# Chapter 5: Telescopes

## Telescopes

• Galileo made the first astronomical telescope in 1609 ﻿$\rightarrow$﻿ revolutionary impact back then.
• Telescopes are still being improved today.
• 10 largest telescopes have all been built since 1993.
• Much larger telescopes are being built today.
• We can also send telescopes into space and to objects in the solar system.

### Seeing Light

• We see by the light that enters the pupil of our eye.
• Pupil is a circle with an area = πR2pupil.
• Our pupil’s maximum diameter is about 8 mm when it expands - “adapts” to total darkness.
• Pupil’s maximum area is about 50 mm2.

### The Light “Gain” of a telescope

• A telescope gives us a pupil bigger than our eye.
• Today’s largest telescope’s diameter = 10.4 m = 10,400 mm (same unit as our pupil).

### Resolving Power

• Resolving power means seeing finer details, NOT just larger images = magnification.
• Chapter 2 we learned that we observe angles.
• Resolving power means that we can separate features just a tiny angle apart - “angular resolution”.
• Angle depends on (the wavelength)/(diameter of pupil or telescope).
• Bigger diameter ﻿$\rightarrow$﻿ smaller angle - better resolution.

### Units for Angles

• We normally measure angles in degrees with 360o = full circle - one degree is a very large angle for astronomy.
• Therefore, we create smaller angular units.
• Minutes of arc = 1’ = 1o/60.
• Seconds of arc = 1’ = 1’/60 = 1o/3600.
• Measuring:
• Angles in seconds of arc.
• Wavelength in nm.
• Telescope diameter in centimeters.
• Angle of Resolution (“) = 0.025*(𝛌(nm)/D(cm))
• NOTE: Using a shorter wavelength and/or a larger diameter gives a smaller angle - better resolution.

### Resolving Power

• Using the pupil of our eye we can resolve angles ≥ 1’.
• Using small telescopes (Dtel >> Dpupil) we can resolve angles ≥ 1” = 60x better.
• This is how Galileo made his important discoveries beginning in 1609.
• Using today’s telescopes (D is about 10 m).

### Atmospheric Blurring

• Light passing through Earth’s atmosphere is constantly shifter by movement of the air.
• This causes the stars to twinkle.
• Through the telescope the images are blurred preventing us from seeing finer detail ﻿$\rightarrow$﻿ increased brightness is the major gain of telescopes on Earth.

### Types of Telescopes

• Lens telescope - Galileo’s telescope.
• Light transmitter from air to glass/water/plastic changes direction - “refracts”.
• Shaping the glass into a lens brings the light to a focus at a point - refracting telescope.

### Refracting Telescopes

• Reflecting telescopes have several problems:
• A lens with a diameter more than 1 m is so heavy it sags and cracks under its weight.
• Thick glass absorbs light.
• A lens focuses light of different colours at different locations.
• Therefore, astronomers developed another telescope:
• Mirror telescope - reflecting telescope.
• Light reflects from a shiny surface - mirror.
• Shaping the mirror into a curve reflects light to focus.
• NOTE: The shiny surface is on the front of the mirror so the light does not go through any glass.
• Because the light does not go through the mirror, it can be supported on the back as well as the sides ﻿$\rightarrow$﻿ much larger mirror diameter.

### Other Wavelengths

• In chapter 4 we learned that astronomers are observing all forms of light from radio waves to gamma-rays.
• Reflecting telescopes are used for all forms of radiation.

### Location of Observatories

• Putting telescopes in space is VERY expensive.
• For telescopes on Earth to be as productive as possible, they must be at the very best locations.
• Far from cities to avoid “light pollution”.
• Light pollution includes city lights.
• Very dry locations with few clouds.
• High mountains to be above as much atmospheric blurring as possible.

### Today’s Telescopes

• Largest possible single mirror has a diameter of 8.4 m.
• Largest diameter are too heavy to support.
• To construct larger mirrors:
• Use many smaller (about 1m) mirrors to assemble a large mirror - like a puzzle or mosaic - a “segmented mirror” because you have a lot of segments.
• Use 2 or more large mirrors to combine.
• Recall: Resolution angle depends on the wavelength divided by the diameter of the telescope.
• Problem: radio waves are very long, need an impossibly HUGE diameter to resolve detail.
• Solution: combine separate telescopes to observe one object - interferometer.
• Combined together their separation - “diameter” of a single telescope.
• So, resolution angle depends on wavelength divided by separation of the telescope.