Elementary Optical Concepts - The University of...

Objective lensEyepiece lens

Image

Elementary Optical ConceptsA Simple Keplerian Telescope

Inverted imageObject

Elementary Optical ConceptsA Simple Camera

f

ab

Thin lens formula: 1/f = (1/a) + (1/b)

O�-axis light raysEntrance pupil

Exit pupil

Field plane(or image plane)

Pupil diameter = D/Mag

(Diameter = D)

feyepiece fobjective

Telescopes magnify angles

Mag = fobj/feye

Collimated light: light rays from individual image points are parallel.

Telescope magni�cationFields and PupilsThe exit pupil is an image of the entrance pupil formed by the eyepiece lens.

Basic layout of a Keplerian telescope.


Image



Exit pupil



(Diameter = D)



Mag = fobj/feye



Basic layout of the Keplerian telescope.


Image



Exit pupil



(Diameter = D)



Mag = fobj/feye



Image



Exit pupil



(Diameter = D)



Mag = fobj/feye


Telescope magni�cation


Image



Exit pupil



(Diameter = D)



Mag = fobj/feye


Telescope magni�cationFields and Pupils


Image




f

ab



Exit pupil



(Diameter = D)



Mag = fobj/feye

The eye is an integral part of the “telescope system”





Image


Entrance pupil

Exit pupil



(Diameter = D)



Mag = fobj/feye



Image




f

ab



Exit pupil



(Diameter = D)



Mag = fobj/feye

The eye is an integral part of the “telescope system”




“Night glasses” and “richest -!eld’ telescopes

• If the exit pupil of a telescope is bigger than about 7mm, all the lightwon’t enter our dark-adapted eye

• A “rich-field” telescope has just the right focal length eyepiece lens togive us all the light we can use and no more

• The objective lenses of 7x50 binoculars are just the right size to giveus all the light we can use from a seven-power telescope

• Questions– What is the magnification of the telescope in our example?– What’s the “richest-field” objective lens diameter for our telescope?– Why might we want to use a higher power eyepiece, even if it gives us an

exit pupil smaller than 7 mm?– What power eyepiece would give us “richest field” performance with an

8-inch diameter telescope with a focal length of 40 inches?– What would be the focal length of a “rich field” eyepiece for our 8-inch,

f/5 telescope?– Why wouldn’t we want binoculars with larger diameter objective lenses

than 50 mm?

Elementary Optical Concepts - The University of...

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