tag:blogger.com,1999:blog-4843157625045099797.post2225308245111435342..comments2024-03-27T23:56:25.286-07:00Comments on Cabinet of Wonders: The Allure of Hot GlassHeather McDougalhttp://www.blogger.com/profile/09683209580852572301noreply@blogger.comBlogger3125tag:blogger.com,1999:blog-4843157625045099797.post-33254508420576575162007-08-12T21:38:00.000-07:002007-08-12T21:38:00.000-07:00You are absolutely right. I was taught by someone...You are absolutely right. I was taught by someone who was more interested in the casting than in the result, and on research, I find that indeed you are correct.<BR/><BR/>In fact, the Hale telescope (Mount Palomar, outside LA) mirror was more than 16 feet across. Later, blanks were cast in a rotating furnace (moving at 8 rpms) so as to get a more concave surface, to bring the annealing time down into the months instead of the years, and to avoid a lot of tedious grinding.<BR/><BR/>Thanks for the correction! And I'm glad you liked the entry.Heather McDougalhttps://www.blogger.com/profile/09683209580852572301noreply@blogger.comtag:blogger.com,1999:blog-4843157625045099797.post-66028065473015667702007-08-12T21:18:00.000-07:002007-08-12T21:18:00.000-07:00"the large lenses they once used in observatory te...<I>"the large lenses they once used in observatory telescopes, which are often some fifteen feet across"</I><BR/><BR/>I think you're referring to <I>mirrors</I>, not lenses. It proved impossible to make high-quality lenses larger than about 40 inches across, which is why since about 1900 all astronomical telescopes are the reflecting (not refracting) type.<BR/><BR/>Telescope mirrors are made from cylindrical "blanks" of glass, whose top surface is then slowly polished into a parabolic shape, then coated with silver. The backs of the blanks have a lot of hollowed-out space to reduce weight, but the glass is still very thick and I know it takes years to cast and polish them.<BR/><BR/>Even mirrors have hit practical size limits at about five meters, so new telescopes (like the Keck) are now built using an array of smaller mirrors, which are continuously adjusted by a computer to maintain a single focus.<BR/><BR/>(Thanks for the article! The Prince Rupert drops are amazing.)Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-4843157625045099797.post-73216531619066703032007-08-08T09:41:00.000-07:002007-08-08T09:41:00.000-07:00You have said previously that your French wasn't p...You have said previously that your French wasn't particularly good, so I would not expect you to pounce on it right now but if Bernard Tirtiaux's book "Le Passeur de Lumière" ever becomes translated into English, I strongly recommend you read it. <BR/>It is a haunting story about glass artists working on the cathedrals across Europe during the 12th century. <BR/>The writer is a master of glasswork (maitre verrier) and his way to describe the skill and the quest for new colours is magic.<BR/>As far as I can judge it only has been translated into Spanish thus far.spacedlawhttps://www.blogger.com/profile/12462723005560128474noreply@blogger.com