1. Chapter 5 Telescopes modified for Geology 360/Physics 380 Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Tools of the Trade: Telescopes Stars and other celestial objects are too far away to test directlyStars and other celestial objects are too far away to test directly –Astronomers passively collect radiation emitted from distant objects –Extremely faint objects make collection of radiation difficult Specialized Instruments RequiredSpecialized Instruments Required –Need to measure brightness, spectra, and positions with high precision –Astronomers use mirrored telescopes and observatories Modern Astronomers are rarely at the eyepiece, more often they are at a computer terminal!Modern Astronomers are rarely at the eyepiece, more often they are at a computer terminal!

3. The Powers of a Telescope Collecting Power –Bigger telescope, more light collected! Focusing Power –Use mirrors or lenses to bend the path of light rays to create images Resolving Power –Picking out the details in an image

4. Anatomy of the Human Eye Leads to the occipital cortex at the posterior (back) of the brain

5. The Macula The macula lutea is the small, yellowish central portion of the retina. It is the area providing the clearest, most distinct vision. When one looks directly at something, the light from that object forms an image on one’s macula. A healthy macula ordinarily is capable of achieving at least 20/20 1 (“normal”) vision or visual acuity, even if this is with a correction in glasses or contact lenses.retinavisual acuity 1 Vision sharper than this sharp may be due to there being more cones per square millimeter of the macula than in the average eye, enabling that eye to distinguish much greater detail than normal.

6. fovea centralis The center of the macula is called the fovea centralis, an area where all of the photoreceptors are cones; there are no rods in the fovea. The fovea is the point of sharpest, most acute visual acuity. The very center of the fovea is the “foveola.”conesrods Because the fovea has no rods, small dim objects in the dark cannot be seen if one looks directly at them. For instance, to detect faint stars in the sky, one must look just to one side of them so that their light falls on a retinal area, containing numerous rods, outside of the macular zone. Rods detect dim light, as well as movement.

7. The astronomical telescope The Refractor

8. Light Gathering Power Light collected proportional to “collector” area –Pupil for the eye –Mirror or lens for a telescope Telescope “funnels” light to our eyes for a brighter image Small changes in “collector” radius give large change in number of photons caught Telescopes described by lens or mirror diameter (inches)

9. Focusing Power Refraction –Light moving at an angle from one material to another will bend due to a process called refraction –Refraction occurs because the speed of light is different in different materials

10. Refraction

11. Refraction is also responsible for seeing –Twinkling of stars –AKA Scintillation Temperature and density differences in pockets of air shift the image of the star

12. Atmospheric Refraction Distortion of Sun near the Horizon

13. How your perception may be fooled. From Explorations An Introduction to Astronomy 3 rd ed, Thomas Arny p 123 Both circles in the sky and the bottom circle look smaller than the circle on the horizon. Indeed all the circles are the same size! 6

14. Refracting Telescopes A lens employs refraction to bend light Telescopes that employ lenses to collect and focus light are called refractors

15. Disadvantages to Refractors Lenses have many disadvantages in large telescopes! –Large lenses are extremely expensive to fabricate –A large lens will sag in the center since it can only be supported on the edges –Dispersion causes images to have colored fringes –Many lens materials absorb short- wavelength light

16. Reflecting Telescopes Reflectors –Used almost exclusively by astronomers today –Twin Keck telescopes, located on the 14,000 foot volcanic peak Mauna Kea in Hawaii, have 10-meter collector mirrors! –Light is focused in front of the mirror

17. Reflecting Telescopes A secondary mirror may be used to deflect the light to the side or through a hole in the primary mirror Multi-mirror instruments and extremely thin mirrors are two modern approaches to dealing with large pieces of glass in a telescope system

