Transcript Land Based Telescopes 10-17-12

Land Based Telescopes
Telescopes: "light buckets"
Primary functions:
Gather light from a
1. ___________
given region of sky.
Focus light.
2. ______
Secondary functions:
Resolve detail in image
1. ____________
Magnify angular size
2. _______
of objects.
Optical Telescopes
wavelengths of light
• Designed to collect ____________
visible
that are ________
to the human eye.
• Data observed by human eyes or recorded
on photographs or in computers.
The Human Eye: Shortcomings
size
• Eye has limited _____.
– limited light gathering power.
frequency response
• Eye has limited ___________________.
– only detects E-M in visible wavelengths.
multiple
• Eye distinguishes new image _________
times/second
_____________.
– cannot be used to accumulate light over
long period to intensify faint image.
cannot store
• Eye _____________
image for future
reference.
– unlike photographic plate or CCD.
Optical Telescope Design
• Basic telescope has two parts:
Objective
1. _________
gather light
Function: to ____________
Lens/mirror of longer focal
Materials: __________
length & larger diameter than the
eyepiece
Eyepiece
2. _____________
magnify image made by
Function: to ____________
objective
Material: ____
lens with a shorter focal length
than the objective
Optical Telescopes
•
___________
Refractors
refraction
–Focus light with _________:
bend light path in transparent medium
–Use _______
lenses
–First kind made, used by Galileo
•
__________
Reflectors
reflection
–Focus light by __________:
bounce light off a solid medium
–Use ________
mirror
–First designed and created by Sir Isaac Newton
•
____________
Catadioptric
lenses and _______
mirrors
-Uses both ______
First Optical Telescopes:
Refractors
Image of source is formed on focal
plane and magnified by eyepiece.
Refracting Telescopes
The Yerkes 40” Refracting Telescope
Refractors: Disadvantages
high tolerance
• Quality optics require ______________
perfect
– surfaces must be _______
absorb light, especially IR and UV.
– glass will _______
– changes in orientation,
_____________________
temperature may flex
lenses
Large size very heavy, hard to support
– __________
aberration
• Chromatic
___________________
–light passes through glass
–refraction a function of wavelength
–all wavelengths focus different distances from
lens
–correctable with compound lenses, expensive
Chromatic Aberration
• Dispersion of light through optical material
causes blue component of light passing through
lens to be focused slightly closer to lens than red
component.
• Known as chromatic aberration.
Reflecting Telescopes:
Designs
Why build reflectors instead of
refractors (advantages)?
1.Mirrors don’t have _________
chromatic
aberration
__________.
absorb light
2.Mirrors don’t ____________
(especially infrared and UV).
3.Mirrors can be supported by their edge
____
and back; lenses by ONLY their edge.
one surface to be
4.Mirrors have only ____________
machined correctly; lenses have two.
Why build reflectors instead of
refractors?
5. Telescopes made with mirrors
compact in design;
can be _________
reflectors cannot.
6. Telescopes using mirrors can
have ______________
larger objective ends
(because they have bigger
mirrors), which means
_______________________.
more light-gathering power
Powers of the Telescope
1. Magnifying Power
The ability to enlarge an image.
2. Light Gathering Power
The ability to see faint objects.
3. Resolving Power
The ability to see fine details.
Magnification and Focal
Length
focal length
of objective
focal length
of eyepiece
focal length of objective
Magnification =
focal length of eyepiece
Light-Gathering Power
•The objective’s area collects light.
•The larger the area,
the greater the light-gathering power of
telescope.
Light-gathering power
proportional to
(objective diameter)2
___________________.
diameter
Resolving Power
• Varies directly with the ____________
diameter of
objective
_________.
• Also depends on
wavelength of light being observed and
– ___________
atmospheric seeing conditions.
– __________________
Resolving Power:
Diameter and Wavelength
Site Selection
• Where are the best places for
ground-based observatories?
• Important factors
dark/light pollution
– __________
– good
_____ weather
– dry
___ air
air turbulence
– _____________
Closer to Sea Level, More air to
pass through
Higher Altitude, telescopes in
the high mountains
Earth At Night
U.S.A. At Night (circa 1994-95)
Detection
• Collected light detected in many
ways.
–image observed and recorded
• eye, photographic plate, ____
CCD
–measurements
• intensity and time variability of
source
–___________
photometer
• spectrum of source
–____________
spectrometer
CCD Imaging
• A charge-coupled device(CCD)
– Wafer of silicon divided into a twodimensional array of many tiny elements,
known as pixels.
– When light strikes a pixel, electric charge
builds up on device.
– Charge buildup monitored electronically.
Radio Telescopes
larger than
•Much _______
reflecting optical
telescopes
•Resemble satellite TV
dishes
•Used to collect radio
waves from space
•AM, FM, and TV
signals interfere, so
must be in a radio
“protected” area
Radio Astronomy:
Wavelength Advantages
•
•
•
•
dependent on time of day/night
NOT __________
NOT as __________
dependent on weather
Use of interferometry
visible light
Information other than _____________
– Quasars, pulsars
• Generally _____________
not absorbed traveling space
– pass through clouds of interstellar dust in our galactic
plane
Accuracy of dish shape not as hard to
• __________
create or maintain
– not need to be highly polish, often light weight
Arecibo Observatory:
Largest Radio and Radar Dish
• 1000-ft radio dish
• Used to
–create maps of Moon, Venus, and Mars
–discover pulsars and galaxies
–measure the rotation
rate of Mercury
–discover planetary
systems outside of
our solar system
Very Large Array(VLA) in New Mexico
27 antennas, each 25 m in diameter
Effective diameter = 36 km
Yields radio-image details comparable to optical resolution
Interferometry
• Two
___________
or more telescopes used
–to observe same object
–at same wavelength and
–at the same time.
• Uses wave interference to
yield high resolution.
Cheaper than one
• ________
(impossibly) large telescope.
• Farthest 2
_ telescopes act like
the end of one telescope.
–Baseline:
• distance between 2 farthest
‘scopes.
• equals the relative ‘scope size.