ASTR 1210 (O'Connell) Study Guide
Summit of Mauna Kea, Hawaii
The telescope is the single most important invention for astronomy.
It is a beautiful example of the interplay between technology
(fabrication of quality glass, optics design, polishing techniques,
large mechanical structures, computers) and basic science.
This lecture describes the main features of optical-band
telescopes---i.e. those which operate in or near the part of the EM
spectrum to which our eyes are sensitive. This is the only kind of
telescope which was in widespread use before 1950.
Since that time, astronomers have developed other types of
"telescopes" to exploit a large part of
the whole electromagnetic
spectrum. Cosmic sources produce radiation across the entire
range of this spectrum. Some telescopes for other spectral bands
(e.g. the ultraviolet and near-infrared) are quite similar to optical
telescopes. Others (e.g. for the radio and gamma-ray bands) use
very different technologies.
A. Introduction and History
The telescope was invented in 1608 by Lipperhey in Holland.
Although lenses had been used in eyeglasses for several centuries,
realization of higher precision optical devices suitable for viewing
distant objects depended on improvements in lens-grinding
technology. Details of the early work of Lipperhey, Galileo, and
Note: the microscope was invented in 1654, also in Holland, and was
responsible for opening up a second kind of "invisible world." Modern
medicine would not exist without the microscope.
The first astronomical use of a telescope was by
Galileo, in Italy in 1609.
The telescope instantly and utterly transformed astronomy
(see Study Guide 7).
- Collect more light: in order to detect fainter objects. This is the
most important function of telescopes.
- Light gathering power depends on the telescope
- Thus, a 10-in diameter telescope collects (10/5)2 =
22 = 4 times as much light as a 5-in telescope.
- An 8-in telescope (widely used by amateur astronomers) collects
1600x more light than the human eye. Because there are many more faint
stars than bright ones, an 8-in scope can detect over 2000x as many
stars (10 million compared to 5000) as the unaided eye.
- Resolve sources better: provide sharper
images, permit seeing more detail. Resolution depends on both the
diameter of the telescope and its optical quality
- Magnify sources: make the images of distance objects
larger for easier study
- An objective or "primary" optical
element forms an image (i.e. an accurate
representation of the original scene) at a
usable focus, where it can be studied by eye, recorded
by film or other detectors (as in a camera), or fed into yet other
The objective element can be either a lens or a mirror.
Types of telescopes
There are therefore two basic types of telescopes:
- Refracting telescopes: the objective is
a lens (shaped, transparent glass)
which refracts (or bends) incoming light rays to a
common focus. The image at the left below shows how a flat glass
surface bends light rays. In this case, two flat surfaces at a angle
have been combined to make a prism. The shorter the
wavelength of the light, the stronger the bending.
The image at the right below shows how a glass surface can be
continuously curved to bring all the light rays passing through
it from a given point on a distant object to a common focal
Galileo's telescopes were refracting but only about one inch in
diameter. The largest
refractor ever built was 40 inches in diameter (Yerkes
Refraction of Light By a Prism
(click for a descriptive animation)
Shaped Convex Lens
- Reflecting telescopes: the objective is a
shaped mirror coated with a metallic film,
which reflects light rays off its front surface to a common
focus. See the picture below. Invented by Gregory (1663); improved by
Many early reflecting telescopes employed
metal mirrors, but almost all modern designs use glass mirrors.
At first sight, a reflecting design is counterintuitive because the
focus is in a position where placing equipment would interfere with
the incoming light beam. This is true, but the effects, both in terms
of blocking the light and diminishing the image quality, are small if
the mirror is large enough. Most reflecting telescopes use a
"secondary" mirror to redirect the light beam outside the main
body of the telescope. There are many different
designs for reflecting systems, each
with its own advantages and disadvantages.
The mirror is easy to support from behind, unlike a lens which
must be supported from its edges and tends to sag. Reflectors have many
other advantages. (Click here for more information on these.)
All large telescopes are therefore reflectors. The
largest reflector is 400-in (10-m)
in diameter (built 1993); a telescope 1550-in (39.3-m) in diameter is
now under construction at the European Southern Observatory.
Reflection of Light by a Figured Mirror
Applet. Here is a Java applet illustrating the
differences between refraction, reflection, and diffraction.
- For distant objects (including all astronomical objects), the
incoming rays are parallel to one another. Such rays are focussed in a
plane which is one focal length from the objective. This is
called the "focal plane." Click on
the button below for a Java applet illustrating image formation for objects
at different distances.
- In the focal plane, the light rays from a distant object form a
one-to-one representation of the distant scene which is called an
- Ordinarily, a camera or other instrument in placed at the focal
plane. For "visual" use of a telescope, an eyepiece
(a smaller lens or combination of lenses) can be used to magnify the
focal plane image so it can be viewed by the eye. See the
illustration above. The magnification of the image seen
through the eyepiece is equal to the ratio of the focal length
of the objective to the focal length of the eyepiece.
