chapter 4
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ONE OF THE TWIN KECK TELESCOPES ON THE SUMMIT
OF MAUNA KEA, HAWAII, DURING A TOTAL ECLIPSE OF
THE SUN. THESE ARE THE TWO LARGEST INDIVIDUAL
OPTICAL TELESCOPES IN THE WORLD.
10-meter Keck Telescope at the W.M. Keck Observatory.
3 distinct types of telescopic power
1. Collecting Power (also called light
gathering power or light grasp)
2. Magnifying Power
3. Resolving Power
1- COLLECTING POWER
= the amount of light the telescope is
able to focus into the eyepiece
• The more light it collects, the brighter
the image
• Stars will always look like points of
light, but you will be able to see
more (fainter ones) and they will be
brighter
=This is the most significant factor
Collecting Power• Light-collecting ability varies with the square of the
aperture.
• Thus, a 90mm telescope (a little under 4”) collects
only 1/5 as much light as an 8” telescope
The larger the diameter of the lens
or mirror used in a telescope:
- The greater its ability to
gather light
- The finer the detail that the
instrument will show
Telescopes - Refractors
Use two or more
lens to bend
(refract) the light,
so it focuses on
the eyepiece at
the end of the
telescope.
Telescopes - Reflectors
Gather light at the primary
mirror (curved) on the far end
of the tube, which focuses the
image onto the secondary
mirror (flat), that redirects the
light at a right angle into the
eyepiece, mounted on the side
of the telescope.
REFLECTORS:
Also called Newtonian Reflectors
- First designed by Isaac Newton, around 1670
The best light-gathering capability
Quality of mirrors very critical
Good for astronomy viewing only-They invert the image (objects look
upside down)
How a curved mirror focuses light
Mirrors that are made of glass that has been
shaped to a smooth curve, polished and
then coated with a thin layer of aluminum or
some other highly reflective material
Dobsonian mounts
were designed for
larger reflectors
that were too
unstable on
tripods.
• They sit on the
ground
Why Reflecting Telescopes are
Preferred over Refracting
• A large mirror can be thin but a large lens must be thicker thus heavier.
• A lens has two surfaces that must be cleaned and polished; a mirror only has one
• Glass absorbs light! The thicker the light the more absorption.
• Lenses need to be supported only around the outside; mirrors can be supported by the back
• For large lenses, glass deforms under its own weight; thus changing the lenses’ properties.
• In a lens, different colors are refracted by different amounts. (Chromatic Aberrations). Lenses are corrected for chromatic aberrations and are called achromats.
Cassegrains telescopeCassegrains
have a compact
tube which
incorporates
primary and
secondary
mirrors that fold
the light path
and focus the
light into the
eyepiece at the
end of the tube.
Calculating MagnificationMagnification = Telescope focal length
Eyepiece focal length
Telescope focal length = The distance from the center of a curved mirror or the center of the lens (where light passes through the first element of the telescope) to the focal point.
Orion Catalogue
Orion AstroView Meade LX200R
Calculating Magnification
Magnification = Telescope focal length
Eyepiece focal length
Eyepiece focal length = The distance from the center of the field lens (where light passes through the first element of the eyepiece) to the focal point.
• Given in millimeters
Magnification = Telescope focal length
Eyepiece focal lengthExamples
• 750 mm = 30x
25 mm
3048 mm = 122x
25 mm
OR
750 mm = 100x
7.5mm
3048 mm = 406x
7.5 mm
3. Resolving Power• Resolving Power = the ability of the
instrument to discriminate fine detail.
• How sharp or fuzzy the image is
• The limitation on resolving power is imposed by
the interaction of light and optics.
• The quality of the optics is a major factor
• Lens, mirrors, eyepieces, etc.
• Usually, you get what you pay for
• The turbulence in the air column you are
looking through, moisture in the air, etc. also
effect this.
The Earth’s atmosphere
interferes seriously with
observing.
It absorbs and totally blocks
many wavelengths.
It blurs details.
Interferometers enhanced the resolving power
in sources.
Astronomers use special-
purpose telescopes to
observe non-visible
wavelengths. Many of
these are in orbit, so they
are not affected by blurring
or atmospheric absorption.
“The Hubble Space Telescope orbits far above the
distorting effects of the atmosphere, about 600
kilometers above the Earth. This perch gives
astronomers with their clearest view ever, but it
also prevents them from looking directly through
the telescope. Instead, astronomers use Hubble's
scientific instruments as their electronic eyes.”
Upper Left: Closer View
“This color image of Saturn was taken with the HST's Wide Field and Planetary Camera (WF/PC) in the wide field mode at 8:25 A.M. EDT, August 26, 1990, when the planet was at a distance of 1.39 billion kilometers (860 million miles) from Earth.”
Credit for picture and text: NASA
“This enlargement of the Saturn image reveals unprecedented detail in atmospheric features at the northern polar hood. Saturn's north pole is presently tilted toward Earth by 24 degrees”
Courtesy for
picture and text:
NASA
Telescopes – Eyepieces• An eyepiece brings
the light rays
gathered by the
telescope into sharp
focus.
• The eyepiece
determines the
magnification, as
well as its
brightness and
contrast
Your eye
Telescopes – Eye Relief
• Eye Relief is the distance between the eyepiece lens and your eye, when the image is in focus
• Eyeglass wearers need at least 15mm
Telescopes – Eye Relief• Shorter focal-length
eyepieces tend to have
shorter eye relief than
longer focal length
eyepieces.
• Smaller lens openings
• You need to get closer
to them to see the
image
• Like looking through a
peek-hole. 17mm 6 mm
NightWatch, p. 76
Telescopes – Finder scopes
• Finder scopes are
miniature telescopes
mounted parallel to
the main tube that
allow easy alignment
of the target object.
• Object is centered in
the crosshairs of
finder scope
• Has to be adjusted
to the main
telescope
NightWatch, p. 67
Finder scopeEyepiece
Telescopes – Tripods/Mounts
• A sturdy tripod and mount are essential
to jiggle-free viewing
• Especially at higher magnifications
• The slightest movement is greatly
magnified
• The mount also has to be able to
smoothly adjust for the Earth’s rotation
• Especially at higher magnifications
• The objects move quickly through
the field of view
Accessories
• Electronic drive
systems
compensate for
the rotation of the
Earth and keep
the image
centered in the
telescope
www.meade.com
Accessories
• “Go to” computer controllers
• A computer is built into the telescope base
• Once the computer is aligned, the system is able to find thousands of celestial objects
www.meade.com
Accessories
• Filters
• Moon – too bright when more than half the moon is lighted
• Planets – different colors enhance the image
• Sun – large filter that goes over the front opening
• Otherwise, you could “cook” the inside of the telescope
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