©JParkinson1TELESCOPES INCREASE THE ANGLE THAT AN OBJECT SUBTENDS AT THE EYE REFRACTORS REFLECTORS THE OBJECTIVE IS A LENS THE OBJECTIVE IS A MIRROR NEWTONIAN CASSEGRAIN

©JParkinson2 MAGNIFYING POWER M DEFINITION The telescope is said to be in normal adjustment when the final image is formed at infinity This allows a fair comparison of performance to be made between the telescope and the naked eye - which has to look at an object, which itself is at infinity.

©JParkinson3 EXAMPLE A telescope with a magnifying power of 40 is used to view the Moon which subtends an angle of to the eye.What will the angle subtended to the eye by the final image, if the telescope is in normal adjustment? The angle will be 40 x = 20 0

©JParkinson4 FORMATION OF THE FINAL IMAGE Rays from bottom of object Rays from top of object Intermediate Image where focal points coincide F o & F e Rays emerge parallel because final image is at infinity To top of final image at 

©JParkinson5 MAGNIFYING POWER FORMULA fofo fefe   h = height of intermediate image For small angles in radians Guide ray // // When the telescope is in normal adjustment h and TELESCOPE LENGTH = fo fo fo fo + fefefefe

©JParkinson6 The Eye Ring or Exit Pupil This is the image of the Objective Aperture in the Eyepiece Lens Objective lensEyepiece lens object FeFe EYE RING RAY DIAGRAM CONSTRUCTION

©JParkinson7 Objective lensEyepiece lens Guide line Eye Ring FORMATION OF THE EYE RING

©JParkinson8 u = f o + f e v DoDo D o = Diameter of Objective Lens D ER D ER = Diameter of the Eye Ring BUT Magnifying power M EYE RING FORMULA

©JParkinson9 EXAMPLE 1 A telescope consists of two convex lenses, one of focal length 500mm, the other of focal length 50mm. The telescope is in normal adjustment. (a)What is the focal length of the eyepiece? (b)What is the separation of the lenses? (c)Where is the final image located? (d)Is this image erect or inverted? (e)What is the magnifying power? (f)Where should the pupil of the eye be placed to obtain the best view through the telescope? f e = 50 mm;f o = 500 mm f e + f o = = 550 mm At infinity in normal adjustment It is inverted M = f o / f e = 500  50 = 10 u = 550 mm, f e = 50 mm, find v v = 55 mm

©JParkinson10 EXAMPLE 2 A small telescope, of length 330 mm, is rated as 10 x 50 (a)What does this mean? (b)What is the focal length of the eyepiece? (c)What is the focal length of the objective? (d)What is diameter of its eye ring? (e)What is the instrument’s relative light gathering potential? M = 10 & the Diameter of the Objective is 50 mm So f o = 11 f e. Then 11 f e = 330; f e = 30 mm f o = 300 mm M = D o / D ER ; so 10 = 50  D ER ; Eye Ring Diameter = 10 mm Diameter of Objective  Diameter of Eye Ring = 50  10 = 5 times

©JParkinson11 CHROMATIC ABERRATION LENSES ACT LIKE PRISMS AND DISPERSE LIGHT A lens has a shorter focal length for blue light than it does for red This causes image to be blurred and to have coloured edges FBFB FBFB

©JParkinson12 SPHERICAL ABERRATION Only rays of light parallel and close to the principal axis pass through the focal point, F, of the lens. Principal axis F Rays farther from the axis strike the lens with a greater angle of incidence and consequently have a greater angle of refraction. USING A LENS WITH A WIDE APERTURE BLURS THE IMAGE SPHERICAL ABERRATION

©JParkinson13 SPHERICAL MIRRORS ALSO SUFFER FROM SPHERICICAL ABERRATION FfFf

©JParkinson14 PARABOLIC MIRRORS ELIMINATE SPHERICICAL ABERRATION FfFf IN ADDITION MIRRORS DO NOT SUFFER FROM CHROMATIC ABERRATION ALL LARGE TELESCOPES ARE PARABOLIC REFLECTORS i.e. There objective is a parabolic mirror

©JParkinson15 NEWTONIAN REFLECTOR Convex Eyepiece Parabolic Concave Mirror Plane Mirror at 45 0 to the axis F

©JParkinson16 CASSEGRAIN REFLECTOR F site of 1st intermediate image Auxiliary convex mirror - in practice very much smaller than the objective Parabolic Concave Objective 2nd intermediate image Convex lens eyepiece, acts like a magnifying glass The Final Image is at Infinity

©JParkinson17 Advantages of Reflectors Less loss of light in reflection compared with refraction Do not suffer from chromatic aberration Parabolic reflectors do not suffer from spherical aberration Diameter can be much greater because a mirror - can it be supported from behind - “glass sags” * There is only one surface to be ground No worry about trapped air bubbles in the glass * Telescopes have large apertures 1.To gather more light 2.To increase their Resolving Power