ASTRO Module 6: Capturing Light-Technology of the Telescope
Giant Magellan Telescope
(A) Artist's concept of the completed Giant Magellan Telescope which will be situated in the Atacama Desert some 115 km (71 mi) north-northeast of La Serena, Chile. (GMTO Corporation) (B) The GMT is composed of seven 8.4-m mirror segments, each of which has twice the collecting area of the Magellan telescopes. The resolving power of the telescope is equivalent to that of a single mirror 24.5 meters (80.4 ft) in diameter. The Magellan 6.5-m telescope is shown on the left at the same scale as the GMT. The collecting area of the GMT will be 12 times that of a Magellan telescope, and its spatial resolving power will be 10 times that of the Hubble Space Telescope.
Refraction: The Bending of the Wavefronts
(A) As a light beam enters the water, it bends or refracts. (B) As wavefronts enter a medium of higher density, one side of the wavefront slows down, and the entire train of fronts twists.
Concave Mirrors
(A) Parallel rays coming into a parabolic mirror are focused at a point F. (B) A concave mirror focuses incoming light rays at the focal point. (C) A parabolic solar dish concentrates the Sun's rays on the heating element of a Stirling engine. This dish is located in the Mojave Desert, San Bernardino County, California. (D) A parabolic dish focuses the Sun's rays and ignites a piece of wood. (L. Black)
Controlling light pollution
(A) Proper shielding of outdoor lights can reduce light pollution. (B) A comparison of the view of the night sky from a small rural town and a metropolitan area
Relative Size of Celestial Object
(A) Relative angular size of objects in our solar system. Locally, the Sun and Moon dominate our night sky. (B) Relative angular size of some of the more famous Messier objects. These objects need a telescope to be able to see them, but at any magnification (e.g., 100X) the relative size difference is maintained. The white circle represents 0.5° (or 30 arcmin or 1,800 arcsec).
Modern & Classic Reflectors
(A) The 8.1-m (26.6-ft) Gemini North primary mirror is an example of a modern very large reflector. Note the Cassegrain design. Gemini North is located on Mauna Kea, Hawaii. (Gemini Observatory) (B) Replica of the 160-mm (6.2-in) reflector with a speculum mirror that William Herschel used to discover Uranus in 1781.
Optical Interferometry
(A) The Very Large Telescope (VLT) operated by the European Southern Observatory in Chile. Movable auxiliary telescopes allow the four large telescopes to operate as an optical interferometer. (ESO) (B) VLT takes the first detailed image of a disc around a young star.
Light pollution
(A) The constellation Orion, imaged at left from dark skies, and at right from Orem, UT, a city of about half a million people. (B) The Empire State Building, New York City, NY, at night shows the immense amount of light a large city produces.
Lens of the Eye
(A) The lens of the eye is transparent tissue filled with a liquid that refracts light rays to a focus on the retina. (B) A raindrop similarly focuses light and can act like a magnifier.
Diffraction and Resolution
(A) Two point light sources that are close to one another produce overlapping images because of diffraction. (B) If the images are closer together, they cannot be resolved. [Bottom] Airy diffraction patterns generated by light from two points passing through a circular aperture, such as the pupil of the eye. (C) Points far apart or (D) meeting the Rayleigh criterion can be distinguished. (E) Points closer than the Rayleigh criterion are difficult to distinguish. (Spencer Bliven)
Convex Lens
(A) Using the principle of refraction, a convex lens will make parallel rays of light come to a focus. (L. Black) (B) An animation of incoming wave fronts being refracted and focused to a point by a convex lens.
Fermi Gamma-ray Space Telescope
(A)The Fermi satellite is also known as GLAST—Gamma-ray Large Area Space Telescope. It was launched June 11, 2008. (NASA) (B) To detect gamma rays, the LAT (Large Area Telescope) portion of the spacecraft has 16 banks of detectors (NASA) (C) Active galaxies called blazars constitute the single largest source class in the second Fermi LAT catalog, but nearly a third of the sources are unassociated with objects at any other wavelength. Their natures are unknown. (NASA/Goddard Space Flight Center)
The Two Most Important Properties of a telescope
1.Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time. 2.Angular resolution: Telescopes that are larger are capable of taking images with greater detail. Computer rendering of the proposed Thirty Meter Telescope that should be operational by 2022. The primary mirror will be 30-m (98.4-ft) across. This new giant telescope will be built atop Mauna Kea in Hawaii. (TMT Observatory Corporation)
Magnification Examples
8′′ Newtonian reflector of FL = 1,000 mm (f/5) will give 250X (1000/4) with a 4-mm eyepiece; 40X with a 25-mm eyepiece. 8′′ SCT of FL = 2,032 mm (f/10) will give 508X (2032/4) with a 4-mm eyepiece; 81X with a 25-mm eyepiece. Under "perfect" sky conditions, the maximum magnifying power per inch of objective is 50X-60X. Thus a 3′′ aperture can deliver a maximum of between 150X and 180X; a 10′′ can deliver 500X-600X. We see here the effect of magnification on an extended object, the planet Jupiter. Because stars are point sources, they cannot be magnified.
Focusing Light & Recording Images with a Digital Camera
A camera focuses light like an eye and captures the image with a detector. The CCD detectors in digital cameras are similar to those used in modern telescopes. (A) Digital cameras detect light with charge-coupled devices (CCDs). (B) A cutaway of a digital single lens reflex (DSLR) camera. The CCD sits at the focal plane where film used to be located. (C) A cutaway of an Olympus E-30 DSLR. Note the multiple lens elements that make up a modern camera lens.
Focus of a Lens
A lens made of clear glass or plastic can bring incoming light rays to a focus. (A) A collimated beam of light traveling parallel to the lens axis and passing through the lens will be converged, or focused, to a spot on the axis—the focal point—at a certain distance behind the lens, known as the focal length (f ). In this case, the lens is called a positive or converging lens. (B) A demonstration of incoming light rays being focused by a lens.
