Chapter 6
Optics and Telescopes
Telescopes, and the detectors to go with them, are
important in ASTR 1100, so the important points are
covered here.
1. Telescopes are simply extensions of the eye. They
gather more light than the eye does because of their
large apertures. Magnification is a consequence of
different focal lengths for the telescope lens/mirror and
eyepiece.
2. More important are the devices used by astronomers
to image the sky as well as spectra of astronomical
objects. The human eye just won’t do !
Telescopes take advantage of the optical properties of
lenses or mirrors, i.e. refraction or reflection.
The bending of a beam of light
by refraction in glass. Properly
figured surfaces on a convex lens
bring the light beam to a focus.
Telescopes can be thought of as “light buckets.” They
gather light in proportion to their collecting aperture
surface area, D2, be it lens, mirror, radio dish, or an
array of hexagonal mirrors. Thus, a 16-inch telescope
has four times the light gathering power as a 8-inch
telescope.
Light from distant objects enters telescopes in a plane
parallel beam.
Basic Optical Principles:
Each point in a distant object is the source of a
spherically expanding wave front, but the light
arrives as the telescope as plane parallel wave
fronts, lightly distorted by Earth’s atmosphere.
An ideal telescope would focus the incident wave
fronts to a single point, the focus, irrespective of
wavelength and position on the wave front.
A prism (upper right) shows how
it is possible to deflect a beam of
light, albeit with some dispersion
resulting from the wavelength
dependence of nλ. A thin lens
(lower right) can be thought of
as simply a series of small prism
segments deflecting the light to a
single point, the focus, a distance
f from the lens.
A convex lens (a) focuses light in
a converging beam, while a
concave lens (b) defocuses light
in a diverging beam.
A simple refracting telescope consisting of two lenses: a
primary lens and an eyepiece lens. The resulting
magnification is given by the ratio of the focal lengths:
Magnification 
f objective
f eyepiece
e.g. old Burke-Gaffney telescope.
Original effective focal length of the 0.4m
telescope was f(telescope) = 4780 mm.
When used with an eyepiece labelled as
having a focal length of 40 mm,
Magnification 
f objective
f eyepiece
4780mm

 119.5
40 mm
In other words, that combination produced
a magnification of 119½ power (119½).
DAO Director Jim Hesser modelling Galileo’s telescope.
Examples:
Chromatic aberration occurring in glass optics is a
serious problem for refracting telescopes, which is why
reflectors are superior.
The advantages/disadvantages of telescope types.
Refracting telescopes: (i) suffer from chromatic
aberration that requires a doublet main lens
rather than a single lens, (ii) four surfaces need to
be figured, (iii) beyond ~40 inches, a lens sags
under its own weight.
Reflecting telescopes: (i) spherical mirrors suffer
from spherical aberration, parabolic mirrors do
not, (ii) parabolic mirrors suffer from “coma,”
that requires special shaping of the mirrors, (iii)
mirrors need only be figured on one side, (iv)
mirrors can always be supported from the back,
(v) the material of mirrors need not be
transparent since it will be aluminized, (vi)
composite mirrors can be built to do the job of
one large mirror.
A mirror system focuses light without suffering from
chromatic aberration.
A modern view of the Plaskett Telescope.
Reflecting telescopes also suffer from light aberration.
Telescope resolution is established by the diameter of the
collecting aperture, the larger the better, except for the
effects of atmospheric seeing. The diffraction image of a
star is surrounded by the “Airy disk” in excellent seeing.
Larger Size = Better Resolution, θ
Up to a point.
Atmospheric seeing is the limit for groundbased telescopes.
1.22 
For θ in radians,

D
1.22  206265
For θ in arcseconds,  
D
5
2.5  10 

D
These devices are also telescopes.
Image Scale of a Telescope.
The image scale of a telescope links the angular
separation of objects in the sky to their linear separation
on an image obtained with the telescope. As illustrated
below using the “chief ray” from an object passing
through the centre of the lens, by geometry:
The image scale is therefore defined as:
For θ in arcseconds rather than radians, and y in mm:
Example.
The ST8 CCD camera at the old BGO operated at an
effective telescope focal length of f = 4924 mm. The image
scale for the CCD camera was therefore:
It seems, at first sight, that one could achieve arbitrarily
large separations between image points merely by
increasing “f.” But images of Jupiter (θ = 49".7 near
opposition) have a size, X, given by:
The value corresponds to ~66 18-micron pixels on the
ST8, for 22 binning. In other words, the size of the
detector elements governs the resolution.
Earth’s atmosphere is opaque to gamma rays, X-rays,
ultraviolet light, some infrared light, microwaves, and
long wavelength radio waves, so observations in these
spectral regions can only be done from space.
Chandra X-Ray Orbiting Telescope
Compton Gamma-Ray Observatory
COBE Satellite
Hubble Space Telescope (from Shuttle)
Many modern telescope mirrors are equipped
with adaptive optics.
Adaptive optics “off.”
Adaptive optics “on.”
James Webb telescope mirror test.
Keck telescope “mirror,” full scale.
A CCD chip (used for imaging).
The inner workings of a CCD camera.
CCD images of the same field: negative image in B&W
(left) and colour image (right).
A photographic image of the same field for comparison.
Telescope spectrographs (used to image spectra of stars
and galaxies) must include a prism or grating.
Spectrograph on the DAO Plaskett Telescope.
Modern observing is done from “warm” rooms equipped
with computers to control telescope and imaging devices.
Telescopes are housed in domed structures to protect
them from the elements. DAO Plaskett Telescope Dome.
DAO 1.2-meter Telescope Dome.
Victoria, B.C., from Little Saanich Mountain (DAO).
Mount Baker from the DAO.
Astronomical Terminology
Refractor. A telescope that uses lenses to focus light.
Reflector. A telescope that uses mirrors to focus light.
Light Gathering Power. A measure of the collecting area
of a telescope’s primary element.
Atmospheric Window. A region of the electromagnetic
spectrum in which Earth’s atmosphere is
transparent.
Optical Telescope. A telescope designed to gather light in
the optical band.
Radio Telescope. A telescope designed to gather light at
radio wavelengths.
Seeing. The fluctuating effect on telescope images caused
by turbulent bubbles in Earth’s atmosphere
randomly deflecting light paths from the source.
CCD = Charge-Coupled Device. An ultrathin wafer of
silicon divided into a 2-dimensional array of
picture elements (pixels) that collect electrons
produced by light falling on the element.
Sample Questions
1. “Twinkle, twinkle, little star. How I wonder
what you are.” Explain why stars twinkle.
Answer: From ground-level we observe stars
through Earth’s atmosphere, where convection
and air density differences create small bubbles
of air that generate random scattering of the
starlight and produce “seeing,” the irregular
wobbling and intermittent size changes in star
images that we call twinkling.
2. Why are the world’s largest telescopes
located on high mountains?
Answer. Earth’s atmosphere is very
detrimental to astronomical observations.
Thermal currents in the atmosphere make
seeing blurry, water vapour absorbs
infrared frequencies, and light pollution at
the surface limits deep sky observing.
Placing telescopes on mountaintops reduces
thermal currents and situates telescopes
above 90% of the water vapour and well
above urban light pollution.