Astronomy A BEGINNER S GUIDE TO THE UNIVERSE EIGHTH EDITION

Transcription

1 Astronomy A BEGINNER S GUIDE TO THE UNIVERSE EIGHTH EDITION CHAPTER 3 Telescopes Lecture Presentation

2 3.0 Imaging the universe Our original observations of the universe depended on our eyes! What other way(s) do you now use to obtain and store images? While your eyes are indispensible, what are the advantages of modern optical instruments?

3 3.1 Optical Telescopes In astronomy we call the light collecting systems telescopes! But just like our eyes, the goal of telescopes is to gather light and bring it to a focus. Images can be formed through reflection or refraction. Reflecting mirror is shown here:

4 3.1 Optical Telescopes Refracting lens is shown here:

5 3.1 Optical Telescopes Light from slightly different directions is focused to slightly different positions. So where would you put the CCD (or film) to record the image of the comet?

6 3.1 Optical Telescopes Typical amateur reflecting and refracting telescopes

7 3.1 Optical Telescopes Modern research telescopes are all reflectors because refractor (telescopes) have some nasty issues including: Light traveling through lens is refracted differently depending on wavelength Some light traveling through lens is absorbed Large lens can be very heavy, and can only be supported at edge A lens needs two optically acceptable surfaces; mirror needs only one But modern binoculars and cameras and Galileo s telescope are/were refractors!

8 3.1 Optical Telescopes For a variety of reasons there are many versions of reflecting telescopes: in all cases the telescope primary (the element that collects the light) is a reflecting mirror:

9 3.1 Optical Telescopes Details of the Keck telescope: a modern research telescope. Do you see the person kneeling in c)?

10 3.1 Optical Telescopes Image acquisition: Charge-coupled devices (CCDs) are electronic devices that can be quickly read out and reset. Q. How many mega-pixels are in your cell phone camera?

11 3.2 Telescope Size Light-gathering power: Improves our ability to see the faintest parts of this galaxy Brightness is proportional to square of radius of mirror so bigger is very important. In the figure, part (b) was taken with a telescope twice the size of (a). So which image (a) or (b) shows the most detail?

12 3.2 Telescope Size Resolving power: this measures how well a telescope can separate two objects that are in almost the same direction in the sky. This is also related to how sharp an image looks. The figure shows the same two sources viewed by 3 telescopes: the telescope with the top image has the best resolving power.

13 3.2 Telescope Size Telescope resolution because light is a wave! Diffraction is an intrinsic property of waves (see Chapt 2), and limits telescope resolution depending on light wavelength and telescope size (mirror diameter). For the best resolving power we want the telescope angular resolution to be as small as possible.

14 3.2 Telescope Size Resolution is proportional to wavelength and inversely proportional to telescope mirror size again bigger mirrors are important! In the figure: (c) has the smallest angular resolution: the blob (in (a)) becomes two distinct sources! So we would say that the telescope for image (c) has the best resolving power.

15 3.2 Telescope Size Effect of decreasing angular resolution: (a) 10 ; (b) 1 ; (c) 5 ; (d) 1 (one arc second is written 1 and one arc minute, written 1, is 60x larger). Image (d) with the smallest angular resolution has the best resolving power (and the image looks sharpest!)

16 3.3 High-Resolution Astronomy Atmospheric blurring due to air movements is called seeing. Twinkle twinkle little star is an example of seeing!

17 3.3 High-Resolution Astronomy To reduce (or eliminate) the seeing issue: Put telescopes on mountaintops, especially in deserts. Put telescopes in space. Use adaptive optics control mirrors by bending them slightly to correct for atmospheric distortion: a) is without and b) is with adaptive optics!

18 3.3 High-Resolution Astronomy Adaptive optics: Track atmospheric changes (e.g. with laser) then adjust mirrors in real time very clever and it works!

