TELESCOPESAstr 221 Lec 3 Spring 2015(Ch. 5 in Nightwatch)

TELESCOPES

Outline:1. Types of Telescopes2. Most Important Properties3. Sky Limitations and Overcoming Them4. What Kind of Telescope Is Best for YOU?

What are the two basic designs of telescopes?

• Refracting telescope: Focuses light with lenses

• Reflecting telescope: Focuses light with mirrors

The Galilean Telescope (1609)

Kepler’s Modified Refractor

Refracting Telescope

• Refracting telescopes need to be very long, with large, heavy lenses

Reflecting Telescope

• Reflecting telescopes can have much greater diameters

• Most modern telescopes are reflectors

AstronomialTelescopes invertAnd sometimes flip(reverse) images

Binoculars - “inverting the inverted” image

What are the two most important properties of a telescope?

1. Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time.

2. Angular resolution: Telescopes that are larger are capable of taking images with greater detail.

Bigger is better

Mirrors in Reflecting Telescopes

Twin Keck telescopes on Mauna Kea in Hawaii

Segmented 10-meter mirror of a Keck telescope

Light Collecting Area

• A telescope’s diameter tells us its light-collecting area: Area = π(diameter/2)2

• The largest telescopes currently in use have a diameter of about 10 meters

• Mag Limit = 9.5 + 5logD(inches) (7.5) (cm)

Angular Resolution• The minimum

angular separation that the telescope can distinguish.

Angular Resolution• Ultimate limit to

resolution comes from interference of light waves within a telescope.

• Larger telescopes are capable of greater resolution because there’s less interference

Angular Resolution• The rings in this

image of a star come from interference of light wave.

• This limit on angular resolution is known as the diffraction limit

Close-up of a star from the HubbleSpace Telescope

Angular Resolution

Dawes Limit = 4.56arcsec/D(in) (theoretical) = 11.6arcsec/D(cm)

Rayleigh Limit = 5.5arcsec/D(in)(observational) = 14arcsec/D(cm)

e.g., RUCO 16”: Dawes = 0.28 arcsecRayleigh = 0.34 arcsec

Interferometry

• Interferometery is a technique for linking two or more telescopes so that they have the angular resolution of a single large one

Twinkling and Turbulence

Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to appear to twinkle

Star viewed with ground-based telescope

Same star viewed with Hubble Space Telescope

Adaptive Optics

Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence

Without adaptive optics

With adaptive optics

Two More Important Things About Telescopes

3. Magnification:

m = f.l. objective / f.l. eyepiece (variable ==> good eyepieces!)

4. Mount:

(a) sturdy but portable(?)

(b) Fine motions (knobs or motors)

(c) “Clock” drive

Mounts are veryImportant!

Light Pollution!

Calm, High, Dark, Dry

• The best observing sites are atop remote mountains

Summit of Mauna Kea, Hawaii

Why do we put telescopes into space?

Transmission in Atmosphere

• Only radio and visible light pass easily through Earth’s atmosphere

• We need telescopes in space to observe other forms

Future of Astronomy in Space?

• The Moon would be an ideal observing site

OK, What telescope should I get?

f ratio = focal length / diameterof objective

f/10 == “slow” (fl=2000mm, d=200mm)(big but faint images)

f/5 == “fast” (fl=1000mm, d=200mm)

Usually “slow” telescopes are long and used to only view bright objects like moon and planets.

For deep sky objects, “fast”telescopes are better (~f/5)

Answer: Depends on what you want to observe…and your pocketbook!

Refractor or Reflector?

DOBSONIAN

“LIGHT

BUCKETS”

If you want to see faint deep-sky objects (starclusters, nebulae, andgalaxies) and want a simple to set up and use telescope -- “Dobs”are for you! ($500 willget you an 8inch)

Computerized “go-to” telescopes: $400up (~$1500 really)

Reggie’s Steps to Buying a Telescope

• Read his handouts from Astronomics!• Get a copy of Sky&Tel or Astronomy magazine• Visit Land, Sea, & Sky on Richmond Ave. (tell

Darren or Art that Reggie sent you!)• Check out Astronomics.com or Telescopes.com• Look through lots of telescopes before buying• Visit George Observatory (Astronomy Day is

Saturday, October 8th)• Buy the best you can afford…