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discardedastro

Having a go at making a mirror

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Since my observatory project is on hold pending funds and some warmer weather, and the setup I have is working okay, I figured I'd try my hand at making a telescope. My plan is to do this in stages - first a mirror, since that's quite cheap to start on, and then I can use my existing tube etc (same mirror size). Then I could get a carbon tube and new mirror cell and assemble that later down the line as a further upgrade. I wanted to do a mirror because the process has fascinated me and I do like the idea of doing my imaging through something I've made from scratch. It's also one of the cheapest, longest-running "projects" I can think of in the hobby!

So I've done my research and ordered a 25mm thick 8" borosilicate blank from http://www.stathis-firstlight.de/spiegelschleifen/material.htm as well as a 21mm 8" blank as a tool, and a set of abrasives etc. Stathis stocks good Schott borofloat glass - I didn't want to do plate/regular glass since the price difference isn't significant and if I'm going to put the effort in I'd rather end up with something as close to perfect as possible!

I've been watching Gordon Waite's videos (and others) on YT and doing my research. For coating I figure I can either use Orion Optics or I have some contacts at Viavi who might be able to arrange something, well down the line. I've also gotten a few books on the topic, so I'm reading up.

Feels like it should be a fun little project and I can start to build up my mirror test/inspection tools - a cheap little Bath interferometer, focault tester etc.

I'll post in here as I go and keep a bit of a record...

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Good luck.  I totally respect those who grind their own mirrors.

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Yes, good luck indeed.

Why haven't you started grinding your own Gina..?..😊

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10 hours ago, Chriske said:

Why haven't you started grinding your own Gina..?..😊

I might do one day but I've got too much on my plate at present.

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Followed.  Go for it Discarded.  Make a 10" and I can sell you a carbon tube! 😁

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On 04/10/2019 at 10:49, Robert72 said:

Followed.  Go for it Discarded.  Make a 10" and I can sell you a carbon tube! 😁

Tempting - if I manage to make an 8" I'll probably want to leap to 12" thereafter though!

The blank kit has arrived in the UK, so might even arrive this week. I've been buying supplies in the meantime:

  • 3M 6500 half-face respirator and P3 particulate filters - don't fancy silicosis from handling fine abrasives and glass dust, though obviously wetting everything is the main defence there...
  • Nice big tupperware boxes to keep the mirrors-to-be in
  • Some small colour-coded bottles I can put abrasive/water mix in
  • Clean spray bottle for water dispensing
  • Sponges for wiping down/removing swarf/chips
  • Big potting tray - this I figured was the easiest option for a good waterproof tray to do everything in. I'm not 100% sure it'll be big enough and might take some adjustment, and I might have to fit some rubber matting or clamp points to secure either the tool or mount
  • 40x optical loupe for surface inspection - I actually want to get a stereo microscope which would be ideal but it's on my "when I have some money in the bank" list :) so a £7 little LED-equipped inspection magnifier will do for now
  • Thick waterproof painter's tape

While I bought a 25mm and 21mm set of blanks with the intention of using one as a tool I've decided I'll start off by making a tile tool and see how I fare with that. Couldn't find dental plaster on Amazon but I found some "herculite" "high-strength" plaster which I'll try for starters along with some bog standard (hexagonal, for easier tessellation) ceramic tiles. If the tile tool works I can either make a second mirror from the 21mm once I've cocked up the 25mm, or pass it on if by some miracle I don't make a mess of the first one!

Also starting to shop around for the bits required to assemble a Focault tester and Bath interferometer, though it'll be a while before I need them - mostly nice and easy, but I want to make my measurements nice and repeatable/recordable, so designing everything around cameras.

I managed to nab a Mitutoyo digital indicator off eBay for £50 (new in box!) - it's only good to 0.01mm but that's plenty for basic measurements of mirror curvature. My friend with a CNC machine is going to turn up a basic spherometer (i.e. reasonably thick bit of metal with hole in middle and grub screw to clamp the indicator) with a few sets of holes I can use for different measurements. Failing that I'll rig something up from some carbon fibre pultrusion stock I've got lying around which should be nice and stiff.

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It's all arrived! Grinding and polishing supplies, two blanks, and a sharpening stone. I've also got almost all the bits for making a tile tool, so can have a crack at that this coming weekend.

