ASTR 1230 (Whittle) Lecture Notes
2. INTRODUCTION TO TELESCOPES
Summit of Mauna Kea, Hawaii, world's
largest astronomical observing complex
The telescope is the single most important invention for astronomy.
Without it, we would have little of the profound understanding we
have obtained over the last few hundred years about the physical
nature of the universe and its history.
This lecture describes the main features of the design and operating
principles of telescopes.
A. THE ELECTROMAGNETIC SPECTRUM
Maxwell (1865) discovered that electric and magnetic forces can
propagate through space at the speed
of light. The immediate inference was that light is an
electromagnetic disturbance. The propagating disturbance moves
through space like a water wave through water and is called an
electromagnetic ("EM") wave.
EM waves are characterized by their wavelengths, or distance
between peaks where the EM forces are strongest. All wavelengths from
zero to infinity are possible for EM waves, and this total range is
called the EM spectrum. From longest to shortest
wavelengths, the EM spectrum includes: Radio, microwaves,
infrared, optical light, ultraviolet light, X-rays, and gamma rays.
The human eye is directly sensitive only to a very small
range of wavelengths in the EM spectrum. This is called the
visible or optical region (see figure below). Within
this region, the wavelength of the light determines the sensation of
color produced in our eyes. Shorter wavelengths correspond
to bluer colors.
Full EM Spectrum with Visible Spectrum Enlarged
- Wavelengths of optical light are
conventionally measured in units of "Ångstroms" (where 1 Å
= 10-8 cm).
- The optical band extends roughly from
wavelengths of 4000 Å in the deep violet to 7000 Å in the
deep red. Green light has a wavelength around 5000 Å, or about
0.0005 mm---far smaller than sizes encountered in everyday life.
Because the wavelength of optical light is so small, we are not
conscious of light's wavelike character.
are microns. 1 micron = 10-4cm = 10,000 Å)
The Earth's atmosphere is opaque to most wavelengths in the
EM spectrum. This is good for lifeforms on Earth's surface, because
the more energetic types of EM radiation are harmful. But, obviously,
it is not convenient for astronomers who want to monitor the universe
across the full EM spectrum. (This is the main motivation for space
astronomy.) The chart below shows the ability of different wavelengths
to penetrate the atmosphere. (Click for enlargement.)
B. TELESCOPES: GENERAL
The telescope is a beautiful example of interplay between technology
(fabrication of quality glass, polishing techniques, large mechanical
structures, computers) and basic science.
- Invented: 1608 (Lippershey, Holland). [Note: microscope invented
1654, also in Holland.]
- First astronomical use: 1609, by
Italy). Utterly transformed astronomy.
In honor of the 400th anniversary of Galileo's introduction of the telescope
to astronomy, 2009 has been designated the International Year of Astronomy.
Click on the logo below for more information.
- Collect more light from source
This is the most important attribute of a telescope (because most astronomical
sources are so faint)
- Magnify source
"Magnify" means to make the source appear larger in angular size
- Resolve more detail in source
"Resolution" is distinct from magnification. A higher resolution image looks
more sharp, less blurry, regardless of how large it is.
- 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
Objectives: Two Types
Reflection of Light by a Figured Mirror
- Lens: transparent glass shaped to refract (or bend)
light rays to a focus. The image at the left below shows how a flat
glass surface bends light rays (in this case, two flat surfaces at an
angle combine to make a prism). The shorter the wavelength,
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 distant object to a common focal
point. Each element of the lens acts like a small prism.
Refraction of Light By a Prism
(click for descriptive animation)
Shaped Convex Lens
- Note that light rays travel in straight lines through
empty space or through any medium (air, glass, water, etc) that has
uniform properties. It will only be at the boundaries
between two uniform media that light rays can be deflected or "bent."
The optical elements of a telescope therefore only change the
directions of light rays at their surfaces (which represent glass/air
- Mirror: shaped glass which reflects light rays off its
front surface to a common focus. A mirror shaped like a parabola will focus
all rays that are parallel to its optical axis to a single point. See picture below.
Applet. Here is a Java applet illustrating the
differences between refraction, reflection, and diffraction.
- Note: Because the primary optical element is the most important
element of a telescope, the "size" of a telescope is
characterized by the diameter of its primary.
Thus, the "26-in"
McCormick refracting telescope has a primary lens that is 26 inches in
diameter. The 200-in Palomar telescope has a primary mirror that is
200 inches in diameter.
- For distant objects (including all astronomical objects), the
incoming rays from each point on the source are parallel to each
other. In this case, the image is formed at a position which is one
focal length from the objective.
- For nearer objects, the
image is formed at a larger distance from the objective. Click on
the button below for a Java applet illustrating image formation.
- Note that the image is inverted with respect to the
original, as in the drawing below:
- In order to look through a telescope, a small lens called an
eyepiece is used to magnify the image at the focal point and
make the rays parallel again. This allows the eye to form a sharp
image of it.
(Although the lens of your eye is partially adjustable and can adjust
to focus on nearby or distant objects, the light beam from the
objective of a telescope is converging or diverging too strongly to
correct in the absence of an eyepiece.)
