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Astronomical refracting telescope

Consists of two converging lenses:

Needs a ray diagram to explain – revise ray diagrams. 

General points:

o       Rays from a distant object (like a star) are always parallel – they will all be refracted through the focal plane of the “objective lens”.  This is the lens that points at the star – the “eye lens” is the other lens – the one we look through.

o       The two lenses have “mutual focal points”  - this means that they are put at just the right distance apart so that the focal point of the objective lens is in the same position as the eye lens

o       The first point (above) works in reverse – rays coming through the focal plane of the eye lens will be refracted parallel to a ray through the centre of the lens.

o       The eyepiece works like a magnifying glass – with the image at infinity


Angular magnification in normal adjustment

Text Box: 	      Angle subtended by image at eye
M= 	Angle subtended by object at unaided eye



This can be related to the focal lengths of the lenses: 

Look at the above diagram again – for angles qo and q

M  = = = small angles (ignore cos) = 

Cancel to give:

Do question 4.6 from P. 49 - Ingham

The eye ring

This helps us to position our eyes to collect most light coming from the objective lens.

See P. 49 – for ray diagram

Explain how refracting telescopes are designed with an eye ring to gain the best observations

Do question 4.7 from P. 50 – Ingham.

Attempt ASSIGNMENT - using the pair of binoculars from P. 50 – Ingham.

List the limitations of a refracting telescope – P. 50 – Ingham.

Example question: a telescope has an objective lens with a focal length of 100cm and a diameter of 5cm.  If the eyepiece has a focal length of 20cm and the telescope is used in normal adjustment, calculate

a.      the magnifying power

b.     the diameter of the eye ring

c.      the separation of the lenses