360mm First time mirror grind.

[Rustysplit]

Right, well here we go then.....

My plan is to to build a 360mm/14" extremely portable truss dob along the lines of the Sumerian Canopus. My home location is not great for observing, but just about ok for imaging, Trouble is, I have found that I find the processing side of imaging a bit dull. I miss the simple thrill of just looking for stuff. So a decent aperture, portable scope seemed the logical way to go.....

Today I received my float glass blank from Image Optics. Along with a "kit" of grinding powders, polishing powders and pitch. I need to buy some course grinding grit 80 and 120, as the kit starts at 180grit. The other thing that bothers me slightly is the quantities supplied, I only have about 200g of 180 and 100g of 400/303 and cerium oxide? Does that sound about right?

[Glasspusher]

Sounds like you could use a little more abrasives, the courser grades can be found on e---y, search under 'lapidary'.

How are you going to hog out the curve to the required depth? Might I suggest a 6/7 inch barbell weight...the type with a raised ring near the edge?

Good luck, and I look forward to hearings about your progress.

John

[Rustysplit]

Mmmm. A bit stingy on the amounts I thought. I saw your tip about the dumbbell on your thread, and I am still looking for one. Roger at Galvoptics also suggested that car brake discs could also be used quite effectively.

Alan

[Rustysplit]

Oh, and also that a "green grit" grinding wheel would also work. The type that is used on double ended bench grinders!?

[Glasspusher]

Yes, but the grinding tool needs to be about 50% the diameter of the mirror, a brake drum might be a little large in this case..not sure, depends on what you can find.

John

[Astrobits]

There are a number of options for the tool.

If you use one made of steel then you must use a smaller diameter bit of steel than the mirror as the steel will not grind away as fast as the glass. I made a steel tool when I was making numerous duplicate mirrors. The first 8/10" was ground with a glass tile tool to the required curve and then I made a plaster cast on that curve and stuck bits of steel ( obtained free from a local heavy engineering shop- they don't drill holes in 1/2" thick steel but punch them out- I got as many 3/4" and 1" dia "holes" as I needed) onto the curve with hard pitch. Those tool made many mirrors with the same curve and hardly got worn at all. Steel is very fast for grinding, Using 80 grit I would have the required curve in 1 1/2 hours on an 8" and 2 1/2 hours on a 10". Many years ago a mirror maker made his tools by putting scrap nuts, bolts and any other steel bits onto the pre-curved mirror and poured concrete onto the lot.

You can always use a glass tile tool. Starting with a flat plaster cast on the blank, stick "squares" of glass at least 10mm thick onto it. When the glass tiles have worn away it would be best to make another tool with the new curve rather than trying to replace the tiles on the old tool. The "squares" of glass need not be square and should be approx 1" across. Just get the scrap glass from the waste bin in your local glass merchant ( ask first of course ). Some mirror makers have used glazed tiles successfully.

You could, of course follow the old books and buy another blank and use that.

Nigel

[Rustysplit]

Thanks for the tips, Nigel. I am scouring the local adds for a barbell. If that proves fruitless then I may well try either the glass or ceramic tile tool.

Purely for the sake of the "story", the fine untouched blank before I turn it into an extra from the "Hall of Mirrors"

[wurzelmike]

looking forward to following and learning from this thread, best of luck Alan

[Daniel-K]

good luck

[Astrobits]

Mmmm. A bit stingy on the amounts I thought. I saw your tip about the dumbbell on your thread, and I am still looking for one. Roger at Galvoptics also suggested that car brake discs could also be used quite effectively.

Alan

Quite agree.

Don't know how far they thought 200gms of 180 grit would get you as a starting grit. My reference mirror making book suggests that 500gms of 80 grit will be needed for a 6" mirror thereafter about 100gms for the rest of the grits. Unfortunately I don't know how much I used as I bought my grits in 5-25Kg lots and used as required.

The polishing pitch also looks a bit light on quantity but it is difficult to estimate the amount from the pic. I recon that you need at least 300ml to cover a 14" tool with a 3mm coating, Less, of course, if you use a sub-diameter polishing tool as I always did with any mirror over 12".

For the extra grits I suggest that you try VCSM ( aka Vacuum Coatings ) www.scientificmirrors.co.uk they do from 60 grit up at a reasonable price.

Nigel

[Glasspusher]

Alan, just noticed that you say Roger mentioned the use of 'brake disks' for roughing out, I think he meant brake drums. The important thing being that the tool has a raised rim round the edge, just like using a short length of pipe. I got a 6 inch 2Kg weight with a raised rim from Tesco, that was some time ago and they might not stock that type now.

