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Introduction
1. Optical Astronomy
There are two basic types of optical telescopes, the reflector and refractor...
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FIGURE 1: Refractor Telescope Diagram
From: Stellafane Web Site
The refracting telescope (FIGURE 1 above) is the oldest type and relies on an objective lense (located at the "front" or "top" of the telescope) to concentrate the incoming light onto the eyepiece, which is selected for based on the desired magnification of the image. Focusing the image is accomplsihed by moving the eyepiece closer to or farther from the objective lens. This type of telescope has some practical limitations:
1- Since magnification depends on focal length, the distance between the lense and the eyepiece, a high-magnification refractor would have to be very long, and
2- Since field of view depends on the size of the lense, you need a large lense to see a large piece of the sky. Large glass lenses tend to diffract light (results in a rainbow effect) towards the edge of the lense.
Using this type of telescope to observe the planets in 1609, Galileo discovered craters on the Moon, four of Jupiter’s moons, sunspots on the Sun, and that Venus has phases like those of the Moon.
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FIGURE 2: Newtonian Reflector Telescope
From: Stellafane Web Site
The reflector telescope (FIGURE 2 above) uses a concave mirror (curved inward) at the bottom of the telescope tube to collect and focus light onto a second mirror located at or near the top of the telescope. This second mirror is set at an angle in order to reflect the focused light onto an eyepiece usually mounted on the side of the telescope. Focusing the image is still accomplsihed by moving the eyepiece closer to or farther from the "objective lens" (concave mirror), with the focal length being the sum of the distances between the mirrors and eyepiece.
There are several variations of the reflector telescope that vary in the number and location of mirrors depending on the location of the eyepiece. The Prime Focus Reflector has the "eyepiece" in front pof the primary mirro working in the same way as a radio telescope "dish" does (more on that later in this discussion), and the Cassegrain (FIGURE 3 below) uses a secondary mirror to reflect the image through a hole in the primary mirror.
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FIGURE 3: Cassegrain Reflector
From: Caltech WIRE Web Site
Yes, there are also hybdids that combine features from both the refractor and reflector. Catadioptic telescopes (FIGURE 4 below) use both mirrors and lenses in such a way as the size of the telescope is reduced, but the field of view is large and crisp.
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FIGURE 4: Catadioptric Telescope
From: Stellafane Web Site
Electromagnetic Spectrum & "ROY G. BIV"
Mirrors and lenses serve to gather and focus the visible portion of the Electtromagnetic (EM) Spectrum, what we call light. As previosuly described, the visible spectrum has wavelengths between ~700 nm and ~400 nm (inclusive) that corresponds to what the human eye can detect, and what the Human brain interprets as color: ROY G BIV (Red, Orange, Yellow, Green, Blue, Indigo, and Violet):
Red - Longest wavelength we can see, Violet - Shortest wavelength we can see
FIGURE 5: Electromagnetic Spectrum
From: NASA Explores Web Site
As it so happens, the visible spectrum is a very small part of the total EM spectrum (FIGURE 5 above). It also happens that the physical dimensions of the wavelengths of visible light happen to be about the size of a human cell, more especailly the receptor cells (rods and cones) in our retina at the back of our eyes. In another remarkable coincidence, the majority of energy output from our Sun that reaches the surface of the Earth corresponds with wavelengths inclusive of the visible spectrum. So, our eyes are pretty well suited to the environment we live in - Planet Earth. However, other creaters on Earth "see" wavelengths different from what we Humans do, and their world can look vaery different from ours. For example, Humans can see red light, but bees can't; bees can see ultraviolet radiation, but Humans can't.
2. Radio Astronomy
Non-Visible Electromagnetic Radiation
We are already familiar with the advantages of using non-visible EM radiation. Day 1 of this Geoscience Tour, Climate & Weather made use of satellite imagery comprised of non-visual information, such as Infrared data showing water vapor content in the atmosphere not visible as clouds are in the visible satellite products.
In the same way that satellites help show information ordinarily not visible to Humans, Radio Astronomy is another way of looking at what isn't visible to us in the night sky.
As previously described, radio wavelengths are too long to be detected by our eyes. Radio wavelengths include Microwaves (around 1 cm from wave-peak to wave-peak), FM Radio wavelengths (around 3 meters or ~117 inches), and AM Radio wavelengths (around 300 meters). Just as our Sun emits wavelengths of radiation we can't see, so too do countless objects in the Universe. To fully understand these objects, just as we work to fully understand weather and climate, radio telescopes capture the non-visible EM spectrum and present the information contained therein in a way Humans can use.
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FIGURE 6 - Radio Telescope
From NASA JPL Website
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FIGURE 6 (left) shows the familiar radio "dish" or "antenna" used to collect and focus radio energy. The curved reflector (a.k.a. "dish") reflects the incomming EM waves to a focal point, the pime focus, where the receiver "horn" coverts the waves into electrical signals processed by electronic circuits and computers. It is important to note that, with trivial changes, FIGURE 6 also diagrams a prime focus optical reflector - replace the dish with a similar mirror and the horn with an eyepiece, and you are back to the visible spectrum!
Radio telescopes tend to be very large because of the nature of the EM radiation they collect. First, the wavelengths are large, so you need a large dish to collect enogh of this energy to equal an optical telescope. In fact, the diameter of a telescope's "objective lense" must be greater than the wavelength of the EM radiation it is try to collect. Optical telescopes's objective lense must be larger than 700 nm, and a radio telescope's reflector must be larger than the longest wavelength of interest. Second, radio telescopes need to be large so that they can detect very faint radio waves.
