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About Astrobits

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  1. 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
  2. Don't forget that the central zone plays virtually no part in forming the image, most of it being covered by the shadow of the secondary. Therefore , as your first mirror, errors in the central area can be left alone in preference to getting the edge good. Of course we all want to get perfection over the whole mirror but a classical cassegrain mirror performs perfectly as a Newtonian and that has a rather more drastic hole at the centre. Nigel
  3. Your 8" mirror with a f/ratio of less than 5 will need fairly long strokes to figure so the W stroke with about 1/3 dia length should get you back on track. Nigel
  4. I have done some aluminium soldering on small parts for years. I saw a demonstration at a hobby show over 10 years ago and purchased a kit from the stall. The technique that was shown to me at the time was to tin the joint components first by heating the separate bits up to melt the rod and then use the stainless steel brush to push the molten solder through the oxide coating to get the key to the base metal over the area to be joined. Then do the normal soldering of the bits together. With a background as a research chemist, I know that aluminium will start oxidising immediately on exposure* thus the technique shown in the one video from the above post that I looked at, where brushing preceded the assembly and soldering, will possibly produce weaker joints than the pre-tinning that I do as there will inevitably be a layer of oxide between the solder and aluminium. * Many years ago in one of my chemistry classes we were tinning pennies with mercury just by rubbing them ( no elf 'n safety in them days). One of my classmates decided to tin his medallion which was made of aluminium. the result was that the mercury prevented the protective oxide layer from building up on the surface and the oxidation of the aluminium went ahead unhindered. The medallion got too hot to hold within seconds and we watched, fascinated, as the medallion turned into a heap of white powder in less than a few minutes. Nigel
  5. There is no way you should use more than three blobs to hold a mirror in place. Any more than three, and that includes extended areas, will almost certainly lead to astigmatism. An anulus of adhesive will act as many points and should not be used on anything where distortion will be unwanted. My 16" mirror has been held by 3 blobs for the last 15 years or so and the 70mm ma diagonal with just one blob for a similar length of time without any problems. Nigel
  6. Your bevelled edge looks a bit small to me, you can very quickly grind that away getting the right curvature. Regarding the lifetime of a glass tile tool it depends on the relative working area compared to the mirror. You will remove roughly the same VOLUME of glass from both the mirror and tile, after all, the grit cannot distinguish which side to chip away at so will remove from both sides equally.. Consequently with a smaller area of glass on the tool the thickness that will be removed will be greater than the mirror, the less the glass area the more thickness lost. Different materials for the tiles will have different thicknesses lost depending their relative hardnesses. I have tried making an 8" f/5 mirror with 6mm thick glass tiled tool and needed a second tool as the thickness of the glass at the edge of the tool got so thin that I was concerned that those edge tiles would break up. I have also tried using a 10mm thick glass disc as a stand-alone tool but that failed because the curvature of an 8" f/5 took the mirror edge down to touch the retaining wedges before I got to the correct depth. Mounting it onto a plaster casting worked fine and I subsequently polished the tool to produce a handy 8" dia plano-convex lens with a focal length of about 56". Nigel
  7. Have you tried SRB PHOTOGRAPHIC in Dunstable. They used to make 1 offs of adaptors to customer specs. Nigel
  8. Louise, Have a look at Ian Poyser's website. irpoyser.co.uk. He has some lenses that might fit your requirements. Nigel
  9. To see when you have got to your curve ( before you get the spherometer) you can try using drills to measure the gap under a strait edge. Rather a rough method as common drills are usually in 0.5mm increments. Another method is to cut a piece of card to the correct curve and use that to check progress. You can also use a depth measuring vernier with the strait edge and measure the difference between the centre and edge of the mirror. Nigel
  10. I have not used ceramic ( household wall/floor type ) tiles for just the reason Rusted mentioned: thin hard layer overlying a softer base. I just used 10mm thick glass. I did try 6mm but needed to do another tool when the edge glass tiles got very thin with a risk of them cracking up. As I mentioned earlier I used steel tools with one tool doing many mirrors but always of the same focal length because steel doesn't wear anywhere near as fast as glass. Rusted's mention of using a flat steel plate would produce a curve on the glass but the tool will remain essentially flat so that it would be very tricky to get a smooth curve. Other metals can be used instead of steel, I do know that 2p pieces have been used which I understand worked well but all metal tools must be pre-curved to the radius of curvature wanted in the final mirror. When grinding glass on glass there will be equal amounts ( volume ) of material taken from both surfaces. Thus using tiles of less surface area than the mirror will use much more thickness of the tiles and they will then need replacing, preferably on a new plaster cast from the now curved mirror. Your tiles are well separated and I suggest that you try to get as many as possible on the tool while allowing sufficient space to clean between them. Rusted mentioned that he had difficulties cleaning up the tools made of plaster. My tools were varnished after drying and the tiles were put on with hard pitch covering the while surface. The tool was then covered with PVC electrical tape over the bottom and sides and proved very easy to clean. The I used "tiles" were steel "holes" from a local engineering shop. Heavy engineering shops don't always drill holes but punch them out with a big press and I used the waste steel from the holes, most of them were 1/2" thick! Nigel