19 3.3 High-Resolution Astronomy Adaptive optics: Originally designed by Air Force to monitor space satellites

20 3.3 High-Resolution Astronomy Adaptive optics: now used on many ground based telescopes to improve image resolution These images show two more examples of the improvements possible with adaptive optics: (left) image(s) are without adaptive optics and (right) image(s) are with adaptive optics.

21 3.4 Radio Astronomy Radio telescopes: Similar to optical reflecting telescopes Design is prime focus (for astronomers but too detailed for us) Less sensitive to imperfections (due to longer wavelengths) thus can be made very large

22 3.4 Radio Astronomy 2 nd largest radio telescope: 300m dish at Arecibo, Puerto Rico. Largest, 500m, is in Pingtang county, China.

23 3.4 Radio Astronomy Big problem: longer wavelength means poorer resolution but solved by interferometry Advantages of radio astronomy: Can observe 24 hours a day. Clouds, rain, and snow are not an issue. Observations at an entirely different frequencys get totally different information: Radio emission (R)à sees very different structure VS the visible galaxy (V) image. The

24 3.4 Radio Astronomy Interferometry: Combines information from several widely separated radio telescopes as if it came from a single dish. Resolution will be that of a dish whose diameter = largest separation between dishes.

25 3.4 Radio Astronomy Interferometry requires preserving the phase relationship between waves over the distance between individual telescopes see Chapt 2 for a short discussion on wave interference.

26 3.4 Radio Astronomy Using radio interferometry, radio telescopes can get radio images whose resolution can be close to that of optical telescopes: a) is radio (R) image b) is optical (V) image of same (probably colliding) galaxies.

27 3.5 Space-Based Astronomy What about telescopes at other wavelengths? Infrared (I) telescopes can often image where visible (V) radiation is blocked and can generally use optical telescope mirrors and lenses. The a) and c) (left) images are in the visible (V); the b) and d) (right) images are in the infrared (I). It is hard to believe these are the same photos taken at different wavelengths!

28 3.5 Space-Based Astronomies Infrared telescopes can also be in space or flown on balloons.

29 3.5 Space-Based Astronomy What about telescopes at other wavelengths? Ultraviolet (U) observing must be done in space, as the atmosphere absorbs almost all ultraviolet rays.

30 3.5 Space-Based Astronomies What about telescopes at other wavelengths? X-rays and gamma rays must also be done in space. X-rays will reflect at a very shallow angle and can therefore be focused, are shown here (for astronomers but too detailed for us)

31 3.5 Space-Based Astronomies X-ray (X) image of supernova remnant Cassiopeia A

32 3.5 Space-Based Astronomies Gamma (G) rays are the most high-energy radiation we can detect. These are even harder to image this supernova remnant would be nearly invisible without the Fermi satellite and its gamma-ray detector. (for astronomers but too detailed for us)

33 3.5 Space-Based Astronomies Much can be learned from observing the same astronomical object at many wavelengths. Here is the Milky Way seen at 5 different parts of the EM spectrum: R, I, V, X and G. Looks like 5 different objects!

34 Summary of Chapter 3 Refracting telescopes make images with a lens. Reflecting telescopes make images with a mirror. Modern research telescopes are all reflectors. CCDs are used for data collection. Data can be formed into images, analyzed spectroscopically, or used to measure intensity. Large telescopes gather much more light, allowing study of very faint sources. Large telescopes also have better resolution.

35 Summary of Chapter 3 (con t) Resolution of ground-based optical telescopes is limited by atmospheric effects. Resolution of radio or space-based telescopes is limited by diffraction. Active and adaptive optics can minimize atmospheric effects. Radio telescopes need large collection area; diffraction is limited. Interferometry can greatly improve resolution.

36 Summary of Chapter 3 (con t) Infrared and ultraviolet telescopes are similar to optical. Ultraviolet telescopes must be above the atmosphere. X-rays can be focused, but very differently from visible light. Gamma rays can be detected. This must be done from space.