Chapter 3 Telescopes The tools of Astronomy

Transcription

1 Chapter 3 Telescopes The tools of Astronomy Very Large Array (VLA), National Radio Astronomy Observatory (NRAO), Socorro, New Mexico (Radio telescope: 27 antennas, Y configuration, 25 meters diameter each) Reading assignment: Chapter 3

2 Let s start with something basics How does your eye forms an image? Light is refracted (bend) by the lens (converging lens) and forms an image in the retina. The pupil control the amount of light accepted by the lens.

3 What is refraction? Incident ray Reflected ray surface Refracted ray Refraction is the bending of light when it passes from one substance into another (substances with different refraction index). Example: Light passing from air to glass. For the ray to bend, it must hit the surface at an angle less than 90 degrees from the surface. The lenses in your eyes uses refraction to focus light and produce an image.

4 Refraction in a prism A simple model of a converging lens Refraction and dispersion of light by a prism An approximation of a converging (convex) lens by prisms and sections of prisms Keep in mind that light of different wavelengths (colors) are refracted at different angles. This is known as dispersion

5 Focusing light with a converging lens Refraction can cause parallel light rays to converge to a focus and form an image.

6 Image Formation The focal plane is where light from different directions comes into focus and form an image. The distance between the lens and the image is the focal length of the lens The image behind a single (convex) lens is actually upside-down!

7 Focusing light, recording an image Digital cameras detect light and record images with an electronic device called Charge- Coupled Device (CCD). A camera focuses light like an eye and captures the image with a detector. The CCD detectors in digital cameras are similar to those in cameras used in modern telescopes. But there are some important differences: Detector in CCD used in astronomy are less noisy, have larger number of pixels and have a high quantum efficiency

8 What are the two basic designs of telescopes? Reflecting telescope: focuses light using mirrors. The curved (concave) mirror reflect light and forms an image Refracting telescope: focuses light using lenses. The lens bends light by refraction and forms an image The main (or primary) mirror or lens is also known as the objective of a telescope

9 eyepiece Reflecting Telescopes opening Secondary mirror Primary Mirror (Concave) A reflecting telescope has a primary mirror and secondary mirror. The secondary can be flat in a Newtonian or curved (convex) in a Cassegrain. The primary mirror is curved (concave) and can be supported from the back in several places so it can maintain the curvature. The primary mirror can be spherical or parabolic. Some spherical mirror requires a corrector plate located at the opening (Example: 8 and 14 Meade at Teaching Observatory) Reflecting telescopes can have much greater diameters in comparison with refractors. All modern telescopes used is research are reflectors.

10 Primary Mirrors in Reflecting Telescopes Twin Keck telescopes on Mauna Kea in Hawaii. Each mirror as an equivalent diameter of 10 meters. Segmented 10-meter mirror of a Keck telescope. The main mirror is composed of 36 hexagonal segments

11 Refracting Telescopes A refracting telescope uses a lens for the objective instead of a mirror Some disadvantages of refracting telescopes: The lens separate light into different colors. It focuses light at different distances along the optical axis. This effect is known as chromatic aberration. To correct for chromatic aberration it is necessary to use an objective composed two or three elements Refracting telescopes need to be very long (large focal length), with large, heavy lenses. Heavy lenses can be supported only from the edges. The weight flexes the lens and distort the shape causing aberrations (distortions) in the images. Light passing through a lens can get absorbed. Absorption can be severe at UV and IR wavelengths To manufacture a lens it is necessary to machine and polish at least two surfaces or more if the objective has 2 or 3 elements. More expensive. The largest refracting telescope is the 40 inch diameter Alvan Clark telescope at Yerkes observatory The UF Campus Observatory owns an 8 refracting telescope. It was manufactured about 90 years ago. The lens was built by the Alvan Clark company, the same company that build the 40 at Yerkes.

12 The telescope size or diameter What are the two most important properties of a telescope? 1. Light-collecting area: The objective in a telescope collect light from the objects. Telescopes with a larger mirrors or lenses (larger diameter) have a large collecting area. It will act as a light bucket. A large collecting area can gather a greater amount of light in a shorter time. A telescope with a large collecting area can observe fainter object 2. Angular resolution: A telescope must resolve small details in an objects. Telescopes that have larger mirrors or lenses are capable of resolving smaller detail or take images that show greater detail in an object. 3. Both the light-collecting area and the angular resolution are increased by a larger lens or mirror.

