Published 3 years ago
- Galileo made the first astronomical telescope in 1609 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.
- 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 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 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.
- 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.
- 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).
- 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 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.
- 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 much larger mirror diameter.
- 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.
- 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.