Transcript Presentation: Light and Telescopes

•
What we call “light” is only one type of …
Electromagnetic Radiation – a way in which energy moves through
space.
Do not confuse EM radiation with Particle radiation.
We’ll start by discussing the visible light spectrum.
When white light is passed through a prism, a “rainbow”-like band of
color is seen.
ROY G. BIV … Ever met him? 
Red
Orange
Yellow
Green
Blue
Indigo
Violet
This is the order from longest to shortest wavelength.
This order of colors never changes!
What is wavelength?…
Wavelength (λ) – length over which the wave repeats.
Or…
Wavelength (λ) – Distance between two crests of a
wave.
Light wavelengths are very short!
λ measured in Angstroms (Å). Named after 19th century
Swedish physicist A.J. Angstrom. 1 Å = 1x10-10 m
Approximate sensitivity limit of the human eye.
4000 Å - 7000 Å (or… 400 nm - 700 nm)
λ > (longer than) 7000 Å
λ < (shorter than) 4000 Å
- Infrared
- Ultraviolet
•
Electromagnetic Radiation – a way in which energy moves through
space.
Speed of light in a vacuum c = 3 x 108 m/s
= 186,000 miles/sec
7 times around the Earth in 1 second!!
“Electromagnetic Radiation” gets its name from the fact that it is a
manifestation of rapidly varying Electric & Magnetic fields.
•
There are seven different regions of the E-M Spectrum
They are, in order of increasing wavelength;
Gamma Rays
X-Rays
Ultraviolet
Visible
Infrared
Microwave
Radio wave
(Be able to describe applications of each region, or where we encounter them in life.)
All of these regions can be used in astronomy to measure objects in the
universe.
Different methods are needed to detect different wavelengths of radiation.
Only certain parts of the E-M spectrum can penetrate the Earth’s atmosphere.
• The parts that get through are called windows of transparency.
• Earth’s atmosphere has two main windows in visible and radio.
TELESCOPES
For this discussion, we will use the word telescope to mean
Optical Telescope
These are the types of telescopes you (the student) are most likely to use.
What are the most important things telescopes do in astronomy?
1. Gathering light
(Our eye’s pupil is only about 8 mm wide)
2. Magnification
3. Increasing light-gathering time
Using CCD’s or film
Our brain “refreshes” about 30 times/sec.
No such thing as a “long-exposure” brain images!
4. Increase resolution or resolving power.
Resolution  [1/(diameter of optical element)] (see Dawes Limit)
Resolution inversely proportional to diameter – The larger the optical
element, the smaller the features that can be resolved.
So What Does a Telescope Do?
• Gathers light – it’s
most
important job!
– Allows us to see faint,
distant objects.
– Doubling the diameter
quadruples the light
gathered.
Since A = r²
– (1)² / (2)² = ¼
– Thus, the bigger one
gathers 4x the light!
Radius = 1
Radius = 2
What a Telescope Does:
• Magnifies
– Important when
viewing planets,
nebulae, and
other objects in space.
Magnification equals the focal length of the telescope divided by the
focal length of the eyepiece. Or, in equation form…
Mag = FLT ÷ FLE
The longer the eyepiece focal length,
the less the magnification.
What a telescope does:
• Resolves
– Separates two objects at a distance so they appear
separate.
Resolution
In the figure right, two lights are
imaged by telescopes of different
apertures. Apertures are decreased
by a factor of two each step to the
right.
• Notice the decrease in the ability
to resolve the two lights as
separate objects.
Bigger aperture = Better resolution
• Note also the wavelength
dependence.
Bluer light = Better resolution.
This is due to the Dawes Limit
r (arcsec) = 2 x 10-3 λ/d
λ - wavelength in Angstroms
d - diameter of the aperture in cm.
Important: Because of air turbulence, the best resolution from groundbased telescopes is limited to ½ arcsec.
1degree = 60 arcmin
1 arcmin = 60 arcsec
½ arcsec = the width of a human hair across two football fields
(~ 200 yards).
The limiting factor of air turbulence in Earth’s atmosphere is why the
Hubble Space Telescope (and other orbiting observatories) are
placed into space.
Refractors vs Reflectors
• Primary/Objective lens
• Secondary lens or
eyepiece
• Primary/Objective
mirror
• Secondary mirror
• Eyepiece
RefractingTelescope
Refraction – the bending of light, as when it travels from one
medium to another.
Focal Length – Distance at
which the light rays converge.
Chromatic Aberration – Different colors are focused
at different points. This causes blurry images.
Chromatic Aberration can be solved using compound lenses,
but this costs light and makes images dimmer. It also
makes telescopes heavier and more expensive.
Reflectors Rule
But Why??????
• Large diameter refractors are expensive to make.
• Mirrors are easier to mount. Large mirrors can be
supported from behind.
• Refractors suffer from chromatic aberration.
• Also, large lenses sag under gravity.
However, reflectors can suffer from
Spherical Aberration
Parallel light rays do not image at the same point.
Parabolic
mirrors solve
Spherical
Aberration
Types of Reflectors
Newtonian – Eyepiece on the side of the tube
near the front aperture or opening.
Types of Reflectors
• Cassegrain – Eyepiece at the rear of telescope.
Light travels through a hole in the primary mirror.
Types of Reflectors
Schmidt-Cassegrain – Similar to a Cassegrain, but
with a spherical mirror and a lens on the front called
a corrector plate. Maksutov telescopes are also similar.
Telescope Mounts
• Altitude-azimuth mounts
Simple up-down, side-to-side motion
• Equatorial mounts
Oriented to track the stars
German
Equatorial
Dobsonian
Classic
Alt-Az
Fork
Equatorial
Manual Telescopes
• More mirror for the
$$$$$.
• Suitable for all ages.
• You must learn your
way around the night
sky.
Automated Telescopes
• Ease in finding
objects.
GPS system
alignments
• A necessity for
astrophotography.
• Expensive.
• Not suitable for
children.
Manual star
alignments