Telescopes: Refractors
A reflecting telescope uses a mirror to reflect light and bring it to a focus rather than a lens that refracts light. Most reflectors have parabolic mirrors that bring all incoming light rays to the same focus, thus eliminating the chromatic aberration found in most refractors. (A) Schematic of the light path and two mirror system employed by the Newtonian reflecting telescope. (B) A 10-inch Newtonian reflector on dobsonian (alt-az) mount. Newtonian reflectors offer large apertures at affordable prices
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. The first spectrographs used photographic paper as the detector. The star spectral classification and discovery of the main sequence, Hubble's law, and the Hubble sequence were all made with spectrographs that used photographic paper. More recent spectrographs use CCDs which can be used for both visible and UV light. The slit on the spectrograph limits the light entering the spectrograph so that it acts as a point source of light from a larger image. This allows an astronomer to take a number of spectra from different regions of an extended source such as a galaxy or of s specific star in the field of view. Light is collimated (made parallel) before it hits a diffraction grating. This disperses the light into component wavelengths that can then be focused by a camera mirror into a detector such as a CCD. By rotating the grating, different parts of the dispersed spectrum can be focused on the camera. The comparison lamp provides spectral lines of known wavelength at rest with respect to the spectrograph, allowing the spectrum of the distant source to be calibrated and any shift of spectral lines to be measured. (© Copyright CSIRO Australia)
Observing non visible light
A standard satellite dish is essentially a telescope for observing radio waves. The dish acts like a primary mirror, reflecting radio waves toward the receiver. The receiver (at the end of the arm) acts like the secondary mirror, sending radio waves to an electronic device that converts the signals to images and sound.
Absolute Magnitude
A star's absolute magnitude is the apparent magnitude a star would have if it were observed at a distance of 10 parsecs from Earth.
Magnification and Eyepieces
A telescope is really composed of two parts: an objective (lens or mirror) at one end and an eyepiece at the other end Eyepieces (EPs) help focus and magnify the image captured by the objective Magnification = FL (scope) mm / FL (EP) mmWatch the video (6m24s) "Magnification and Focal Ratio" A telescope is really composed of two parts: an objective (lens or mirror) at one end and an eyepiece at the other end. Eyepieces (EPs) help focus and magnify the image captured by the objective. Magnification = FLscope mm / FLEP mm With appropriate EPs, two telescopes with different objective apertures can deliver the same magnification; an image seen through the larger objective will appear brighter.
Light Collecting Area
A telescope's diameter tells us its light-collecting area: Area = π(diameter/2)2The largest telescopes currently in use have a diameter of about 10 meters. Even larger telescopes are planned or are in early stages of construction. Single 8.4-m (27.6-ft or 331-in) mirror for proposed GMT telescope with a man for comparison. (U. Arizona Steward Observatory Mirror Lab)
FAST: World's Largest Single- Aperture Radio Telescope
China has finished building the Five-hundred-meter (1,640-ft) Aperture Spherical Telescope (FAST), the world's largest single-aperture telescope.
Keck looks at Neptune
Adaptive optics use computer controlled sensors to move a specially shaped mirror to compensate for atmospheric distortions that lower a telescope's resolution.
Equatorial Motions and Mount
An equatorial mount has one axis (right ascension) fixed, pointing to Polaris. The other axis (declination) follows the movement of the stars with one motion. A clock drive attached to that axis will automatically move the telescope to compensate for the Earth's rotation. (A) A German equatorial mount (GEM). (B) The green equatorially mounted telescope rotates at the same rate as the earth but in the opposite direction, while the red telescope is not driven.
Human Eye
An organ that reacts to light and has several purposes. As a sense organ, the mammalian eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can distinguish about 10 million colors. Eyes contain pupil, lens, and retina. Light-sensitive cells—cones and rods—are in the retina. Light is bent (refracted) by the lens and comes to a focus at the back of the eye—the retina—in the focal plane. The image formed by the lens in the eye is upside down. The brain flips the image right side up. The pupil acts to change the aperture of the eye. In the dark, your pupil opens up, allowing more light to enter. To adapt your eyes fully to the dark can take up to 45 minutes. The eyeball is curved when viewed from the side.
Angular Resolution
Angular resolution is the smallest angular separation between two points that one can see at a particular distance. The human eye has an angular resolution of about 1 arcminute (1/60°), meaning that two stars will appear distinct if they lie farther than 1 arcminute apart in the sky. Larger telescopes have better resolution than smaller telescopes, and resolution ultimately is limited by the properties of light—known as the diffraction limit. As one moves toward longer wavelengths, aperture size of the telescope must increase in order to have the same angular resolution (diameters of radio telescope dishes are much larger than diameters of optical telescope mirrors). (A) An example of angular resolution can be found in an approaching automobile at night with its headlights on. When the car is very far away from an observer, the car's headlights will appear very close together (or even merged into one light.) As the car gets closer, the headlights appear more widely separated. For the approaching car, the angular separation between the headlights increases. We can measure this angle in terms of degrees (°), minutes (′), and seconds (′′) of arc. This principle is used to measure and denote the separation of double or binary stars. (B) Cor Caroli (α Canum Venaticorum) is the brightest star in the constellation Canes Venatici and is a binary star with a separation of 19.6 arcseconds. (C) Albireo (β Cygni) is the fifth brightest star in the constellation Cygnus and is a double star with separation of 35 arcseconds.
Two Point Sources
Any light entering a telescope is going through a circular aperture, and like light going through a slit, a diffraction pattern will result. When viewing two close together "point" stars, both will turn into circular disks surrounded by rings. As shown in the figure, as the stars move closer together, their diffraction patterns begin to overlap. If they become much closer, as shown at the bottom of the figure, they will become indistinguishable. The generally accepted minimum angular separation that is resolvable is given by θ = 1.22(Wavelength)/D, where 1.22 results because the opening is a circle instead of a slit, D is the diameter of the lens, and the wavelength of the light.