18. Styles of Refractors

19. Basic Type of Telescopes Basic Diagram of Schmidt-Cassegrain Technology 11

20. A classical Newtonian reflecting telescope. (Image by Duncan Kopernicki.) Small reflectors are often in a Newtonian configuration (shown above). They have a paraboloid primary mirror which brings the light of any object in the field of the telescope to a focus near the top end of the tube, called the prime focus. A flat mirror is placed at 45  to the axis of the tube and reflects the light out to an eyepiece at the secondary focus. 9

21. Resolving Power A telescope’s ability to discern detail is referred to as its resolving power Resolving power is limited by the wave nature of light through a phenomenon called diffraction Waves are diffracted as they pass through narrow openings A diffracted point source of light appears as a point surrounded by rings of light

22. Resolving Power and Aperture Two points of light separated by an angle  (in arcsec) can be seen at a wavelength (in nm) only if the telescope diameter D (in cm) satisfies: D > 0.02 / 

23. What causes diffraction in telescopes? From Hyperphysics web site http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html

24. Text Problem in Resolving Power

25. Problem at the End of Chapter 4, Resolving Power

26. Magnifying Power (not discussed in detail in text) This is only for your information—not in exams “The ability of a telescope to enlarge images is the best-known feature of a telescope. Though it is so well-known, the magnifying power is the least important power of a telescope because it enlarges any distortions due to the telescope and atmosphere. A small, fuzzy faint blob becomes only a big, fuzzy blob. Also, the light becomes more spread out under higher magnification so the image appears fainter! The magnifying power = (focal length of objective) / (focal length of eyepiece); both focal lengths must be in the same length units. A rough rule for the maximum magnification to use on your telescope is 20 × D to 24 × D, where the objective diameter D is measured in centimeters. So an observer with a 15-centimeter telescope should not use magnification higher than about 24 × 15 = 360-power. “ from http://www.astronomynotes.com/ Nick Strobel’s Astronomy Notes

27. Increasing Resolving Power: Interferometers –For a given wavelength, resolution is increased for a larger telescope diameter –An interferometer accomplishes this by simultaneously combining observations from two or more widely-spaced telescopes

28. Interferometers The resolution is determined by the individual telescope separations and not the individual diameters of the telescopes themselves Key to the process is the wave nature of interference and the electronic processing of the waves from the various telescopes

29. Observatories The immense telescopes and their associated equipment require observatories to facilitate their use and protection from the elements Thousands of observatories are scattered throughout the world and are on every continent including Antarctica Some observatories: –Twin 10-meter Keck telescopes are largest in U.S. –The Hobby-Eberly Telescope uses 91 1-meter mirrors set in an 11-meter disk –Largest optical telescope, VLT (Very Large Telescope) in Chile, is an array of four 8-meter mirrors

30. The Horsehead Nebula in Orion. This image, approximately 1.5° across, was obtained with the UK Schmidt telescope at the Anglo-Australian Observatory. (Image Credit: David Malin, Anglo Australian Observatory/Royal Observatory Edinburgh.) For photography of large areas of the sky the primary mirror is made with spherical curvature and an aspheric `corrector plate' is placed at the top end of the telescope tube. There are three large Schmidt telescopes in the world with fields about 6° across (the Moon's apparent diameter in the sky is half a degree). The oldest of these is the Palomar Schmidt (not to be confused with the Palomar 200-inch) and the other two are the ESO Schmidt in Chile and the United Kingdom Schmidt in Australia. These have been used to produce photographic charts of the whole sky. The Schmidt Telescope 11a

31. The “OWL” Telescope

32. Detecting the Light The Human Eye –Once used with a telescope to record observations or make sketches –Not good at detecting faint light, even with the 10-meter Keck telescopes Photographic Film –Chemically stores data to increase sensitivity to dim light –Very inefficient: Only 4% of striking photons recorded on film Electronic Detectors –Incoming photons strike an array of semiconductor pixels that are coupled to a computer –Efficiencies of 75% possible –CCD (Charged-coupled Device) for pictures

33. Nonvisible Wavelengths Many astronomical objects radiate in wavelengths other visible –Cold gas clouds radiate in the radio –Dust clouds radiate in the infrared –Hot gases around black holes emit x-rays

34. Radio Observatories

35. Radio Observations False color images are typically used to depict wavelength distributions in non- visible observations

36. Gamma Rays Bursts Exploring New Wavelengths: Gamma Rays –Gamma-ray astronomy began in 1965 –By 1970s, gamma rays found to be coming from familiar objects: Milky Way center and remnants of exploded stars –1967 gamma-ray bursts from space discovered by military satellites watching for Soviet nuclear bomb explosions –Source of gamma-ray bursts is likely due to colliding neutron stars!

