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
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
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
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 the
Looking at Venus
Jupiter and its satellites
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