C. Image Quality
The crispness of images made by a telescope depends on several factors: fabrication
of the optics, the size of the telescope compared to the wavelength of light,
and the Earth's atmosphere.
The "resolution" of a telescope image is quantitatively defined to be
the smallest measurable detail in an image (in seconds of arc).
Diffraction of light waves
- Light is a wave. In order to produce a good image,
telescope optics must be figured to a minimum tolerance of about
1/4 of the wavelength (distance between crests) of the light they
are intended to focus. For optical telescopes, this is about
- The intended shape of an optical surface, e.g. the curve in the convex
lenses shown above, must be reproduced to high precision in order to
obtain good image quality.
- Scale comparison: if a 320-in (8-m) diameter telescope mirror were
scaled up to the size of the continental United States, i.e. about
3000 miles diameter, then the maximum ripple allowed in its polishing
would be only about 2 inches!
- A fundamental limit on resolution is set by the physics of light.
Since it is a wave phenomenon,
light spreads out or diffracts when it passes through
an aperture (like ocean waves around a breakwater). This smears
is a nice interactive Java applet illustrating diffraction.
- Diffraction is worse the longer the wavelength of light and the
smaller the telescope aperture. Click
for an illustration of how telescope size affects resolution.
- A 10-in diameter telescope with perfect optics can resolve 1
arc-sec at optical wavelengths. A 100-in diameter telescope could
resolve 0.1 arc-sec.
- Note that almost all stars are so distant that they are smaller in
angular size than 0.1 arc-sec and therefore appear as point
sources in such a telescope. Only a handful of stars can be
resolved by even the largest telescopes.
"Seeing" Produced by Earth's
- The Earth's atmosphere also refracts light; and
because it is constantly moving, there is always a blurring and
jittering of images in a telescope. Astronomers call this "seeing."
Seeing actually dominates diffraction in most cases and usually limits
telescope resolution in practice to 0.5-2 arc-seconds.
- Above is an enlarged image of the bright star Betelgeuse seen
though a large telescope. It is a large blob, broken up into smaller
near point-like units. Click on the image for a
video of the seeing effects. In
the absence of the atmosphere, the image would be steady with all of
the light concentrated into one of those point-like structures.
- To overcome seeing effects, special equipment such as
adaptive optics, which can sense and partially correct
for atmospheric blurring, can be used. Or, telescopes can be placed
in space (where there's no atmosphere), though this is much
Mirror blank for one of the two mirrors of the Large
Click for enlargement.
D. Current Telescope Milestones
The Hubble Space
Telescope: 94-in reflector in space (launched 1990)
HST is not a large telescope by modern standards. But it has
produced the highest resolution images yet obtained at visible
wavelengths, with blur sizes of only about 0.05 arc-seconds. This is
because of its high quality optics and the fact that it
is outside the Earth's atmosphere, so it does not have to
contend with seeing. Its high resolution and the absence of
the natural and artificial atmospheric background also allows
it to detect very
Here is a composite
version of some of HST's best images.
Two 400-in "segmented mirror" telescopes (1993, Hawaii). The
collecting area of each consists of 36 independent 36-in hexagonal
mirrors. See image at right and
The Very Large Telescope (VLT): Four
320-in monolithic mirror telescopes (2001, Chile)
Large Binocular Telescope: two 330-in (8.4-m) diameter
monolithic mirrors on a common mount, providing the largest existing
collecting area. One of the mirrors is shown above. UVa is a
partner in this project.
Large mirror technology
Observatory) is a brief description of how the latest
generation of huge telescope mirrors are manufactured:
The Magellan main mirrors are f/1.25 paraboloids and a radical
departure from the nearly solid-glass mirrors of the past. Each is
21,000 pounds of borosilicate glass with a lightweight honeycomb
structure inside. It took 6 months to build the mold for each mirror,
2 days to fill it with chunks of glass, 1 week to melt the glass and
spin it into shape (in a specially designed rotating oven), and 3
months for the glass to cool. Each was then polished for 8 months
while its surface was constantly tested for accuracy. Relative to
their size, the main mirrors are about as thin as a dime.
The aluminum surface of each mirror is a mere four-millionths of an
inch (0.1 micron) thick. Each also sits in a "cell" that peforms two
important functions. First, the cell's thermal control systems prevent
warping from thermal expansion and contraction. Second, the support
systems in the cells maintain the mirrors in their proper shape, so
there is no distortion or cracking. The actual shape of the mirror
surface is controlled to within two-millionths of an inch (0.05
Other EM spectral bands
The telescopes we've discussed so far operate only in the optical
(or "visible") and adjacent spectral bands, but astronomers now
exploit most of the full electromagnetic spectrum. The first
instruments outside the visible range were radio telescopes (1950's).
