Astronomy & Space Science Association
University of Kelaniya
Astro Night 2014
Telescopes and Optical
Hon. mentd Dip in Astronomy & Astrophysics ( IOAA – China )
National Representative at Space Generation Advisory Council ( UN )
Sri Lanka national Astrophysics Olympiad Team Trainer at Institute of Physics Sri Lanka
Why we use telescopes?
• To see better detail.
• To see fainter objects.
• To observe objects by using various wavelengths.
• To detect special elementary particles
Telescopes can be divided into several types
based on few classifications
• 1) Based on detecting wavelength
( Optical Telescopes , Radio telescopes, IR Telescopes etc)
• 2) Based on optical mechanism
– I ) Optic System ;
Refracting Telescopes i) Astronomical Telescope
ii) Galileo Telescope
Reflecting Telescopes i) Newtonian Telescope
ii) Cassegrain Telescope
iii) Gregorion Telescope
– II) Catadioptric System ; i) Schmidth Cassegrain Telescope
ii) Maksutove Cassegrain Telescope
Astr121 Lecture 5 4
Basic telescope type #1
Use lenses to create the image.
Refract = To bend the light as it passes through
some material. (glass, plastic)
Astr121 Lecture 5 5
Basic telescope type #2.
Use curved mirrors to create the image.
Reflect = To bounce the light off the surface
of some material. (glass, metal)
Disadvantages of Refracting Telescopes
• Chromatic aberration: Different wavelengths
are focused at different focal lengths (prism
effect). Can be
by second lens
out of different
Difficult and expensive to
produce: All surfaces must be
perfectly shaped; glass must
be flawless; lens can only be
supported at the edges
Reflecting vs. Refracting
Problem #2: Weight:
Supported on the edges.
Can be thin.
Supported on the back.
Largest refracting (lens) telescope:
Yerkes 1 meter
Reflecting vs. Refracting
Problem #3: Manufacturing:
Thin lenses = very long focal lengths.
Problems designing and using such long telescopes.
Thick lenses = short focal lengths.
Lots of glass: must be free of defects, will be heavy.
Bigger is Better
Keck Telescope (actually 2 of them)
10 meter mirror
Located on Mauna Kea,
a dormant Hawaiian volcano,
about 14,000 feet high.
Reason #1 for big telescopes:
Collect more light.
The larger the opening,
the more light collected.
Larger opening more light
detect fainter objects.
Astr121 Lecture 5 35
Reason #2 for big telescopes:
Resolving power: the ability to distinguish detail.
Means how closely two objects can be seen separately.
Not Resolved Barely resolved Resolved
360 degrees in a circle 1o
60 arc-minutes per degree 1’
60 arc-seconds per arc-minute 1’’
Or use radians:
2p radians = 360o
α = 2.3 ´ 10 5 λ
Astr121 Lecture 5 37
Resolution of a telescope is determined by the size of its aperture (opening).
If D = opening diameter,
Smallest angular separation:
If l and D are in same units a is in arc-seconds.
Larger diameter telescope = Smaller resolution
α = 2.3 ´ 10 5 λ
For l = 500 nm (visible):
D a Golf ball at:
Human eye: 0.5 cm 23 arc-sec 300 m
Small telescope: 10 cm 1 arc-sec 7.5 km
Keck telescope: 10 m 0.01 arc-sec 725 km
Magnification = ‘Zoom’ factor for a given image, set by eyepiece.
It’s important, but it can’t overcome low resolution.
– Information limited by resolution and ‘seeing’.
– Magnification useful below these limits.
Example: TV or newspaper picture made of tiny dots.
– Dot size = resolution
– Zooming in doesn’t make more detail, just bigger dots.
at 14,000 feet
– Above the weather.
– Above a lot of turbulent air.
– Above a lot of absorption.
Seeing: how stable and clear the atmosphere is.
Turbulence in the atmosphere causes distortions:
‘Twinkling’ of stars.
Left side: bad seeing.
Right side: good seeing.
Typical atmospheric limit is 1’’.
At high altitudes you are above much of the turbulent air.
Certain wavelengths are strongly absorbed, so must get
to high altitude to see them. Especially IR, UV.
Astr121 Lecture 5 43
Populated areas emit a lot of light, making the sky appear less dark and blocking
out faint objects.
Telescopes often located in remote, unpopulated areas.
Bigger telescopes better because:
• Collect more light see fainter objects
• Better resolution see smaller detail
• “Seeing” air turbulence
• Absorption atmosphere opaque at some wavelengths
• Light pollution blocks faint objects
Solution: telescope at high altitude, usually away from population.