Wednesday November 2 2011

**************************** Electromaganetic radiation**************

EMR spectrum (TA36)

EMR spectrum, optical and radio "windows"

EMR spectrum, more detailed infrared transmission.

These 3 diagrams show the electromagnetic spectrum and the wavelength bands over which the Earth's atmosphere allows radiation to pass (primarily in the "visual window" and "radio window"). The visual window wavelength range can also be called the "visible window" or the "optical window". Of course our eyes sense the different wavelengths over the visual range as different colors, as shown in the color bar. The wavengths in the color bar are given in nanometers (nm). 1 nm= 1 billionth of a meter. Note that red light has the longest wavelength in the visible range and violet the shortest wavelength in the visible range, but other forms of EMR have wavelengths far longer than red light and far shorter than violet light. Hence the names "infrared" (infra- mean "beneath" or "below"), or light below red (in frequency) and "ultraviolet" (ultra- means "beyond" or "above"), light above violet in frequency.

The first graph (TA36) shows the frequency (in Hz = Hertz, or cycles per second) on top and the wavelengths (in meters) on bottom. Note that the numbers on top get larger as you move to the right, while the numbers on the bottom get smaller as you go to the right (the arrow on the bottom is somewhat confusing- it does NOT point in the direction of bigger numbers but of smaller numbers "Decreasing wavelength").

The different graphs give somewhat different regions called "radio waves", "infrared" etc. There are no sharp divisions between these different forms of EMR- they only differ in wavelength and frequency from each other, so the distinctions between, say, radio and microwaves is somewhat arbitrary.

The inverse square law. As light (or any EMR) travels away from its source, it spreads out to cover a larger and larger area. Thus, the flux, or amount of light per unit area, drops with distance. The flux goes as the inverse of the square of the distance- for example if we have two light sources with the same power or luminosity, but one is 3 times as far from us as the other, the flux we measure from the more distant one will be 1/9 that of the closer light source.

The flux is what we see as brightness- a bright star has a high flux, while a faint star has a low flux.

Luminosity, flux and distance Just another way of looking at the inverse square law. The luminosity of a source of EMR is the total power emitted. This is measured in units of Watts (W). At a distance d from the source, an observer measures a flux (f) of power from the source. Flux has units of watts per square meter (W m ^-2). If the source emits isotropically (equally in all directions) and there is no absorption of photons between source and observer then the same amount of power (equal to the luminosity) must pass any sphere at any distance (assuming the EMR has had time to get to the distance d).

In astronomy we measure f (using a telescope and detector- usually over only a limited wavelength range), try to get d (from various techniques, depending on the type and distance of object we are studying), then calculate L.

Apparent magnitudes Apparent magnitudes (table) Astronomers usually talk about the flux of a star in terms of its apparent magnitude. This idea goes way back to Hipparchus, a Greek astronomer who lived more than 2100 years ago. The scale appear to go "backwards"- bigger number mean smaller flux (fainter stars). The scale is also a ratio scale- a change of 5 magnitudes corresponds to a change in flux of a factor of 100.

The flux of objects shown on the graph covers an enourmous range- from the Sun to the faintest objects we can see with the largest telescope spans a range of about 50 magnitudes. Each 5 magntudes is a FACTOR of 100 in flux, so a difference of 50 magnitudes corresponds to a factor in flux or brightness of 100x100x...x100 (100 multiplied by itself 10 times)= 100 million trillion!

Continuous spectra of blackbodies of various temperatures Every solid or dense gaseous body radiates a continuous spectrum called a blackbody spectrum. The word "blackbody" is quite confusing- the Sun radiates a blackbody spectrum, but it sure isn't black! The wavelength of the peak of the emission depends on the surface temperature of the body. The hotter the body, the shorter the wavelength of the peak emission. The Sun, with a surface temperature of about 5700 Kelvin, radiates mostly visible light (visible EMR). A hotter star (an example of a 50000 Kelvin star is shown) would radiate mostly ultraviolet EMR. You and I, with a body temperature of 98.6 Farenheit (or 310 Kelvin) radiate or emit infrared EMR. Of course, our eyes are not sensitive to infrared EMR, which is why you can't see yourself glowing if you are in a dark room (one with no sources of visible EMR). In your normal everyday life, you see people and objects near room temperature only by visible light that is REFLECTED by the body- not EMITTED by the body. The visible light is usually from a hot body (Sun or lightbulb filament, but there are also "cool" things that emit visible light- such as LEDS - but these do not emit blackbody radiation, but a form of emission line spectrum).