No optical system can produce a perfect image of a scene.
Departures from a perfect image are called aberrations.
One important aberration for any refractive optical element is the
fact that different wavelengths are deflected by different amounts.
In any refractive telescope (or eyepiece), this means that different
colors of light come to a focus at different places. See the illustration
below. This is called
chromatic aberration. You will experience it as a faint halo
of color around any object viewed through your telescopes.
Other aberrations are caused by the fact that no simple lens or mirror
can precisely focus rays coming from different directions within the
field of view to a single flat focal surface. Parabolic mirrors, for
instance, produce a blur called coma that increases
with off-axis angular distance. This kind of aberration can be
reduced, but never completely eliminated, by using multi-mirror,
multi-lens, or lens+mirror designs.
C. CELESTRON TELESCOPES USED IN ASTR 1230
The telescopes you will use in this class are Celestron 8-in Schmidt-Cassegrain
reflectors and use an equatorial fork-mount.
What this means is explained in the rest of the lecture.
D. TELESCOPE PERFORMANCE CHARACTERISTICS
Focal ratio (or "f/ number")
Magnification or "power"
- f/ = (Objective Focal Length)/(Objective Diameter).
- The smaller is the focal ratio, the more
concentrated is the light in the focal plane and the
easier it is to see faint extended objects like nebulae.
[Same as for the aperture setting on a camera---the smaller
the f/ number, the higher is the brightness in the focal plane.]
- Typical small telescopes have f/ numbers in the range 5-20. The Celestron
8-in telescopes are f/10 with focal lengths of 2032 mm.
Field of View
- Defined to be the increase in the apparent angular size
(measured, e.g., in degrees) of the image
- Mag = (Image Size in Degrees) / (Object Size Without Scope in Degrees).
- For a given telescope, magnification is determined by the
focal length of the eyepiece:
Mag = (Focal Length Telescope) / (Focal Length Eyepiece)
For the Celestron 8-in scopes: Mag = 2032 mm/FLE. Thus:
Note: high powers (> 150x) are not necessarily better, except
for specialized applications (e.g. planets). High magnification is
susceptible to image blurring by atmosphere, telescope vibration.
- A 40 mm eyepiece yields 50 power
- A 20 mm eyepiece yields 100 power
Light Gathering Power
- Defined to be the original angular diameter
of the region viewable through the telescope. Field of view
decreases as magnification increases.
- E.g. with a 20 mm eyepiece, the Celestron 8-in scopes produce
a field which is 20 minutes of arc in diameter. (Slightly smaller than
the full Moon, which is 30 min of arc diameter.)
Resolution: Optical Figuring Tolerance
- Most important attribute of a telescope
- Light collected is proportional to the area of the objective;
think of the telescope as a "light bucket."
The area of the objective is proportional to its diameter2.
- Comparing the capability of the 8-in Celestron scope to the human
- Assume pupil diameter of dark-adapted eye is 5 mm. Celestron
objective is 203.2 mm.
- Light gathering capability of the
Celestron is therefore (203/5)2 = (40)2 = 1600x larger than
- Therefore, with the telescope, you can see objects which are 1600x fainter
than with your naked eye. If your eye can detect magnitude 5 stars,
the Celestron will detect stars of 13th magnitude.
There are over 5,000,000 of these, compared to the
2,000 or so visible with the naked eye!
- For more information on the human eye as an astronomical
detector, see Lecture 3.
Resolution: Diffraction of Light Waves
- Resolution is quantitatively defined to be the smallest
measurable detail in an image (in seconds of arc). In practice,
this depends on telescope optics, the wave nature of light, and
the Earth's atmosphere.
- To produce a good image, telescope optics must be fabricated to
match their designed surface shape within a minimum tolerance of about
1/4 of a wavelength of light in their operating band. For optical
telescopes, this is 10-5 cm. Very small! but relatively
easy to achieve with current technology.
- Good polishing/test techniques were not developed until the 19th
century. To overcome limitations in optical technology, early
astronomers favored telescopes with very long focal lengths (click here for an
example), which reduced chromatic and spherical aberrations.
- A fundamental limit to the resolution of any optical system is set by
the physics of light: since it is a wave phenomenon, light
spreads out or diffracts.
The picture above shows how light diffracts when passing through a
narrow aperture, such as the objective lens of a telescope.
Because of the interference of light waves from different parts of the
aperture, the larger the aperture, the more concentrated the
emerging beam, implying better resolution.
- The resolution is proportional to (Wavelength/Objective Diameter).
The larger the telescope, the better the resolution.
picture illustrates the effects of telescope diameter on the size
of the image of a binary star.
(Note that all stars are so distant that they appear as
point sources in smaller telescopes. You cannot
actually detect any detail on star surfaces except in the largest telescopes).
- A 10-in diameter telescope with perfect optics will produce a 1
arc-sec diameter image of a star (assuming a perfectly stable
The Earth's atmosphere also refracts light, and
because it is constantly moving, there is always a blurring and
jittering of images in a scope. Astronomers call this "seeing."
