AY 101 Lecture Notes - Lecture 9: Primary Mirror, Active Optics, Curved Mirror
Telescopes and Observatories
The primary purpose of a telescope is to collect light over a large surface area and
secondarily to produce a magnification of the image of the objects under study. The site
at which one or more telescopes have been constructed is known as an observatory.
Refracting telescopes
A refracting telescope, or refractor, uses the property of refraction to bend the paths
of light entering at every position on the objective lens to bring it to a common focus
(see Figure 1). The basic function is similar to that of the eye of a camera. The largest
existing refractor is the 1.02‐meter‐diameter Yerkes telescope in Wisconsin. Refractors
have gone out of favor for astronomical work due to the enormous size of the glass
lenses (the Yerkes objective lens weighs 2 tons), the bending of the long tube of the
telescope due to its weight, their awkwardness because of the length of the tube, and
the cost of the large enclosure needed to shelter such a telescope. Also, as objective
lenses are made larger, they become thicker, causing progressively significant loss of
light by absorption in the glass.
Reflecting telescopes
A reflecting telescope, or reflector, uses a curved mirror as its light gathering surface
( primary mirror) and other mirrors and optical elements to bring all incident light to a
common focus (see Figure 2). Modern reflectors have a number of advantages over
refractors in that they can be constructed with larger light gathering surfaces (they can
be structurally supported on the back of the mirror), are mechanically more stable, are
physically smaller instruments (multiple reflections allow the light path to be folded
over), require a smaller and less expensive observatory building, and are much easier
to use. These reflectors do have the disadvantage that they require more critical
alignment of the optical elements (the mirrors); but with modern computers and laser
technology, alignment can be easily corrected. The most recent large telescopes are
also able, under computer control, to modify the surface configuration of their primary
mirrors ( active optics) to correct for the visually blurring effects of the atmosphere.
Detectors
Although the original detector of light was the human eye, astronomers now rely upon a
wide variety technological devices for the recording of electromagnetic radiation. These
devices are not only far more sensitive than the eye (a fully dark‐adapted eye detects
only 2 to 3 percent of the incident photons, whereas a modern electronic light detector
records 90 percent or more of the photons), but often can add (integrate) the energy
received over long periods of time and in wavelength regions other than visible light.
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