Chapter 4 - TELESCOPES

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

Astronomers

use telescopes

to gather light

and thereby

make dim

objects

observable

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.

How a lens focuses light

A lens bends (or refracts) the

light and focuses it on a point

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.

2- MAGNIFYING POWER

the number of times a

telescope can increase

the apparent size of an

object.

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.

Resolving Power

http://physics.uoregon.edu

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

View of a colliding galaxy dubbed the "Tadpole" (UGC10214): Photo Courtesy NASA Hubble

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