So you really want to build a Telescope Mirror.
Bob May

Here's another way to do the work along with some digressions on doing things related to making mirrors.

Another work in progress although it's complete enough to make a mirror at this time.

 I'm just adding thoughts as I think of them. Last update is June, 2000.

If you want to move the source of these pages to your own website, I encourage it! Just make sure that you take the pictures also. I also recommend that you come back on occasion for any updates.

      I assume from your looking for this page that you probably want to make a mirror for your new telescope. The first thing is to understand that you will be spending a fair bit of time on this project. If your time is money then you would be better off buying a nice mirror from someone else, but then you will really have no real pride that you'll have gotten if you had done the job yourself which is part of the enjoyment of a project that you do yourself. If you understand this and that you may not do the best job the first time around then lets get with the program and grind and polish a mirror!

       I'm going to talk a lot and about a number of different things concerning the grinding and polishing of a mirror so you might as well settle back and read through this entire set of pages and get familiar with them. I also suggest that you find other pages describing the same processes on the web and compare them with what's here and remembering that while you do this comparing that all of the various methods that you will read are essentially the same process expressed in different ways. People just say things in different ways and they think that certain parts of the process of grinding and polishing a mirror are more important than others.

      Remember that you can't really destroy the mirror unless you drop and break it or otherwise take a big ding off of it by hitting it with something. The mirror is a glass, so treat it that way. This whole process of making a lens or mirror surface was originally discovered way back in the 15th century (or even earlier) and is really a very crude process that does some very precision machining JUST BECAUSE THAT'S JUST THE WAY THE PROCESS WORKS!

       The emphasis above is because the process of making a mirror surface is really pretty simple and some people want to make it a lot harder than it needs to be to get good work done. The most important thing is to keep your equipment clean of dust and dirt as this will cause problems with the polishing and the finer grits but again, don't go overboard in this department unless you are built that way.

       Lets get all the stuff together that we need. This list is what's needed for making a mirror and it's a pretty complete list so let's the stuff together and get to the work at hand, namely grinding the mirror.

       Pull out all the parts of the mirror kit (if you got one) and put the various grits in the jars. Start with the finest grit and pour it into a jar and mark it with the grit on the glass and put a piece of clear tape (that protects the writing from your abrasive hands) over it and also put the name on the top. You can mix up the tops if you don't watch out and mix grit sizes as a result and thus end up with no end of problems. Leave the Cerium Oxide in the shipping container for now. You will want to be very careful with the Cerium Oxide as that part of the making of a mirror is a different process from the grinding of the mirror and you don't want to get the abrasive grits mixed up with the Cerium Oxide which is to be used as the polishing compound. The reason for starting with the finest is that it's OK to mix fine grits with the coarser grits but not the other way around and that will happen if you're not extremely careful with the handling of the grits and starting with the coarse grits makes it a lot easier to spread them around and destroy the usefulness of a pile of grit.

       Next thing to do is to setup the grinding platform. There's many ways of doing the grinding platform from just a basic piece of plywood clamped to a table to a 55 gal. drum with sand or other things in it and a plywood cover over the top of it to work on. First is to put the grinding platform where you plan on using it and put the tool blank (or mirror in the case of the Plaster Tool method) on it and screw (don't nail as you're going to have to remove the cleats when you change grits) the 3 pieces of wood for the cleats equidistant around the tool so that they restrain the tool without being too tight. Remember that you're going to be pulling the tool on and off a fair number of times and you don't want to have to get out a screwdriver and loosen something to get it off of the grinding platform. When I want a snug fit, I use a small wedge of wood between a cleat ant the tool to hold things snug.

       Now you are almost ready to start grinding. Before you start you need to figure out how deep the hole is that we are going to need. The formula for this is pretty simple. You need to know the diameter of the mirror (D) and the Focal Length (F) that you want to make and you get the Sagitta (S) which is the depth of the hole that you need to carve into the glass. The formal formula is:

S = 2F - sqrt( (2F)2 - (D/2 )2)

The second way (an approximation) to calculate the sagitta with this formula which is probably a lot easier to calculate:

S = (D/2)2 / 4*F

"Sqrt" is the square root of the number inside of the brackets. Sagitta is how deep the curve of the mirror is going to be in the center of the glass. The more accurately you calculate and measure this dimension, the closer you will be to the Focal Length you want when you get done. Either formula should provide the same basic answer unless you're doing a fast mirror.