  11. Lung disease in optical workers wasn't caused by making a mirror or two using the typical amateur wet process of slow hand or machine grinding. When making a larger number of mirrors as a business the primary curve generation was/is done by high speed diamond tooling which generates a fine mist of glass powder and water. Working in this environment every day as a job is hazardous and can lead to lung disease. Well, I used ordinary cheap plaster for my tools. The grinding tool was "tiled" with steel and was used many times for mirrors without the need for repair or replacement. Steel is not an option for the first time mirror maker as it wears extremely slowly and you will not get a satisfactory curve from a flat steel tool. If you are going to make more than one mirror the same diameter and focal length then the first mirror will be made with ordinary solid glass or tiled tool and once the curve has been generated a steel tool can be made against that curve which will grind as many mirrors as you like but they will all be the same focal length. I could, and did, produce mirrors within a millimeter focal length of each other. O.k for Binocular or multi-mirror set-ups but not so useful for the amateur as they will be wanting something different every time they make a mirror. Nigel
  12. Focal Ratio has nothing to do with field of view (fov). Fov is entirely controlled by the focal length of the telescope for any given eyepiece. Your 115mm f/7.8 Vixen has a focal length of 897mm (115 x 7.8) while a 6" f/5 is 750mm. The ratio 897:750 gives you the difference in field of view and will remain at that ratio between the two telescopes for any given eyepiece. Using a simple ratio you will get between 16 and 19% ( depending on which way you work it ) extra linear field of view with the 6"f/5 using your existing eyepieces. Personally, I would go for wider field eyepieces rather than the Newt as they will give bigger and more pleasing images. More field stuffed into narrow field of view eyepieces makes everything smaller. Nigel
  13. You will find that the dry grits are very heavy and do not readily get suspended in the air. Even the polishing Cerium Oxide doesn't become airborne very easily. However it is best to make up the polishing slurry well in advance to allow any agglomerates to break down before you use it. Nigel
  14. A few thoughts. Having made a number of mirrors I can confirm that keeping things wet will totally eliminate any dust to the extent that you do not need any eye or face protection. Nothing flies up from steady wet working. Don't try to make a slurry of the coarser grits in a bottle, it will just settle far too quickly and become more of a hindrance than a help. I only made slurrys with the two finest ( aluminium oxide ) grits that I used. The coarser grits ( silicon carbide) were applied to the work from a shaker jar, a coffee jar with holes drilled in the lid, or alternatively just using a spoon to ladle from the bulk container and sprinkled over the tool. I don't know how thick your tiles are but you better plan on making a second tile tool when the first one wears out. If the tiles are about 6mm thick they are unlikely to last to the finest grit. You will then need another to finish off. This second one can be made to the curve that was generated by the first but will still need bedding in. I used 10mm thick glass tiles and they will last for one mirror 8" mirror. When placing your tiles don't use a regular pattern, that will almost certainly lead to a mirror with zones, Also make sure that there are no very narrow spaces between the tiles where grit can become difficult to remove. I usually filled the spaces between the tiles with candle wax ( melted in a pan and spread around where necessary with a hot air gun ) which could be melted out at the end of a grit size ( using the hot air gun ) bringing any embedded grit with it, then fresh wax was poured on sealing in any wayward bits of grit. Just make sure that the level of the wax is below the level of the tiles if you do this. As regards plaster then any plaster will do, just make sure that your casting is allowed to thoroughly dry ( it takes days. However, beware-- all my faster drying attempts using heating in an oven resulted in the tool cracking) and then waterproof with varnish. Wet plaster is not as strong as dry plaster. When casting a plaster tool you can use surprisingly flimsy appearing material for the circular dam wall. I have used polythene bags ( the thicker ones like the large dog food bags) cut to size and held with sticky tape around the mirror blank. The advantage of using a flexible material is that when you pour the plaster into the mould the shape naturally becomes round whereas a rigid dam will not. For cleaning the tool and mirror during grinding I do not think that sponges are the best choice. Just dunk them in a bucket of water and use your hand to brush the water over the mirror surface ( a finely ground mirror surface feels wonderful). More care needs to be taken with the tool as there will be sharp edges. Here a nail brush is most suitable to get between the tiles. For the work area then copious water flow would be best. For examining the ground surface a 10X loupe is sufficient even at the finest grit sizes. The higher the power, the smaller the area in view and the longer it will take to examine the entire surface to find that one rogue pit. More important than magnification is method of illumination. I made a light box with a glass top. A piece of black card with holes in it ( approx 1" square chequerboard style) was on top and the mirror placed on that. It is easy to focus on the pits at the (out of focus ) boundary between dark and light. I am sure that you will have many questions once you get going. Have fun. Nigel
  15. I note that your location is Fishguard. Is the deposit on your equipment salt spray deposit? If so you will have to dissolve it off with water with little or no rubbing in the first instance. You can follow that with Isopropyl alcohol/Baader fluid and finally distilled water. Nigel
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