A Simple Telescope: Measuring Magnification


Many people had some trouble seeing the test target used to estimate the magnification of the simple telescope. Some found it hard to use both eyes at the same time; others may have been a bit confused by what they saw. This page provides an opportunity to measure the magnification without any eye-strain!

The target seen without the telescope. The target seen through the telescope.
Fig. 1. The target seen without the telescope. Fig. 2. The target seen through the telescope. Note that the image is inverted, and a bit fuzzy.

The two photographs above show the test target set up for a magnification measurement. Fig. 1 shows what you might see with your naked eye. Fig. 2 was taken with the simple telescope set up in front of the camera; this closely approximates what you can expect to see looking at the target through the telescope.

The target seen with both eyes.

Fig. 3 shows what you will see if you can look with both eyes at the same time: the two images are superimposed on each other. Some people had trouble with this; the brain expects both eyes to be looking at the same thing, and tends to `tune out' one eye or the other if they're not. That's why one image or the other sometimes seems to fade out. It takes practice to comfortably view both magnified and unmagnified images at the same time!

Fig. 3. The target seen with both eyes.

The basic idea of the magnification measurement is to compare the size of one of the red bars seen through the telescope with the same bars seen without the telescope. You do this by counting the number of unmagnified red and white bars which appear superimposed on the magnified red bar. Why count both red and white bars? Notice that both red and white bars have the same height; the unmagnified bars serve as a kind of ruler which you are using to measure the magnified red bar. For example, suppose the telescope magnified exactly two times, so the magnified bar was twice as high as the unmagnified bars. In that case exactly two unmagnified bars -- one red, one white -- would fit within the image of the magnified red bar, and counting both bars gives you the correct magnification.

Close-up of the target.
Fig. 4. Close-up of the target.

Now look at Fig. 4 and count the number of red and white bars you see superimposed on the large red bar. That number is a direct measurement of the simple telescope's magnification.

Of course, any measurement has some experimental error. What are the possible errors associated with this measurement? One source of error is obvious from Fig. 4: the image seen through the simple telescope is somewhat fuzzy, so you may find it hard to tell just where the magnified bar begins and ends.


Joshua E. Barnes (barnes@ifa.hawaii.edu)
Last modified: September 22, 2005
http://www.ifa.hawaii.edu/~barnes/ASTR110L_F05/simplescopehelp.html
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