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Galileo's Telescope


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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


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