PHO 101 Photonics Concepts
Three Rivers Community College ONLINE
Home Lab #7
Before proceding. look at Figure 26 on page 44 of your notes. Notice that the telescope is viewing a very distant object (the rays entering the objective lens are parallel). The eye lens and objective lens are separated by a distance equal to the sum of their focal lengths.
The first thing you will need is a "very distant" object, one that has a definite orientation. I used the jack-o-lantern on the door of the house across the street. (I wouldn't recommend using a neighbor's windows, however!)
Keplerian Telescope: Place the long focal length "objective" lens ("A) on a firm support at a comfortable viewing level, facing the object. (I set mine on a couple of physics books.) Now set the short focal length eye lens ("B") at the same height, a distance in front of the objective lens equal to the sum of their focal lengths. While looking though both lenses, move the eye lens "B" back and forth until the object comes into clear view.
What does the image look like? Is is upright or inverted? Record your observations.
Galilean Telescope Replace the converging eyelens with the diverging lens from your kit "C". Experiment until you find the location of lens "C" that produces a clear image. (Lens C will need to be closer to the objective lens A.)
What does the image look like? Is is upright or inverted? Record your observations. Why is this version better for terrestrial viewing?
Keplerian telescope: Description of image _____________________________________________________________________________________________
Galilean telescope: Description of image _____________________________________________________________________________________________
© J Donnelly 2001