Name:
_______________________
AST-130
Date: __________
Astronomy Lab
TELESCOPES
In this lab, you will have the chance to build your own telescope and explore some
properties of lenses. Let us start with some basic principles of optics.
The figures below are called Ray Diagrams. Light rays are reflected off an “object” and
pass through the lens. Where the rays cross locate the “image” which can be seen with
your eyes. Ray Diagrams can predict where that light converges (forms a focused image)
for a lens.
The line drawn through the center of each lens below is the principal axis of that lens.
Figure 1 illustrates a set of rules to predict the location of an image based on an object’s
position and the lens’ focal length:
1. Rays parallel to the principal axis pass through the lens’ focal point.
2. Rays that pass through the center of the lens continue on their path, unaltered.
3. Rays that pass through the corresponding focal point on the object’s side of
the lens emerge parallel to the principal axis.
Use the Three Ray Diagram rules to locate the image in the following diagrams
(Figures 1, 2, 3).
Figure 1.
Draw the image arrow. Is the image upright or inverted? Is the image larger or smaller
than the object?
_____________________________________________
1
Figure 2.
Draw the image arrow. Is the image upright or inverted? Is the image larger or smaller
than the object?
_____________________________________________
Figure 3 requires a modification to our Ray Diagram rules. After carefully drawing rays
following rules 1 and 2 note that they do not cross on the right side of the lens. Extend
both rays backwards on the left side of the lens. Where they cross will be the image of the
arrowhead.
Figure 3.
Draw the image arrow. Is the image upright or inverted? Is the image larger or smaller
than the object?
_____________________________________________
2
The Telescope Kit
Step 1: Measuring Focal Lengths
Hold the lens above the table. Move the lens up and down to focus the overhead
fluorescent light onto the table. Measure the distance from the lens to the image formed
on the table. Use the formula for the image location, “i ” of distant objects, from the
previous page, to determine the focal length of the lenses:
Large lens focal length (= image distance from lens) _______________ cm
Small lens focal length (= image distance from lens) _______________ cm
Step 2: Building the Telescope
Place the large lens (called the "objective lens") over one end of the larger tube, and
slide the plastic cap over the end of the tube to hold it in place.
1.
2.
Slide the smaller tube into the larger.
Push your small lens into the hole in the foam lens
holder. It should be near one end, and nice and straight.
3.
Push the little cardboard tube spacer in behind it, to keep it
stable and aligned.
4.
Put the eyepiece assembly (foam, lens, and spacer) into the open end of the smaller
tube: the round part of the eyepiece lens should face the big objective lens, and the
cardboard spacer should face toward the outside, where your eye will go.
5.
3
Step 3: Observe and Estimate Magnification of Telescope
Look through the telescope at a small distant object (a small picture on the projector
screen on the far side of the room). You will need to slide the inner tube in and out to
focus the image. Look at an object on the far wall
Is the final image you are looking at upright or inverted? __________________
Using geometric optics, one can show that the theoretically calculated magnification of
your telescope should be
f
calculated magnification: M  1  _____________________
f2
Where f1 is the focal length of the objective lens, and f 2 is the focal length of the
eyepiece from Step 1.
Aim the telescope towards the ruled scale mounted on the lab wall. Use one eye to look
through the telescope, and the other eye to simultaneously look at the wall scale.
The magnified image of the scale as scene through the telescope will be visually
superimposed on the unmagnified scale, as depicted in the figure. Count the number of
divisions on the unmagnified scale that overlap one division on the magnified scale. This
number is the telescope’s magnification.
Record the estimated magnification below.
Estimated telescope magnification: ____________________
How does this compare to the calculated magnification?
4