       The first thing we're going to do in the grinding is to put a chamfer on the edge of the mirror. Grab the knife sharpening stone and put a flat on the edge you're going to grind on. This edge should be at about 45° and the proper way is to grind along the edge rather than across the edge as this will make a lot less chips off of the glass. Put about a 1/16" - 1/8" or so chamfer evenly all around the glass and make sure most of the chamfer stays during the coarse grits. If you're going to use the glass tool as a tool then you will need to put the same chamfer on it also. Often the cast mirror blanks have a nicely rounded edge. If you're not going to grind until the cows come to my home (forever), then that is usually about the right amount of an edge and you can let the curve work. Be aware that the shallower the angle to the top, the less you're going to have to worry about chipping the edge. This chamfering is done because glass loves to break off nice sharp slivers because of the high pressure that can be applied at the corner when a piece of grit presses near the edge and that the glass will keep breaking off and eventually make a big hole in the edge of the mirror. If, at any time a chip flakes off, immediately grind a good chamfer with the stone on the chip area and go all the way around the mirror (or glass tool) and refresh the chamfer. As long as there's a angle on the glass, then it shouldn't chip off unless you really press hard on just the edge or tip the mirror or tool and press on it (never a good idea).

       There are four major stages in making a mirror:

 First carve a hole in the mirror to the basic depth that you need. This part is called coarse grinding.

 Second is to smooth out the pits to the point where the mirror can be polished. This part is fine grinding.

 Third is the polishing of the mirror. This part removes all of the pits and makes a smooth surface on the glass.

 Fourth is to figure the mirror. This part puts a particular shape on the mirror so that it conforms to the requirements of the design. For Newtonian telescopes, this figure is a paraboloid figure.

       We can take to two different directions for the initial rough grinding. If you have a desire to make a plaster tool, then go here for the Plaster Tool method or if you are going to use the glass tool supplied with the kit, then you should go here for the Glass Tool method. In either case, you will end up carving a hole in the mirror with about 90% of the depth of the curve desired as the next stage of grinding (where you go down through the grits) you will gradually get to about the desired Focal Length. With the rough grinding, feel free to apply a lot of pressure (watch out for putting excessive pressure on the edge of the mirror as you will quickly get a big chip off of the edge of the tool) to your mirror to get the grit to cut better. This works only for the rough grinding and when you start using the finer grits, you will need to lighten the pressure until the mirror's weight is doing most of the work. Don't worry too much about going too far or not far enough with the digging of the hole with the coarse grit as I've seen people go past the Radius of Curvature on the rough grinding and then end up with the correct Radius of Curvature by turning over the mirror and tool so that the radius gets larger. Remember that the principle states that whichever piece of the work is on the top has it's curvature go towards the concave direction. If you want to make a mirror that has too short of a radius to a longer radius then you merely put the mirror on the bottom.

      The other thing that's important is to rotate the mirror as you are grinding. This motion is standard all along the process of grinding and polishing as you are trying to make a figure of a revolution and not a groove in the mirror. You also need to walk around the barrel or rotate your tool so that it too gets to be worn about equally well in all directions. Grinding/polishing machines work by rotating the bottom piece (mirror or tool) and letting the top piece freely rotate about it's central axis as it desires to do. As you watch a machine work, the upper piece will rotate at about 1/2 to 1/10 the speed of the bottom piece. You can make the ratio anywhere you feel comfortable and you don't even have to keep it constant as there should be a bit of randomness to the whole procedure otherwise patterns may become produced by the precision workmanship. It's odd that the sloppier worker actually does a better job in this regard.