13 Light-Collecting Area ( Or Light-Gathering Power) A telescope s diameter tells us its light-collecting area: A = π (D/2)² A= Area D= Diameter The diameter of the telescope D (the diameter of the lens or mirror) is normally referred as the aperture of the telescope The light-gathering power is proportional to the area A of the objective The light-gathering power is proportional to the diameter square The largest telescopes currently in use have diameters in the range of 8-10 meters. The largest single mirror telescope is the 10.4 meter diameter telescope in the Canary Island (Spain). The mirror is composed of 36 small hexagonal mirror. UF is a partner in the Gran Telescopio de Canarias (GTC)

14 Question How does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope? Possible answers: a) It s 5 times greater. b) It s 10 times greater. c) It s 25 times greater.

15 Question How does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope? a) It s 5 times greater. b) It s 10 times greater. A = π (D/2)² c) It s 25 times greater. Ratio of the diameters is 10/2 = 5 The ratio of the collecting area is 5² =25

16 Angular size and angular separation Read More precisely 0-1, Angular Measure (See page 12) Angular size of an object depends on two parameters The physical size of the object The distance to the object Angular size is measured in units of angle (degrees, arcmin and arcsec) Angular size = Physical Size Distance Approximate formula (valid for small angles): Angular Size = Physical size 360 degrees 2 π x distance Angular size = Physical size x 360 degrees/ (2 π x distance) Example: Calculate the physical size of the Moon Given the: Angular size = 0.5 degrees Distance = 380,000 km

17 The resolving power of a telescope and the angular resolution The resolving power is the minimum angular separation that the telescope can distinguish

18 Angular Resolution The ultimate limit to resolution comes from interference of light waves within a telescope. Larger telescopes (larger diameter of lens or mirror) are capable of greater resolution. The angular resolution is proportional to the wavelength and inversely proportional to the diameter D of the lens or mirror Angular resolution = 0.25 ( /D) in micrometer (10^-6 m), D in meters High resolution means that the telescope can distinguish details that are separated by small angular distance Angular resolution is given in arc seconds. A small number means good resolution

19 The effect of the telescope aperture (diameter of lens or mirror) in the resolution Example: Resolving a binary star Small aperture Medium aperture Large aperture

20 Airy disc Diffraction rings Close-up of the image of a star taken by the Hubble Space Telescope Angular Resolution Most of the stars are so far away that their image should look like point source (not possible to resolve the disk of the star) The image of a star produced by a telescope is not a point source The structure of the image of a star shows a central bright spot and rings around it. The image of a star is produced by the phenomenon called diffraction of light (Light behave like a wave) Light passing through a circular aperture produce this diffraction pattern. The opening of a telescope is circular The central bright spot, called the Airy disc increases in diameter when the diameter of a telescope decreases. Larger diameter telescopes produce a small Airy disc and have better resolving power There is a limit in the angular resolution that a telescope can achieve. It depends on its diameter. This limit in the resolution is known as diffraction limit. Angular resolution = 0.25 ( /D) The angular resolution of a telescope is given in arc seconds, smaller numbers, better resolution

21 Airy discs and the resolution of a telescope Airy disc produced by a green laser beam on a circular aperture. In a telescope, the diameter of the Airy disc produced by a star increases if the diameter of the aperture decreases Intensity distribution of Airy discs produced by two sources (stars) Airy discs patterns and resolution. An example: stars in a stellar cluster

22 An example of images of the Andromeda Galaxy with different resolutions (Using telescopes of different apertures) Aperture of a telescope: the diameter of the mirror or lens. 10 arc minutes (10 ) (very small aperture, bad resolution) 1 arc minute (1 ) 5 arc seconds (5 ) Resolution of the human eye: ~0.5 arc minutes (0.5 ) 1 arc second (1 ) (Large aperture, better resolution)

23 The focal length of a converging lens f is the focal length f is the distance from the lens to the focal point, where the image forms. Valid for parallel rays (distant objects)

24 The magnification of a telescope The magnification of a telescope is how many times bigger an object looks through a telescope compared to how it looks with the naked eye. The focal length of a telescope (Ft) is the equivalent distance from the lens or mirror to the plane where the image forms. The focal length of an eyepiece (Fe) is the distance between the lens of the eyepiece and the point where the image forms The magnification of a telescope is the ratio M = Ft/Fe The magnification is a number, it has no units The magnification of a telescope can be changed by changing the eyepiece. Eyepieces of different focal length (Fe) provide different magnifications Magnifications around are a practical limit for a telescope. Under good sky conditions and with good optics, it can be as high as 600 Important: Increasing the magnification of a telescope does not increase its resolution