Telescope Eyepieces: AFOV
Apparent field of view (APOV) is a measure of the angular size of the image viewed through the eyepiece, in other words, how large the image appears (as distinct from the magnification). This is constant for any given eyepiece of fixed focal length, and may be used to calculate what the actual field of view will be when the eyepiece is used with a given telescope. The measurement ranges from 30º to 110º.The typical low- to mid-priced eyepiece (Plössl) has an apparent field of view (AFOV) of between 50º and 55º. Premium eye-pieces have larger apparent fields of view. Superwides" typically offer 68º-70º AFOV. "Ultrawides" offer 82º AFOV. Recently, 100º AFOV eyepieces have been created. Wider AFOV eye-pieces show much more sky to the observer.
Stellar Brightness: Magnitudes
Apparent magnitude is the relative brightness of stars as seen with the naked eye from Earth. Hipparchus (135 BC) was the first to classify stars from magnitude 1 (bright) to magnitude 6 (dim). Magnitude 1 is 100X brighter than magnitude 6, so each magnitude change is ≈ 2.512 (1001/5 ≈ 2.512). In modern times, we know of stars brighter than magnitude 1, so we use 0 and negative numbers to indicate the brightest stars and numbers greater than 6 to indicated dim stars. Under very dark skies, the human eye can see to magnitude 6.5-7.0. Hipparchus of Nicaea (c.190-c.120 BC) was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry but is most famous for his incidental discovery of precession of the equinoxes. He compiled the first comprehensive star catalog and ranked stars in six magnitude classes (from 1 to 6) according to their brightness.
George Ellery Hale (1868-1938)
As an undergraduate at the Massachusetts Institute of Technology, George Hale invented the spectroheliograph. He worked in his private Kenwood observatory two years before joining the faculty of the University of Chicago, for which he built the Yerkes Observatory and its 40-inch refractor. To expand solar observations and promote astrophysical studies he founded Mt. Wilson Observatory, where he discovered that sunspots were regions of relatively low temperatures and high magnetic fields. He hired Harlow Shapley and Edwin Hubble as soon as they finished their doctorates, and he encouraged research in galactic and extragalactic astronomy as well as solar and stellar astrophysics. Due to ill health, Hale retired from the Mt. Wilson Observatory in 1923 and spent most of his remaining years on solar research at his private Hale Laboratory in Pasadena. Hale planned and raised funds for 60-, 100-, and 200-inch reflectors, the last completed on Palomar Mountain in 1948 and named for him after his death. George Ellery Hale (1868-1938) played a leading role in developing, funding, and building the world's largest telescopeFOUR times.
Galileo's Telescopes
As early examples of refractors, Galileo's telescopes suffered from chromatic aberration, which is the inability of a lens to focus light of different wavelengths—red, green, blue—at the same focal point. Galileo's telescopes from around 1609-1610. His most powerful telescope could magnify 30X but had a very narrow field of view.
Our Eyes and Astronomy
Astronomers use red lights to see because cones in the center of the retina are sensitive to red while the rods are not. This is why our eyes, even aided by a telescope, are not very good for detecting colors and other details in objects such as nebulae, clusters, and galaxies. Our eye-brain takes a continuous series of snapshots; we cannot "take" a long exposure to build up detail and color as when using film or CCDs.Quantum efficiency of the eye-brain is very low at about 1%-5%; film is lower at 1% (or less) but has superior integration time; electronic detectors (CCDs) are 60%-80%. Astronomers use red flashlights as they do not seriously affect night vision.
Imaging
Astronomical detectors generally record only one color of light at a time. Several images must be combined to make full-color pictures. Astronomical detectors can record forms of light our eyes cannot see. Color is sometimes used to represent different energies of nonvisible light, such as in this X-ray image of a supernova remnant. Though consumer digital cameras (imaging devices) employ a full color CCD, professional imaging cameras typically use a monochrome CCD. Images are taken through various filters and then combined into a final image. This produces a higher quality image. Shown here is M16, the Eagle Nebula. (Pete Challis/Harvard-Smithsonian Center for Astrophysics) A "true color" Chandra X-ray image of supernova remnant (SNR) N132D located 160,000 l-y distant in the Large Magellanic Cloud. The colors represent different ranges of X-rays, with red, green, and blue representing, low, medium, and higher X-ray energies, respectively. (NASA/SAO/CXC)
Atmospheric Windows
Atmospheric windows only allow some types of light to get to us on Earth. Other parts of the EM spectrum can only be "seen" from above the atmosphere—in outer space. Ultraviolet and infrared light can be collected using conventional optical telescopes equipped with special sensors. Some infrared telescopes can be located on Earth (on high mountains) while others are in space; all ultraviolet telescopes have to be space-based. Only radio (except for extremely long wavelengths) and visible light pass totally unobstructed through Earth's atmosphere.
