Telescopes represent an effective way of producing magnification without changing the working distance. Disadvantages They have a restricted field of view Often used to focus on objects closer than infinity Can be modified to correct for the Px’s refractive error.
There are two basic kinds of Telescope. Keplerian or Astronomical
D C B A Keplerian or Astronomical Telescopes f o ’ f e t F e F o A B C D
+ve Objective lens (F o ) and +ve Eyelens (F E ) Focal points of these lenses are coincident The image produced by a Keplerian Telescope is inverted Prism are required to re-invert the image The exit pupil is formed outside the instrument Keplerian or Astronomical Telescopes Specified in terms of the magnification and the diameter of the objective lens (e.g. 4 x 12)
Galilean Telescopes A B f o ’ t f e A B F o F e
Galilean Telescopes +ve Objective lens (F o ) and -ve Eyelens (F E ) The image produced by a Galilean Telescope is erect The exit pupil is formed inside the instrument The length of the Galilean Telescope is shorter than the Keplerian Specified by the magnification 2x 2.5x etc.
M = angle subtended at the eye by instrument image angle subtended at the eye by object Magnification in Telescopes M = Fe Fo M = size of entrance pupil size of exit pupil
So far we have dealt with afocal telescopes considering objects placed at optical infinity and viewed by emmetropic eyes. Afocal – the tube length of the system is fixed and cannot be adjusted for different viewing distances and refractive errors. Afocal and Focal Telescopes
In practice telescopes are used by ametropic eyes to view objects that are not at optical infinity. There are 3 ways by which a telescope can be adapted to compensate for spherical ametropia: 1. Add full Rx to eyepiece 2. Partial Rx correction of the objective 3. Changing the length of the telescope Telescopes which can be adjusted in these ways are known as focal
Field of View with Telescopes A telescope gives maximum field of view if the objective lens is as large as possible. To gain maximum benefit, the Px’s pupil should be as close as possible to the exit pupil of the telescope. This is much easier for a Keplerian telescope than for a Galilean.
Telemicroscopes The accommodation required to view an object at a particular working distance through a telescope is given by: Accom n = Working Dist. (in Dioptres) x (M) 2 e.g. to view an object @ 40cm using a telescope with M = 4x Accom n = 2.50 x (4) 2 = 2.5 x 16.00 = 40 Diopters! Vergence Amplification (Freid 1977)
Even modest working distances can make high demands on the accommodative effort. In order to be useful for near work telescopes have to be modified <ul><li>This can be achieved in two ways: </li></ul><ul><li>Increase the tube length </li></ul><ul><li>Introduce a reading cap into the device. </li></ul><ul><li>- this is a +ve lens of power equivalent to the required </li></ul><ul><li>working distance </li></ul>
RC F RC Telemicroscope Telescope Telemicroscopes F o F e
The magnification provided by such a system is the product of the individual components: M total = M telescope x F RC / 4