25 What do astronomers do with telescopes? Imaging: taking images of object or large area of the sky Spectroscopy: breaking light into spectra and analyze the spectral lines Photometry: measure the intensity of light and variation in the intensity or brightness of the light from an object. Obtaining the light curve of an object. The brightness of an object can be expressed in an scale called magnitude. Astrometry: Measure the position (distance and angle) of an object in the sky or the position respect to another object Timing: measuring how the light output from an object varies with time and keeping a precise timing of those variations

26 A CCD (Charge Coupled Device) used in astronomy Electronic cameras in astronomy use a CCD s The small elements in a CCD sensitive to light are called pixels. A CCD has millions of pixels When light strike a pixel it will develop an electric charge proportional to the intensity of light. The charge in each pixel is read by the electronics controlled by a computer and stored as an array of numbers An image of a CCD

27 How a CCD store the information and produces an image? The charge in each pixel is read and stored as an array of numbers in a computer To display the image, the array of number is send to a computer monitor or screen. The value of each number is converted into intensity and displayed in the screen. The intensity is proportional to the value read from a pixel. Numbers in a two-dimensional array resulting from reading the charge in the pixels of an image

28 Imaging At the present the astronomical detectors for taking images in astronomy are CCD (Charge Couple Device) No more use of photographic plates in professional astronomical observatories. Astronomical detectors generally record only one color of light at a time. They generate a black/white image. Several images taken with color filters must be combined to make full-color pictures. Normally the color filters are red, green and blue (RGB) The software combining the images assign the colors to each of them and produce a color image.

29 Imaging in IR, UV, X-rays and Gamma rays Some astronomical detectors can record forms of light our eyes can t see. Example: X-rays, Gamma rays, IR, UV Color is sometimes used to represent different energies of non-visible light. Some of the color images may not represent actual colors. Different wavelength (Or energies) are assigned different colors. Images at some wavelengths can be at radio, UV, X-rays or Gamma-rays An example: An image of an object in X-rays. Colors represent energies (photons at different wavelengths). E ph = h f = h c/

30 Example of color images of Mars taken by the MAVEN spacecraft after it entered orbit in September, The images were taken in UV light reflected from Hydrogen and Oxygen

31 Spectroscopy A spectrograph separates the different wavelengths of light before they hit the detector. A diffraction grating is used to separate the colors or wavelengths. A diffraction grating has a few thousand lines per mm The diffraction grating shown here in the spectrograph is a reflection diffraction grating

32 Spectroscopy Graphing the relative brightness of light at each wavelength shows the details in a spectrum.

33 Photometry and timing The example to the right is of the light curve of a star with variable brightness. The period (331 days) and the amplitude of the variations can be measured from the light curve amplitude A light curve represents a series of brightness measurements made over a period of time. Photometry uses a reference star that show no change in brightness. The brightness of the variable star is calibrated against the reference star This technique can detect variable stars and variability in astronomical objects An important application: Measuring the variation of brightness from a star allows the detection of the transit of an exoplanet in front of a star. The exoplanet will deem the light of the star by a very small amount (Planet much smaller than the star) Exoplanet: Planet in orbit around other stars

34 A summary What are the two most important properties of a telescope? Collecting area (Light Gathering Power) determines how much light a telescope can gather. Angular resolution is the minimum angular separation a telescope can distinguish. Both properties, collecting area and angular resolution improve when the diameter of the lens or primary mirror increases What are the two basic designs of telescopes? Refracting telescopes focus light with lenses. Reflecting telescopes focus light with mirrors. The vast majority of telescopes used at the research observatories are reflectors.

35 Summary What do astronomers do with telescopes and the associated equipment? Imaging Spectroscopy Photometry Astrometry Timing

36 Atmospheric Blurring How does Earth s atmosphere affect groundbased observations? The best conditions for ground-based sites to locate astronomical observatories are: Calm atmosphere (not too windy, atmosphere less turbulent) High altitude (less atmosphere to see through, less absorption) Dark sky (far from city lights, low light pollution) Dry (Low humidity, few cloudy nights)

37 Light Pollution Scattering of human-made light in the atmosphere is a growing problem for astronomy. Light pollution on the UF campus is bad. Lots of light fixtures pointing up, where there is no need of light

38 Effect of atmospheric blurring in the formation of an image The light coming from a star comes from a point source, the rays will add (constructive interference) or subtract (destructive interference), causing the image to twinkle The light coming from a planet comes from many points in the disk. Planets normally do not twinkle Rays of light are distorted by the turbulence in the terrestrial atmosphere. The rays are deflected and travel using slightly different path. The rays interfere, adding or subtracting which causes the image to twinkle And individual image forms, each lasting for a fraction of a second but its position changes continuously In a long exposure, the individual images form a disk. If the atmosphere is turbulent, the diameter of the disk is large ( A condition called bad seeing)

