I believe Galileo's first telescope was 40mm, but he had to mask it down to 25mm due to poor lens design.
The two extant scopes have a lens that is 51mm masked down to 26mm (FL 1330mm), and 37mm masked down to 16mm (FL 980mm). The eyepiece lens was 11mm FL -94mm and 16mm FL -47.5mm respectively.
It was not poor lens design, but the limited knowledge of the time about how to make good lenses, or even to fully understand what was possible. We have the advantage that the worst lens we have ever seen, or can obtain, is better than any lens that was made prior to 1650 (or probably even later). When you are accustomed to (in comparison) high quality lenses in cheap children's toys, it is difficult to appreciate that in early times it was not known with certainty that making a perfect lens was even possible.
As Rolf Willich makes clear in his The Long Road To The Invention of the Telescope the breakthrough that created the telescope was the discovery that an aperture mask could produce a legible magnified image instead of one that was an amorphous blob.
If you have a long focus refractor of any kind a 16-26mm aperture mask could be used to an image similar to what Galileo saw (though it would be far sharper).
You can make a similar instrument for less than $5 of parts.
This 50x1000mm lens at SurplusShed is $1.50 and a close match to the larger scope objective:
For the EP lens either of these 38mm lenses (-100mm at $1.5 0 and -50mm at $2.40):
could be used, adding a second aperture mask at the lens to reduce to the same aperture as Galileo.
Or if you want to splurge you could blow $4.50 on a lens that is a very close match to either:
You just need a cardboard tube - which would be historically accurate as that is the type of tube Galileo used.
Edited by careysub, 21 May 2022 - 03:58 PM.