Using a Galilean Telescope

We encountered several difficulties using our Galilean telescope. Here are some tips for getting the best observations possible:

Build a good mounting!

Galileo didn't tell us about what kind of mounting he used. All that we have left of Galileo's handiwork are two telescopes and a broken lens. The mounting that we used in the first half of the semester really gave us problems. It did not remain positioned on a particular field of view making it very difficult to draw what one saw through the telescope. In fact, even when someone tried to hold it still we couldn't keep it steady making it impossible to draw the craters on the moon, or the phases of Venus, or even a star map. Stars seemed to jump around. There was even one evening when the whole telescope kept falling down. With our new mounting however, we were able to watch the stars without the aid of another person stabilizing the telescope. The telescope would remain in position as different people looked through it to see a particular object. Our mounting allows us to position our telescope almost anywhere within a half sphere with a base at the base of the telescope. The new mounting allows us to swivel the telescope 360 degrees along the base and swivel it up and down. Take a look at our new telescope. Of course, it is much more difficult to move around, an important consideration since we have to transport it in a station wagon to go out into the country and take it up onto the roof of Fondren Library at Rice University, too. Galileo might not have had our mounting, but he must have had a good one to be able to create the star maps that he did.

Make sure your lenses are good!

If your lenses are crooked, cracked, or otherwise defective, then you will see dancing stars with lots of distortion around the edges. As pretty as the colors you get may be, it is awfully frustrating to try to focus on anything or draw anything. If you can't seem to be able to focus on anything, it may be that your lenses have been knocked slightly out of place, an easy thing to do if your telescope is made of cardboard tubing like ours. So be careful transporting it!

Set the focus!

Experiment with your telescope to get the optimum focus on the moon by pulling the tube all the way out and pushing it in slowly. Once you have gotten the optimum focus, put duct tape around the tube so it will remain in this position. This is beneficial because not only does it save you time, it keeps the field of view constant for all of your observations.

Experiment with the magnification!

Galileo built several telescopes and did not actually see the satellites of Jupiter until he had built his 16X telescope. Because a higher magnification telescope has a smaller field of view, you may not always need to use the highest magnification telescope that you have. A small field of view is one of the biggest flaws of the Galilean telescope. Although Galileo probably made a telescope of higher magnification, 30X perhaps, these telescopes were not the ones with which he made his most significant findings. Indeed, it was this flaw that allowed the Galilean telescope to become quickly outdated as the newer Keplerian telescope with its larger field of view introduced the great telescope race.

Experiment with apertures!

An aperture stop is a round piece of paper or cardboard with a hole cut out of the middle like a doughnut. Certainly Galileo did not use the same aperture stop for all his observations. Experimenting with differently sized apertures, 15mm-25mm, is essential when viewing objects of different magnitudes. Smaller apertures let less light in, but do not reduce the field of view significantly since the objective lens is not the limiting factor with regards to the field of view. When viewing an object like Venus, it is important to have a small aperture since the brightness of Venus tends to expand outside of its borders making it impossible to describe the phase. Reducing the brightness helps one to see the outline of Venus much more clearly. Of course, when you are mapping the stars of the Pleiades, you don't want to decrease the amount of light coming in since some of the stars are very faint.

Leave the city!

Galileo did not have to contend with city lights. To reproduce his results, it is very important to get as far away from the city lights as possible. Even our best-eyed person was unable to map the Pleiades and Orion as Galileo had done on the roof of Fondren Library in Houston, Texas. However, approximately thirty-five miles south of Houston, the sky appeared to have many, many more stars since higher magnitude (fainter) stars could be seen. It was from this remote location only that we were able to make our observations of Orion, Pleiades, and the satellites of Jupiter. With the moon and Venus, however, brightly lit skies are not a problem since these objects are so bright anyway. Because Galileo did not have the problem of bright lights, he was able to see many stars of high magnitude, many more than we were able to see, even with his lower powered (8X) telescope. Even from our remote location, we still could see the city lights...

Plan for Bad Weather!

Even Galileo had to deal with cloudy nights and rain! We had a whole semester of classes to observe the night sky, but the weather took away a lot of our opportunities. Remember that even some cloudiness can really ruin a viewing opportunity, especially compounded with the city lights. Also, the telescope must be kept inside or covered.

How did Galileo use the Galilean telescope?

Galileo did not tell us much about this. His writings about the telescope tell us nothing about materials used for the tube or the mounting. He does not tell us when he used a 10mm aperture or when he used a 25mm aperture. Galileo's writings and discoveries come before the establishment of an experimental, reproducible science. Indeed, he left little evidence for having performed experiments on motion. Likewise, Galileo probably did not meticulously take notes on the improvements and adjustments he made to his telescope.

From our troubles and tribulations as well as from our successes with our Galilean telescope, it is obvious that Galileo did not not just build a nice lens and look at the sky and immediately see the mountains of the moon, the phases of Venus, the stars of then-thought-to-be nebulas, and the satellites of Jupiter. Galileo must have spent much time perfecting his telescope ... the lenses, the tubing, the mounting. Surely, he experimented with different magnifications and apertures. Perhaps the biggest conclusion that we have drawn from our research is that Galileo was not only extraordinarily skillful, but amazingly persistent. He undoubtedly improved upon his telescope throughout his observations, much like we did. See how we were able to hypothesize what some of these improvements and adjustments were.

Looking at Orion

Looking at Pleiades

Looking at the Moon

Looking at Venus

Looking at Jupiter and its satellites

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