How Astronomers
Collect Information

Disadvantages of Refraction

1. 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

2. Some of light passing through the lens is absorbed by the glass. This is important for IR and UV observations.

   • No such effect occurs for mirrors

3. Lenses are heavy

4. Both sides of lenses have to be processed but for mirrors only one side has to be managed.

Telescopes using Reflection

1. Prime Focus

   • Single reflection from the primary mirror. Difficult to orient and observe

2. Newtonian

   • Double reflection. The light beam is directed outside of the tube, just before it is focused using a secondary mirror.

3. Cassegrain

   • 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.

4. Nasmyth / Coude

   • Triple reflection. The same as Cassegrain however the light beam is deflected outside the tube before it reaches to the primary.

Faster Collection: Mirror is said to be collecting light fast if its mirror size is large

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.

  • Its pointing is very limited due to its heavy mirror; its curvature is deformed when tube is tilted too much.

• 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

• Segments are combined in an hexagonal (like a honeycomb) construction and they are aligned 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:

  • twin Keck telescopes (Mauna Kea, Hawaii - 36 x 1.8 m = 10 m)
    ➤  constructed in 1992/1996 (first modern segmented mirror telescope)

  • GTC (Canary Island, Spain - 36 segments = 10 m)
    ➤  constructed in 2009