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This answer to What (the heck) is a Hamiltonian telescope? Is this one? confirms that the telescope in the question linked there is indeed as described and that the first lens is a full 65 cm aperture lens, the second element is a full 65 cm negative meniscus back-silvered, and some corrector lenses are embedded= within a hole in the large primary lens.

Optically I can imagine that it might be possible to let the light transmit again through the primary and still compensate, and mechanically that seems more attractive than polishing a double-sided transmission lens with a hole through the center.

But apparently that's what's been done.

Question: How to make a 65 cm lens with a 20 cm hole in it for a Hamiltonian telescope? I'm thinking about issues including the following:

  • Is the blank cast with a hole already, or is it drilled?
  • If drilled, is that before the first side is polished, before one side and after the other, or after both sides?
  • After drilling does one need to anneal the glass again?

Glass can experience strain-induced birefringence among other things, so I am really interested in finding out how optical surface figures are applied to both sides of this lens with a big hole in it without causing optical problems within the bulk of the glass.

from https://astronomy.stackexchange.com/a/39704

Image from this answer to What exactly is a Hamiltonian telescope? Is this one?

uhoh
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    An amateur way of making a mirror. The use of tools is not very clear. http://www.astronet.ru/db/msg/1262317 – A. Rumlin Nov 06 '20 at 16:34
  • @A.Rumlin ya this is even harder because it's a lens and residual stress is a problem. just fyi I've updated the question slightly and added a bounty with an explanation message as well. – uhoh Nov 15 '20 at 23:52
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    This question is more about manufacturing than astronomy. If you really want an answer, you'll have to specify things like tolerance levels, physical characteristics of the glass material, machinery available, etc. – Greg Miller Dec 15 '22 at 06:28
  • @GregMiller disagree! Check the site, there are plenty of questions about manufacturing optical components of telescopes here! Just for example Why telescope mirror glass shipped as randomly broken chunks of a constant size in cardboard boxes? and Has anyone ever tried to make a simple telescope using ice? and Would it be possible to create a telescope from a hanging reflective cloth? – uhoh Dec 15 '22 at 06:39
  • @GregMiller and and Can a material other than glass be used for making telescope mirrors? Tolerances of optical surfaces is exactly what people who make amateur telescope mirrors always talk about. We have several questions about making and testing mirrors here as well. – uhoh Dec 15 '22 at 06:41
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    As a maybe helpful point of information, last month I attended the Stellaphane convention. Stellaphane was founded a century ago by telescope-making enthusiasts and the convention was lots of fun: people brought their homemade telescopes, some of which were quite large, e.g., 40 cm or larger mirrors. They ran mirror making workshops, etc. In the excitement of wandering around in a crowd of telescopes and makers, I forgot to ask about anyone the question you posed. I am going next year, if at all possible, and will ask one of the mirror making experts. – Ed V Sep 11 '23 at 13:23
  • @EdV Oh lucky you! Sure growing up poring over the latest issue of Sky & Telescope each month, the issue covering Stellaphane was always the most exciting and dream-inspiring. I'm curious, did you see any "ultralights"? Something a backpacker could bring a long? Or anything like this? – uhoh Sep 11 '23 at 21:09
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    Unfortunately, there was nothing a backpacker could bring along. The variety of telescope and mounts was incredible and everyone was very passionate about the scopes they built, the mirror grinding they did, the clever support schemes, the paints used, etc. There was even a scope with major external parts 3D printed in pink. The biggest telescopes were on trailers! On the other hand, I saw only one telescope with a DSLR camera attached and no computer-assisted astronomy. I could not stay overnight, but many did and there were all night star parties. And the swap tables were very cool. – Ed V Sep 11 '23 at 21:33
  • @EdV this answer to Can a material other than glass be used for making telescope mirrors? just got pinged - did you see it? I think the first person to find a way to make thin silicon carbide telescope mirrors for ultralight portable telescope could have a nice little business. It may require having someone else making them and being a reseller (for financial and/or IP reasons) – uhoh Sep 11 '23 at 22:20
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    I just upvoted there! You always provide interesting relevant links and I learn something following them. Your suggestion about thin SiC telescope mirrors is interesting, but I know nothing about that, sadly. – Ed V Sep 11 '23 at 22:24

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Drill or core the blank (anneal if needed ... most likely not if annealed to begin with) use pitch to glue a plug of the same material in the hole grind, polish and figure as normal then remove the plug.

tomc
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    Right now this is unsourced and it's impossible to know if it is correct or not, or if it's fact or speculation. Is it possible to add some supporting material? Thanks! – uhoh Nov 16 '20 at 04:25
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    I think this can be a great answer, all it needs is a supporting source or cited reference, perhaps with a short quote. – uhoh Feb 05 '21 at 01:40
  • @uhoh you won't find many sources as most such processes are taught to apprentices by master lens makers through hands on training. It's how I learned it... – jwenting May 15 '23 at 08:00
  • @jwenting you mean you know "how to make a 65 cm lens with a 20 cm hole in it" but you're not at liberty to say? Not even the basics? – uhoh May 15 '23 at 10:37
  • @uhoh no, I mean that you're not going to find a website telling you how to, or a "making a telescope mirror for dummies" book – jwenting May 15 '23 at 13:33
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    Actually, there is a website that does this: https://stellafane.org/tm/atm/index.html. – Ed V Sep 11 '23 at 13:46