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Astronomy 1010
Planetary Astronomy
Fall_2015
Day-21
Course Announcements
•
SW-chapter 6 posted: due Fri. Oct. 23
•
Exam-2 will be returned on Friday
•
1st Quarter Observing night: Tuesday, Oct. 20; 7:30pm
Course Announcements
•
CHECK YOUR LAB GRADES IN D2L. If you are missing
grades, and have gotten the lab back, return it so we can
enter the grade. (mostly Scientific Methods, but might be
others as well.)
Telescopes & Instruments
 The telescope is the
astronomer’s most
important tool.
 Purpose: to gather
light of all kinds.
 Two kinds of optical
telescopes:
reflecting and
refracting.
 Invented in 1608 by
Hans Lippershey.
Telescopes come in three
general types
Reflectors use mirrors to reflect
the light to a focus
Refractors use lenses to bend
the light to a focus
Catadioptric telescopes use both lenses and
mirrors
Telescopes

Telescopes have three functions:
1.
Gather light

2.
Resolve objects

3.
LGP ∝ Area = πR2
Θ = 2.06 X 105 (λ/D)
Magnify EXTENDED objects
The most important property of any
telescope is to gather large amounts
of light and concentrate it to a focus.
Light Gathering Power  Area  radius 2
Refraction is the bending of
light when it goes from one
medium to another
“n” is the
index of
refraction.
c
n
v
Refraction is
governed by
Snell’s Law:
n1 sin 1  n2 sin 2
If we curve the surface and
make a lens, we can get the
light to concentrate to a point
 Refracting telescopes
use lenses.
 Objective lens: refracts
the light.
 Aperture: size of the
objective lens (larger
aperture gathers more
light).
 The objective lens is
placed in the aperture.
The refracting telescope
uses two lenses
Since the eye already has a lens, the eyepiece is needed to bring the light
rays back to parallel for the eye to see
Large refractors
can be very long
and bulky
The Largest Lens is 40”
Built in the late 1890’s, it is the last great refracting telescope.
Lenses and refractors suffer
from Chromatic Aberration
This applies to camera lenses, your
eye, telescopes and anything else that
uses a lens to focus light
Correcting for Chromatic
aberration can be expensive
The compound lens takes two lenses of different materials and combines
them to correct for color distortion
Color separation is useful in a
prism so that we can obtain a
spectrum of light
Since it is meant to
be separated we
don’t call it an
aberration.
Instead, it is called
dispersion
A diffraction grating works on
interference of light waves
Diffraction is much
more efficient at
separating light
into its colors than
dispersion
Unfortunately, diffraction
also leads to problems
Look closely enough at stars and they aren’t just
points of light but rings, too
i_Clicker Question
 Telescopes and Astronomical Instruments:
 Refraction – Option 1
 Lenses – Option 1
 Reflecting telescopes use mirrors.
 There are primary and secondary mirrors.
 Focal length is determined by the path the
light takes reflecting off the mirrors.
Reflection is the bouncing of
light off a surface
i  r
Mirrors do not suffer from chromatic aberration and they do not
cut off long or short wavelengths
A concave
mirror
focuses light
to a focal
point
Telescope mirrors are made so
that the focus is a plane
instead of a point
Reflecting Telescopes Suffer
From Spherical Aberration
There are
several
types of
reflecting
telescopes
i_Clicker Question
 Telescopes and Astronomical Instruments:
 Reflection – Option 1
 Spherical Aberration
LSST Site – Artist Concept
LSST Site – 1st Blast – March 2011
Steward Observatory
Mirror Lab
LSST M1 in polishing build-up
Coming “soon”
The 30m Tele.