Research Telescopes
Because a mirror requires only one side to be ground and polished, reflecting telescopes are less expensive to build than refractors. Because a mirror can be supported from behind (rather than along its edge), mirrors can be made much larger in diameter than lenses. All large modern research telescopes are reflectors and most are Cassegrains. (A) Schematic of the light path and mirrors in a classic Cassegrain reflecting telescope. Note the hole in the middle of the primary mirror. (B) The 2.1-meter (84-inch) telescope at the Kitt Peak National Observatory, near Tucson, Arizona. The white cylinder at the bottom of the telescope is an infrared spectrograph. (NOAO/AURA/NSF)
Binoculars
Binoculars are twin telescopes placed side by side. Prisms in binoculars bend light to give a stereo upright view. The two main types of binoculars are based on the types of prisms in each. Porro prism binoculars bulge at the sides and are easier to build. Roof prism binoculars are slimmer, but good quality ones are more expensive to build than those with porro prisms. For astronomical use, the objective lenses should be at least 50 mm and a magnification of 7X to 10X (described as 7 X 50 or 10 X 50) "Astronomical" binoculars usually have objectives of 60 mm-100 mm and magnifications of 12X to 25X For astronomical use, binoculars should be tripod mounted to steady the image
Same Aperture, Different Focal Length
Both of these Orion reflecting telescopes have 4½-inch (114-mm) apertures, but they differ in one key respect. The SkyQuest XT4.5 Dobsonian (left) has a relatively long 910-mm focal length (f/8), making it a better choice for higher-power views of the Moon and planets. The focal length of the StarBlast (right) is only 450 mm (f/4), making it better suited to low-power, wide-field sky sweeping, and it is easier to carry and store. A 10-mm eyepiece will produce 91X in the SkyQuest (left) and 45X in the StarBlast (right).
Telescopes: SCTs and Maks
Catadioptric scopes such as a Schmidt-Cassegrain (SCT) or Maksutov-Cassegrain(Mak) incorporate a main mirror and a secondary mirror to fold the light path, which gives a short tube. The glass plate at the front of the telescope acts something like a lens.
Refracting Telescope
Classic large refracting telescopes need to be very long, with large, heavy lenses. These telescopes employed a two-lens arrangement called an achromat. The very long tubes indicate a long focal length, which helped to reduce chromatic aberration. (A) An achromatic magnifier (10X). (L. Black) (B) Yerkes 1-m (40-in) refractor is the world's largest and began operation in 1893. (C) Zeiss 254-mm (10-in) refractor at The Franklin Institute. Both of these telescopes are on equatorial mounts. (L. Black)
Four factors that limit Earth-bound visual observations
Cloud cover—when it's cloudy, you can't see the stars! Clear sky is cloud-free. Transparency—this involves the amount of water vapor in the atmosphere; more water vapor can degrade the view. Seeing—involves turbulence in the upper atmosphere (what makes stars twinkle); limits maximum angular resolution of ground-based telescopes to 0.5 arcsecond. Light pollution—scattered light from other Earthbound sources.
What are the two most important properties of a telescope?
Collecting area determines how much light a telescope can gather. Angular resolution is the minimum angular separation a telescope can distinguish.
Angular Resolution and Diffraction
Diffraction also limits our ability to resolve two point sources. Consider two point sources (e.g., stars) with angular separation α viewed through a circular aperture or lens of diameter D. The angular separation can be calculated using the following equation: a(lower c)=1.22xwavelength/D The Rayleigh criterion defines the images to be resolved if the central maximum (c) of one image falls on or further than the first minimum of the second image. (A) The angular separation, α, between two point sources can be represented by the wavelength of the incident light rays coming from those sources. (B) Graphs of the central maximum of each source.
Diffraction Spikes
Diffraction spikes are lines radiating from bright light sources in photographs. They are artifacts caused by light diffracting around the support vanes of the secondary mirror in reflecting telescopes. No matter how fine these support rods are they diffract the incoming light from a subject star and this appears as diffraction spikes. (A) "Spider" vanes holding the secondary mirror in a Newtonian reflector. (B) HST image showing diffraction spikes from stars. (NASA, ESA, and H. Richer) (C) Comparison of diffraction spikes for various strut arrangements of a reflecting telescope—the inner circle represents the secondary mirror.
Your Eye: Telescope "1"
Each eye is really a one-power 7-mm (slightly more than a ¼-inch) telescope Your two eyes give you built-in binoculars Dark adapting your eye allows your eye's pupil to open to its full aperture Our brains process the image we "see" Telescopes and binoculars gather more light (their primary function) for our eyes to transmit to our brain Eyepieces in telescopes and binoculars magnify images
Spectroscope
Early spectroscopes were simply prisms with graduations marking wavelengths of light. Modern spectroscopes generally use a diffraction grating, a movable slit, and some kind of photodetector, all automated and controlled by a computer. Joseph von Fraunhofer developed the first modern spectroscope by combining a prism, diffraction slit, and telescope in a manner that increased the spectral resolution and was reproducible in other laboratories.
Atmosphere limits resolution
Earth's atmosphere is constantly moving, and different layers bend the light from a star in different directions, blurring our view from the ground. Stars twinkle because air pockets distort incoming light rays, which create blurry images.
X-Ray Telescope Design
Focusing of X-rays requires special mirrors in nested arrays. Mirrors are arranged to focus X-ray photons through grazing bounces (grazing incidence) off the surface. (A) X-ray telescope mirror module. (NASA) (B) Schematic of grazing Incidence X-ray mirrors. Mirror elements are 0.8-m long and from 0.6-m to 1.2-m diameter.
Refractors
Give the sharpest images but cost the most per inch of aperture. (A) Refractors employ a closed-tube design that uses either 2 lenses (achromatic) or 3 lenses (apochromatic, which gives color-free images). (B) A 6-inch (150-mm) refractor on a motorized go-to equatorial mount and tripod.
Green Flashes
Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the Sun, or a green ray shoots up from the sunset point. Green flashes are actually a group of phenomena stemming from different causes and can be observed from any altitude (even from an aircraft). They are usually seen at an unobstructed horizon, such as over the ocean, but are possible over cloud tops and mountain tops as well. Refraction of light in the atmosphere causes a green flash: Light moves more slowly in the lower, denser air than in the thinner air above, so sunlight rays follow paths that curve slightly, in the same direction as the curvature of the Earth. Higher frequency light (green/blue) curves more than lower frequency light (red/orange), so green/blue rays from the upper limb of the setting Sun remain visible after the red rays are obstructed by the curvature of the Earth.
