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Astrobits

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About Astrobits

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  1. If I was designing something like this I would make sure that it was easy to maintain/repair using the simplest, readily available, components possible as a first requirement. Any friction implies that there will be wear that will eventually cause the component to malfunction so the easier it is to replace the better. Nigel
  2. While it is fun/instructive to design the telescope structure and analyse it's performance on a computer, I would have thought that the fundamental question in this project is "can you make a mirror to the specifications desired?" Personally that is the first thing that I would do.....make the mirror. If it works out fine then it will wait for the rest of the structure to be made. If you make the structure first, or alongside the mirror making, and the mirror is not up to your specs then it could be a big disappointment. Nigel
  3. I'd scrub option 2. If you are going to slump it then low expansion will be a safer bet than float. Low expansion can be cooled a bit more quickly than float. With the development of toughened and laminated glass the thickest float glass now commonly produced is 19mm although 25mm is listed by some Chinese suppliers. Really thick stuff can still be had when aquaria replace their very big glass tanks with acrylic and occasionally when old ships are broken up. Unfortunately most of these sources have already gone, so these are not very easy to come by but there might be some available from someone who just grabbed some when the going was good. You will need to experiment with the support during grinding/ polishing as I am sure that you will get some astigmatism at first. I tried supporting a 40mm thick 500mm dia mirror on bubble wrap and got triangular astigmatism because I didn't rotate the mirror adequately ( I was in a hurry and bubble wrap has a triangular pattern of bubbles). As for testing I think that you will need to make a tester specifically for this mirror. There is plenty of literature on test equipment. Nigel
  4. Thinking about it I have a local secondhand wood yard who often have damaged worktops. Now, Formica is not damaged by water but the chipboard it is stuck to is. Can you see where I am going with this? I will have to see what I can do when the yard is open for business, or perhaps speak to my local builder for some from refurbished kitchens when he gets going again. Nigel
  5. If you can locate a local worktop manufacturer they might be able to supply offcuts at a reasonable price ( possibly even free from their rubbish bin). Nigel
  6. Individual lenses made by these top end companies always have a ridiculous price tag. If your requirements are very specific then there is nowhere else to go. At the cheaper end is Edmond Optics which will sell you a 50mm achromat for £120 +vat However if your requirements can fit around particular specifications then you can get achromatic lenses 50mm or larger for much less. A pair of 50mm achromats with about 250mm focal length can be had for way less than £100. You will get two eyepieces thrown in for free. They are called binoculars. Nigel
  7. The edge bevel definitely should be more than 1.5mm, you will loose a little more in the finer grits which could leave you with 1mm or less as the final bevel if not enlarged. A shrinking bevel size is a common problem in many mirror making efforts especially when more coarse grinding than expected has been needed to get to the curve. You can try to only grind the edge on the stroke away from the surface, lifting the grindstone away from the glass for the backstroke. This will hopefully leave chips only on the outer edge of the bevel but be prepared to go back to coarser grinding if there are more mirror surface chips. nigel
  8. As the May issue of S & T is current I will not scan the article to post here. That said, I will try to give the flavour of the test report by Dennis di Cicco. Overall I think he was impressed. He tried it with the following telescopes: 2.4, 4 and 6 inch refractors 12" f/5 and 18" f/4 Dobsonians 8" and 16" Schmidt Cassegrains 6" f/12 Mak-cass 8"f/3 Newtonian and with and without a coma corrector on the reflectors "It worked well on all of them." He also used a pair of 26mm Plossls with 2x, 3x and 5x barlows without any problem. There were no back focus issues, and the field of view and faintest stars visible were the same as with the eyepiece alone. His last paragraph : "If you've ever wanted a binoviewer but were put off by the need for lots of back focus or the expense of sets of matched eyepieces, then now might be the time to consider the Orion BinoViewer. It certainly changed my concerns about these issues" I expect the Orion web site to link to the S&T test report at some time, they'd be silly not to. Nigel
  9. There is a review of the Orion Premium Linear BinoViewer in the May issue of Sky and Telescope. An interesting item as it fits any telescope that works with a normal 1 1/4" eyepiece without extras of any kind ( just more eyepieces!) , even low profile focussers. Nigel
