Telescopes

Refractors and ReflectorsVarious Designs

Important PropertiesModern Advances

Beyond Visible Light

Basic Optics - Refraction

• Light is bent or refracted when it changes speed going through a lens or prism.

• Where the refracted rays meet is the “focal point.”

• Distance from the lens to the focal point is the “focal length.”

Basic Optics - Reflection

• When light strikes a mirror it is reflected at an angle equal to its incident angle.

• Rays of light striking a curved mirror will meet at a “focal point.”

• The distance from the vertex of the mirror to the focal point is the “focal length.”

Reflectors and Refractors

• A reflector telescope has a mirror as its objective.

• A refractor telescope has a lens as its objective.

• The main function of a telescope is to “gather light.”

Chromatic Aberration• Different wavelengths focus at different

distances• World’s Largest Refractor - Yerkes

Observatory

Reflectors

Why Mirrors?

• Light gathering power - size matters!

• lgp increases with the square of the radius

• More lgp, more resolution!

• Magnification is third!

200’ Hale Telescope Mt. Palomar

Comparing lgp

• lgp depends upon the area of the lens or mirror; A = r2

• lgp = (Dbig/Dsmall)2

Calculating Magnification

M =fobj

feye

Mmax = 20 x Dcm

Practical Limit of Magnification

• Smaller focal length eyepieces give greater magnification.

• They also limit light & increase diffraction.

• At some point the image is too poor to be useful.

Mmax = 20X____cm

x D

Resolving Power

• The ability of a telescope to distinguish two objects.

• Measured in arcseconds ( 1 arcsecond is 1/3600)

a =11.58_______

D (in cm)

Modern Advances• Binocular Telescopes• LBT - two 8.4 m mirrors with adaptive

optics• LGP of an 11.8 m mirror• Detail of a 22.4 m mirror• Surpasses HST in clarity

Adaptive Optics• Correct for atmospheric turbulence• Use the light from a guide star or laser

to determine what the atmosphere is doing to the light passing through it.

• Distortable mirror will change shape to correct the image.

Beyond Visible LightRadio Telescopes

VLA Telescope

UV and X-Ray Telescopes

SOHO

Chandra

HST and James Webb