How/why does the metal grid in the door of a microwave oven—which we can look through—protect us from the microwave radiation released inside of the oven that cooks our food?
Grid in a microwave oven.Microwaves used have a wavelength of ≈4 cm (1.6 in)—40 times larger than the openings in the grid (1 mm) —so the microwaves "see" the grid as a solid wall. Visible light has a wavelength of a few hundred nanometers—about 2,000 times smaller than the openings in the grid. The grid allows us to see easily into the oven's interior while protecting us from the microwave radiation.
Spectrum of a star
HD 21619 (HIP 16426) is a white main-sequence star in the constellation Perseus. It lies 638 l-y away, shines at an apparent visual magnitude of 8.78, and is of spectral class A3. This is a spectrum of the star HD 21619. The dark bands are hydrogen absorption lines. The temperature of this star is about 9000 K. (Top) This is the photographic form of the spectrum. (Bottom) This is a line intensity trace.
Spectroscopy and Resolution
Higher resolution spectra reveal more information. The strong, resonant Mg II line is easily identified in the spectra of quasars. The top panel of the figure is a R = 3000 spectrum of Q1206+4557, obtained by Steidel & Sargent (1992) on a 4-m class telescope. A Mg II doublet line at redshift z = 0.93 is shown, which places it at the observed wavelength of ~5400 Å. By the 1990's, 10-m class telescopes brought about high-resolution spectra. The lower panel shows a Keck/ HIRES spectrum of the high-lighted wavelength region (Churchill, 1997). The resolution is R = 45,000. (C. Churchill/PSU)
Time Machines
Humans have longed to invent machines that can take them backward or forward into time. (A) The Time Machine was originally a novel written by H. G. Wells and published in 1895. (B) The Tardis from the British TV series Dr. Who. (C) The time-traveling DeLorean from the Back to the Future movies.
Uses of Telescopes
Imaging: making pictures of objects using various filters and recording the image on film or CCDs. Spectroscopy: dispersing light into spectra via a diffraction grating. Timing: Measuring how light output varies with time, which can be shown in graphs known as light curves
Light Curves and Timing
In astronomy, a light curve is a graph of light intensity of a celestial object as a function of time. The light is usually in a particular frequency interval or band. Light curves can be periodic, as in the case of eclipsing binaries, Cepheid variables, other periodic variables, and transiting extrasolar planets. Or, the curves can be aperiodic, like the light curve of a nova, a cataclysmic variable star, a supernova, or a microlensing event. The study of the light curve, together with other observations, can yield considerable information about the physical process that produces it or constrain the physical theories about it. The bottom scale is in days and one full rotation runs from about -0.14 to 0.02 (or about 3.75 hours). The light curve has two maxima and two minima because 201 Penelope has (to the first order) an ellipsoid shape. It rotates about its short axis, so the minima are when we see it end on, while the maxima are when we see it side on.
IR & UV Telescopes
Infrared and ultraviolet-light telescopes operate like visible-light telescopes, but they need to be above the atmosphere to see all IR and UV wavelengths. A) SOFIA (Stratospheric Observatory for Infrared Astronomy) is a NASA airborne infrared telescope. (NASA) (B) Spitzer is a space-based infrared telescope.
Interferometry andRadio Telescopes
Interferometry is easiest to do with radio telescopes because the relatively long wavelengths are easier to work with. Though less common and more difficult to implement, interferometry is now becoming possible with infrared and visible-light telescopes. The Karl G. Jansky Very Large Array is a radio astronomy observatory located on the Plains of San Agustin, between the towns of Magdalena and Datil, some 50 miles west of Socorro, New Mexico. Each antenna is 25 meters (82 feet) in diameter. The data from the antennas is combined electronically to give the resolution of an antenna 36km (22 miles) across, with the sensitivity of a dish 130 meters (422 feet) in diameter.
Angular Resolution Calculations
Large Optical Telescope: mirror diameter= 1 meter = 100 cm Wavelength of light= 0.5 Micron(visible) Angular Resoultion= 0.25 x 0.5/ 1= 0.125 Arcsec Single Radio-Telescope Dish: Diameter=25 meters Wavelength of Radio Wave=1 meter= 1 x 10(to the 6) microns Angular Resolution = 0.25 x (1 x 10<6)/25= 10,000 arcsec Much worse than an optical telescope Very Large Array: diameter=25 km=25,000m Wavelength of radio wave= 1 m= 1 x 10<6microns Angular Resolution=0.25 x (1 x 10<6)/25,000=10 arcsec Worse than an optical telescope, but not so bad Intercontinental Array: diameter=10,000 km = 1 x 10<7m Wavelength of radio wave= 1 m= 1 x 10<6 microns Angular Resolution= 0.25 x (1 x 10<6)/ (1 x 10<7)= 0.025 arcsec Better than an optical telescope
Magnitudes Continued
Larger apertures can see dimmer stars, i.e., stars with higher magnitude numbers Light pollution limits the faintest stars one can see; suburban skies: magnitude 3-4; dark skies: magnitude 5.5-7 Some apparent magnitudes: 26.80 Sun 1.46 Sirius 12.74 Moon (full) 0.74 Canopus 04.60 Venus (at max) 0.03 Vega 02.80 Mars (at max) 1.97 Polaris Apparent magnitudes and distances of the 10brightest stars: Sirius −1.46 (8.6 l-y); Canopus−0.72 (74 l-y); Alpha Centauri−0.27 (4.3 l-y); Arcturus−0.04 (34 l-y); Vega 0.03 (25 l-y); Capella 0.08 (41 l-y); Rigel 0.12 (1,400 l-y); Procyon 0.38 (11.4 l-y); Achernar 0.46 (69 l-y); Betelgeuse 0.50 (1,400 l-y)
Lens Sag
Lens "sag" limits the size of refractor one can build. The largest diameter lens ever placed in a successfully functioning telescope is 40 inches (1 meter) found in the Yerkes telescope. (Slide 51)
Radio telescope
Like a giant mirror that reflects radio waves to a focus.