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The two mirror blanks (I've already smeared fingerprints over the 25mm, which I got flattened). I got 10" supply kits, just to give myself a bit of a supply buffer.

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Was all very well packaged from Germany in about 6 layers for the glass and 4 layers for everything else - very well protected.

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Good luck with your project. Have you decided on the speed of the mirror?  David

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Just now, davidc135 said:

Good luck with your project. Have you decided on the speed of the mirror?  David

Since it's my first one I'm sticking to an easy focal length - the one I've already got a telescope for, 1000mm, so an f/5. So if all goes well I can pop this in my 200PDS as an initial base.

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Figured I'd have a go at laying up the form for a tile tool even though I don't have all the bits yet. I'm leaning towards Gordon Waite's method of laying up tiles in the base of a form on the mirror (plus a layer of clingfilm) and pouring directly onto that, since it'll fit the tool to the mirror exactly. I've got some Dentstone KD dental plaster on the way as I'm a bit wary of the "best hobbyist stuff from Amazon" option. I've used some waterproof 3M tape and the remains of dinner to make the form around the mirror - I think it'll hold.

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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

 

 

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Thanks Nigel - tons of useful tips!

Ref dust, absolutely - I'm mostly taking precautions for handling of abrasives before they're wet and on on the tool/mirror, cleaning up, etc. Won't be wearing my mask while grinding.

I'll definitely try and get my hands on some thicker tiles. Glass tiles I hadn't considered but should be able to get those in quite good thickness. I've got some epoxy on the way to coat the tool with once it's had a week or so to dry out.

The light box trick sounds interesting - essentially a sort of contrast imaging source. I'll have to give it a go.

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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

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Figured I'd do a pour with the too-thin hex tiles and my non-preferred plaster (some "herculite" from Amazon, mixed to 42% hydration as recommended) just to get a feel for things and let me cock everything up on something that doesn't matter much!

It all worked out pretty well for a first attempt, I think. I clearly need to make the clingfilm over the mirror perfectly flat - if you zoom into this photo you'll spot there's some gaps and voids in the surface, which isn't ideal. I'd put down a layer of Castrol multipurpose grease (what I happened to have lying around) underneath the clingfilm just to ensure there was a surface between the glass and everything else but I think I'll omit this next time as it made positioning the clingfilm taut across the mirror quite tricky - air bubbles were hard to avoid.

Otherwise the mold didn't leak at all and the depth feels pretty good - might do a little more for the next tool, but it's there or thereabouts (~1kg of plaster, so I'll probably go for 1.5kg or so in the real tool).

The sides are pretty good - not perfect, but perfectly acceptable.

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The risk of lung disease from making a mirror, or two, pales into insignificance when optical workers, who used rouge, could be followed home just by their red foot prints. :icon_santa:

 

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I used a weatherproof 'Cassini' plaster at a 28 (water):100 mix. Hard, strong and not affected by water, although I did coat it. Loses very little weight on drying at 40 to 50 degrees. I'd have thought 'herculite' would be better than ordinary plaster.  David

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I couldn't get any better tiles but got some more promising (much finer) plaster, so mixed that up and poured a fresh tool; the plaster is Dentstone KD yellow. It mixed up a bit thicker than I'd anticipated and set fast, so I had to do some sanding to get the back reasonably flat once it had hardened. My 80 grit and tungsten carbide sanding tool both appear to have gone walkies so I ended up using some 120 grit - took some time but got it pretty well tidied up. Weatherproof plaster with a slightly lower hydration is probably ideal - I'm not sure how the Dentstone would hold up very wet. Might mix up a small billet and soak it, see how it copes.

I've done a first coat of epoxy using a polyester resin for the sides/back, so that should do for waterproofing and help level the back - I'll see how it looks after that coat and give it another coat if it needs it. Messy process so far, so the potting tray is paying off; the respirator, while not (as many have noted) a critical safety tool does help a bit with all the dust kicked up by sanding and the smell of the epoxy. I've gotten plenty of use out of my stash of nitrile gloves.

If the epoxy's set well and completely covering the tool then tomorrow will be the first day of grinding!

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On 19/10/2019 at 08:52, Rusted said:

The risk of lung disease from making a mirror, or two, pales into insignificance when optical workers, who used rouge, could be followed home just by their red foot prints. :icon_santa:

 

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.

