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Glasspusher

Making a 12 inch Mirror

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What a great thread, I doubt I'd have the patience to make my own mirror but anything DIY in astronomy and I'm interested :) I'll be following this avidly ...... lots pf pictures please :)

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This thread is just great. I love the look of your tile tool. Can't wait to see and read more.

Andy

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Thanks for the comments guys, Yeti congrats on your purchase. I think I saw that blank it is quiet thick if memory serves me, will make an excellent mirror.

John

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Almost 60mm thick, so the chap says. It did cross my mind to grind it into a HUGE magnifying glass, but only for a fleeting moment. I'm looking forward to the challenge, but wondering if perhaps I aught to start smaller first.....

Nah!

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Here is a nice video showing the process of roughing in the curve with a sub diameter tool, hope this helps clarify my ealier post on this subject. You do not need a powered rotating table for this process!!!

John

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I'll have to wait until I return to UK to watch, as YouTube, along with farceberk, is banned here in China.

My family are already convinced that I have become obsessed with astro, when I turn up with a HUGE glass paperweight, they'll be left with no doubt whatsoever.......... the disappearing into the shed for days on end bit, of course, won't be at all unusual.

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Yeti, shame about the situation where you are, let’s hope you can access the link in the next exciting chapter!!!!

It is now time to start grinding with the full sized tool. Start using 120 grit silicon carbide, we have two objectives at this stage, one is to bed in the tool to ensure good contact with the mirror and the other is to ensure that the mirror is spherical and not deep in the centre as a result of grinding with a sub diameter tool. We also have the opportunity to change the radius of curvature of the mirror if we need to. If we wish to deepen the curve in the mirror we work with the mirror on top (MOT) and use a chordal stroke. In this stroke the centre of the mirror sits near the edge of the tool (about 1 inch in) and the mirrors prescribes a series of chords round the edge of the mirror. As with all strokes the mirror is rotated and the worker walks from one side of the workbench to the other. This is maintained until the mirror reaches the correct depth. If we want to reduce the radius of curvature of the mirror the same course of action is taken, but with the tool on top (TOT). After either of these actions, or indeed after roughing in the curve with a sub diameter tool we must take steps to ensure that the mirror has a spherical curve on it, to do this we use a 1/3 centre through centre stroke. In this stroke the centre of the mirror passes through the centre of the tool, and the total travel of the mirror is about 1/3rd the diameter of the mirror, in the case of a 12 inch mirror this is 4 inches. Looking at it another way the overhang at the end of each stroke is 2 inches. The mirror is rotated during this stroke and the worker walks to either side of the work bench. It is good practise to work 50% of the time with MOT and 50% of the time with TOT during this phase. Here is a good description of the various mirror making strokes together with videos.

Stellafane ATM: Guide to Strokes

As the grinding with 120 progresses it will be seen that the glaze is ground off the surface of the tiles, this is not a problem. It can be helpful in that it allows us to see the state of the contact between tool and mirror. The first attached image shows the tile tool after 1 hours grinding with 120 grit, the second after 2 hours and the third after 3 hours. If the removal of the glaze is fairly uniform it tells us we have good contact. Grinding with 120 grit will take a few hours. At periodic intervals the mirror and tool should be washed in water to remove the slurry build up. If an abrasive coarser than 120 grit was used during curve generation we can check the mirrors surface with a lens and light source to assess its state. A jeweller’s loupe or inverted eyepiece can be focussed on the mirrors surface, a light source such as a bright led torch helps. The surface is scanned to see if the pits resulting from grinding are uniform. Larger pits from previous work must be removed. It is important to do this carefully as you do not want to have to re-grind at some later time. Better do an extra hours work now if you are not certain than have to go back to grinding from the polishing stage.

Any questions? Please ask.

John

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Hi John you got a really good thread going here, very good of you to give your time to teach others. Your tiling looks great, can you come over and give me a hand with my kitchen lol ,,,Paul.

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If we wish to deepen the curve in the mirror we work with the mirror on top (MOT) and use a chordal stroke. In this stroke the centre of the mirror sits near the edge of the tool (about 1 inch in) and the mirrors prescribes a series of chords round the edge of the mirror.

Should that be "the mirrors prescribes a series of chords round the edge of the tool" ?

Still can't view vids, thanks to the GFWoC! But I'm due to travel tomorrow, weather permitting, so can view later in the week.

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Paul, I'll tile your kitchen and you can help me sort my autoguiding out!!

Yeti, nice to know that someone is reading carefully, yes you are right it should be tool. Thanks for that. The tool is re-useable until the tiles are too thin, you can probably get at least 3 mirrors from the same tool.

