1. The Simple Astronomical Telescope

2. Angular Magnification by Astronomical Telescope
The angular magnification, M, (also sometimes called magnifying power)
produced by an optical instrument is defined as

3. The Light From A Distant Object
Remember that the light rays from any given point This is why the moon appears to stay in the same place in the sky as we travel on a train.
However for us to see a distant object there must be an “angular distance” between the top of the object and its base
Notice that the rays from each point on the object are parallel to each other

4. eye
Light rays from the top of the distant object
Light rays from the base of the distant object α α α
The angle α between any light rays coming from the top of the distant object and the light ray coming from the base of the object is the same at any point on the eye’
α is called the angle “subtended by the object” at the eye.

5. Here the eye has been replaced by the OBJECTIVE lens (the main “front” lens) of a telescope
The light arriving parallel to the principle axis is focussed at F F
focal plane
Parallel light arriving from the top of the object is focussed below F on the focal plane.

6. The Telescope in Normal Operation
Light rays emerge parallel from the eyepiece lens
Focal plane of the lens
Focal length of eyepiece lens
In normal operation the eyepiece lens of the telescope is placed so that the principal focus of the objective lens is at the principal focus of the eyepiece lens
Focal length of objective lens

7. angle α is the angle subtended by the object with the unaided eye
o’ represents rays from the top of the distant object o’
angle β is the angle subtended at the eye using the telescope o’ β o’
Rays from the top of the object (o’) appear at infinity. This is a virtual image
The angles are exaggerated so we need to extend the eyepiece lens

8. Light from object A (blue lines) meets at the principal focus of the objective lens.  It then spreads out until it meets the eyepiece.  The eyepiece is set at the focal length away from its principal focus.  Parallel rays emerge from the eyepiece.
At the same time parallel rays from object B arrives at the objective at a small angle a to the axis.  The light is focused onto the focal plane.  It then passes through the eyepiece to emerge as parallel rays.  The angle of these parallel rays is b to the parallel rays from A.

10. At the same time parallel rays from object B arrives at the objective at a small angle a to the axis.  The light is focused onto the focal plane.  It then passes through the eyepiece to emerge as parallel rays.  The angle of these parallel rays is b to the parallel rays from A.

11. A simple expression for the magnification of the telescope
= β/α fo α fe β α β h
Using the small angle approximation

12. Concave mirriors

13. Parallel rays are brought to a principle focus by a concave mirror

14. The Cassegrain arrangement of mirrors
to eyepiece

15. The Newtonian arrangement