John

[Ajohn]

You can buy coarser grits off ebay intended for pressure washers if you need a lot. There was some one selling smaller bags but the minimum seems to be 10kg now. This is usually aluminium oxide. I've always wondered if this would be better than silicone carbide as it doesn't break down so easily.

I wouldn't try green grit wheels personally as I feel they would break down to slowly and would clog up quickly. A general purpose silicon carbide wheel would be better.

I reckon you will have problems testing a mirror of that size in 1in plate. It will sag under it's own weight. Be interested to hear how you get on with that. I understand people get round that by rotating the mirror and seeing if the shadows remain in the same place rather than move with the mirror.

When it comes to the mirror cell this might prove useful

http://www.atmsite.o...ptimized_cells/

Or here

http://www.davidlewistoronto.com/plop/

John

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[nytecam]

Good luck with your project - mine of 1/2 century ago were small eg 4-1/2" then 6"

[Ajohn]

Should have said 8kg not 10.

Some people were finishing fine grinding with a tube of toothpaste not many years ago. Seems it does reduce polishing time and they do contain a very fine abrasive. The same abrasive may be available separately off ebay as well. I believe it's white aluminium oxide, the purer grade.

John

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[Astrobits]

I have always switched from Silicon Carbide to Aluminium Oxide after 320 grit. Al oxide gives a smoother finish as it breaks down to a less angular grain than Si Carbide but it cuts slower so less useful for the coarser grits.

1" plate is useable to at least 16-18" and possibly even larger without problems during testing. The mirror will not sag sideways if it is mounted vertically so that testing can be done horizontally, along a bench for example. I have a very simple jig for testing, see pics showing an 8" Suprax blank in position on it. Note that the support blocks are tapered to provide maximum support to the front edge of the mirror thus causing the mirror to preferentially lean back against the back support. Please be aware that with a proper bevel on the mirror the jig supports only the front edge of the side of the mirror, not the edge of the optical surface itself. This jig has done many mirrors from 8" dia. and 19mm thick up to 20" dia and 40mm thick and I have not had any signs of problems.

Nigel

[Glasspusher]

I agree Nigel, 16 - 18 inch by 25mm mirrors are doable without too much difficulty when working by hand, things get a little more complicated when working with a machine. I have successfully figured a 22 inch x 25mm mirror by machine, but it was no easy task. Your test stand looks good, current thinking is that the two support points should be 90 degrees apart. I still test with the mirror supported in a sling and I have no reason to think that this does not work well.

John

[Rustysplit]

Wow, so much encouragement! I am starting to feel the pressure to deliver now

Ajohn, I have already had a play with the Plop software, and have found that an 18 point with differing angles seems to give the best support..

John and Nigel, Glad to hear that testing should not be an issue with the 1" blank, that would have been a real downer at this stage.

I had a bit of time at lunch today and Heath Robinson'd a grinding bench together out of a couple of old thinners cans, some abandoned observing chair test pieces, a bit of old ply and some lid closer straps. The bottom can will be filled with wet sand and I think will be just the job. If not I will fit a large stirling board disc to the bottom and I can stand on it as I walk round to give it max stability.

"she 'aint exactly pretty...."

[SimonfromSussex]

Good luck Alan, following this with great interest!!!

[Ajohn]

I based my comments on mirror sag around a lot of comments on the atm_free yahoo group. These days it should be called the bath interferometer group as little else crops up. Could be that they are over stating the problem as they have software that removes it from the test results. It can also be a rather accurate test. Easy to make the tester as well apart from taking photo's. Ideal thing for that is a web cam and a lens on it that causes the image of the mirror to more or less fill the view. Not easy to arrange.

I would guess a coudre mask would mostly remove any problems with sag anyway. I've been collecting the bits needed for a grinding and polishing machine for some time now. Now I have most of them I found this guys video's on youtube

http://www.youtube.c...donWaite/videos

and found that I don't need most of them! What he does makes a lot of sense. He has also posted a good video on using a mask for testing which you might find of interest,

When you get round to making a tester it's worth working out how accurate it needs to measure before actually making it. The faster the mirror the more precise it needs to be. For me the only good book on testing is Texereau's "How to Make a Telescope". That can be downloaded here

http://archive.org/d...oMakeATelescope

He is about the only person that mentions slit width and diffraction effects that I'm aware of. Loads of people have found the need to build a tester along the lines he suggests. Some use a dial gauge for measurement. Some use a screw. It's possible to use piece of wood etc as a base with a strip on top to use as a guide for a block with the knife edge mounted on it. The knife edge needs to be square to the slit so one of them needs to be adjustable. Pin holes are round so that aspect isn't a problem but adjusting the size is a pain.