There are two choices for collecting very long wavelength radio waves: build a really large dish, or use interferometry - "connect" more than one radio dish together (IMAGE 1 below). By spacing telescopes a set distance from each other, or knowing the distance between already existing observatories, and using computers to correlate the signals being received at each telescope, astronomers can "stitch together" a detailed image of the object of interest (IMAGE 2 below right).
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IMAGE 1 - Very Large Array
From: National Radio Astronomy Observatory Website
IMAGE 3 is essentially no different from the enhanced weather images we worked with during the Climate & Weather Field Trip from day one, specifically that of IMAGE 17. In both cases, imformation from the visible spectrum is combined with information from other parts of the EM spectrum to produce a tool that conveys much more information than would othersie be availble.
3. Astronomical Coordinates
Wether your are using an optical or radio telescope, you need some way to locate the part of the sky you are interested in observing. Astronomers use a system known as Right Ascention & Declination, abbreviated as RA and Dec respectively. Dec and RA are often compared to the coordinate systems used on the surface of the Earth, lattitude and longitude, but they are not the same system.
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The Celestial Sphere (FIGURE 7 right) is an imaginary sphere surrounding the earth upon which celestial objects are mapped. Dec and RA, like latitude and longitude, are the coordinates that locate a given object on the sphere.
The Celestial Equator is an imaginary circle that results from projecting the Earth's equator onto the celestial sphere, and the celestial sphere has a similarly located North and South Celestial Pole. Declination is the angle formed between the object being viewed and the celestial equator. The North Celestial Pole has a Dec of 90° N, the South Celestial Pole has a Dec of 90° S, and the Celestial Equator has a Dec of 0° (this is basically latitude).
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[Click Image to Enlarge]
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FIGURE 7 - Celestial Sphere
From: University of Virginia Astronomy Dept. Web Site
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Right Ascention is often confusing until thought of as a type of longitude that is fixed on the celestial sphere rather than the familiar longitude being fixed on the surface of the Earth. A brief digression to review longitude is in order...
The Prime Meridian (0° Longitude) in Greenwich England is the starting point for the longitude system used on Earth. Meridian is a Latin derivative meaning "middle of the day", or "noon". A given longitude, therefore, locates where on the Earth 12:00 Noon will be and thus divides the day into AM (anti, or before meridian), and PM (post, or after meridian) at that location. Moving West or East of the Prime Meridian changes when the Sun will rise and set and also gives us the various time zones we are familiar with (subtact 5 hours from Greenwich Time, and you get Eastern Standard Time). The distance between two lines of latitude vary depending where you are on the surface of the Earth (unlike latitude). Dividing the circumference of the Earth by 360° gives a separation of 111 km for each degree of longitude at the equator, but this distance gets smaller as latitude increases until reaching 0 km at 90° N. The upshot of this nonlinear relationship of longitude with latitude requires the use of a very accurate clock when trying to determine longitude. For more background on this topic, refer to WGBH's NOVA site Lost at Sea - The Search for Longitude.
Just as the longitude system involves time, so too does Right Ascention. The difference is that RA is based on when the Vernal Equinox takes place - when the Sun's path (the ecliptic) crosses the celestial equator. RA is divided into 24 hours because it takes 24 hours for the Earth to rotate 360°. The Earth, therefore, will rotate 15° every hour (360°/24 hours). The 0 hr of RA is defined as the 1950 Vernal Equinex, which is the first point in the constellation Aries, just as the Prime Meridian is defined to be 12:00 Noon in Grenwich England. So how does RA actually work to help locate celestial objects? Say you are interested in observing the constellation Cancer. You know its RA is 8, but you don't have a planishpere handy to tell you when to go out and make your observations. If you see the constellation Orion directly overhead and know its RA to be 5, you can be assured that Cancer will be overhead in 3 hours (8-5). This isn't very different from using longitude (time zones) to determine the best time to telephone someone overseas so as not to wake them up.
Fortunately, observing the night sky is made easy by using tools that have already calculated the coordinates of the various stars and constellations.
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The easiest tool to use is the Planisphere (IMAGE 4 right). A planisphere is essentially a star chart that is covered by an overlay that has an oval-shaped hole in it. The star chart has the month, day, and RA (in both hours and degrees) printed around its edge and is visible through the overlay outside the red line in IMAGE 4. The overlay has the local time and cardinal compass points on it.
To use the planisphere, rotate the overlay until the local time lines up with the month and day on the chart beneath. Next, turn and face North and hold the planisphere over your head, aligning the N compass point on the planishpere with North. The stars visible in the planisphere's oval "window" will match the stars you see in the night sky (assuming you arn't beneath a street light!).
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[Click Image to Enlarge]
IMAGE 4 - Plansiphere
From: Firefly Web Site
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A planisphere and your eyes are all you really need to make observations. Binoculars are the next tool, followed by ever more powerful, and expensive, equipment.
Summary
The background information presented here covers the basics that will help you get more out of the field trip. We covered the electromagnetic spectrum and how optical and radio telescopes work. The celestial coordinate system was introduced, as well as the planisphere, as you will need to make use of these during the evening of the field trip.
What Next?
The information presented above is meant to provide a context with which to better appreciate astronomy. During the field trip, additional information detailing the locations we will visit will be provided, as well as a step-by-step guide for observing the night sky.
Continue on to the
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IMAGE 2 - NGC 5128 radio image
IMAGE 3 - NGC 5128 Composite
