ASTR 1210 (O'Connell) Study Guide
Summit of Mauna Kea, Hawaii
Spring 2014: this guide is optional reading only
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 "telescopes" to exploit a
large part of the whole
electromagnetic spectrum. Some of those (e.g. for the
ultraviolet and near-infrared) are quite similar to optical-band
telescopes. Others (e.g. for radio and gamma-ray) are very
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 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 instruments
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
combine to make a prism). The shorter the wavelengthof the
light, the stronger the bending.
The image at the right 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. The largest refractor ever
built was 40-inches in diameter (1896).
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 reflecting material,
which reflects rays off its front surface to a common
focus. See picture below.
Invented by Gregory (1663); improved by Newton (1669).
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.
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).
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 can
be used to magnify the focal plane image so it can be viewed
by the eye. See the illustration above.
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 improves resolution.
- A 10-in diameter telescope with perfect optics can resolve 1 arc-sec.
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
the seeing effects:
- To partly overcome seeing effects, special equipment such as
adaptive optics can be used. Or, telescopes can be placed
in space (where there's no atmosphere).
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 faint sources.
Two 400-in "segmented mirror" telescopes (1993, Hawaii). Mirrors
consist of 36-in independent hexagonal mirrors. See image at right
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.
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
The human 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.
- Film was the main astronomical detector used between 1900 and 1980.
- Detects only 1-2% of incident photons (not much better than
the eye), but allows long integrations
- Requires chemical development of image after exposure
- Provides permanent storage of image information, but is not digital
- Extends the observable EM wavelength range to regions where
the eye is not sensitive
- Large formats are possible (up to 20-in square for astronomy)
- Nonlinear, nonuniform response makes determination of incident
light energy difficult
Charge-Coupled Device Architecture
- CCD detectors are a type of solid state electronics; widely used
in still & video cameras.
- 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).
- Works well at both very short (TV) and very long (astronomy) exposure times
- 50-100x more
sensitive than film
- Provides digital image storage for immediate computer processing
- Determination of incident light energy easier
- Only small
formats available (2-in typical); but one can "mosaic"
CCDs to create large area detector surface
- Now are the standard detectors used in astronomy
Many other types of electronic detectors based on similar concepts are
used in the UV, IR, and X-Ray bands of the EM spectrum
Sunset over the William Herschel Telescope (La Palma,
Spain; N. Szymanek)
(Optional) 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
June 2014 by rwo
Text copyright © 1998-2014 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.