Astrobits

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Have you tried SRB PHOTOGRAPHIC in Dunstable. They used to make 1 offs of adaptors to customer specs. Nigel

Louise, Have a look at Ian Poyser's website. irpoyser.co.uk. He has some lenses that might fit your requirements. Nigel

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

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

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

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

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