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

TELESCOPES Outline: 1.Types of Telescopes 2.Most Important Properties 3.Sky Limitations and Overcoming Them 4.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

Astronomial Telescopes invert And 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 = logD(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 Hubble Space 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 arcsec Rayleigh = 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 opticsWith 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 very Important!

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 / diameter of 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 (star clusters, nebulae, and galaxies) and want a simple to set up and use telescope -- “Dobs” are for you! ($500 will get 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!)Land, Sea, & Sky Check out Astronomics.com or Telescopes.comAstronomics.comTelescopes.com Look through lots of telescopes before buying Visit George Observatory (Astronomy Day is Saturday, October 8th) Buy the best you can afford…