15 hours ago, davidc135 said:

I used a weatherproof 'Cassini' plaster at a 28 (water):100 mix. Hard, strong and not affected by water, although I did coat it. Loses very little weight on drying at 40 to 50 degrees. I'd have thought 'herculite' would be better than ordinary plaster.  David

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

 

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Well, today the mirror tool's epoxy had cured and I thought I'd have a go at this actual grinding lark.

The epoxy pour had worked well in terms of coverage but had leaked round just a little bit (I elevated it on three more tile segments) leaving a couple of raised spots - I ground these off with a sanding block prior to starting.

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I marked my mirror with sharpie just to give me a view on coverage, added some 80 grit and some water, and off we go...

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I only did a short session this evening before rinsing it all off, just to get my head around it all and go re-read all my texts/books with some practical knowledge in hand. The contact seemed pretty good by the end of the 10 minutes or so. I was quite gentle with the amount of force I applied, so wasn't expecting a lot of material removed. I used a ~40% chordal stroke, randomly moving the tool and more frequently randomly moving the mirror (mirror on top).

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This was the surface I was left with, which was quite uniform. So that appears to be working okay - just need to do some more now I think!

The fact I'm now removing material has brought into slightly sharper focus my lack of measuring tools. I've got straight edges, of course, but my spherometer is still in my friend's lathe backlog. I'll have to fab something else instead at this rate. I need to get a surface plate in either case, which will have to wait for next month (for boring, practical financial reasons) - I'm aiming for a decent-spec 400x400 granite plate which should do for all the potential mirrors I'd make.

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A decent straight edge and feeler gauges will put you close to your desired sagitta.
I just used a suitable length, stainless steel rule on edge and measure the gap in the middle of the mirror.
Try two or three rules in the shop edge to edge against the light.
Reverse the test edges in turn. It's a null test for straightness if several edges match.

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The edge test is a good trick! I'll have to give that a try.

Spent some more time grinding today, tool and mirror definitely feeling more closely aligned now. Tool showing wear already - getting through the glazing on the ceramics pretty quickly, wear distributed largely on the edge.

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I also realised I'd forgotten to put a bevel on my edge, so did that this evening. Still nothing measurable in terms of sag, but I've only done ~15-20 minutes of grinding, and the way the table's set up I can't put loads of force on things. I've taken about 4 layers of Sharpie off, or thereabouts, and while the whole mirror's getting some abrasive action the sharpie in the middle is going first, which I take as a good sign.

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Adding anything on top of a curved tool will increase its radius of curvature.
It follows that grinding will remove the outer regions first.
Ceramic isn't that hard under the glaze. Steel washers and Araldite used to get a good press. :wink2:
I cut up thick glass into squares. Then used pitch to stick them onto a 16" pre-curved, dental plaster tool.
The plaster was too soft even after gentle baking and a nightmare to clean up between grades.

A metal disk from a scrapyard would be my choice of tool these days if I couldn't afford a plate glass tool.
Plate glass has the advantage of being softer than "Pyrex." So "boulders" will be crushed into the plate glass surface first.
Use less abrasive for a sharper but shorter cutting action. Too much powder and it just grinds together and turns to mud.
Be guided by the noise. If there is any with your plaster and ceramics? Glass on glass rings nicely.
You could play it by ear. Even if you were tone deaf.  :smile:

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Plenty of noise when grinding so far at least! And yes, expected the outer to go first but quite satisfying to see it. I've uploaded a short (sequences reduced, as the advertisers say) clip of my grinding - well aware my technique needs work (not least rotating the tool more frequently, which I got to around the end) but gingerly uploading this for the purposes of constructive criticism - be gentle!

I do think that my next tool I'll go for epoxy-onto-plaster rather than embedding, at least to see how it differs - as much a learning process as anything. The plaster seems to be holding up very well though so far - no chipping or disintegration at least.is

I did buy a second (thinner - 21mm) blank as a tool but I figured I'd hang onto it as a spare/second mirror instead. It's Borofloat as well so not got that nice hardness differential advantage you've pointed out!

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I'd use less than a quarter as much powder as that. Probably even less. :wink2:
Halve your stroke speed and spread your hands flat on the back of the mirror.
If you let your hands curl over the edge in later stages you will expand the glass and grind or polish it away.
A pencil might be better than a Sharpie if the ink is water soluble?

Edit. My wireless connection was dropped.

Edited by Rusted
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