Magnifer
Like the water droplet, a semispherical transparent substance like glass or acrylic can act as a magnifier. (L. Black)
Galaxy M51 Imaged at Different Wavelengths
M51, the Whirlpool galaxy, is shown here imaged at different wavelengths, mostly in the infrared. The Webb telescope will enable astronomers to obtain higher resolution images of galaxies near and far in the infrared. Webb's primary imager will be the Near-Infrared Camera (NIRCam), which will operate at wavelengths 0.6-5 microns. Webb's Mid-Infrared Instrument (MIRI) will operate in the 5-micron to 28-micron wavelength range.
Mirrors in Modern Giant Reflecting Telescopes
Modern very large reflecting telescopes use light-weight multiple mirrors rather than one heavy large mirror as in the past.
X-Ray Telescopes
Need to be above the atmosphere.
Newton's Telescope
Newton's reflector of 1668 employed a metal speculum mirror that tarnished quickly. Speculum is an alloy of copper and tin that could be cast, ground, and polished to make a highly reflective surface. Silvered glass mirrors, invented in the 1850s, replaced speculum mirrors by the late 19th century. Aluminum-coated mirrors became common in the 20th century. Early 20th century glass mirrors were made of plate glass. By the 1930s, low-expansion glasses like Pyrex were used to construct large telescope mirrors.
Focal Length & Focal Ratio
Note that a shorter focal length telescope will produce less magnification than a longer focal length telescope with the same focal length eyepiece.
Look-Back Time
Our look-back time is the observable universe, which we know to be 13.8 billion years old. The world's largest telescopes currently can look out a little over 13 billion light-years or back in time 13 billion years. As new large telescopes are built in the coming decade, astronomers should be able to see further back in time Look-back times to the Sun, closest star, and closest large galaxy
Index of Refraction
Ratio of light's speed in a vacuum, c, to its speed in a medium, v, is the medium's index of refraction, n—i.e., n = c/v. n is approximately 1.5 for typical glass, so the speed of light in glass ≈ 200,000 km/s. Because the speed of light changes as it enters a medium, light that enters glass at an angle is bent. The change in direction of the light ray is called refraction. (A) A schematic of a series of wavefront sentering a new medium. (B) An actual light ray entering and leaving a medium (glass block). Light waves are refracted (bent) when entering a medium with a higher index of refraction (glass block). They are refracted again as they reenter the medium with the lower index of refraction (air). (L. Black)
Flattening of the Sun at sunset
Rays from top of the Sun are also refracted but not as much because they enter the atmosphere at a less oblique angle. Thus, the top of the Sun is also flattened but not as much as the bottom. (A) The setting Sun's rays (lower image) are refracted by the atmosphere, making the Sun appear higher in the sky. The bottom limb is lifted more than the top, making its image oval. (B) The disk of the Sun has nearly set, and the Sun appears oval as a result of refraction.
3 Basic Telescope Designs
Refracting telescope (refractor): operates more like the eye; uses transparent glass lenses to focus light. Reflecting telescope (reflector): uses a precisely curved (parabolic) mirror to gather and focus light. Catadioptric (Schmidt-Cassegrain or Maksutov-Cassegrain): uses a system of mirrors and a correcting plate (type of lens) to gather and focus light. The three basic designs of telescopes. (A) A refracting telescope uses lenses. (B) A reflecting telescope uses mirrors. (C) A catadioptric (such as the Schmidt-Cassegrain shown) uses mirrors and a front corrector that acts like a lens.
Refraction
Refraction is the bending of light when it passes from one substance into another. Your eye uses refraction to focus light. (A) The principle of refraction. (B) An animation of a sinusoidal traveling plane wave entering a region of lower wave velocity at an angle, illustrating the decrease in wavelength and change of direction (refraction) that results. (C) Ice-land spar, a transparent variety of calcite, is remarkable for its birefringence, which means that the index of refraction of the crystal is different for light of different polarization. A ray of unpolarized light passing through the crystal divides into two rays of perpendicular polarization directed at different angles, called double refraction. (L. Black) Refraction of light waves in water will cause a pencil or straw to appear bent or broken. (A) The complete pencil represents the actual position of a pencil sitting in a bowl of water. The partial pencil represents the apparent position of the pencil. Notice that the end (x) looks like it is at (y), a position that is considerably shallower than (x).The straw in the glass seems to be broken, due to the refraction of light as it emerges into the air from the water. Air has a refractive index of 1.0003 while water has a higher refractive index of 1.33.
Image Formation
Refraction of parallel light rays through a lens can cause the light rays to converge to a focus. The focal plane is where light from different directions comes into focus. The image behind a single (convex) lens is always inverted or upside-down. The lens in one's eye functions exactly like a glass or plastic lens. An inverted image of an object is formed at the focal plane.
Radio Interferometry
Single radio telescopes have poor resolution due to long wavelengths. Interferometric arrays combine the signals from many telescopes, which increases resolution. (A) Schematic of how an interferometer works. Interferometry is a technique for linking two or more telescopes so that they have the angular resolution of a single large one. (B) The Very Large Array (VLA) in New Mexico functions as a large interferometer with 27 independent antennae, each of which has a dish diameter of 25 m (82 ft). Recently upgraded, it has been renamed the Karl Jansky Very Large Array.
Chromatic Aberration
Single-lens or two-lens refractors suffer from chromatic aberration where light of different wavelengths comes to a focus in different places. It causes bright sources of light to have blue edges on one side and red edges on the other, as in this picture of the Moon. In modern refractors, using special glass can greatly lessen chromatic aberration.