Spectrum of the filament in an ordinary incandescaent light bulb.. An ordinary (incandescent) light bulb produces light when a metal filament (made of the metal tungsten) is heated to about 2300 degrees Kelvin by passing an electric current though the filamant. The filament gives off blackbody radiation, with the maximum emission at a wavelength (lamda- max) of about 1250 nm. As seen in the graph, only a small fraction of the light's EMR is in the visible region of the spectrum, from about 400 to 700 nm, where our eyes are sensitive. Thus, most of the electrical energy is wasted, as it produces infrared radiation which can not be detected by the human eye. The infrared radiation is absorbed by the bulbs surroundings and turned into heat, which may be fine in the winter, but in the summer this wasted energy is doubly bad- you have to pay more for your airconditioning bill to get rid of the useless heat generated by the filament. If we could run the filament at a higher temperature, then the bulb would make a higher fraction of useful light, but then the filament would melt.

Light bulb and Wien's Law Here is a practical application of Wien's Law. The spectra show the spectral energy vs. wavelength for an ordinary incandescent light bulb. Note that the peak is in the infrared, and only a very small fraction of the emitted EMR is useful light that our eyes can see. (Much of the EMR is *worse* than useless- in a building, the IR energy heats up the building and often one must pay more in A/C bills to remove the heat!) Why? Application of Wien's law shows that the filament temperature is 2320 K. To get more useful light, the filament would have to be made to run hotter. This is possible, but then the filament would rather quickly evaporate.

Spectral energy distribution of Sun The spectral energy of the Sun can be fitted with a 5800 K blackbody curve over a wide range of wavelengths, particularly in the visible and infrared where most of the Sun's energy is emitted. The Sun has much more x-ray emission than can be accounted for by a 5800 K source. These x-rays come from the hot corona, a very hot (2E6 K) but very optically thin gas surrounding the optically thick photosphere of the Sun.

********************Telescopes*********************

Image formation by a lens A lens forms an *image* of an objects by bending light rays from the object. If we put a piece of photographic film (or nowadays an electronic imaging detector) where the image is we have a camera or telescope.

Focal length of lens The distance from the central plane of the lens to the place where the image forms (for a very distant object) is called the focal length. The two most important properties of a lens are its diameter and its focal length.

Measuring angles: degrees and radians The common way to measure angles is with degrees. A more natural way is to use radians.

Measuring angular area or solid angle

Image size vs fl The size of the image depends only on the focal length of the optical system (here a simple lens) NOT on the diameter of the lens. Here a small diameter, long focal length lens makes a larger image of a distant galaxy than a much larger lens with a shorter focal length.

Image size: the math This is the quantitative relationship between angular size of the object, the focal length of the lens, and the size of the image of the object in the focal plane.

Visual telescope. When you think of "telescope", you probably think of a tube that you look through. A simple visual telescope uses a lens or mirror to form an image, which is then visually inspected with a small magnifying glass, or eyepiece. However, most astronomical telescopes are used as giant cameras. There is no need for an eyepiece, we put our detector directly in the image plane.

Spherical and chromatic aberration in lenses.. (Above line): A lens which has a curve that is part of a sphere will not bring distant light to a good focus due to spherical aberration. A lens with a special non-spherical (an aspherical lens) can bring distant light rays to a sharp focus.

(Below line): Different color light is bent slightly differently when passing through a lens. This defect is called chromatic aberration. This means that there is no common focus for the light, and images will look fuzzy. By combining two lens, made of different types of glass, we can reduce the effects of chromatic aberration.

In a reflecting (mirror) optical system, the light does not pass thorugh the mirror, and there is no chromatic aberration. This is a big advantage of reflecting over refracting telescopes.

Spherical aberration in mirrors. Just as in a lens, a spherical - curved mirror exhibits spherical aberration (but not chromatic aberration). By making the curve on the mirror parabolic in shape, spherical aberration is eliminated.

Refracting and reflecting telescopes A refracting telescope uses a lens as its primary light collecting optical element. A reflecting telescope uses a mirror as its primary light collecting optical element.

Cassegrain telescope There are many different optical configurations for telescopes. This one uses 2 mirror, the primary (largere) and the secondary (smaller). The focal plane is behind the primary mirror. The primary has a big hole in the middle.

Schmidt and Schmidt-Cassegrain telescopes The Schmidt camera is a telescope that has a mirror AND a refracting (transmissive) corrector plate. This combination refractor/ reflector is often called a catadioptric system.

Some common astronomical telescope variants

Yerkes 40 inch refractor. This telescope was completed in 1897. It remains the largest successful refracting telescope (lens as primary light gathering element) ever made (a larger lens made in France in 1900 never worked). The telescope is located at Williams Bay, Wisconsin, which is not a very good observing site by todays standards! Note how long and thin the telescope is- the focal length is about 19 meters, and the lens is 1 meter in diameter, so its an f/19 telescope.

