Galileo's Telescope
I doubt there are many amateur astronomers out there who are unaware that this year
marks the 400th anniversary of Galileo's discovery of the craters and mountains
of the moon, the phases of Venus, and the satellites of Jupiter. But the number
of American observers who have traveled to Philadelphia this year to see
one of his original telescopes is much lower.
Because of remodeling work being done at the museum in Italy where the Galileo
exhibit normally lives, it is on temporary loan to the Franklin Institute. It
is unlikely to cross the Atlantic again in my lifetime, so I made a pilgrimmage
there last weekend to see the foundation of modern astronomy. I regret that
pictures were not permitted, so I can't illustrate this article. You'll have to
follow along in your mind's eye.
Much of the exhibit revolves around the Medici dynasty and how they developed
their power and patronage of the arts and sciences. It is interesting to history
buffs, but probably less so to astro-nuts. However, it is important historically
to realize why Galileo lived and worked where he did, and who it was that was
able to fund his work of discovery (once he left behind his teaching in Padua).
Near the end of the historical exhibit we see one of the two remaining telescopes
build by the master telescope maker himself. The focal ratio is unbelievable to
modern amateurs. I didn't have a tape measure with me, but it appears to be a
little less than a yard long, with an OTA about 2" in diameter and an aperture of
about 1". That's a focal ratio greater than 30! Why is the aperture so small
and the focal ratio so high?
These questions and others are answered in the final section of the exhibit, which
explains the actual optical design and includes several hands-on modern
reproductions. The visitor can look through several telescopes who images
illustrate the limitations of 1609. Most amateurs will have heard of chromatic
aberration and the color fringes that it adds to images, but some may not realize
that Galileo did not have access to achromatic objectives, much less modern
apochromatics. This dictated a very long focal ratio to suppress the color as
much as possible. (Apochromats were not invented until 1733, over a hundred
years later.)
Another problem was the poor figuring quality of the lenses, which caused much
blurring and distortion in the outer half of the lens diameter. This issue was
corrected with a trick of which John Dobson would approve--a diaphragm is placed
over the objective to block light from passing through the outer section. (Hence
the 1" aperture in a 2" OTA.) One of the hands-on exhibits lets the visitor
look through a 'scope while adjusting a variable diaphragm on the objective.
The image is not sharp by any means with the objective stopped down, but unstopped,
it provides only a featureless blur. Of course, stopping down the eyepiece
increases the focal ratio still further. You have to actually look through a
replica to realize just how tiny the field of view really is. And after looking
at a simulated image of Saturn, you'll never again wonder how Galileo could
have been fooled into descriptions of the rings as "cup handles", or three bodies
in a row. (I had always wondered about this, since I can see the rings very
clearly even at extremely low magnification. I had never considered that the
resolving power might be limited by optical quality rather than design.)
The design of the optical system seems strange to modern eyes. The objective
is a single plano-convex lens, and the eyepiece a single plano-concave lens.
Although the use of a convex eyepiece was invented by none other than
Johannes Kepler in 1611, the optical figuring capability of lens-makers was
not adequate to make the Keplerian telescope a practical tool until the mid-17th
century. This is a good reminder that it's not enough to have the
right theory; you also have to have the tools that you need to make the tools.
If you can make it to the Franklin Institute before the September 7 closing of
the exhibit, I encourage you to pay a visit. It's well worth the trip, and
greatly increased my appreciation of the challenges and difficulties that Galileo
had to overcome on the first steps toward our modern optical wonders. More information
is available at the following URL:
Galileo at the Franklin Institute
Cathy James |