Spectra of SNR N132D
Supernova remnant N132D resides in the Large Magellanic Cloud, which is located 170,000 l-y distant. The Cosmic Origins Spectrograph (COS) aboard NASA's Hubble Space Telescope detected pristine gas ejected by the supernova that has not yet mixed with the gas in the interstellar medium. The resulting spectrum, taken in ultraviolet light, shows glowing oxygen and carbon in the remnant. (NASA, ESA, and the Hubble SM4 ERO Team)
Observing with a Telescope
Telescopes typically gather more light than binoculars and can change magnification through interchangeable eyepieces. Astronomical telescopes are less portable than most binoculars and always require a mount. Telescopes have a narrower true field of view than binoculars—typically 1°. Binoculars can have fields of view that range from 8° to 2.5°, depending on the magnification of its oculars (the higher the magnification, the smaller the field of view). Simulated true fields of view through optical instruments are compared here using the Pleiades (M45). With relatively narrow fields of view, telescopes typically have a field of view of 1°. Binoculars, which were originally designed for terrestrial use, have much wider true fields of view, such as the 6° shown.
The 100-m Radio Telescope
The 100-meter (328-ft) Green Bank Radio Telescope is the largest single-dish fully steerable radio telescope. The device weighs over 30 times more than the Statue of Liberty, and yet can point anywhere in the sky more precisely than one thousandth of a degree. (C) The main parts of the telescope.
The First Green Bank Radio Telescope
The 300-foot radio telescope taken on November 15, 1988, before the collapse. This radio telescope was not fully steerable.
James Webb Space Telescope
The 6.5-m James Webb Space Telescope (JWST) is a large, infrared-optimized space telescope, scheduled for launch in 2019. The HST has a primary mirror 2.4-m (7.9 ft) in diameter, while the JWST has a primary mirror 6.5-m (21.3 ft) in diameter. (B) A side-by-side comparison of the mirrors of the two telescopes. Note the size of the mirrors in relation to a standing human figure.
Telescopes in space
The Hubble Space Telescope (HST) is a space telescope that was carried into orbit by a Space Shuttle in 1990. A 2.4-meter (7.9-ft) aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared spectra. The telescope is named after the astronomer Edwin Hubble.
Newtonian Reflector
The Newtonian telescope is a type of reflecting telescope invented by Isaac Newton (1642-1727), using a concave (parabolic) primary mirror and a flat diagonal secondary mirror. They are free of the chromatic aberration found in refracting telescopes. (A) Schematic of a Newtonian reflecting telescope showing the light path. (Tamasflex) (B) A modern commercial Newtonian optical tube. Note that because a Newtonian has two mirrors, the objective mirror is often also called the primary mirror.
Refraction: At Sunset
The Sun appears distorted at sunset because of how light bends in Earth's atmosphere. Here, the Sun sets at Paranal, Chile, location of the Very Large Telescope (VLT). (Stéphane Guisard/ESO)
Resolution of a telescope
The ability to resolve two points (stars) close together is related to the amount of light-gathering power (aperture). A greater aperture can see a smaller angular separation as well as dimmer objects Typical resolving abilities of telescopes: 4′′1.36 arcsec 12.5 limiting magnitude 5′′1.10 arcsec 13.0 limiting magnitude 6′′0.92 arcsec 13.4 limiting magnitude 8′′0.68 arcsec 14.0 limiting magnitude 10′′0.54 arcsec 14.5 limiting magnitude
Light has a Speed Limit
The answer lies in the light that we collect with our telescopes from the distant reaches of our universe. Light takes a finite time to travel through space. A composite visible and infrared image of M104, Sombrero Galaxy, which is 29.3 million l-y (9.0 Mpc) distant in the constellation Virgo. (NASA/JPL-Caltech and The Hubble Heritage Team-STScI/AURA)
Angular Size M31
The apparent size of M31 is 190 arcmin (11,400 arcsec) long, or about 6 times the diameter of the Moon, which is ½° or 1,800 arcsec in diameter. Therefore, 1,800 arcsec ×6 = 10,800 arcsec or 180 arcmin. (Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA /NSF)
How does Earth's atmosphere affect ground-based observations
The best ground-based sites for astronomical observing are: Calm (not too windy) High (less atmosphere to see through) Dark (far from city lights) Dry (few cloudy nights)
Spectra of Stars of Different Temperature
The hotter the star, the greater the intensity of its peak radiation, and the shorter the wavelength at which that peak occurs. The graph shows spectra of four stars. All of them have hydrogen in their outer atmospheres, which absorbs light at a series of wavelengths at about 486 nm, 434 nm, 410 nm, and so on. This series of lines, called the Balmer series, is formed when atoms that are in the first excited state absorb a photon and jump up to a higher excited state.
The human eye as a camera
The lens in the eye, as well as a camera's lens, inverts the image of what is seen. (A) Light rays bounce off a tree, enter the eye's lens, are refracted, and an inverted image forms at the back of the eye. The image is then transmitted via the retina and optic nerve to the brain where we perceive the image a right side up. (B) Light rays bounce off a tree, enter the camera's lens, are refracted, and an inverted image falls at the back of the camera on the focal plane. The focal plane is where film or an electronic detector (CCD) is located that records the image.
Function of a Telescope
The primary purpose of a telescope is to gather light—not to magnify an image. Light-gathering power is directly related to the diameter of the telescope's objective lens or mirror. Gran Telescopio Canarias (GRANTECAN or GTC) is a 10.4-m (410-in or 34.2-ft) optical telescope at the Roque de los Muchachos Observatory, La Palma, Canary Islands. The GTC is currently the world's largest optical telescope. (H. Raab)
Correcting Chromatic Aberration
The severe chromatic aberration (CA) produced by a single lens (left) can be greatly corrected by using an achromatic doublet (right) made of two different kinds of glass. Though this innovation improved the quality of refractors, long focal lengths (f/12 to f/15) were still needed to help manage CA for short wavelength (violet) light. Three-lens apochromatic refractors were subsequently made that eliminate all CA and permit shorter focal lengths (f/6 to f/8), but such lens systems are expensive to build and generally are not employed for apertures beyond 8 inches.