       One of the things that is nice to learn how to do is to measure the Radius of Curvature (which is twice the Focal Ratio). This is easily done at any time with a small flashlight and a mirror wetted with some water (another use for the spray bottle) so that is will reflect the light back to you. The process of actually finding the radius is actually pretty simple - take and put the flashlight near your eye and alongside your head and shine it onto the glass so that you can see the reflection of the light from the flashlight. The picture above shows the flashlight above a head but it's all the same. The rule is that if you are inside of focus, you will see the light move on the glass in the same direction as your head and flashlight moving around and when you get outside of focus, the light moves in the opposite direction as your motion. When you are at the center of curvature the light will seem to blink on and off as you move your head. When you have the mirror blinking nicely you can then put the flashlight down at that point and measure from the lamp to the mirror. You will probably be within a fraction of an inch of the proper radius. The process gets a lot easier with the finer pits as you go through the grinding process. You can even use a card in place of your eye if you do the job in a darkened room and thus get a more accurate distance if you use a small source like a small bright LED and get a good image of that source on the card. A mask over the light to make a figure of some kind will also help in determining the exact distance.

       The other way to check for the Focal Length of a mirror is to take it out into the sun and find out when the sun image is most focused (it won't be a very good focus because you have a very poor reflection (it's a spherical surface rather than paraboloid and is a very irregular surface) on a shaded wall or roofline. Remember that the closer that the image of the sun is to the actual source path, the more accurate you're going to be with this method. This method works for a fair approximation of what the Focal Length is. My biggest problem with this method is that it's hard to measure the distance without setting up a test station so that you can measure the distance with your tape measure without having to hold the mirror in place.

A big word of caution

If you are doing this in the sun - be sure not to aim the mirror towards anyone's eyes, even the uncoated mirror gives an extremely bright reflection which can damage one's eyes very quickly I might also remind everyone that the retina does not have any pain nerve endings so you will not realize that you have damaged your eyes until it is too late.

      The problem with any of these tests is that you will only be close to the truth but it really doesn't matter much until you get to the figuring that you will get to the real exact focus of the mirror. The coarse grinding will, of course, give the poorest accuracy as the water is trying to fill in huge holes in the mirror's surface.

       Go do your initial grinding with one of the two methods given above and then come back to here. You should have a sort of spherical hole that goes all of the way out to the edge of the glass in the mirror when you return back to this page and you will be ready to make the pits in the glass finer in size until you're ready to polish the mirror.

       Now that you're back from getting a curve into the glass and cleaned the workplace of any of the grits that may be around, it's time to go down through the grit sizes. First we are going to put a pinch of the next size grit and water on the tool, spread it around and, as we place the mirror down on the tool, we will hold the mirror lightly off of the tool and gently move the grit around with the mirror so that the larger pieces and agglomerates of the grit all get a chance to be broken up before you really start grinding. Especially in the finer grits, this procedure is important as you don't want to get a scratch from a strange piece of grit which will cause you to have to grind extra long to get rid of the scratch and you don't want to do that. The image to the right is the stroke that you now want to do. It doesn't try to carve the hole deeper as much as the initial stroke does and tends to make the mirror surface more spherical as you go along.

       A little question that I have heard many times from firsttimers is about the air bubble that tends to be in the middle of the mirror. Don't worry about it as it is indeed grinding there properly as well as any other place on the surface. It's just that the clearance is a bit larger in the center than at the edges and it will go away as the grit gets smaller. If the mirror sticks to the tool, immediately hit the mirror with the palm of your hand back the way it came from with a firm stroke as you have gotten a good vacuum seal between the mirror and the tool and the only way it's going to release quickly is by going back the way it came. Immediately pull the mirror off of the tool and make sure there's enough grit and water on the surfaces. This problem is most prevalent when you're done doing a roughing stroke and the center is ground out more than the edges on whichever is on top. It also happens with the finest grits as the distance between the tool and mirror is very small and any slight differences in spherical will quickly form a nice vacuum. You need to work these little sticky spots (as opposed to a tired wet) out of the mirror when they do happen. This problem should be very rare with the tile tool as you can put some little grooves in the plaster to relieve the vacuum that may get produced.

       If you get the mirror and tool stuck together and can't get it loose by the method of quickly whacking it back the way it came from, put the mess in a bucket of hot water and hope that it will come loose. A cold bucket following this should pop them loose. If not, then take and put it back into the holder and place a 2x4 on the mirror side, hit it with a wood mallet or (be very careful here you don't hit anything but the wood) a regular hammer. When hitting you should always try to drive the mirror and tool opposite the way that they went when they got stuck together.

