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 concentrate 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"

Telescope Types

Instrument Types

2. Telescopes

Refraction vs Reflection

Effect of Refraction. Red/Blue light focuses at different points after it is refracted. This is called chormatic aberration.

Types of Telescope Designs

Special Telescopes

Telescope Size

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

Resolving Power

Finer Resolution

➤ Better distinguishing the objects

Objects "close together"

➤ Objects are separated by "a small angle"

Effect of Improving Resolution. Resolutions of 10 arcmin, 1 arcmin, 5 arcsec, 1 arcsec from (a) to (d), respectively.

Resolving Power. From (a) to (c) resolving power of the telescope increases.

Limit of Resolution

➤ beam is not focused to a sharp point

➤ a fuzziness created

Degree of Fuzziness   ➤   Determines the angular resolution of telescope


➤ diffraction increases

➤ angular resolution (as a value) increases

therefore resolving the objects gets worse

4. Very Large Telescopes

How to increase the diameter of a telescope today?

© 2023 ATASAM - DAG (Doğu Anadolu Gözlemevi - Eastern Anatolia Observatory). 4 m Telescope with 2 Nasmyth Platforms.

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".

➤ the stellar image dances around on the detector (or on our retina)

➤ creating twinkling of stars.


This creates what is called seeing:

Current best resolutions

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 (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:

Resolution ~ Wavelength / Diameter

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:

7. Interferometry

8. Multi-wavelength Astronomy

9. Space-based Astronomy