Galileo’s telescope
Refractor or Reflector
Refraction
• Refraction is the
bending of light
when it passes
from one
substance into
another
• Your eye uses
refraction to
focus light
Refraction makes a
pencil appear to be
bent when placed
in water.
When light passes
through a glass slab it
first refracts towards
the normal then away
from the normal.
Example: Refraction at Sunset
• Sun appears distorted at sunset because of how
light bends in Earth’s atmosphere
Refraction telescope images
The main lens in a refracting telescope is called the primary
lens (or the objective lens). This lens is a part of the telescope
and is fixed.
Problems with refractors.
Light of differing frequency travel at different speeds in
glass. This results in a spectrum from a prism and leads
to chromatic aberration in lenses.
Large achromatic lenses are expensive.
Lenses must be supported at the edges. Large lenses
sag in the middle under their own weight.
Long focal length= long tubes. Costly mounts and
domes.
Chromatic Aberration
Achromatic Lenses
Yerkes Observatory
Williams Bay Wis.
Worlds largest at
40 inches in
diameter
63ft long
Reflectors
Most large telescopes are reflectors.
Isaac Newton presented a reflecting telescope to the Royal
Society in 1671.
Mirrors avoid chromatic aberration.
Objective mirror instead of an objective lens.
Early reflectors were made of polished metal alloys. Tarnished
rapidly dimming images.
Reflectors 2
1850 method for depositing silver on glass.
Most telescopes became reflectors
Still tarnished, normal glass changes shape
with temperature changes
1940’s technique for casting Pyrex glass and
aluminum coatings (more durable)
Now Pyrex or Fused Quartz
Palomar 200 inch mirror took 11 years to
build and required 5 tons of glass.
Reflecting telescope images
Light in a reflecting telescope does not have to pass
through glass at all.
The main mirror in a reflecting telescope is called the
primary mirror (objective mirror).
Images are inverted
Types of reflectors
Prime or Cassegrain
Prime focus is often
used in photography.
It places the
observer in a cage.
The Cassegrain focus is at the bottom and
requires a secondary mirror.
Cassegrain Telescopes
18” Telescope
41” Telescope
Earl of Rosse’s mid –nineteenth century
telescope
Observers stood at the prime focus.
Newtonian
A Newtonian
focus is
inconvenient for
large
telescopes.
SCT
Telescope Mounts
The choice of mount depends on the
telescopes main function and size.
Good optics are useless if you can’t control
the telescope.
Good control is useless if the optics are poor.
Alt-Azimuth Mounts
Equatorial Mounts
Equatorial Mounts
200in
Russian 6m
First large telescope to
use alt-azimuth mount.
A key move to the new
generation of
telescopes.
For a given aperture this
permits a cheaper
lighter structure in a
more compact dome.
Telescope Rating Characteristics
Powers of a telescope
Light gathering power
Resolving power
Magnifying power.
Low
High
Light Gather Power
Low
Magnification
Low
Resolution
High
High
Light Gathering Power
A telescope is a light
bucket.
Like a bucket catching
rain the diameter is
the determining
factor in hiw much
light gets caught.
Area= π r2
The large the radius,
diameter, the greater
the area.
Bigger is better. The more light you collect the
farther out you see.
Resolving Power
… ability of a telescope to see fine
detail.
Defined as the angular distance
between two objects that are barely
visible as separate.
Due to the wave nature of light
magnified images have diffraction
fringes.
Diffraction Limits
The edge of a
telescope acts as
slits causing
diffraction patterns.
These are only seen
at the highest
magnification for a
telescope.
Overlap
Closely spaced
objects begin to
overlap, becoming
indistinguishable.
Increasing resolving power.
The smaller the number the better the
resolution.
Resolution formula

(

m
)
Angular resolution = 0.25 *
D ( m)
For visible wavelengths
in the middle of the
visible spectrum
“ α” ≈
13.7
D(cm)
Note aperture is in the
denominator. Large D smaller
resolution.
Bigger telescope is better.
Magnifying Power
Usually the big sell. Least important. It can be changed.
M=
fo
fe
Magnification is calculated
by the focal length of the
objective divided by the
focal length of the eyepiece.
To Increase magnification
just change to a shorter
focal length eyepiece.
Maximum Magnification
• As magnification of an instrument is increased
the image will be dimmer.
• As a rule of thumb the maximum
magnification of a telescope can be found by
• Mmax = 20(X/cm) *D(cm)
F5_4 Focal Length of a Lens
The focal length of a lens or mirror is the distance
from the center of the lens to the image formed
from an object placed at a great distance.
Observing Problems: Light Pollution
Many of the most interesting objects are dim. Bright skies wash out the
images. The darker the sky the better.
Seeing
Seeing
To reduce the
effects of the
atmosphere
observatories are
placed at high
elevations and in
regions where the
air is dry.
Paranal Observatory ESO
The best seeing is from remote, high and dry locations.
Telescopes
New Generation telescopes use
advances in technology to correct
images for bad seeing conditions.
New Generation Telescopes
Large mirrors had to be made thick to avoid sagging.
A mirror can be supported from the back. Traditional
telescopes were big, heavy, and expensive. Control devices
had to be massive too.
High speed computers have helped to reduce costs and
improve performance.
Computer control makes alt-azimuth mounts usable.
Computer control makes it possible to control the shape of
thin mirrors rapidly. This reduces the costs of making the
mirror, smaller mounts and allows for…
Thin floppy mirrors
Mirrors are backed by movable
pistons able to change the shape
of the mirror quickly under
computer control.
B.
One of the Keck hexagonal
mirror segments.