Kitt Peak National Observatory, Arizona. This national (governement funded) observatory was founded in the late 1950s as a response to Sputnik. Access to telescopes here is open by competition to all qualified astronomers in the US. The observatory is located at 2200 meter elevation, about 80 km from Tucson. Because of growth of Tucson and Phoenix, the site is no longer completely dark, but its still a decent site. The largest telescope is a 4 meter (on left side of image). The odd triangular structure on the right is a telescope designed to study the Sun. Larger U. S. National Observatory telescopes are now located in Hawaii and Chile. I have spent over 300 nights of my life on this mountaintop.

4 meter telescope DrBill near Cass cage 4 DrBill with spectrograph The 4 meter RC telescope at Kitt Peak. This telescope was built in the 1970s. The black cylindrical structure is the "flip cage", which can be flipped end over end. One end of the tube has a prime focus, the other a secondary mirror to send the light to a standard RC focus behind the primary mirror.

Palomar 200 inch (5 meter). This telescope was built over 50 years ago, and remained the world's largest telescope for decades. It is located northeast of San Diego, California.

WIYN telescope mirror support. This is the back end of the WIYN (Wisconsin- Indiana- Yale- NOAO) 3.5 meter telescope on Kitt Peak. The primary mirror is thin, so that it can reach thermal equilibrium with the surrounding air quickly. This helps the seeing, or image sharpness. However, the mirror is so thin that it could not hold the proper shape by itself. Therefore, an active mirror control system pushes on the back of the mirror to bend it into the proper shape. The blue cylinders are part of the pistons that push on the mirror,.

Mauna Kea, Hawaii Keck telescope Mauna Kea The 4200 meter elevation summit of Mauna Kea, on the Big Island of Hawaii, is one of the worlds best observing sites. Note the complete absence of green plants- that is a good thing in an observing site, as it suggests lack of rainfall (and hopefully lack of clouds!). The silver structure to my right is the Japanese 8.4 meter Subaru telescope. The two white domes house the twin Keck 10 meter telescopes.

It does snow in Hawaii The white stuff in front of domes is snow, which is not uncommon at 4200 meter altitude in Hawaii. The white stuff behind the silver dome are clouds below the summit. The dark mass behind the silver dome is Maui, about 130 km (80 miles) from Mauna Kea, across the Alenuihaha Channel (yes, this will be on the test- just kidding).

Keck Headquarters building. Most astronomers who use the Keck tekescopes actually control the telescope from this building, which is about 30 km from the telescopes, at a much lower altitude! The telescopes can just be seen as white dots at the apex of Mauna Kea, which dominates the horizon in this image.

Tired astronomer in Keck control room. Here I am after a LONG nights work! Not that I am complaining. It was clear!

Side view of Keck telescope Looking into dome of Keck The Keck telescopes don't look as impressive as the Palomar 5 meter, even though each Keck has a mirror about twice diameter of the Palomar telescope. The Kecks are "short and fat", meaning they have a much faster (lower f/ ratio) primary than does the Palomar telescope.

Back end of Keck telescope. The primary mirror of the Keck telescopes are made up of 36 hexagons, each about 2 meters across. At each corner of each hexagon, pistons push and pull the individual mirrors to keep the overall mirror in the proper shape. An engineering marvel! (And it works!)

*********** Fighting atmospheric "seeing" (improving angular resolution)******

The atmosphere smears the light from distant objects, and makes them look "fuzzy". The technical term for this is "seeing". If we can get better seeing, we can see finer detail and also fainter objects. There are two basic ways we can improve the seeing (besides going to a good site): (1) adaptive optics (2) getting above the atmosphere.

Adaptive optics (I talked about this topic when I talked about binary asteroids.)

Deployment of Hubble Space Telescope from Shuttle bay. Here, in 1990, the Hubble Space Telescope (HST) is being moved out of the Space Shuttle cargo bay to be left in space.

HST cartoon. As soon as HST returned its first images, it was clear (or maybe "unclear" would be a better word) that something was very wrong. Turned out the primary mirror had been made to the wrong shape, so there was spherical aberration which resulted in greatly diminished angular resolving power. Hubble became the butt of many jokes- this is just one example. It was a major PR disaster for NASA.

Fixing HST. The main mirror on the HST (2.4 meters in diameter) was made to the wrong shape. This caused images to be about as sharp as images obtained from the ground, rather than much sharper as expected. In 1993, corrective optics were installed by astronauts, and the telescope has worked pretty well since then. Here an astronaut works in space to fix the Hubble's optical problems by inserting a module with "corrective lenses".