39 Twinkling and Turbulence Bright star field viewed with ground-based telescope Same star field viewed with Hubble Space Telescope The turbulent air flow in the Earth s atmosphere distorts our view, causing stars to appear to twinkle. In a long exposure, the image of a star looks bigger due to the random motion of the instantaneous location of the image around a central location

40 An example of bad seeing The turbulence in terrestrial atmosphere distort the image of these lunar craters. The large crater is Clavius

41 Adaptive Optics A technique to reduce the effect of the atmosphere A technique to reduce the effect of the atmosphere is called Adaptive optics. It consist in rapidly changing the shape of a telescope s mirror to compensate for some of the effects of turbulence. The mirrors in modern telescopes are thin, its shape can be changed. Without adaptive optics With adaptive optics

42 Location of observatories: in a calm, high, dark and dry place Summit of Mauna Kea, Hawaii Altitude 14,000 feet (4,000 m) Some of the telescopes: Gemini North: 8.1 m, Keck twins: 10 m, Subaru: 8.3 m The best observing sites are atop remote mountains. Some of the best places are: Hawaii (Mauna Kea) North of Chile (Cerro Tololo, Cerro Pachon, Cerro Paranal, Cerro La Silla, Cerro Armazones) The Canary Island South-West US, Arizona, Texas, New Mexico (Kitt Peak observatory) South Africa

43 Why do we put telescopes into space? Telescopes in space are not affected by the blurring effect, absorption of light (Specially at IR and UV) and the light pollution of the terrestrial atmosphere The Hubble Space Telescope In orbit since Still in operations 2.4 m diameter mirror (solid) It can observe in near UV, visual and IR Cost about 2.5 billions US $ Next space telescope: James Webb Telescope 6.5 m diameter, 18 hexagonal mirrors Projected launch date : March 2021 It will have instruments mainly for the IR Estimated cost: 9 billion US$ (maximum authorized by Congress)

44 Hubble Ultra-Deep field Image of a field taken by Hubble Telescope in the constellation Fornax. The size of the field is about 1/10 the size of the full Moon A good example of the high resolution and long exposure that can be achieved in space. Very faint galaxies can be imaged (no light pollution!). Total exposure time around 11.3 days

45 The TMT (Thirty Meter Telescope) telescope The primary mirror is composed of 492 segments with an equivalent diameter of 30 meter It will be located at the Mauna Kea observatory (Hawaii) Projected date for beginning of operations is Cost: around 1.2 billion US dollars A comparison of the size of the mirrors in the 30 m TMT, the 10 m Keck and the 5 m (200 inch) Hale (Mount Palomar) telescopes

46 The ELT 39 meter diameter European telescope in Cerro Armazones (Chile) Comparison the size of the mirrors of the largest existing and projected telescopes

47 Transmission of light in Atmosphere Only the longer wavelengths of radio and the visible light pass easily through Earth s atmosphere. Telescopes high in the atmosphere are necessary to observe in the IR part of the spectrum To extend the observations into the UV, X-Ray and Gamma ray it is necessary to have telescopes mounted in balloons, rockets and even better in space

48 Summary How does Earth s atmosphere affect ground-based observations? Telescope sites are chosen to minimize the problems of absorption of visible light at shorter and longer wavelengths, minimize effects of light pollution, atmospheric turbulence, and bad weather. Why do we put telescopes into space? Some wavelength of light other than radio and visible do not pass through Earth s atmosphere. Much sharper images are possible because there is no atmospheric turbulence. No light pollution from light scattered in the terrestrial atmosphere allows long exposure for recording very faint objects

49 6.4 Telescopes and Technology Let s explore the following topics: How can we observe invisible light (radio, IR, UV, X-rays and Gamma rays)? How can multiple telescopes work together and what is the advantage of multiple telescopes working together?