John

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hi glasspusher and anyone else watching this mirror thread, it is brilliant to see this tutorial in such an easy to follow style, can't wait for the next part !!, i am hopeing to finish off a part done 8.75" mirror shortly, and i have a question or 2, can a scratched mirror be reground /repolished to a different focal length relatively easily i have measured it to be f5 can it be made into an f6/f7 , and another 8.75"mirror i have is still not polished out yet, but saggita puts it at around f3.5 -f4 at the moment so i would like to make this one f5 ish, i want to buy the grit etc any advice welcome before i order the kit, ? looking forward to your replies, Tony

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Tony, lengthening the focal length of both mirrors should not be a problem. Make a full sized tile tool as described in a previous post. Work with the tool on top of the mirror and use a long stroke centre thru centre (about 1/2 D) with 120 grit. Check the sagitta regularly, when you get to the required depth reduce the stroke to 1/3 D to ensure both mirror and tool are spherical. Be aware that this process can reduce the focal length slightly. You will then need to grind with 400 and 600 grits as I will outilne in my next post. Hope this helps, please get back to me if anything is not clear.

John

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John thanks for the reply , I will order my grits etc and get stuck in following your tutorial and advice , Tony

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Most of the standard books on telescope making recommend a grinding sequence using the following abrasives: 80,120,220,320,400,600 and 800 before going onto polishing. It is not necessary to following this sequence; I regularly use 120, 400 and 600. The 120 and 400 are silicon carbide but the 600 is white aluminum oxide. The trick is to spend longer at each stage to ensure thorough grinding. Silicon carbide is very hard and fractures the surface more than aluminium oxide this is why it is better to finish fine grinding with aluminum oxide. The finer the abrasive the more likely for scratches to occur which is why I like to finish on 600. Finishing on say 800 or 100 aluminium oxide can reduce polishing time, so you pay your money. After thorough grinding with 120 and a careful examination of the surface to ensure there are no pits remaining if a coarser abrasive was used in roughing in the curve it is time to wash up. The tile tool should be scrubbed thoroughly with a stiff brush and the mirror should be washed off. The next stage is to grind with 400 silicon carbide. The stroke is a simple 1/3rd centre over centre stroke as used previously with 120 grit. Half of the grinding time with 400 should be done MOT and half TOT. It is always difficult to say exactly how long should be spent at each grinding stage but a total of 4 hours on 400 grit should do. The process of grinding 50% MOT and 50% TOT will ensure that the radius of curvature remains constant. Look at the videos showing the various strokes in a previous post. With 400 grit I like to mix it with water in a washing up liquid bottle and give it a good shake before squirting the mixture onto the mirror or tool. Using a small washing up bottle I would add 6-7 teaspoons of 400 grit and fill the bottle to ¾ full. It is very important not to let the mirror and tool dry during the grinding process, too wet is better than too dry. It is difficult to say exactly how much abrasive will be used at which stage, as we are using only 3 abrasives in our grinding sequence more will be required that with a traditional sequence. One possible source for abrasives at a reasonable cost is from lapidary suppliers. Try searching for them on the World Wide Web. Internet auction sites are worth searching too. You need to satisfy yourself that the abrasives that you are buying are not contaminated, may be worth contacting the seller and ask if the abrasives are suitable for optical work. On completion of grinding with 400 grit it is time to wash up thoroughly again. Satisfy yourself that the mirror is fully ground out at 400 grit. The surface MUST be uniform when viewed with a loupe or inverted low power eyepiece. If you are happy with the grind the whole process is repeated with 600 aluminium oxide.

Sorry, no picture with this post but the next stage, making the pitch lap, will have lots. If you have any questions shoot away!

John

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As mentioned, if we grind glass on a tile tool with finer and finer abrasives the chances of scratching increases, this is why we have to polish glass on pitch. Pitch is not easy to source, just about the only place to buy it in small quantities is here:

http://www.galvoptics.fsnet.co.uk/telescope4.htm

If anyone knows of any other sources please let us know. A fellow SGL member found pitch from the above source to be very hard and had to soften it using boiled linseed oil, about 10 teaspoons were needed to make the pitch useable. Hard pitch can cause scratches so it needs to be at the right viscosity for use. The hardness of the pitch varies with temperature so working in a shed or garage in the winter is not a good idea. A simple but effective way of judging the hardness of pitch is to use the ‘thumb nail’ test. If the thumb nail is applied to the pitch with a fair amount of pressure and no impression is made then the pitch is too hard. If a slight impression results after about ten seconds continuous pressure then the pitch should be OK. This is not a very scientific approach but it does work. Pitch laps are made with facets, this allows good contact to be maintained between the mirror and lap and allows the circulation of polish. To make the facets a kitchen draining board mat can be used. As you can see in the attached picture I have cut one edge of the mat down to the same width as the middle part. As the mat is smaller than the pitch lap this will allow me to turn the mat round and complete the facets on the rest of the lap.

