Lick Observatory Telescopes Index
Telescopes: Gathering LightTelescopes enable astronomers to gather light from distant celestial objects using a series of lenses and/or mirrors. Early astronomers observed this light directly at the telescope's eyepiece. Nowadays, the gathered light is passed into auxiliary instruments where it can be studied in detail. By analyzing the light, astronomers strive to learn about the nature of the celestial objects that produced it. Refractors vs. Reflectors
The maximum size of a refractor is limited to an aperture of about 40 inches. A lens much larger than this will slump and deform under its own weight. Since light must pass through the lens, it can only be supported around the edges and cannot be reinforced by a backing structure. In addition, there are serious technical difficulties both in casting perfectly clear disks of glass of this size, and in building, supporting, and housing the proportionately long tube structure needed for refracting telescopes.
Reflectors solve many of the problems inherent in refractors. Because reflectors use a mirror at the base end of the telescope, rather than lenses at the top end as refractors use, the mirror can be supported from below. More support means that larger aperture (mirror diameter) telescopes can be built. In addition, since light is reflected, it does not suffer the chromatic aberrations of light passing through a lens, so the object of study appears less distorted, and additional correctional lenses are not needed. This, in turn, decreases the length that telescope tubes need to be. Decreasing the scale reduces the cost and technical concerns of building both telescopes and the domes to house them.
Cameras & CCDs: Recording LightIn the late 1800s, A.A. Common, using the Crossley Reflector, discovered that astronomers could see more detail in the night sky in photographs than they could by direct observation. Similar to making a time-exposure with a camera, more light can be gathered and recorded over a period of time than the eye can see. Glass plates covered with photographic emulsion were used with telescopes much like film in home cameras. The longer the photographic plate was exposed, the more light was recorded. Many thousands of photographic glass plates have been produced at Lick Observatory, using the Great Lick Refractor, Carnegie Astrograph, and Shane Reflector, as well as the Crossley Reflector.
CCDs record light much more sensitively than photographic emulsion. Hundreds of thousands (sometimes millions) of light-sensitive cells called pixels cover the CCD surface. Each pixel is capable of recording many single photons, the smallest unit of light, by producing an electrical charge for each photon. Each pixel develops a total charge that represents the number of photons of light that it has detected. After an observation, a computer is used to read out the number and distribution of photons detected by recording the amount of electrical charge in each pixel. A very precise, high resolution image of the object observed is thus revealed. Astronomical CCDs differ from CCDs in digital cameras in several important ways. Astronomical CCDs:
By today's standards, an 8 megapixel astronomical CCD is small. Astronomical CCDs in the range of 500-1000 megapixels are currently constructed. Because CCDs are most sensitive in the near-ultraviolet to near-infrared range, CCD cameras record mostly visible light plus some ultraviolet and infrared radiation. Infrared (IR) CCDs are now being developed to accommodate growing interest in IR observation and imaging. The CCDs pictured above were built in the UCO/Lick CCD lab located at University of California Santa Cruz. Note the relative size of the quarter. Spectrographs: Analyzing Light
Spectrographs record light (using CCDs), spreading it into its constituent components, and draw a spectrum for the observed object. Spectra obtained at Lick Observatory include the visible colors plus invisible extensions, particularly infrared. The spectrum of an object shows its unique pattern of energy at each wavelength. Dark and bright lines appear on an object's spectrum, indicating absorption and emission of energy by chemical elements within the object. By studying the size, shape, and distribution of these lines, astronomers can determine characteristics of the object such as its temperature, chemical composition, and velocity, and whether it is moving towards or away from Earth. The spectrum of the sun above was generated by synthesizing data recorded with the Fourier Transform Spectrometer at Kitt Peak National Observatory. Image credit: N.A. Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF. Click on image for enlargement. New Tools: Adaptive Optics
Adaptive optics (AO) systems detect and measure how the blurring is affecting the image, and actually warp an auxiliary mirror to compensate. The deformable telescope mirror is almost infinitely adjustable. It changes shape in numerous places many times per second, compensating for changing atmospheric conditions to focus light precisely. Lick Observatory engineered the first astronomical Laser Guide Star (LGS, or AO LGS) system, making adaptive optics more practical. Before correcting the blurring of an observed object, AO systems must measure the atmospheric distortion. Sometimes a nearby bright star can be used as a "guide star," to measure this distortion. More often, no bright star is near enough to the object to be observed. The Lick LGS system actually creates a guide "star" by shooting a laser beam into the sky, making sodium ions in the upper atmospere glow. The atmospheric distortion is measured using this "star," and the deformable mirror is adjusted accordingly to observe the fainter object. This system is now being used at observatories throughout the world. Please see the Adaptive Optics webpage for more details about AO engineering and observation at Lick Observatory. Laser image courtesy of Laurie Hatch. New Methods: Infrared ObservationLight pollution, or light from communities and industries adjacent to observatories that reduces the accuracy of observation, is an increasing problem throughout the world today. Astronomers are increasingly interested in infrared (IR) observation, which measures wavelengths that we perceive as heat, because there is less earth-based "pollution" in this area of the spectrum. Many new IR cameras and spectrographs are being designed and built. Mt. Hamilton is less light-polluted than many observatories, thanks to the city of San Jose. San Jose employs low-pressure sodium vapor streetlights which "pollute" only one small area of the spectrum. However, the night sky over Mt. Hamilton is even more "dark" (without interference) in the IR range. Many astronomical processes create IR radiation that can be observed and analyzed. Examples include star formation, forming and colliding galaxies, novae and supernovae, and the event horizons of black holes. These processes are now being studied and quantified. Astronomers are able to quantify IR radiation from around "room temperature" to several thousand degrees above absolute zero. IR radiation also penetrates cosmic dust clouds more easily than visible light. This makes IR observation superior to visible light observation if the process itself is dusty, such as star formation. IR observation is also practical for observing remote objects when there are cosmic dust clouds between the object observed and the observer. The now-retired Lick Infrared Camera (LIRC-II) was an early pioneeer in IR imaging, used at the Cassegrain foci of both the Shane and Nickel telescopes. The Gemini IR Imaging Camera, built at UCLA IR Imaging Labs, is currently used with the Shane telescope. IR-sensitive instruments are being built for Adaptive Optics (AO), and IR-sensitive CCDs are being developed. Clearly, IR observation and imaging will be widely used in future research at Lick Observatory, and throughout the world.
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The turbulence of the earth's atmosphere creates an inherent problem for earth-based observations. Because the atmosphere is not uniform, pockets of varying density air bend (refract) the light from observed objects. This causes the familiar "twinkle" of stars. While poetic, this twinkling creates havoc with observations which are recorded over a period of time, in effect blurring the observed image.