  10. On the original question in this thread, my first query is just what size the secondary holder is. If that is the same ( or nearly the same) size as the current secondary mirror then that will need to be replaced with one as small as the new secondary mirror. Nigel
  11. I am in the process of making an eight sided telescope tube for a 114mm dia mirror from Sapele Mahogany using my own plans ( mostly in my head, the whole telescope and mount will be in Sapele in the style of the old fashioned telescopes ). The sides are edge jointed together after putting a bevel of the correct angle onto them. I didn't want to fill the large gaps left when joining square edged strips at an angle.To assemble the sides I made a jig to hold them in the correct angle while the glue set. I then reinforced the joins with strips again bevelled to fit. To bevel the edges I made a jig for my router table for use with a straight bit. An alternative procedure would be to make support rings which will also act as baffles in the finished tube as in Louis D's link and attach your strips to that. A couple of pics: Nigel
  12. I might be wrong here but if IIRC Meade and Celestron had two different approaches to making their early SCT's. One would use a mix and match approach with their stock of corrector plates and mirrors until the image was acceptable to them, the other would stick with one item of each component and do some refiguring if needed to achieve their acceptability standard. I can't remember which company did which procedure. This would imply that a stock corrector plate for your scope might not be available or would not give satisfactory images and it might be necessary to send it to Meade for refurbishment ( if that is possible with their current situation). Nigel
  13. Unfortunately, any material used for the body of the tool (plaster, cement) will pick up grit as Glasspusher and Discardedastro have mentioned. The advantage of the wax as I see it is that it is soft and easily scraped out along with any embedded grit. Secondly you can use a hot air gun to melt the wax and seal in any grit as it will sink below the surface of the molten wax. I have found that on initial pouring of the wax the boundaries with the tiles can become a bit depressed (where the wax doesn't wet the tile) which will trap grit if left as is. Going over the tool with a hot air gun can cause the wax to bond well with the tiles removing that little valley. I, too, think that 4mm glass is a bit thin. Try visiting local glass merchants for offcuts of 6mm at least. Should be minimal cost if not free out of their scrap bin. I also used steel tools which are great for doing many mirrors as they don't wear much at all. A steel tool will produce consistent focal lengths time after time but a single tool will not produce different focal lengths very easily. Nigel
  14. That is the problem with tile-cast-in-tool arrangements. Plaster ( and cement ) is fine as long as you stick the tiles on top and fill the space with wax as I do. The plaster tool will have cavities around the edge and underneath as well so I covered the edge and base with PVC electrical tape. The tool is then virtually waterproof so can be dunked into water for cleaning. Unprotected plaster will absorb water and can possibly swell causing the figure on the tiles to change. Another disadvantage of the tile-in-tool is that you have less visibility of the thickness of the tiles, particularly at the edge, and cannot easily see whether they are thick enough to withstand the subsequent operations. My method was to make a plaster cast against the coarse ground mirror. When it was dry arrange the tiles on the curved mirror surface and warm them up with a hot air gun, pour some hard pitch onto the tool in a thin layer and quickly invert the tool onto the tiles. This way the curve of the new tool matched the mirror fairly well. Once you have done the coarse grinding it is possible to use thinner glass as subsequent grits will not remove much glass. This, of course, assumes that you will not be needing the grinding tool for further mirror(s) with the same focal length. When I made the curved plaster cast I always made two, the second one was for the pitch lap. This ensured that I could return to fine grinding should that have been necessary. Nigel
  15. I have not done any definitive work just that the reaction on that aluminium medallion started while the unfortunate owner was still rubbing the mercury on to it, i.e the reaction started immediately the bare surface was available. It got so hot that he had to put it down quickly before he dropped it. The oxidation on the surface of aluminium will slow down as soon as the first reaction provides that protective layer ( provided it is allowed to of course unlike my mates experience ) and will probably continue for years as long as there is aluminium available. Unfortunately, rust provides no protection to Iron. Nigel
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