To make a lap, pitch is melted and poured onto a suitable base, a plywood disk at least one inch thick is OK, it is a good idea to varnish the disk before use. I had a spare 12 inch glass disk available which I used for my lap base as you can see in the pictures. A paper damn is attached to the edge of the disk to prevent the molten pitch from flowing off the base. The pitch is slowly heated until it melts; it is then poured uniformly onto the lap base. The pitch is then allowed to cool; pressing the edge of the paper damn with the finger gives an indication of the viscosity of the pitch. As the pitch cools two channels, at 90 degrees to each other are made using the edge of the kitchen mat as shown in the picture. If the pitch sticks to the mat, which has been kept in warm soapy water prior to use, the pitch is too hot. If the channels stand in the pitch without closing up then the time is right to proceed with forming the facets.

John

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Another source for Pitch is Vacuum Coatings, and also i believe Beacon Hill telescopes sell mirror making kits still.

Its been a few days since I last checked, due to not having internet access at home at the moment, but I am loving this thread.

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The first two channels in the pitch lap are to allow the air to escape when the mirror is lowered unto the hot pitch to make it match the curve in the mirror. It is best to make contact on one side of the mirror and lower the rest onto the pitch to help exclude the air and prevent the formation of bubbles. The mirror is warmed in hot soapy water prior to this, keeping the mirror wet helps prevent sticking. The mirror should be removed quickly form the hot pitch to prevent sticking. If the contact is not good repeat the pressing of the wet mirror onto the hot pitch until good contact is achieved, looking through the wet back of the mirror will reveal the level of contact. The two channels made to allow the air to escape can be allowed to close when the contact is good. Next the kitchen sink mat is placed on the surface of the pitch so that the centre of the lap is not coincident with the centre of one of the squares in the mat. As the mat is not big enough to cover the whole lap begin by covering one half, or a little more. The mat should be kept in hot soapy water prior to use, this helps reduce the chances of the pitch sticking to the mat. The mirror is now pressed firmly down on the mat so that the pitch squeezes up to fill the holes in the mat. Quickly lift off the mirror and peel away the mat to see the facets beginning to form. Switch the position of the mat so that the remainder of the lap is under it and again press down firmly with the mirror. Go back to the first position and repeat the pressing with the mirror, move the mat and repeat in the second position. This process is repeated until the facets are nicely formed. Cool the lap under cold water for about 20-30 seconds; place the wet mirror on top of the lap and removed the excess pitch from the edge of the lap with a chisel as shown in the picture attached. Do not allow the mirror to sit on the warm pitch or it will stick. Dry off the surface of the mirror and pitch lap with kitchen towel. Place a sheet of greaseproof paper over the lap and allow to cool. The greaseproof paper will prevent the mirror and lap from sticking and yet allow good contact to be maintained. A mixture of cerium oxide and water is used to polish the mirror, some workers like a thick mixture, I generally use a thin mixture of about 1 level teaspoon of cerium oxide to a cup of water. This can be kept in a small washing up liquid bottle to made application easy, down forget to shake before using!

After an hour the mirror can be lifted off the lap and the paper removed, squirt some cerium oxide on the surface and replace the mirror and move it around. Wet the back of the mirror and look through to judge the level of contact. It is easy to see where the mirror and lap are not in good contact when the mirror is moved around. If contact is poor place the lap in hot water to soften the pitch, this may takes a several minutes. Next press the mirror on the lap to improve contact. This process may have to be repeated several times if contact is poor. Do not leave the mirror on the lap for long periods without moving it around as sticking is likely to occur. During this process the channels between the facets may start to close up, if this happens they must be remade before they fully close. Channels may be re-cut using a hand saw (medium teeth), care is required, and some chipping of the pitch may occur, if this is excessive warming the lap a little in hot water prior to cutting will help. Lap making is one of the more difficult aspects of mirror making and care must be taken to ensure good contact between the mirror and lap before polishing begins.

Everthing clear so far???

John

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Hi John,

This seems like a nice way of faceting the lap -- I'll have to have a look around for one of those mats!

Quick question -- do you bother putting CeO slurry between the pitch a the grease-proof paper on the initial press, or does it not need it?

I guess the other thing to emphasis is quite how slowly you need to heat the pitch to avoid it getting too hard.

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