37. The Crab Nebula –In A.D. 1054, ancient Chinese noticed bright “new star” in the sky, which then faded from view in just over a year –In 1731, with the help of a telescope, a fuzzy patch was discovered in the area of the former “new star” –In 1844, filaments were noticed that gave the fuzzy nebula the appearance of a crab –In 1921, comparison of photographs lead to idea that the nebula was expanding –By 1928, it was realized that the ancient Chinese had observed a supernova explosion – the death of a massive star – and the nebula was the result

38. The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus. The nebula was first observed by John Bevis, and corresponds to a bright supernova recorded by Chinese and Arab astronomers in 1054. Located at a distance of about 6,500 light-years (2 kpc) from Earth, the nebula has a diameter of 11 ly (3.4 pc) and expands at a rate of about 1,500 kilometers per second. At the center of the nebula lies the Crab Pulsar, a rotating neutron star, which emits pulses of radiation from gamma rays to radio waves with a spin rate of 30.2 times per second. The nebula was the first astronomical object identified with a historical supernova explosion. The nebula acts as a source of radiation for studying celestial bodies that occult it. In the 1950s and 1960s, the Sun's corona was mapped from observations of the Crab's radio waves passing through it, and more recently, the thickness of the atmosphere of Saturn's moon Titan was measured as it blocked out X-rays from the nebula. Crab Nebula catalogue designations

39. Observations of the Crab Nebula Since 1928, Crab has been investigated at all wavelengths: –Powerful source of radio waves –Further radio observations revealed the remnant of the supernova explosion – a rapidly spinning “star” (30 times per second) –Radio waves also indicated that charged particles are moving at near the speed of light –Visible light indicates expansion of nebula at about 1000 km/s –Source of x-rays

40. Major Space Observatories Why put them in space?

41. Atmospheric Blurring –Twinkling of stars in sky, called scintillation, is caused by moving atmospheric irregularities refracting star light into a blend of paths to the eye –The condition of the sky for viewing is referred to as the seeing –Distorted seeing can be improved by adaptive optics, which employs a powerful laser and correcting mirrors to offset scintillation

42. Light Pollution

43. Space vs.Ground-Based Observatories Space-Based Advantages –Freedom from atmospheric blurring –Freedom of atmospheric absorption Ground-Based Advantages –Larger collecting power –Equipment easily fixed Ground-Based Considerations –Weather, humidity, and haze –Light pollution

44. Telescopes Summary Ability to FocusBending of LightIndex of Refraction ( Dependent) Collecting PowerHow Bright!Depends on Collector Area Resolving PowerTwo Objects CloseDepends on size (Ability to Discern)of collector area and quality MagnificationImage Size/Object Size Related Concepts Atmospheric RefractionThe Moon Illusion and distortion

45. Going Observing To observe at a major observatory, an astronomer must: –Submit a proposal to a committee that allocates telescope time –If given observing time, assure all necessary equipment and materials will be available –Be prepared to observe at various hours of the day Astronomers may also “observe” via the Internet –Large data archives now exist for investigations covering certain wavelengths sometimes for the entire sky –Archives help better prepare astronomers for onsite observations at an observatory

46. Computers and Astronomy For many astronomers, operating a computer and being able to program are more important than knowing how to use a telescope Computers accomplish several tasks: –Solve equations –Move telescopes and feed information to detectors –Convert data into useful form –Networks for communication and data exchange