I have acquired and read the book recommended on the recent Galileoscope thread "The Long Route to the Invention of the Telescope" by Rolf Willach, which is quite eye opening. I had read the standard "History of the Telescope" which is a classic work, unlikely to be surpassed as a general history, but the invention of the telescope is mysterious in that work. Indeed, the invention of spectacles is a mystery too.
By examining the actual surviving optical artifacts in Europe, going back to the pre-spectacle era, he constructs a thoroughly documented account of how both spectacles came to be, and then developed, and how the telescope was invented in September 1608.
The breakthrough (as was discussed at the end of the thread) was the aperture mask, that blocked out the really poor outer zone of lenses that then existed, making a magnifying device that actually improved vision.
Willach demonstrates that the fact that lens combinations could magnify images was well known in the previous century, there are many references to experiments with this. But these magnified images were worse than the naked eye, they showed less detail than without it (this is much worse than "empty magnification").
One problem with attempts to understand the development of science and technology is knowing how to discard what we know now and think in terms of what they knew then. Every lens you have ever seen was far better than any lens anyone had ever seen in 1608 (and for many decades after). No one at the time knew what was possible with lenses. They had really poor lenses, but did not know how, or more to the point if, they could be made better. Probably the invention of the aperture mask accelerated lens technology because once they knew what the good central part of the lens could do, they would start thinking about making this zone larger.
Now - to the purpose of this post.
I would like to be able to demonstrate the effect of the invention of the aperture mask.
My problem - there is no source of lenses with really bad outer zones.
If anyone here is in to glass pushing, has a Ronchi screen, and is willing to mess up a lens for me, I would like to send them a lens that can be misground so that I can make a real Galileoscope that requires an aperture mask!
If you don't want to purchase the Willach book (though it is a great book, and anyone interested in this subject should buy it), I can scan and post a couple of Willach's Ronchi images of typical lenses to give you an idea of what the messed up lens should look like (exact match is not required, according to the Anna Karenina principle all perfect lenses are alike, every bad lens is bad in its own way). I would actually like a couple of similar bad lenses to make a masked and unmasked example. I intend to use these in lecture demonstrations.
Edited by careysub, 11 March 2019 - 11:45 AM.