Quantitatively, seeing is defined
to be the diameter of a star image
(in seconds of arc) caused by atmospheric turbulence.
Expect typical seeing of 2-4
seconds of arc at the Student Observatory. Seeing can be measured by
observing a double star of known separation (see writeup for Lab 3).
dominates diffraction in most cases, so having a better telescope often
does not improve performance. Instead, you may need a better observing site.
Below is an enlarged image of the bright star Betelgeuse taken with a
large telescope. It is a large blob, broken up into smaller near
point-like units by seeing effects in the Earth's atmosphere. (The
small "speckles" represent the image size you could see in the absence of
atmospheric effects.) Click on the image for a video of the seeing effects.
for an illustration of seeing effects on an extended object (the Moon).
"Seeing" Produced by Earth's Atmosphere
E. TELESCOPE TYPES
Three basic types of telescope optics:
great variety! Here are four common types of
- Refracting: objective is a lens; bends rays.
Galileo's of this type. McCormick 26-in of this type. Largest: 40-in
diameter (built 1896).
- Reflecting: objective is a mirror; reflects rays. Invented
by Gregory; improved by Newton. All large telescopes are reflectors.
Largest 400" (10-m) diameter (built 1993).
- Catadioptric: combines lenses and mirrors, e.g. to produce
a larger well defined field of view. Most famous: Schmidt wide field
survey telescopes. These use a spherical primary mirror
surface, which by itself would produce serious blurring but add a
specially-shaped correcting lens at the front of the telescope
that eliminates the blur.
Note that in three of the designs shown, a "secondary" mirror at the
top of the telescope tube is used to redirect the light beam. Although
the secondary does block part of the primary, this has only a small
effect on the net image quality. In particular, it does not
produce a "hole" in the center of the image. In the Cassegrain design,
a hole is actually made in the primary itself.
The Celestrons are catadioptric systems. They
combine a spherical mirror and Schmidt corrector plate with a Cassegrain
through-the-primary light beam design. See diagram below:
Mounting designs: again, a great variety.
Two primary types:
Why are all large telescopes reflectors?
- Altitude-Azimuth (Alt/Az) mount: one vertical axis and
one horizontal axis. Easier to engineer, and therefore lower cost.
But these require computer control for accurate tracking
since the telescope must be moved in two axes simultaneously to follow
- Equatorial mount: two axes, but polar axis is tilted
to parallel the Earth's rotation axis. See this
illustration. Motion around this one axis then tracks the stars.
Harder to engineer, easier to operate. Most telescopes use equatorial
mounts but largest ones are Alt-Az. Your 8" telescopes are
- Lenses produce chromatic aberration (see above). Since
light of only a small range of wavelengths is in good focus, this is a
particular difficulty for using modern broad-band electronic
- Mirrors need be figured only on one side
- Mirrors easy to support accurately from behind; lenses require
support at edges, tend to sag.
- Harder to support heavy lens mechanically at top end of tube than
mirror at bottom end.
- Folding action of primary and secondary mirrors (see below) means that
telescope tube is much shorter than in "straight through" refracting design.
- More discussion
Glass mirror blank
for one of the two 8.4-m diameter
mirrors of the Large Binocular
F. TELESCOPE MILESTONES
A binocular is simply a pair of two small, co-aligned refracting
telescopes mounted together in such a way that each eye can look
through one of telescopes.
- Additional optics (prisms---see at right) are used so that the
view is "right side up."
- The view for nearby objects is 3-dimensional; special optics may
be used to increase the separation of the objective lenses for better
- Astronomical objects are so distant that there can be
no 3-D effect. However, views with binoculars can be especially vivid
because simultaneous use of both eyes produces less eye strain, and the
optics of binoculars usually allow for easy centering of the eyes on
the emergent beam. It is much easier to find your target with binoculars
than with a telescope (assuming it's bright enough).
- Binocular optics are usually classified in the form "7 x 35".
Here, the first number is the net magnification of the
binoculars, and the second is the diameter of the primary
lenses in millimeters. Good types for general astronomical
observations are 7x35 or 8x50. Binoculars higher than 10 power require
tripods for stability. The field of view is also often marked on
the binoculars, typically given as the diameter in feet for objects
at a distance of 1000 yards.
- Binoculars produce some of the best views of the Moon, rich star
fields, comets, and the Milky Way.
Sunset over the William Herschel
Telescope (La Palma, Spain)
- Download, print, and read the notes for Lecture 2.
- Read Appendix C in the Manual on "Telescope Basics"
- Complete Lab I (Constellations) at either of the next two observing opportunities
- Begin Lab 2 ASAP. TA's ready to support you after Lab I observations
Prepare for Laboratory 2 by reading the writeup in
Read Appendices D and E on preparing lab reports and filling
out observing forms.
November 2008 by rwo
Text copyright © 1998-2008 Robert W. O'Connell. All rights
reserved. Some images copyright © by Prentice-Hall and by the
University of Tennessee at Knoxville. WHT image copyright © by N.
Szymanek. These notes are intended for the private, noncommercial use
of students enrolled in Astronomy 1230 at the University of Virginia.