Resolution and the face on Mars
The so-called "Face on Mars" from 1976 is the result of a low-resolution image and lighting geometry that fools us into thinking it looks like a face. The object is actually a lava dome that has created an isolated mesa or butte-like structure (i.e., a hill) and is located in the Cydonia region of Mars.
Calm, high, dark, dry
The summit of Mauna Kea in Hawaii is one of the world's top observing sites and home to many big telescopes. The best observing sites are atop remote mountains.
Resolution and Interface
The ultimate limit to resolution comes from interference of light waves within a telescope. Larger telescopes are capable of greater resolution because there is less interference. (A) Simulated interference pattern. (B) Animation of interference of light coming from two in-phase point sources. (Oleg Alexandrov ) (C) Interference pattern in water waves. (ESO/M. Alexander) (D) White light interference in a soap bubble.
Types of Binoculars
There are two major binocular designs. Porro prism binoculars bulge at the sides and are easier to build than roof prism binoculars. Roof prism binoculars (bird watcher) are slimmer, but good quality ones are more expensive to build than those with porro prisms.
Optical Interferometer
This image of the triple star system Eta Virginis is the first ever made by an optical interferometer using the combined beams of 6 separate telescopes. The closer pair of stars have an angular separation of about 5.4 milliarcseconds, equivalent to the size of a US cent coin viewed from a distance of 800 km (500 miles). This level of detail would require a single monolithic telescope mirror in excess of 50 meters in diameter. (U.S. Naval Observatory)
Our Time Machine: The Telescope
This marvelous time travel tool is the telescope. But how does it work to take us back in time? Amateur telescopes. (A) An 18-inch f/4.5 Newtonian reflector on a Dobsonian mount. (Andrew Cooper) (B) A 6-inch f/8 refractor on an equatorial mount. (L. Black) (C) A 6-inch f/10 Schmidt-Cassegrain on an alt-az mount. (L. Black)
World's highest observatory
This panoramic view of the Chajnantor plateau, 5,000 meters (16,400 feet) high in Chile's Atacama Desert, spans about 180 degrees from north (on the left) to south (on the right). Shown are some of the 66 12-m diameter antennas of the Atacama Large Millimeter/submillimeter Array (ALMA). Because ALMA observes at relatively short wavelengths, the array needs to be located in a dry environment or the signals can be blocked by water droplets. (Babak Tafreshi/ESO)
Astronomical Imaging
Today, electronic detectors—CCDs or charge-coupled devices—are used to record images digitally. Pixels are picture elements, which come from a grid of squares etched onto a CCD silicon chip. Modern CCD-based cameras have 12-16 million (or more) pixels. CCDs have these advantages: More sensitive, recording up to 80% of photons striking the chip, as compared to 4-6% for film and 1% for the human eye Much wider dynamic range in recording dimand bright light at the same time Digital images are easily manipulated through techniques of image processing via computer software Fig. 6-16: Array of 30 CCDs used on a Sloan Digital Sky Survey telescope imaging camera, an example of "drift-scanning." (SDSS)
Diffraction Limit
Ultimately, the wavelength of light will limit the ability of an optical instrument (i.e., a telescope's lens or mirror) to focus to a perfectly small point. The interference of light waves creates a diffraction pattern that limits the resolution of the optical system, known as the diffraction limit. (A) The diffraction pattern resulting from a uniformly-illuminated circular aperture has a bright region in the center, known as the Airy disk which together with the series of concentric bright rings around is called the Airy pattern. Owing to diffraction, the smallest point to which a lens or mirror can focus a beam of light is the size of the Airy disk. An optical system in which the resolution is no longer limited by imperfections in the lenses but only by diffraction is said to be diffraction limited. (B) Zeta Boötis imaged with the Nordic Optical Telescope using the lucky imaging method. The Airy discs around the stars are caused by diffraction from the 2.56-m (8.4-ft) telescope aperture.
Maximum Magnification
Under superb sky conditions, the maximum magnifying power per inch of objective is 60X. Thus a 3′′ aperture can deliver a maximum of 180X; a 10′′ can deliver 600X. Do NOT be fooled by claims of high magnification for some low-quality telescopes of small aperture. Some low-quality "department store" 60-mm (2.4-inch) refractors claim magnifications of 400X to 600X with the supplied eyepieces and "super power" barlow lens. The maximum usable magnification for a 60-mm telescope with superior optics is 144X.
Refrating Telescopes
Uses a lens to collect and focus light, forming images in its focal plane. (A) The path that light rays take when they enter a refracting telescope. (Tamasflex) (B) A modern 4-inch (102-mm) refractor. Note the smaller refractor attached to the side of the telescope tube. This small telescope functions as a wide-field finder. (L. Black)
Wormholes and Time Travel
Wormholes offer the possibility of time travel. Though theoretically possible, the creation of a wormhole is far, far beyond our current capabilities. A 2D analogy to a wormhole. The green arrow represents the short way through the wormhole. The red arrow represents the long way through normal space.
Our Tools for Time Travel
Yet, we have a device—invented 400 years ago—that can allow us to travel hundreds, thousands, millions, and even billions of years back in time. At the same time, this wonderful tool allows us to explore the vast, rich wonders of what we call the universe. Galileo's telescope began a revolution in understanding our universe that continues to today. (Inset) A 1636 portrait of Galileo Galilei by Giusto Sustermans.
How does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope?
πr2 = π(10/2)2 = π(25) or 25π for 10-m telescopeπr2 = π(2/2)2 = π(1) or 1π for 2-m telescope25π/1π = 25 times greater