Thin mirrors are lighter require less
support and they change
temperature faster (less trouble
with convection currents at
surface)
Floppy Mirrors
Nordic 2.6 M
Canary Islands 1989
First large instrument whose
dome and primary mirror
shape are continually adjusted
for the sharpest possible
image.
Gemini 8.1 m mirrors
Rapid control of mirror
shapes makes it possible to
correct some distortions
caused by poor seeing.
Real time mirror control
achieved by 120 actuators
under the mirror and 60
around the edge.
Right: The Gemini mirrors
have adaptive optics
Adaptive optics
AO allows this 1.5m telescope to reach 0.1” resolution
Adaptive Optics Corrected Image
Adaptive Optics
Without adaptive optics
With adaptive optics
Rapidly changing the shape of a telescope’s mirror
compensates for some of the effects of
turbulence
Pluto and Charon by adaptive
optics
Star by AO
The central star is
blocked out to show a
disk of matter ..
Inside Gemini
Gemini open
The air inside
and outside
the dome
must be the
same
temperature
so the dome is
opened
shortly before
sunset.
Inside looking out
Mauna Kea Observatory Hawaii
At 4 km altitude sky is clear. Keck 10 m mirror
Keck
Keck (I and II) have 9.8 m segmented mirrors[ 36 of them,
90cm on each side]. A major advance in telescope design.
Keck I Segmented Mirror
Segmented Mirrors
Moons by Keck
Segmented Mirror
Texas
Hobby-Eberly Telescope,
HET:
9.1 effective aperture
mirror is made from 91
spherically segmented
mirrors forming a
hexagon 11 m x 10 m.
Plans
Giant Telescopes are proposed. The California Extremely Large Telescope
would be a 30 m telescope featuring 1080 segments 40cm on a side.
More plans
XLT
Extremely large telescope
The mirror
Enormously large telescope
OWL
Overwhelmingly Large
Telescope
Optical Interferometry
Combining signals requires
control of signal path
lengths to a fraction of the
wave length of the e-m
radiation being combined.
Only radio waves could be
used until recently.
The resulting image has the resolution of the distance between mirrors.
Very Large Telescope Array
Paranal Observatory at Atacama
Chile
Four 8.2 m reflecting telescopes .
Used together have the effective area
of a 16 m .
VLT
Large Binocular Telescope
Binoc mirror
Spinning oven makes mirror closer to end shape, requires less
finish grinding.
Giant Magellan Telescope
CCD (charge coupled device) Images
CCDs can detect both dim and bright objects in the same exposure, are
more sensitive the a photographic plate, and can be read directly into a
computer.
CCDs,2
CCDs can not only image but give relative intensity data that was once
required a photometer. This leads to interesting contour plots and the use
of false color images.
Imaging
• Astronomical
detectors
generally
record only
one color of
light at a time
• Several images
must be
combined to
make full-color
pictures
Imaging
• Astronomical
detectors can
record forms of
light our eyes
can’t see
• Color is
sometimes used
to represent
different
energies of
nonvisible light
•
Early 90s
Spectra
Spectrograph:
Most use a grating
instead of a prism.
Details about the
intensity at specific
wavelengths gives a
wealth of information.
Earth’s Atmosphere
• The Earth’s atmosphere is mostly
transparent for visible light and radio waves.
• For that reason, there are two major types of
telescopes:
• Common Optical Telescopes
• Radio Telescopes
The other window
Radio ?
Why don’t we see in the radio region?
Components of Radio telescope
because you would have to have huge eyes.
Remember the resolution formula ?
Radio telescopes
Still bigger is better
To get the same
resolution radio
telescopes must be
huge.
“α” = 0.25
 ( m)
D ( m)
Green Bank 100 m telescope
Largest steerable radio telescope in the world.
300 m Radio Telescope
Arecibo Puerto Rico
Very Large Array
Radio interferometry increases the resolution, by combining signals. VLA
consists of 27 dishes has the resolution of a 22 mile diameter radio
telescope. Soccoro NM
Radio images
Very Long Baseline Array VLBA
Combines electronically signals from Hawaii and the Virgin Islands. This gives
the resolution of an Earth sized telescope
IR Infrared Telescopes
Top:Longer wave
length light doesn’t
see the smog
particles.
Dust doesn’t block
IR as easily in space
either.
NASA 3 m Infrared Telescope
Water absorbs IR
IR Telescopes must be
at high elevations, in
the mountains, on
balloons or in space.
Still only the near
infrared is visible
under even the thin
atmosphere.
NASA’s Great Observatories
Figure 6.22
• Hubble launched April 1990. Visible light and UV
( Shuttle Discovery)
• Compton Gamma-Ray Observatory.(Shuttle Atlantis). April
1991 to June 2000 (lost gyroscope)
• Fermi Gamma-Ray Observatory (formerly GLAST)
2008 replaces Compton
• Swift. 2004. Has gamma ray detectors, as well as xray and visible light telescopes.
• Chandra X-ray Observatory July 1999 (Shuttle Columbia)
• Space Infrared Telescope Facility (SIRTF), now called
the Spitzer Space Telescope
SIRTF Space Infrared Telescope Facility
[Spitzer]
SIRTF, Spitzer Space Telescope
Was launched by Delta rocket first expected in mid April 2003. Saved a lot of
money, but required a redesign. Was to have been launched by a shuttle.
IRAS
Infrared Astronomy
Satellite surveyed cooler
gas and dust.
Dust gets in the way
Resolution
Orbit
Chandra Xray Telescope
Gamma ray telescopes
Design
X ray mirrors
• From Chandra Ed at Harvard
Extreme Ultraviolet Explorer
F5-23a Hubble
F5-23bMars by Hubble
James Webb Space Telescope –
2013… I mean 2018 launch…