Monday Feb 14 and Wednesday Feb 16 2011

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.

Time for light to reach Moon from Earth This shows the actual time for a beam of light (or any EMR) to get from Earth to Moon- about 1.3 seconds. In this image, the sizes of the Earth and Moon ARE to scale with the distance between them.

Astronomers often use the speed of light, combined with a TIME measurement, as a way to talk about DISTANCES. The distance light travels in one second is called a "light-second". So the Moon is about 1.3 light-seconds from Earth. The Earth is about 8 light-minutes from the Sun. We will see later in course that the stars you see at night are 10s to 1000s of light-YEARS away and that other galaxies besides our own Milky Way are millions to billions of light-years from us.

Ultimate speed limit The speed of light seems to be an absolute limit to the speed at which two bodies can pass each other. Also, more importantly, the speed of light seems to be the ultimate speed at which information can be transmitted from place to place.

Image formation with a lens. You can follow this by tracing rays of light. Always remember that the rays that pass through the center of the lens travel in straight lines. The rays hitting other parts of the lens are bent towards the optical axis of the lens, an imaginary line perpendicular to the lens and passing through its center.

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. A telescope that uses a lens to collect light is called a "refractor", as the lens focuses light by bending or refracting it. A telescope that uses a mirror to collect light is called a "reflector", as the light is focused by reflection from a curved mirror. 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.

Human eye cross section The human eye has a lens that focuses light on the light sensitive organ called the retina. It also has a varaible sized opening (the pupil) in the iris. Under low light conditions, the pupil enlarges to allow more light to enter the eye.

Camera cross-section This is a cross sectional view of a camera. Parallels to the human eye are obvious. The main parts are the lens, which focuses the light into an image in the focal plane, and the image detector, which records the image. In the "old days" when drbill was a boy (and we walked to school uphill both ways) the detector was a piece of photographic film. Now most cameras use an electronic detector. Most cameras also have an iris, an opening which can be made larger or smaller to adjust the amount of light reaching the image and also control depth of field.

In this camera, the lens has 3 separate pieces of glass or 3 elements. Multi-element lens are used to reduce chromatic and other aberrations.

Cassegrain reflecting telescope. Almost all big research telescopes are some variant of this basic Cassegrain design. The light is focused by the primary mirror (on right in this diagram) then it reflects from a smaller mirror through a hole in the primary mirror. The final focus is thus behind the main mirror (where the red X is in this diagram).

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. The 4 meter Cassegrain 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 focus behind the primary mirror.

DrBill posing at 4 meter in his younger days

Snow in Hawaii on Mauna Kea

Mauna Kea, Hawaii. 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.

Hawaii tourist pic

Hawaii tourist pic

Keck Headquarters building. Most astronomers who use the Keck telescopes 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. The Kecks are "short and fat", but have the largest collecting aperture of any optical telescopes.

Posing in front of Keck

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!)

8.4 meter mirror This is the mirror for one of the new large telescopes in Arizona. The mirror is 8.4 meters across. Thats 9.2 yards, or almost a first down. The mirror is not solid (it would weigh way too much) but has a honeycomb structure (visible through the front solid disk of glass here). The mirrors are made in a giant rotating oven located under the seating of the U. of Arizona football stadium.

By spinning the molten glass mirror as it cools, the glass automatically hardens into roughly the correct concave shape for focusing light. The front surface must be further polished to exactly the right shape and smoothness, but having the rough shape from the rotating oven makes this a lot easier. After the right shape is acheived, the front glass surface is coated with a thin layer of aluminum which is what actually reflects the light. Glass is used as the substrate for telescope mirrors, as it is possible to shape glass very precisely. Also, glass holds its shape very well.

A CCD "chip"

CCD vs. photographic plate All telescopes now use electronic detectors instead of photographic film to record images in visible wavelengths. The detector, called a CCD for charge-coupled device, is a light-sensitive silicon chip divided into many little "picture elements" called pixels. The first image shows a typical CCD- it has several million pixels (which you can't see on this image). The second image shows a comparison of of picture of a very small part of teh sky taken with a CCD and with a photographic film. The CCD is about 100 times as sensitive as the plate, so shows many more faint objects, even though the exposure time is much shorter than for the photographic image. The CCD has truly revolutionized astronomy in the past 2 decades.

Large CCD array This is a full-size model of the CCD array for another large telescope. Each little square is a CCD better than the best digital camera you could buy. The entire array will have 3.2 billion pixels- in camera ad terms, its a 3200 Megapixel camera!

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. The telescope is covered with a door that will open to let light into the telescope. The door has a shiny, reflective surface, and you can see the blue ocean and white clouds of Earth reflected in the door.

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 (it had spherical aberration). 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. Artificial lights light up the sky and make it harder to see faint objects, even with large telescopes. So astronomers try to put their observatories far from city lights. In the continental US, the best combinations of good weather and dark skies are found in parts of Arizona, New Mexico, and southwest Texas.

Dark sky OK panhandle / NE NM

Spectacular "shooting star" over Black Mesa 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.

The area around Black Mesa, at the western end of the Oklahoma Panhandle, provides some of the darkest sky anywhere. Each fall the Oklahoma City Astronomy Club (click here to go to OKC Astro Club website) has a week-long star party, called the OkieTex Star Party, near Black Mesa. The second image shows a spectacular "shooting star" (we will talk about those later) over the annual OkieTex Star Party held near Black Mesa. (Note that Orion is visible below the shooting star.) The red lights are from the camps of the astronomers who gather for the event. I was actually present at OkieTex when this shooting star came by, but I was asleep!

Many people have never seen the night sky from a truly dark place. Instead of the dozens or maybe hundreds of stars visible from light-polluted urban areas, the sky at a dark place is ablaze with thousands of stars visible to the naked eye. So, if you get the chance, go out and look at sky from a dark place on a clear moonless night. Summer nights are good, as they are usually not too cold and the Milky Way can be seen in all its glory.

Arecibo radio dish This is the largest single radio telescope in the world. The aluminum dish- essentially the collecting mirror- is 1000 feet across. The dish focuses the radio waves onto the receivers suspended above the dish.

Typical radio telescope This is a typical radio telescope. The dish is about 140 feet across. This dish, unlike the Arecibo dish, can easily be pointed at different parts of the sky.

(1) One of the VLA dishes (2) Very Large Array (VLA) The first images shows one of the 27 radio telescopes that make up the Very Large Array (VLA) in New Mexico. Each telescope is about 25 meters (80 feet) across. The telescopes can be moved aout on a large Y-shaped set of railroad tracks. The VLA telescopes act as a radio interferometer.