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Glasspusher

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About Glasspusher

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  1. Thanks for posting the video, interesting. From what I can see the tool is not free to rotate during the grinding process which it should be. Also to help with controlling astigmatism a larger grinding/polishing tool is advisable, probably around 80% the diameter of the mirror and best used in a 'spin' grinding mode. Hope this helps. John
  2. Hi Robin, The spectrum was taken on Monday 30/11/20 at about 8.45pm. Thanks for the links, weather permitting I will take more images in the future! John
  3. Managed to capture this raw spectrum of Nova Per using a 10 inch Newt and a SA 100, single exposure of 90 secs. H alpha and H beta emission lines shown quite well. John
  4. Thought I would give this thread a bump as it is 15 years since David's untimely passing. I am pleased to say that my paper celebrating David's life and achievements has been accepted for publication in the BAA Journal and will appear this year. In the mean while here are a couple of pictures, the first shows David with a 1.2m mirror for a telescope in India, the second shows the completed instrument. Thanks for looking, John
  5. Found this chart on the web, not sure how reliable it is but you can see that your mirror is close to the red line indicating the difficulty in using a Bath interferometer for this project. Hope you find this of interest. John
  6. As you say the advantage of using borosilicate glass would be reduced time between figuring sessions due to the better thermal properties of the borosilicate glass. Regarding supporting the convex back during working, I ground the convex back to get a smooth regular figure I then cast a support against the convex back to ensure the support matched as well as possible. Despite this and regular rotation relative to the support I still got astigmatism. I don't think a wooden support would be stable enough for a mirror of this size. The stability and uniformity of wood is a concern. The sli
  7. The fire brick mould worked well and survived several firings. The 500mm mirror is plate glass; with thin mirrors like this it is not so essential to use borosilicate glass as the glass cools quite quickly being relatively thin. The mirror was finished and tested but suffered from astigmatism probably due to poor support during grinding/polishing. I should probably go back and have another go at it. The reason that I did not continued with kiln work was that the cost of running the kiln was becoming a little excessive. It was apparent that it would not represent a significant saving, if any, o
  8. Gluing is definitely not the way to go. Fusing and careful annealing will produce a usable result. The Hubble sandwich mirrors are of this type and many are happy with the performance. See also the work of Normand Fullum: http://www.optiquesfullum.com/optical-mirrors/ John
  9. Yes that was Dave Thompson who was part of the group that built the 30 inch scope. The problem in using a Bath interferometer is that the resulting interferogram will have crowded fringes due to the size and speed of the mirror making analysis difficult. You will need to obtain good clean high resolution igrams with such a fast mirror. It has been a few years since I did kiln work. I made the mould from kiln fire bricks glued together with kiln cement as shown in the pictures. I sanded in the curve using a glass disk with the approximately the same curve as required. A circular self
  10. Just few further thoughts regarding the mirror blank. Slumping is very much in it’s infancy as far as amateur telescope makers are concerned. If you choose the slumping route you will have to consider: The cost of a kiln and how to use it effectively. How to make a suitable mould. If the mould is not accurate enough you might have to do extra grinding to get the radius required. This will result in loss of thickness which is not helpful if you are starting with what is already thin glass. Supporting the mirror during grinding/polishing, if not done properly you will end up with
  11. As a mirror maker with many years experience I wonder if I might share some thoughts on your ambitious project. When making a big mirror the first thing to consider is how do I test it? If you can’t test it don’t make it. Why go through all of that work in the hope that testing will be achievable? When testing mirrors the difficulty increases with larger diameter and shorter focal ratio. So a 300mm F5 mirror can be figured in a few hours or less by an experienced optician. An 800mm F3.3 is an entirely different proposition. All forms of testing require a degree of skill which is acquired
  12. To clean the nut tool between grits I scrub with a stiff brush under running water if not satisfied with that I use a wire brush which usually does the job. With tiles or glass chunks water can ingress into the dental stone down the edges which can cause the pieces to become loose and draw in abrasives. You can see this more obviously at the edges of the tool where the tiles are exposed. Always try to set the tiles in from the edge so that they are surrounded by dental plaster and use the thickest tiles you can find, 4 mm is a little thin in my experience. John
  13. Over the years I have tried every type of grinding tool and a nut tool is difficult to beat. I use M16 steel nuts set in dental plaster. Cover the concave glass mirror with something like greaseproof paper, place a damn round the edge, place the nuts on the greaseproof paper and carefully mix up the plaster and pour into the mould. Allow to set and that is it. During use if the tool has dried out immerse in water for a few minutes before use. Worth thinking about! Good luck with your mirror! John
  14. Looking for M90 x1 extension tubes. Thanks for looking. John
  15. Hello, I have slumped my own blanks in a kiln but gave up due to the expense of running the kiln. To properly anneal a large blank takes many hours of controlled cooling which is expensive. You then have the problem of supporting the meniscus blank during grinding which is not straight forward. It is more economical to go the monolithic blank route which is well tried and tested. Hope this helps. John
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