slightly < 1"
If you take the photo of this twinkling (see the Figure):
The disk where the star's light is spread over is called seeing disk.
Q1
If a telescope theoretically has extremely high resolution, why do ground-based observations often still appear blurred?
How to chose observing sites with smaller seeing?
go to mountain tops;
in there the atmosphere is known to be fairly stable;
and relatively free of dust;
and free of moisture;
and away from the light pollution from cities.
Current best resolutions
Hubble Space Telescope - 2.4 meter 0.05"
NTT (Chile) - 3.5 meter 0.5" (active optics)
Keck Telescopes - 10.0 meter 0.25"
Q2
Why are major observatories usually built on high mountains or remote deserts?
2. Modern Solutions
2. Modern Solutions
Active Optics
Control mirrors based on temperature and orientation
Adaptive Optics
Track atmospheric changes with laser; adjust mirrors in real time.
Adaptive Optics in Action (b) These visible-light images were acquired at a military observatory atop Mount Haleakala in Maui, Hawaii. The uncorrected image (left) of the double star Castor is a blur spread over several arc seconds, giving only a hint of its binary nature.
With adaptive compensation applied (right), the resolution is improved to a mere 0.1”, and the two stars are clearly separated.
Q3
What problem does adaptive optics solve, and why is it necessary even with large telescopes?
3. Radio Astronomy
3. Radio Astronomy
Jansky discovered (in 1931) a faint static "hiss" (noise) that had no apparent terrestrial source. It is then identified as a space source which is now known that it was the Galactic Center.
Radio Telescopes collects photons at radio frequencies:
Their working principles are the same as reflecting telescopes.
Radio telescopes use prime focus.
However to change the frequency you have to re-tune the instrument.Since radio wavelengths are longer than visible they are less sensitive to imperfections and therefore they can be made very large
Resolution
The best angular resolution is around 10".
Note that the radio radiation arriving at Earth is < E-12 W.
Arecibo Observatory. An aerial photograph of the 300-m-diameter dish at the National Astronomy and Ionospheric Center near Arecibo, Puerto Rico.
The receivers that detect the focused radiation are suspended nearly 150 m above the center of the dish.
Robert C. Byrd Green Bank Telescope. The 105-m-diameter device at the National Radio Astronomy Observatory in Green Bank, West Virginia, is 150 m tall
Longer wavelength means poor angular resolution. However, radio astronomy has different advantages too:
Observations can be carried out for 24 hours.
Cloud, rain and snow don't interfere the observation
It creates another world at an invisible wavelength to human vision.
Orion Nebula in Radio and Visible. The Orion Nebula is a star-forming region about 1500 light-years from Earth. The bright regions in the picture are stars and clouds of glowing gas. The dark regions are not empty, but their visible emission is obscured by interstellar matter.
Superimposed on the optical image is a radio contour map (blue lines) of the same region. Each curve of the contour map represents a different intensity of radio emission. The resolution of the optical image is about 1”; that of the radio map is 1’.
Q4
If radio telescopes often have poorer resolution than optical telescopes, why is radio astronomy still extremely valuable?
4. Interferometry
4. Interferometry
Combine information from several widely spread radio telescopes as if they came from a single dish (like a segmented optical telescope)
Resolution will be that of dish whose diameter = largest separation between dishes.
Interferometry involves combining signals from two receivers; the amount of interference depends on the direction of the signal.
Interferometry can get radio images whose resolution is close to optical (similar to using adaptive optics)
Interferometry can also be done with visible light but is much more difficult due to shorter wavelengths.
VLA Interferometer. It is located on the Plain of San Augustin in New Mexico, comprises 27 separate dishes spread along a Y-shaped pattern about 30 km across. The most sensitive radio device in the world.
Interferometry Two detectors (left and right) record different signals from the same incoming wave because of the time it takes the radiation to traverse the distance between them.
When the signals are combined, the amount of interference depends on the wave’s direction of motion, providing a means of measuring the position of the source in the sky.
Q5
Why can combining several small telescopes through interferometry produce the resolution of a much larger telescope?
5. Multi-wavelength Astronomy
5. Multi-wavelength Astronomy
Band Earth's Atmosphere Collected at Notes
Radio Transparent (largely) Ground based (1) Can penerate dusty ISM; (2) Can be recorded in day time; (3) Need very large telescopes
Infrared Transparent (partially) Ground + Space based (1) Can penerate dusty ISM.
Visible Transparent Ground + Space based -
Ultraviolet Opaque Space based -
X-ray Opaque Space based (1) Special mirrors are needed
Gamma Opaque Space based (1) Cannot form images
The Milky Way Galaxy as it appears at (a) radio, (b) infrared, (c) visible, (d) X-ray, and (e) gamma-ray wavelengths. Each frame is a panoramic view covering the entire sky. The center of our Galaxy, which lies in the direction of the constellation Sagittarius, is at the center of each map.
Q6
Why do astronomers observe the universe at many different wavelengths, not only visible light?
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