Download Refraction - Geneva 304

Telescopes
Telescopes: Instruments that gather electromagnetic radiation to study
astronomical phenomena.
 The human eye has some shortcomings that ultimately limit that
exploration:
1.
2.
3.
4.
Limited light-gathering power.
Sees only electromagnetic radiation in the visible wavelengths
Cannot be used to accumulate light over a long period
The eye cannot store an image for future reference
 Astronomers have developed a variety of instruments and techniques to
supplement the human eye.
Binoculars
 Collect more light than the eye: Can see things not visible with the eye
 A good purchase before telescopes
 Specifications given in 7x35, 15x45, 10x50
o First number = magnification
o Second number = Width in mm of the
 From 7x35 up to 10x50 power are the best for hand held observations
(anything more powerful would shake the images around too much due to hand
movements)
 Can get a good pair from $70 and up
Telescopes
 Optical telescopes may be divided into two general categories:
o Refracting telescopes - use lenses
o Reflecting telescopes - use mirrors
 The main purpose of a telescope is to gather
light, i.e. to collect and focus photons. We can
think of a telescope then as a "light bucket"
Refraction
 Refraction: The change in direction of light propagation at the boundary of
two media having different densities.
 The direction of light
propagation is changed
at the boundary of glass
and air by refraction.
 Lenses having the right
curvature, this principle
can be used to gather
and focus light
Principle of refraction and the refracting telescope
 Here are Java applets illustrating image formation by a converging lens, and
by a diverging lens.
Chromatic Aberration
 Different wavelengths focus at slightly different points. This causes objects
like stars to be surrounded by fuzzy, rainbow colored halos.
 Can be corrected by using a second carefully designed lens mounted behind
the main objective lens of the telescope to compensate.
Types of Refracting Telescopes
 Achromatic - basic refractor; 70 to 100 mm objectives
 Apochromatic - use two or three corrective lenses; up to 180 mm objective;
very high quality image
Reflection
Reflection: The angle of incidence (measured from the perpendicular to the
reflecting surface) is equal to the angle of reflection.
Principle of reflection and the reflecting telescope
 Here is a Java applet illustrating the use of a mirror (a diverging or convex
mirror in this case) to form an image.
 The largest optical telescopes are reflecting rather than refracting telescopes:
it is easier to build and support large mirrors of high optical quality than
large lenses.
Types of Reflecting Telescopes
 Various mirror arrangements are used to transport the light from the focus
to an external observer.
o Newtonian (more common, longer tube)
o Schmidt-Cassegrain (a short tube that uses two reflecting mirrors)
o Maksutov-Cassegrain (a variation of the Schmidt-Cassegrain, a bit
smaller and more compact)
o See figures 4.13 - 4.17
Other Observing Instruments
 Observing all wavelengths of electromagnetic radiation
 Includes: Space telescopes; Infrared & Radio telescopes, detectors, and
arrays; Ultraviolet, X-, and Gamma Ray telescopes and detectors
 Famous Telescopes
Atmospheric Conditions for Night Viewing
 Seeing - the steadiness of Earth's atmosphere
 Transparency - clarity of the Earth's atmosphere
 Light pollution - glow from lights in cities and towns that 'bleaches out' the
darkness of the sky
Buying a Telescope
 Using $100 to $250 department store telescopes is an exercise in frustration!
 Good quality telescopes and a minimum array of accessories begin around
the $300 to $400 mark (save up for a good one, don’t waste your money!)
 Things to look for:
o Stable mount (aluminum or wooden tripods, dobsonian)
o Type of movement (pushing tube, slow motion knobs, motor drives)
 Accessories
o Finder scopes (6x30 minimum)
o Filters (solar, lunar, and color filters; solar and lunar recommended)
o Eyepieces
 Magnification = focal length of telescope divided by the focal
length of the eyepiece
 Example: 2000mm telescope divided by 25mm eyepiece length
=
2000 mm / 25 mm
=
80x magnification
 Usually about 1.25 inches wide; some larger telescopes have 2
inch wide eyepieces
 $40 to $120, depending on brand
 Barlow lens - extension attachment for eyepieces - magnifies
eyepiece by 2x up to 5x
 Diagonal - an adapter for viewing at 900