04 Observational Techniques
1. Recording Light
A light collector whose main function is to capture as many photons as possible from a given region of the sky and concantrate them into a focused beam for analysis.
point ➤ collect ➤ focus
Optical Telescope: It is the one designed to collect wavelengths of radiation that are visible to human eye.
Mirror size "increases" ➤ amount of collected light "increases"
- Refracting: a lens focuses the light
- Reflecting: a mirror focuses the light
- Measures "light". It measures the total amount of light received in all or part of the image.
- Measures "spectrum". It distributes the total energy into its wavelengths creating a spectrum.
Refraction vs Reflection
- Large lenses cannot be constructed
- the lens in a refracting telescope focuses Red and Blue light differently. This deficiency is known as chromatic aberration.
- You can correct but cannot eliminate
- As light passes through the lens some of it is absorbed by the glass. This is important for IR and UV observations.
- No such effect occurs for mirrors
- Lenses are heavy
- Both sides of lenses have to be processed but for mirrors only one side has to be managed.
Effect of Refraction. Red/Blue light focuses at different points after it is refracted. This is called chormatic aberration.
Types of Telescope Designs
- Prime Focus
- Single reflection from the primary mirror. Difficult to orient and observe
- Double reflection. The light beam is directed outside of the tube, just before it is focused using a secondary mirror.
- Double reflection. The same as Newtonian however the light beam is reflected back to the primary which passes through the hole at the center of the primary.
- Nasmyth / Coude.
- Triple reflection. The same as Cassegrain however the light beam is deflected outside the tube before it reaches to the primary.
- A coma (bright central object having a tailed structure) appears as we move away from the center of the field of view. Its size increases further away from the center.
- To correct the coma a correcting lens introduced right before the beam enters to the tube: Schmidt Telescopes.
- Their constructions are expensive (mirror + lens). However, produced image quality is high. They are mostly used in surveys and astro-photography.
Mirror size "increases" ➤ amount of collected light "increases"
Observed brightness ∝ Area of the mirror ➤ D x D
- Faster Collection: Mirror is said to be collecting light fast if its mirror size is large
Size Comparison. (b) Taken with a mirror size twice as (a)
3. Power of Telescopes
- The ability of any optical device to form distinct, separate images of objects lying close together in field of view.
➤ Better distinguishing the objects
- Angular Resolution: It is the factor that determines our ability to see the fine structure.
Objects "close together"
➤ Objects are separated by "a small angle"
- is proportional with wavelength and
- is inversely proportional with to mirror size
Effect of Improving Resolution. Resolutions of 10', 1', 5", 1" from (a) to (d), respectively.
Resolving Power. From (a) to (c) resolving power of the telescope increases.
Limit of Resolution
- Diffraction (bending of light around corners) causes parallel beam of light to spread out slightly
➤ beam is not focussed to a sharp point
➤ a fuzziness created
Degree of Fuzziness ➤ Determines the angular resolution of telescope
Amount of Diffraction ∝ Wavelength of Radiation / Diameter
- As wavelength increases (ie. observing the objects in red part of the spectrum)
➤ diffraction increases
➤ angular resolution (as a value) increases
➤ therefore resolving the objects gets worse
4. Very Large Telescopes
- Mirrors are made from quartz blocks
- One needs years of engineering to construct just the mirror of the telescope.
- The largest single mirror telescope is BTA-6 (6 meters) in Zelenchukskaya, Caucasus build in 1976. Next largest is Hale Telescope (5 meters) in Palomar Observatory, California constructed in 1948!
How to increase the diameter of a telescope today?
- Use smaller sized mirrors as segments
- Combine them in an hexagonal (like a honeycomb) construction and align them to focus like a single mirror.
- This type of telescopes are called segmented mirror telescopes.
- Using this type of construction mirror size can be increased tens of meters. Examples:
- twin Keck telescopes (Mauna Kea, Hawaii - 36 x 1.8 m = 10 m) - constructed in 1992/1996
- GTC (Canary Island, Spain - 36 segments = 10 m) - constructed in 2009
5. High Resolution Observations
In theory you can reach 0.02" with a 5 meter telescope. But in reality you cannot do better than 1".
- Reason: Earth's turbulent atmosphere which blurs the image even before the light reaches our instrument.
- Blurring: The light from the star is refracted slightly in the atmosphere.
➤ the stellar image dances around on the detector (or on our retina)
➤ creating twinkling of stars.
- on a good night
- at the best observing site
- the best angular resolution is slightly < 1"
This creates what is called seeing:
- 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.
- So, to achieve the best possible seeing, telescopes are sited
- on mountain tops,
- in regions of the world where atmosphere is known to be fairly stable and
- relatively free of dust,
- 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"
Control mirrors based on temperature and orientation
Track atmospheric changes with laser; adjust mirrors in real time.
Adaptive Optics in Action (a) The improvement in image quality produced by such systems can be seen in these images acquired by the 8-m Gemini telescope atop Mauna Kea in Hawaii. The uncorrected visible-light image (left) of the star cluster NGC 6934 is resolved to a little less than 1”. With adaptive optics applied (right), the resolution in the infrared is improved by nearly a factor of 10, allowing more stars to be seen more clearly. (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.
6. Radio Astronomy
Radio Telescopes collects photons at radio frequencies:
- Jansky discovered (in 1931) a faint static "hiss" that had no apparent terrestrial source.
- It is then identified as a space source which is now known that it was the Galactic Center.
- 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.
- However, since radio wavelengths are longer than visible they are less sensitive to imperfections and therefore they can be made very large:
Resolution ~ Wavelength / Diameter
- 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.
Longer wavelength means poor angular resolution. However, radio astronomy has different advantages too:
- Observations can be carried ou for 24 hours.
- Cloud, rain and snow don't interfere the observation
- It creates another world at an invisible wavelength to human vision.
- Definition: 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.