Now astronomers operate not only radio telescopes
(e.g.the National Radio Astronomy
Observatory, with headquarters in Charlottesville) but also
microwave, infrared, ultraviolet, X-ray, and gamma-ray telescopes.
Because the Earth's atmosphere screens out many parts of the the EM
spectrum (see Study Guide 10),
telescopes for the gamma-ray, X-ray, ultraviolet, and parts of the
infrared and microwave spectrum must be placed on spacecraft outside
E. Next Generation Telescopes
Two very large telescopes based on the segmented-mirror concept
of Keck have been designed:
Thirty Meter Telescope
Extremely Large Telescope (39-meters or 1550-in). The ELT is now
under construction in Chile, but the TMT is embroiled in a dispute
over environmental and cultural impacts at its preferred Mauna Kea
Giant Magellan Telescope
(at right), also
under construction in Chile, is a multiple-mirror design with
7 8.4-meter spin-cast mirrors and an equivalent collecting area
of a single 22-meter mirror. The six off-axis segments are
challenging to figure to the correct surface shape.
Another large telescope with a very different design and operations
mode is the Large Synoptic Survey
Telescope. To achieve a wide field of view (3.5o) it
employs a unique 3-mirror design in which the primary and tertiary
mirrors have been figured on a single piece of spin-cast glass
8.4-meters in diameter. The telescope is intended to repeatedly image
the entire usable sky every three nights, searching for transient or
moving targets (including near-Earth asteroids) while building up an
ultra-deep combined image of the sky. Continuous output from its 189
imaging CCDs (see below) will generate an unprecedented data volume
(15 TB/night). Under construction in Chile, LSST should be in
operation by 2021.
The James Webb Space
Telescope, expected to be launched in 2021, is the follow-on to
HST. It features a 6.5-m diameter primary mirror (a 25 square meter
collecting area, 5.5 times that of HST) composed of 18 hexagonal
segments that must be deployed on-orbit. JWST carries four imaging
and spectroscopic instruments and is optimized for the near-infrared
spectral region. A large sunshield permits a low overall structural
operating temperature through cooling to space. JWST will orbit
around the Earth-Sun L2 point, about 900,000 km
eye is a sophisticated, auto-focus, auto-exposure, electrical
camera system. However, for all its versatility and importance to us
in everyday life, it is a seriously limited astronomical detector: it
is small, its maximum integration time is only about 0.1 sec, and it
has low sensitivity. Astronomers have long sought more capable
detectors to use with telescopes. Descriptions of the two most
important kind of imaging detectors are given next:
- Film was the main astronomical detector used between 1900 and 1980.
- It detects only 1-2% of incident photons (not much better than
the eye) but allows long integrations (hours)
- Requires chemical development of image after exposure, a
- Provides permanent storage of image information---a tremendous
benefit. But information on film is not in digital form.
- Extends the observable EM wavelength range to regions (the
near-UV and near-IR) where the eye is not sensitive
- Large formats are possible (up to 20-in square for astronomy)
- Nonlinear, nonuniform response makes quantitative measures of incident
light energy difficult
Charge-Coupled Device Architecture
CMOS solid state detectors, with characteristics similar to CCDs but less
suitable for astronomical use, are employed in commercial still and
video cameras and cell phones. These are mass-produced in quantities
Many other types of electronic detectors are now used in the UV, IR,
and X-Ray bands of the astronomical EM spectrum.
- CCD detectors are a type of solid state electronics, using
a light-sensitive silicon wafer fabricated with embedded
microelectronic integrated circuits
- See image above. The CCD surface is composed of millions of
independent, light-sensitive pixels.
- After exposure, pixel
contents are shifted in 2 dimensions across the surface to an output
amplifier and storage device.
- Astronomical applications were pioneered during development of
the Hubble Space Telescope (1974-85).
- CCDs work well at both very short (TV) and very long (astronomy)
- They are 50-100x more
sensitive than film
- They provide digital image storage for immediate computer
- Determination of incident light energy is easier
- Only small
formats are available (2-in typical); but one can "mosaic"
CCDs to create a large area detector surface
- CCDs are now the standard detectors used in optical-band astronomy.
Sunset over the William Herschel Telescope (La Palma,
Spain; N. Szymanek)
Reading for this lecture:
Study Guide 14
Bennett textbook Chapter 6
Reading for next lecture:
Bennett textbook: pp. 203-204; Secs. 9.3, 9.5.
Study Guide 15
July 2018 by rwo
Text copyright © 1998-2018 Robert W. O'Connell. All
rights reserved. These notes are intended for the private,
noncommercial use of students enrolled in Astronomy 1210 at the
University of Virginia.