Transcript Optical Telescopes (visible light)

Telescopes
What is a telescope?
• It is a “light bucket”, gathers/collects light.
– The purpose of a telescope is NOT TO
MAGNIFY, but to collect light. Eyepieces do
the magnifying.
• The more light it collects, the better the
quality of the image.
– Bigger telescopes = better quality
– Longer exposure = better quality
Refracting Telescopes
• uses a lens to
refract (bend) the
light.
Largest Refractor
• Largest is the
102 cm (40 in)
diameter at
Yerkes
Observatory in
Chicago.
Problems with Refractors
1. Expensive
2. Chromatic aberration
–
–
Not all colors of light focus at same focus point
(different wavelengths).
Need an extra lens, called an Achromatic lens, to
compensate.
3. Size restrictions
–
–
Lens will sag due to lack of support
Unlike reflectors, you cannot put a support behind
the lens, since light must move through it.
Chromatic Aberration
Reflecting Telescope
• uses a mirror to reflect the light.
Biggest?
• The Large Binocular Telescope (LBT),
in Arizona.
• 11.7 meters
Electromagnetic Spectrum (EmS)
• classifies light (energy) by its wavelength.
• Less than 3% of ALL light is visible, that means 97% of the universe
is invisible!
Who cares?
 It helps determine what’s in stars!
 Spectrum- when light undergoes diffraction, it
will split into individual wavelengths (colors of
light)
Spectroscopy
 the study of the ways matter emits and
absorbs radiation (light). Each element
has a unique spectrum
Hydrogen
Carbon
What else can spectroscopy do for
me?
 What can’t it do?
 Allows us to study the makeup of stars
AND helps us determine movement of
galaxies!
How does it help us determine
movement of galaxies?
• Doppler Effect- The APPARENT change
in wavelength of any observed wave
(light or sound).
• For an object moving away from wave
source, spectral lines appear to shift
towards the Red portion. Called Redshift.
• Object moving towards is called
Blueshift.
Redshift
• Spectral lines are
shifted towards the
red.
•The distance between
lines stay the same, so
we can identify the
elements.
Cool info
• ALMOST all galaxies are redshifted!!!
Universe is expanding!
• Andromeda (400 bil stars & 2 mil ly away)
is blueshifted. Due to collide with Milky
Way in 2 bill years.
ARTIST’s
conception.
NOT viewable from
ANY mountain range.
Telescope for all light!
• ONLY visible light and radio waves can travel through
the atmosphere.
• All other “telescopes” must be viewed in outer space or
very high altitudes.
Frequency Detection:
• Optical telescopes detect ALL visible
frequencies simultaneously (all wavelengths
of visible light)
• Radio light emits a HUGE amount of light in
a LARGE array of wavelengths.
– Thus radio telescopes detect different radiation
at very narrow bands of frequency (separate
wavelengths)
Arecibo
Advantages of Radio Astronomy
• Observe 24 hours a day (Sun is a weak
source, does not interfere much).
• Observe on cloudy days
• Radio waves can travel extremely far
distances without being “blocked” by
interstellar stuff (like gas, dust, stars,
called Interstellar Medium).
– *** For example, we cannot “see” the center
of our galaxy with visible light, BUT we can
with radio!
Disadvantages
• Lacks good angular resolution (hard to focus
on a single point)
• to resolve problem, technique known as
interferometry is used.
• Uses 2 or more telescopes to observe the
same object at the same wavelength at the
same time.
• Combined apparatus is called
interferometer.
• The effective diameter is equal to the
distance between the outermost telescopes!
Very Large Array
Allen Telescope Array (used by SETI)
•
Currently
only 42
operational
antennas,
but will
eventually
have 350
(if funding
allows)