Download Telescopes

Reminders!
Website: http://starsarestellar.blogspot.com/
Lectures 1-3 are available for download as study aids.
Reading: You should have Chapters 1-3 read, and
probably Chapters 4-7 by the end of this weekend.
Homework: Save the chapter 4 problems for the next
homework set. Just do the chapters 1, 2, and 3
problems on homework #1.
Homework #1 is due this Wednesday at the
BEGINNING of class.
Discussion: Today from 1-2 pm is the last math review
class. For Tuesday and Wednesday, we’ll have open
office hours.
Telescopes: Extending Our Senses
Today’s Lecture:
• Finish discussing light
Why is the sky blue and sunsets red?
• The tools of astronomers (Chapter 3, pages 38-63)
Introduction to telescopes
Different telescopes for different types of light
• Phases of the Moon (Chapter 4, pages 64-68)
Explain the phases as a geometrical effect
Show that the phase is related to the position of
the Moon in the sky
Why blue sky and red sunsets?
Blue light is preferentially scattered. Away from the
Sun, the sky looks blue.
As the light path through the atmosphere
increases, the Sun looks progressively more
yellow, orange, and red and it approaches
the horizon.
Telescopes
• The primary purpose is to collect more light
• Typical large telescope: D ≈ 6 m mirror
• Dilated eye pupil: D ≈ 6 mm
• A = area = πR2 = πD2/4, since R = D/2
• The ratio of areas for two telescopes is
D1
D2
Telescopes (continued)
If D2 = 6 m and
D1 = 6 mm = 6 x 10-3,
then the ratio of areas
is (1000)2 = 106!
• A 6 meter telescope can see a million times fainter than the
human eye!
• A large telescope also provides higher resolution -- the
ability to see finer details.
• But turbulence in the Earth’s atmosphere blurs the image,
and limits the detail that can be seen (especially at optical
wavelengths)
Angular Resolution
Resolution ≈ 0.002λ/D
Resolution in arc seconds, λ in Å, and D in cm
Refracting Telescopes
• Invented in Holland in the 1600s
• Light is bent (or “refracted”) by a lens and brought
to a focus.
Lens
Light rays
Focused
to a point
Light rays from a star are
essentially parallel
Stars are very, very far away, so the rays reaching
us diverge only very slightly. They are PARALLEL
for all practical purposes.
Refracting Telescopes (cont.)
• Simple to make, but they suffer from several
problems, including chromatic aberration
• This is a property of all lenses…including your eyes!
• Lenses are also heavy and absorb light
Reflecting Telescopes
• Use a mirror to bring light to a focus.
spherical
mirror
parabolic
mirror
• A spherical mirror does not focus the light to a single
point, which leads to spherical aberration.
• A parabolic mirror does focus the light to a single point.
• This was in fact the problem that Hubble first had.
Common Reflector Designs
Telescopes in space
• no distortion (blurring) by the atmosphere
• darker sky (especially in the infrared)
• be able to see ultraviolet, X-rays, gamma-rays,
and infrared
ozone (O3) blocks UV radiation at altitude
of 20-40 km
water vapor (H2O) blocks IR at altitude of
2-10 km
other atoms/molecules block X-rays and
gamma rays
Telescopes in space (cont.)
• IR: telescopes on top of high mountains, on
balloons, in airplanes, and in space.
• X-rays, gamma-rays: balloons, rockets,
orbiting satellites
• UV, optical: orbiting satellites (for example,
the Hubble Space Telescope)
Twinkling of Stars
• Air bends (refracts) starlight. Due to the
atmospheric turbulence, the bending varies with
time, so the amount of light hitting your eyes varies.
• Stars closer to the horizon => more air, more
turbulence => more twinkling!
• This causes blurring of the star’s image, so that it
doesn’t look point-like.
Twinkling of Stars (cont.)
• Planets don’t twinkle as much. They are closer,
and thus disks of light (not point-like) and the
randomly varying points average out.
Star (far away)
Planet (much closer)
• But planets DO twinkle when close to the horizon.
• For a telescope larger than 20-30 cm, twinkling is
the limiting factor for resolution (<1’’ resolution).
• How do we get rid of the twinkling?
Adaptive Optics
• Used with ground-based telescopes
• A method to correct for blurring of images due to
turbulence in the Earth’s atmosphere over small
areas of the sky
• Monitor the light from a “guide star” or “laser
guide star”
• Make small changes to the mirror shape many
times per second, and this cancels the smearing
from the turbulence
Adaptive Optics
corrects the
wavefront
Makes for a
MUCH clearer
image.
Phases of
the Moon
The position of
the Moon in the
sky is closely
related to the
phases!
Can you see a
Full Moon at 12
o’clock noon?
What would
someone see
for the phases
of the Earth
from the Moon?