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Astronomy 114
Lecture 26: Telescopes
Martin D. Weinberg
[email protected]
UMass/Astronomy Department
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—1/17
Announcements
Quiz #2: we’re aiming for this coming Friday . . .
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—2/17
Announcements
Quiz #2: we’re aiming for this coming Friday . . .
Today: Optics and Telescopes
Optics and Telescopes, Chap. 6
Tomorrow: Galaxies
Galaxies, Chap. 26
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—2/17
Telescopes
What is a telescope?
Collects light
Focuses (concentrates) the photons onto a
dectector
Goals for telescopes
Make sensitive observations ⇒ distant objects
Make BIG telescopes
Resolve small details on the sky ⇒ distant or
nearby objects
Angular resolution
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—3/17
Types of telescopes
Refracting
Focuses light through a lens.
Examples:
Camera lens
Magnifying glass or eye glasses
Reflecting
Focuses light by reflecting light and changing its
path in a coordinated way
Examples: shaving or make-up mirror
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—4/17
Refraction (1/2)
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—5/17
Refraction (1/2)
Demo
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—5/17
Refraction (1/2)
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—5/17
Refraction (2/2)
Parallel light rays from a
distant point come
together at a focus point.
Parallel rays from another distant point come
together at a different focus point in the same
plane ("focal plane").
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—6/17
Refraction (2/2)
Parallel light rays from a
distant point come
together at a focus point.
Parallel rays from another distant point come
together at a different focus point in the same
plane ("focal plane").
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—6/17
Refraction (2/2)
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—6/17
Refraction (2/2)
Magnification =
A114: Lecture 26—17 Apr 2007
Focal length of objective
Focal length of eyepice
Read: Ch. 6,26
Astronomy 114—7/17
Refraction (2/2)
Magnification =
Focal length of objective
Focal length of eyepice
Focal length of objective
Diameter of objective
Smaller f-ratio → more light reaches focal plane
Focal ratio =
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—7/17
Refracting telescope
Goal of objective lens is to
collect as much light as
possible
Simple design, still used for
small amateur telescopes
Limitations:
Bending depends on
wavelength, focal point
depends on color!
Lens must be supported at
edges. Heavy glass lens
sags under its own weight,
distorts optics.
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—8/17
Reflection
Law of reflection
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—9/17
Reflection
Use reflection to focus light. Invented by Newton.
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—9/17
Reflecting telescope
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—10/17
Reflecting telescope
Largest refracting telescopes: 1-m primary lens
Largest reflecting telescopes: 8-10 m primary mirror
Mirror usually polished glass with thin layer of
aluminum
Can support mirror from behind to prevent sagging
Nearly all modern telescopes are reflectors
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—10/17
Angular resolution
Angular resolution–ability to see structure
Limited by the wavelike properties of light
Image of a point of light is spread by the edge of the
objective mirror or lens
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—11/17
Angular resolution
Angular resolution–ability to see structure
Limited by the wavelike properties of light
Image of a point of light is spread by the edge of the
objective mirror or lens
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—11/17
Angular resolution
Angular resolution–ability to see structure
Limited by the wavelike properties of light
Image of a point of light is spread by the edge of the
objective mirror or lens
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—11/17
Angular resolution
Angular resolution–ability to see structure
Limited by the wavelike properties of light
Image of a point of light is spread by the edge of the
objective mirror or lens
Radius of disk: θ = 1.22λ/D radians
Points closer in angle than this are merged
Examples:
Human eye (0.2 cm pupil): about 1 arc minute
10 cm (4-inch) telescope: about 1 arc second
2.4 m (HST) telescope: about 0.05 arc second
10 m (Keck) telescope: about 0.01 arc second
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—11/17
Atmospheric seeing
Blurring by air currents in atmosphere usually
smears images over several arc-seconds ⇒ seeing
Best sites: 0.5-1 arc second
Atmospheric seeing limits the angular resolution of
ground-based optical telescopes
Solutions:
1. Put telescope in Earth orbit (hard, $$$)
2. Adaptively bend mirror to compensate for
atmospheric motions (hard, $$$)
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—12/17
X-ray and gamma-ray telescopes
High-energies photons interact with most materials
X-ray telescopes use ring-shaped "glancing" mirrors
Made of heavy metals
Reflect the rays just a few degrees
Mirrors are rotated parabolas and hyperbolas
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—13/17
X-ray and gamma-ray telescopes
High-energies photons interact with most materials
X-ray telescopes use ring-shaped "glancing" mirrors
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—13/17
X-ray and gamma-ray telescopes
High-energies photons interact with most materials
X-ray telescopes use ring-shaped "glancing" mirrors
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—13/17
X-ray and gamma-ray telescopes
High-energies photons interact with most materials
X-ray telescopes use ring-shaped "glancing" mirrors
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—13/17
X-ray and gamma-ray telescopes
High-energies photons interact with most materials
X-ray telescopes use ring-shaped "glancing" mirrors
Gamma-ray telescopes give up on focusing entirely
Use coded aperture masks
The pattern of shadows the mask creates can be
reconstructed to form an image
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—13/17
X-ray and gamma-ray telescopes
High-energies photons interact with most materials
X-ray telescopes use ring-shaped "glancing" mirrors
Gamma-ray telescopes give up on focusing entirely
Use coded aperture masks
The pattern of shadows the mask creates can be
reconstructed to form an image
Satellites and balloons . . . why?
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—13/17
Transmittance of the Earth’s atmosphere
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—14/17
Detectors (1/3)
First detector: human eye
Use secondary lens ("eyepiece") to make converging
rays parallel again
Limitations:
Short exposure time (1/30 second)
Have to rely on observer’s description or drawing
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—15/17
Detectors (2/3)
Better: film or photographic plates
Put in focal plane, so extended image forms on plate
Incoming photons cause permanent chemical
change
Long exposure ⇒ increased sensitivity
Permanent record
Inefficient: best emulsions miss 99% of incoming
photons
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—16/17
Detectors (3/3)
State of the art: electronic detectors (CCDs)
Incoming photons knock
electrons out of silicon
Electrons counted
electronically
Excellent efficiency: up to 80%
of incoming photons counted
Digital data, easily analyzed by computers
CCDs have revolutionized optical astronomy (since c.
1980)
In digital cameras and camcorders
A114: Lecture 26—17 Apr 2007
Read: Ch. 6,26
Astronomy 114—17/17