# Galileo's Telescope

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## Introduction: Galileo's Telescope

After reading Galileo's Starry Messenger, I had the desire to really experience what it was like for him to first turn a telescope up to the skies. I decided the best way to do that was to recreate, as closely as possible, his telescope. Except out of aluminum, instead of wood, because I much prefer working in metal.

I am not the first to do this, of course, and I am grateful to those who have made measurements available. The design I used is based on the telescope at the Museo Galileo in Florence. The dimensions I used were based on a paper written by Edison Pettit for the Astronomical Society of the Pacific in 1939, and the investigation done by SciTechAntiques.

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## Step 1: The Optical System

I wanted to use as many off-the-shelf components as possible, particularly the lenses. I'll probably grind my own someday, but not this year. I found a pair from ThorLabs that were very close to Galileo's: Ø25 F=-50.0 N-BK7 plano-concave for the eyepiece, and Ø50.8 F=1000.0 N-BK7 plano-convex for the objective. I didn't get a fancy modern coating on them, but they're still undeniably far higher quality than anything Galileo would have had access to.

Not being particularly confident in my optical design skills, I bought these first and tested them to make sure I hadn't screwed anything up. I found the objective was held very nicely by my PCB soldering clamp. Holding the eyepiece in my hand, I was about to confirm the focal length. If you zoom into the second picture attached to this step you can just see part of the spine of October Sky in the DVD case across the room, magnified. Success!

## Step 2: Making the Telescope

Knowing that the lenses worked, I could finalize the design of the telescope itself. The following describes how I made it using the metal-working tools I have access to, but it could be made from cardboard a lot easier.

I tried to keep as close as possible to the original from the Museo Galileo. I found a section of schedule 80 aluminum pipe at my favorite local metal supplier, which has a 1.9" OD and a 1.5" ID. Perfect! I put it on the lathe mainly to cut it to length and clean up the ends, but I also applied some sandpaper to improve the surface finish. To finished it off, I cut six grooves into the outside. These represent the wire wrappings around the original, which hold the two halves of the wood tube together. They're purely decorative, obviously, but they really add a lot of character.

The objective holder and the eyepiece are both turned from a 2.5" cylinder of aluminum. Which is probably overkill, but turning aluminum is so much fun! Following a suggestion on the SciTechAntiques page, I made the eyepiece much longer than Galileo's original. This allows the focus to be set for much closer targets, which is useful for demonstrations during the day. I also added a series of grooves cut into the shaft, which serve as a seat for o-rings (1-3/16" ID, 1-7/16" OD). This made getting a snug, sliding fit a lot easier, but it turns out I greatly overestimated the number I would need. I only ended up leaving a single o-ring in the groove farther from the lens, and it works great.

## Step 3: The Results

It works! It works surprisingly well, even through the light pollution of Seattle. The weather of Seattle proved far more of an issue. My main conclusion of this project is that if Galileo had lived in the Pacific Northwest, the Scientific Revolution might never have happened.

I set myself four main goals of the most significant discoveries of Galileo's that I wanted to recreate. For all of these, I first made the observation through the Galilean telescope, and only once I had recorded what I had seen would I switch to a 6" reflecting telescope or look up the current state of the objects online. I wanted to keep myself as honest as possible.

The moon: Is it a perfect sphere, as the ancients said? Nope! It has mountains and is definitely not perfect. I've attached the sketch I made of it half full, along with one that Galileo did of the same subject. I realize mine isn't of quite the same quality, but, well, he was a person of the Renaissance, and I'm not.

Venus: Does it have phases, in a way that would support a Copernican world system? Yes! This one was right on the edge of visibility for me, but I could just make out the crescent shape.

Jupiter: Does it have satellites of its own, which would further refute the idea that the Earth is the natural center of the universe? Yes! This one really blew me away. The four largest moons were clearly visible through the telescope. (We now call these the Galilean moons, but Galileo himself called them the Medicean Stars, trying to suck up to the Medici family.) More than anything else, seeing them through this telescope made Galileo's experiences come to life for me. An entire little solar system, spinning there like a toy. I managed to make observations several days in a row, and this time I think my sketches hold up very well to his originals.

Saturn: Is it round like the other planets, or is it weirdly lumpy? It's lumpy! This was another observation right on the edge of perceptibility, but that's okay. Galileo himself never saw the rings as rings. He described what he saw as "ears".

The only thing that I really failed to do was take a picture through the telescope. The one attached to this page is the best I managed with Jupiter, and you can't see the moons at all. I'm sure it could be done, but it would take something more elaborate than holding a smart phone up to the eyepiece. As with the Starry Messenger, you can believe my sketches, or you can go make your own telescope to try it yourself. I highly recommend it!

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## 4 Discussions

Where did you get your lenses?

This is amazing. I wish I had the skills, or rather the tools, to make something like yours. Can I ask roughly how much the lenses cost? I'd like to make one on my own, maybe cardboard or wood.