A Basic Refractor Telescope

The size of the telescope is the diameter of the light-collecting

lens

“Normal” Eye

Refractive Errors

Farsightedness Nearsightedness

Limitations of Refractors

• Lens must be made of high-quality glass with no imperfections

• Larger lenses weigh a lot; lenses can be supported only around their rims

• The lens will sag under its own weight

So, the largest refractor telescope anyone has built is about 1.1 m.

A Basic Reflector Telescope

The size of the telescope is the diameter of the objective mirror

Usefulness of Reflectors

• Objective (mirror) can be made of many things, even plastic or metal

• Only one side of the mirror is polished

• Mirror can be supported from its entire back, not the rim only

So, all large modern telescopes are reflectors because they are easier

to make and maintain.

A Prime Focus Reflector

(some light blocked)

A Newtonian Reflector

(secondary mirror)

A Cassegrain Reflector

(hole in primary mirror)

Example: Hubble Space Telescope

Resolution vs. Magnification

• RESOLUTION: a telescope’s ability to distinguish fine details in an

image.

• MAGNIFICATION:

focal length of objective

focal length of eyepiece

So, magnification depends on the eyepiece used in the telescope.

Hubble Space Telescope