 specialized instruments are located. See the Shane telescope webpage for further discussion of different foci and their associated instruments.
Reflectors solve many of the problems inherent in refractors. Because reflectors use a mirror at the base end of the telescope, the mirror can be supported from below, unlike the lenses at the top of a refracting telescope tube. More support means that larger aperture (mirror diameter) telescopes can be built. In addition, reflecting the light rather than refracting it eliminates the chromatic aberrations of light passing through a lens. The object of study appears less distorted, and additional correctional lenses are not needed. Elimination of the lenses also decreases the length that a telescope tube needs to be. Decreasing the size of the tube reduces the cost and technical challenges of building the both telescope and its dome.
The glass of a reflector's mirror does not need to be as free of optical distortion than the lens glass of a refracto, since the light does not pass through it.. Only the surface of the mirror needs to be perfect. So casting mirror glass is less technically challenging than casting lens glass. Of course, the mirror surface must be painstakingly ground and polished to perfection and the reflective aluminum mirror surface carefully applied. Single mirrors as large as 330 inches have been cast. Even larger reflectors have been built using multiple mirrors nested in a mosaic. The Keck twin reflectors, for example, each use a mosaic of 36 hexagonal mirrors to provide an aperture of 10 meters, or 393.7 inches! Even larger reflectors are now being designed, most notably the Thirty Meter Telescope (TMT). Larger apertures mean more light can be gathered, so fainter objects may be detected and studied.
In 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 is used in 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.
Nowadays, charge-coupled devices (CCDs) are used to record the light from observed objects which the telescope gathers, replacing both photographic plates and film. CCDs are used in home digital cameras as well as astronomical cameras and spectrographs. CCDs produce electrical charges in response to light, recording light much more sensitively than photographic emulsion. Hundreds of thousands (sometimes millions) of light-sensitive cells (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 records the amount of electrical charge in each pixel, producing a map of the number and distribution of photons detected. This map creates a very precise, high resolution image of the object observed.
By astronomical standards, a 10 megapixel CCD is small. Astronomical CCDs in the range of 500-1000 megapixels are commonly 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. High-sensitivity infrared (IR) CCDs are being pioneered at UC Berkeley Labs in conjunction with Lawrence Livermore Labs, to accommodate growing interest in IR observation and imaging. These new CCDs will increase image resolution by a factor of 10.
The CCDs pictured above were built in the UCO CCD lab located at University of California Santa Cruz. Note th