Sideways Orion

Galileo’s Telescope July 30, 2009

Galileo's Telescope

Galileo’s lone surviving in-tact telescope paid a visit to the Franklin institute this year.  The telescope is there until September 7, when it will return to Italy. It is part of a larger (huge even) exhibit on Galileo and the age of the Medici. I have another blog post about the exhibit (with a little about the Franklin too).

Coming up on the ‘scope, the most striking thing (to me) was just how long it was relative to its diameter. I mean, I’ve see pictures, and I know how a Galilean telescope is constructed, but jeez! It looks more like a medieval weapon than a telescope! If you look carefully, you can see that the tube is actually sagging.

A Galilean style telescope is constructed so that the focal point of the objective is at the same position as the focal point of the eyepiece, so the tube length is equal to the sum of the two focal lengths. This way, you get an upright image. Also, it is easy to determine the length of the tube, and there is less travel for trying to focus it when you throw in that third lens (your eye). However, it generally has a tiny exit pupil and a dim image, so it’s a hard style of telescope to use. Astronomers and instrument makers of 400 years ago also had a problem caused by the low quality and strictly spherical shape of the lenses:  chromatic aberration.

Chromatic aberration is caused because the angle the light bends actually depends on the wavelength: blue light refracts a little less that red light, so the focal point for blue light is actually slightly closer to the lens than the focal point for red light. You can really see this looking through something like a crystal ball or round fishbowl. You can usually see straight through the middle, but as you look toward the edges you get a rainbow effect. Galileo was stuck with spherical lenses, so he had to find other ways around the problem.

One way is to make the lens as flat as possible, so the light isn’t being bent as sharply.  This means a really long focal length, and that is why the tube is so long.  There was a sketch in one of the books in the exhibit that appeared to be a telescope of about 3 inches in diameter, but a good 15 feet long!

An aperture mask on the objective reduces chromatic aberration

An aperture mask also helps. The steeper the angle of incidence, the greater the angle of refraction. Light hitting the center of the lens should be coming in at nearly a 0º angle, but the angle increases as you get away from the center.  An aperture mask like this one will block the light that would others wise hit the lens at too steep an angle.

Looking at this end you can also see that the ‘scope is in fact made of wood, and you might be able to see the copper tension ring that holds the lens in place. (more on the assebly in my next blog post)

Supposedly, there is a note in Galileo’s handwriting on the eyepiece aperture mask telling the magnification of the telescope. However the lighting was such that I couldn’t see anything (you pretty much had to crawl on the floor to see the eyepiece end.)

oh, and in case you’re wondering, I don’t know how the focus is, but yes, light does still get through it (you can see the tiny dot to the lower right in the image below)

A tiny point of light in the lower right shows that, at the very lease, light does still get through.