You will have a lab this week on telescopes, in which you will build a refracting telescope. In the first lecture, back in the first week of classes, we already talked about telescopes and adaptive optics. Here we will say a bit more about how telescopes work, so that you will be better prepared for learning about them by building one in the lab this week. 1
This is quite a complicated diagram from Wikipedia, and we will take it just a few light rays at a time in the following slides. 1
Here we can see graphically that the telescope can collect more light from a distant object than the unaided eye. This can make a distant object appear brighter. The objective lens on the left brings parallel rays of light from distant objects into focus on the focal plane a distance f 1 from the lens. The eyepiece has a shorter focal length f 2 which returns the rays to be parallel, so that the eye perceives a distant object.
Parallel rays from a distant object in the lower part of the field of view are brought to a focus at the upper part of the focal plane, from which they are made parallel again by the eyepiece, so that they appear to the eye to come from the upper part of the field 1 of view.
In this way an inverted image of the distant object is created, as indicated by the arrows at 4, 5, & 6. The distant object also seems larger. Parallel rays from a distant object in the upper part of the field of view are brought to a focus at the lower part of the focal plane, from which they are made parallel again by the eyepiece, so that they appear to the eye to come from the lower part of the field 1 of view.
The above diagrams illustrate the function of a telescope in collecting a large amount of light from a distant object, so that faint objects are visible. Also, the diagrams illustrate how a telescope can make distant objects appear larger. A telescope can also produce a sharper image than is possible with the unaided eye, because of a phenomenon called diffraction. 1
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Plane wave fronts enter from the right, hit a slit in a barrier, and a circular wave is transmitted rather than just a section of the plane wave front.
The diffraction of light as it passes through an aperture, such as the aperture of a telescope or of a lens (such as the lens in your eye) causes the paths followed by light rays to spread out, which causes the resulting image to be blurred. Because the aperture of a telescope is much larger than the aperture of your eye, images it forms are much sharper and have far greater detail. Light collection and image resolution of detail are the two most important features of astronomical telescopes. The following slides are a reprise of our discussion in the first lecture of advances in telescope technology, and, in particular, of adaptive optics.
The 36-inch refractor at the Lick Observatory (at left). An example of 19 th century telescope technology. At right, the Mt. Wilson 100-inch telescope, 1920s.
The 200-inch telescope of Palomar Observatory (pointing to the zenith and viewed from the south). This old telescope is still in very active use today.
HST at Lockheed (at left). Hubble Space Telescope before launch. Should be old technology by now! Above it is released from the Space Shuttle (now dead).
Keck 10- meter telescope, Mauna Kea, Hawaii A modern, groundbased telescope, that is much larger than the Hubble Space Telescope.
Adaptive optics, a recent advance, makes it possible for ground-based telescopes to eliminate the blurring of images that atmospheric turbulence otherwise would introduce.
This technology, which is now in digital cameras, was invented in the 1970s by the inventor of the laser in order to allow soldiers to shoot steadily from helicopters in the Viet Nam war. The technology took 15 years to migrate to astronomical telescopes. It caused a veritable revolution in observational astronomy.
In astronomy, the effects of atmospheric blurring can be avoided by going into space. However, facilities like the Hubble Space Telescope are extremely costly to build and operate, and despite their expense, space-based telescopes remain relatively small. To compare HST and the Keck Telescopes, HST cost roughly 20 (?) times more to build and launch, yet Keck has 20 times the light gathering area and -- potentially -- 4-5 times better resolution.
Mars, seen with the 100-inch telescope on Mt. Wilson
Mars, seen with a small telescope with attached video camera by a student, Rolf Karlstad.
Karlstad observing equipment setup. A video camera records the images at a rate of 60 per sec. in a digital format saved on video tape. The camera can take pictures in infrared light. The images are later aligned and composited on a PC. This gives a manual kind of adaptive optics, which, together with the patience and persistence of the observer, explains the image clarity.
An image of Mars taken with the Hubble Space Telescope 8/24/03. This is the sharpest color picture ever taken of Mars from Earth. When viewed through a powerful telescope, surface markings are visible, as well as polar caps, from which the rotation of the planet and the alternation of its seasons can be observed.
Iani Chaos Mars Express An artist s concept of the Phoenix Lander in the arctic region of Mars We get the sharpest images of Mars by simply going there.
The Phoenix Lander site on Mars, taken at 6 AM 8/14/08, with visible frost on the rocks.