Optical light resolution vs. year. This is a schematic diagram showing the best resolution (ability to resolve two objects close in sky into two separate objects). The resolution is shown in arcsec, so smaller (towards top) is better. Before the telescope, resolution was limited by the human eye to 50 - 80 arcsec. The telescope, introduced around 1600AD, allowed a jump in resolving power. Although larger telescope should give better resolution, the Earth's atmosphere limits groundbased observations to about 1 arcsec resolution, so that even though vast improvements were made in telescopes from 1600 to 2000, the angular resolution didn't improve much. The HST (at least after 1993 "fix") is of course not affected by the blurring effects of the atmosphere, so its resolution is limited only by the telescope optics. The resolution of HST is about 0.1 arcsec.

Improved resolution with HST fix. This shows two images, obtained with HST, of the same galaxy. The righthand image was obtained after the 1993 optical fix. Note the great improvement in detail seen, as the resolution was much improved by the fix.

Kuiper Airborne Observatory. Astronomers are always trying to get high - I mean they try to get their telescopes above as much of the Earth's distorting atmosphere as possible. Space is the best place to put telescopes, but that is extremely expensive and time-consuming. To get above much of the atmosphere, telescopes have been placed on airplanes, small rockets, and balloons. Here is a dedicated aircraft observatory, called the Kuiper Airborne Observatory, on a C-141A jet transport aircraft. The black square is an opening through which a 0.9 meter telescope views the sky. This observatory is being replaced by a much larger telescope (2.5 meter diameter) on a 747 aircraft, called (for now) SOFIA- Stratospheric Observatory for Infrared Astronomy.

Infrared Astronomy Satellite (IRAS) telescope. Long before Hubble, many small telescopes were placed into Earth orbit. These were all designed to observe wavelengths that don't penetrate the Earth's atmopshere- x-rays, ultraviolet, many infrared wavelengths. Here is a very small telescope called IRAS that observed the infrared sky from Earth orbit in the mid 1980s. Although it was a very small telescope compared to telescopes on the ground, it made many important discoveries, as it observed wavelengths that simply cannot be studied from the ground. The main mirror was 0.6 meter in diamter (24 inches) smaller than the telescopes of some amateur astronomers!

There are many, many more small telescopes that have been placed in orbit over the past 40 years. This is just one example.

U.S. at night This is an image of the light pollution, caused by man-made lights, across the US.

Dark sky OK panhandle / NE NM The only "darkest sky" in Oklahoma, where there is essentially no problem with light pollution, is the very northwest corner of the Oklahoma panhandle. This is a "negative" image, with areas with darker sky showing as lighter shades of gray.

Large Synoptic Survey Telescope (LSST) The proposed LSST will have a mirror roughly equal in size (8.4 meters) to the mirrors in the largest telescopes now operating, but will operate quite differently from other large telescopes. Current large telescopes can only look at a very small part of the sky at once. The LSST will be able to look at a much larger piece of sky. The LSST will take very detailed pictures covering the ENTIRE SKY every few nights. Current big telescopes would require years or decades to take pictures covering the entire sky. The LSST will allow astronomers to do many types of projects, including searching for asteroids that might someday hit the Earth. If the $300 million can be found to build the telescope, it could be operating around 2014. The data will be made public immediately. This will allow astronomers from anywhere to do their own research using the data. Although there are current astronomical data archives that allow public access to vast amounts of data, such as the HST archives, this telescope should mark a new way to obtain and deal with optical images from large groundbased telescopes. There is, of course, a website: www.lsst.org.

Model of detector for LSST camera The camera for the LSST will use a large number of individual CCDs (electronic "film") arranged in a big array. This is an "actual size" model of the array. Each little square is a CCD bigger and more sensitive than the best digital camera chip. All together, the camera will have about 3.2 Billion pixels, or individual "dots" in the image. In terms of digital camera advertising, this is a "3200 megapixel" camera!! The camera will produce a staggering amount of digital data each clear nite, and the guys from Google are planning to help deal with the tremendous data flow. Each clear *nite*, the camera is expected to generate 25 TeraBytes of digital images!!

CCD versus photographic plate. The vast improvement in quantum efficiency between photographic plates and CCDs is illustrated here. These are images of teh same exact patch of sky. The lower exposure, taken with a CCD and a much shorter exposure than the upper image, taken with a photographic plate, goes MUCH deeper (detects fainter objects) even though it was taken in only 5 minutes, as opposed to 90 minutes for the top image!

WOW!