50 How can we observe invisible light? A standard satellite dish is essentially a telescope for observing radio waves (It is called a radio telescope) The dish capture signals and focus them into the focal point where a preamplifier amplifies the signal. Coaxial cables carries the signal to the receiver

51 Radio Telescopes A radio telescope is like a giant mirror that reflects radio waves to a focus. The image shows the 300 meter (1000 ft) diameter radio telescope in Arecibo (Puerto Rico) Why it is necessary to use a large dish? The wavelengths in the radio part of the spectrum are long (m, cm, mm). Long wavelengths low resolution (large angular resolution value bad resolution) To increase the resolution it is necessary to increase the diameter of the dish (D) Angular resolution = 0.25 ( /D)

52 ALMA (Atacama Large Millimeter Array) The ALMA radio telescope (Interferometer) is located in the Atacama desert (North of Chile) at an altitude of 5,000 meter ( feet). Operated by NRAO with support from NSF and ESO. Collaboration and support from organizations in Japan, Canada, Taiwan, Korea and Chile. It consist of 66 antennas. Each antenna 7 to 12 m diameters. They are used as an interferometer to achieve a resolution about 5 times better than the Hubble telescope. It work in the wavelength range from 0.3 to 9.6 mm NRAO: National Radio Astronomical Observatory NSF: National Science Foundation ESO: European Southern Observatory Cost: about 1.5 billion US dollars.

53 ALMA A view of some of the antennas. In the background, the Magellanic Clouds, the Southern Cross and the Milky Way

54 Infrared and Ultraviolet Telescopes SOFIA (IR) Spitzer (IR) Infrared and ultraviolet light telescopes operate like visible-light telescopes but need to be high or above the atmosphere to see at these wavelengths. The SOFIA telescope has a 2.7 mirror mounted in a modified Boeing 747 airplane. Two UF Ph.D. graduates, Drs. Jim De Buizer and James Radomski are scientists with SOFIA observatory. UV telescopes were carried in Space Shuttle Missions ASTRO 1 and ASTRO 2. The crew member in charge of the telescope on those missions was the astronaut and a UF Ph.D. graduate Dr. Ron Parise (deceased)

55 X-Ray Telescopes X-ray telescopes also need to be above the atmosphere. X-rays are absorbed by the terrestrial atmosphere Chandra X-Ray Observatory (launched in 1999) Named after the Indian-American astrophysicist Subramanyan Chandrasekhar (1983 Nobel prize in physics)

56 X-Ray Telescopes Focusing of X-rays requires special mirrors. Mirrors are arranged to focus X-ray photons through grazing bounces off the surface.

57 Gamma-Ray Telescopes Fermi Gamma-Ray Observatory Launched in 2008 Named after Enrico Fermi, physicist at Chicago University Gamma-ray telescopes also need to be in space. Gamma radiation doesn t reach the ground. Focusing gamma rays is extremely difficult. For the Fermi telescope, the detectors are scintillation detector, an array of 12 crystals (Sodium Iodide) The detectors are arranged in a 3-D matrix. That allow to determine the direction from where the gamma ray came from

58 The Milky way at different wavelengths

59 How can multiple telescopes work together?

60 Interferometry Interferometry is a technique that consist in linking two or more telescopes and combining the signal from them. Important: The light waves coming from each telescopes must be combined in phase. What is the advantage? The resolution obtained is equivalent to the resolution of a single large radio telescope of an equivalent diameter equal to the separation between individual telescopes. A couple of examples: The ALMA radio telescopes The VLA array in Socorro, New Mexico

61 Interferometry Easiest to do with radio telescopes because of the longer wavelengths More difficult in the visible part of the spectrum, the wavelengths are around 400 t0 600 nm. Now it is possible with infrared and visible-light telescopes In radio telescopes operating at long wavelengths the signals are transmitted by coaxial cables or waveguides. In optical, IR and short wavelength radio telescopes, the signals is transmitted by optic fiber Very Large Array (VLA) in Socorro, New Mexico Part of the National Radio Astronomical Observatory (NRAO)

62 Interferometry in visible and IR light wavelengths An example: The ESO (European Southern Observatory) VLT telescopes in Cerro Paranal (Chile) The VLT (Very Large Telescopes) are 4 telescopes that can combine the light to make up a large telescope with an equivalent diameter equal to the separation between them. The primary mirror of each telescope has a diameter of 8.2 meters

63 Future of Astronomy in Space? The Moon would be an ideal observing site. The lack of atmosphere make it ideal for observing at almost all wavelengths (radio, UV,IR, X-Ray and Gamma-Rays) Radio telescopes located in the far side of the Moon will be shielded from radio interference

64 Summary How can we observe invisible light? Telescopes for invisible light are usually modified versions of reflecting telescopes. Telescopes used for observing invisible light such as X- ray, Gamma rays, UV and IR must be in space. How can multiple telescopes work together and what is the advantage? Linking multiple telescopes using the interferometer technique enables to increase the resolution and produce the angular resolution of a much larger telescope.