Slit or pin hole sizes are swings and roundabouts. The finer the more sensitive and accurate the test is but on the other hand diffraction effects get worse. It's worth knowing that even if you don't read the book. I'm going to rebuild my tester mostly because the light source is too big. I'll try a led but wouldn't be at all surprised if I find a prefocus torch bulb better as I have seen the amazing colour effects using a led in the Dall null test. Or maybe one of the more powerful grain of wheat bulbs. It uses a micrometer spindle for measurement but I found that in some ways it's easier to set the knife edge by sliding it by hand. This might be why many people use a dial gauge. I'm going to try to graft in a cheap digital calliper.

The mirror I want to finish is F3. I used a tile tool to smooth it out as it was diamond milled roughly to shape. The tile squares were cut from some large good quality frost proof floor ones left over from doing the porch. I don't think these would have any difficulty hogging out a plate glass mirror. I stuck them to an old plate glass tool with plastic padding. I just smeared a suitable amount all over the tool and placed the tile squares on the mirror and pressed to 2 together. I'd be inclined to do the same thing with a plaster tool. Save all that dust cleaning the channels out.

John

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[Astrobits]

The generally quoted reason to use a slit is that it allows more light through. However, consider that when you put your eyeball up to the tester and cut the beam with your knife edge you will only be using a part of the slit which is about the same size as a pinhole would give. Light from the lower part of the slit will be returned above your eyeball and vice versa. When using a pinhole it can often be difficult to locate the returning cone of light ( I have had this problem sometimes and the questions arise-have the batteries for the bulb died? has the bulb blown? etc.) and the slit would make location much easier.

Another technique for measuring the position of a manually moved knife edge is to put the knife edge on a square block and then tape a piece of paper to the bench where the block will be. Put the block onto the paper and at each zone scribe a sharp pencil line onto the paper against the back or front edge of the block. Although not as accurate as the micrometer/vernier methods it is a way of doing it if you don't have these tools.

As far as the illumination of a pinhole, I use a lens end bulb through a condenser lens made up of another lens end from a blown bulb cemented into a washer with an aluminium foil pinhole stuck on the other side.

I don't understand how You get colour in the Dall Null test. The Dall test uses a plano-convex lens and must have a colour filter in the system to eliminate the chromatic abberation of that lens. Hence- no colour other than the colour of the filter.

Nigel

[Astrobits]

Alan

Can I suggest that when you put your wet sand into the bottom drum you seal the lid with candle wax? This will limit the rusting of the steel can from the inside.

Nigel

[Ajohn]

I used a red led Nigel. Very early led days. The entire mirror seemed to contribute to the view including the interior of the mirror material giving a sort of granular effect. I suspect it was down to the purity of the light given off by the led. I like the test but was put off by the difficulty of obtaining a decent lens. I used one out of an eye cup type loupe. As it turned out results appeared to be the same as I achieved with a mask. These days it's easier to get a known precision plano convex lens - linos for instance and ray trace the set up as a check with osloLT or osloedu both of which are free.

John

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[Rustysplit]

Not too worried about the rust Nigel. This rather sorry looking contraption will hopefully not be around that long.

At this stage, all this technical testing talk is rather going over my head. I will come back to it and read again after I have done a bit of rough grinding. I am really glad of the input though, so please don't stop.

[Glasspusher]

Alan, the technical talk can be a little off putting I agree, we need to focus on the fact that you are new to mirror making. Enjoy your mirror making, that's what it is all about. As you progress we can talk about test methods that are simple to set up and perform and are quite capable of producing a first class mirror.

John

[fwm891]

Wow, so much encouragement! I am starting to feel the pressure to deliver now

Ajohn, I have already had a play with the Plop software, and have found that an 18 point with differing angles seems to give the best support..

John and Nigel, Glad to hear that testing should not be an issue with the 1" blank, that would have been a real downer at this stage.

I had a bit of time at lunch today and Heath Robinson'd a grinding bench together out of a couple of old thinners cans, some abandoned observing chair test pieces, a bit of old ply and some lid closer straps. The bottom can will be filled with wet sand and I think will be just the job. If not I will fit a large stirling board disc to the bottom and I can stand on it as I walk round to give it max stability.

"she 'aint exactly pretty...."

Go careful with that grinding table it looks very 'vertical' and would easily tip over once you start the grinding process, most grinding table have a wider base to counter the side loading at the top when your using it.

If you cant find a steel disc (College Engineering Supplies ( http://www.collegeen....uk/CastBar.htm ) will cut slices of cast iron off standard stock sizes in 1" increments or have faceplate castings...)