       After about 5 wets of grit and water, go look at the pits that are on the mirror with the magnifier. You will notice that there are some big pits and some small pits. Mark two or three of the biggest pits you find with a magic marker on the back but don't be exhaustive about finding the biggest ones as anything fairly big is good enough. Now that you have some idea of where the big pits are, do more wets until those pits are lost in the bunch of any of the other pits that are on the mirror. Please note that you may find some other big pits where there were none before. This is normal as you will have some large pits generated when the glass cracks just right. Next while the mirror is dry, reflect an incandescent bulb off of the mirror (in a grazing angle) and see how orange the light is in the reflection. The search for pits will become more important as you get to the smaller grits because there's always the possibility that you went to the next grit too fast and you will need then to go back a grit and do some more wets.

       The larger the grazing angle you are at, the oranger the image of the light bulb will be. Get a nice burnt orange shade of color in the reflection and move the reflection of the lamp left and right over the mirror and see the different colors in the different zones of the mirror if the pits aren't the same size all the way across. This will be a quick way to see if the entire mirror surface is ground to the same size pits in a very rapid fashion. A properly ground surface will have the same color over the entire surface. This is a nice method of seeing how fine the finish is on the mirror and as the grit gets smaller, you will see that the grazing angle will get larger and you will be able to see the next grit making smaller pits as you go along. Every other grit (or every grit if you wish) check for large pits and mark them and work some more wets until those larger pits are gone. Again, note that there will always be a few pits that will be a bit larger than those around them. This is normal and the important part is to make sure that the ones you initially found in a particular area are gone.

       Another method of finding out if the grinding is well done is to take a marker or pencil and mark the surface of the mirror (you can also use a marking fluid) and grind a wet or two and see if the marking fluid is fully removed from the ground surface. After the second grit, all of the rest of the grits should proceed at a rather rapid rate as you are merely making smaller and smaller pits in the glass from the hilltops of the previous grit. Sometimes 4 wets are enough to do the job if the grit sizes are anywhere near close in size.

       After you have a grit done you should clean the whole grinding platform of any grit that may be on it. Pull the wood cleats off is best and clean underneath them also. Remember that if you pick up a piece of grit from two grit sizes earlier you will get a nice scratch from it which you won't like. After cleaning, put everything back together and proceed to the next smaller grit. As you proceed down through the grits, make your W stroke more and more centered and shorter strokes rather than larger as you don't need to get the radius shorter than what it is. These shorter strokes will make the mirror more spherical than what it was with the larger grits. When you get down to the two smallest grits you may notice that the start of the grinding seems to not be doing anything. Part of this is because the much of the water has to be pushed off of the mirror before any real grinding can happen. Less water is more of the way to go but beware of getting the mirror and tool dry as they will lockup and be very hard to separate. This is also the time when the more center over center strokes are most important as we are very close to having a good sphere formed on the surface and you will also notice that the bubble in the center isn't forming anymore. This is because you are now almost completely spherical and there's no hole in the middle anymore to hold the air bubble. Any bubble that forms will be very thin and will move around a lot as you grind.

       As you go through the various grits, also make sure that you keep a clean worksite and thoroughly wash everything down between grit sizes. You don't need to make a nasty scratch from a stray piece of 320 grit when you're at the 12 micron size. Also, you may want to put some thin epoxy or other waterproof glue or paint in any of the bubbles in the face of the tool if you're working with a plaster tool if you're really afraid of holding grits although that's a bit extreme. I'll leave that up to you as to how much trouble you go through to seal up the plaster of the plaster tool. I've seen good mirrors done with just a carving out of bubbles so they don't hold a grit when washing the tool.

       When you get to the last two grits, you want to make the grinding with a more center over center stroke. This will insure that the mirror is nicely spherical when it comes time to start polishing it. Be careful of sticking the mirror and tool together as you can quickly get the two stuck together if the two pieces aren't basically in a good mating condition as you're proceeding. If you do get a stick, quickly thump the top piece back the way it came from to release the stiction and then work to one side or the other of the sticking point and get the stick out. Be careful as you can quickly end up with an astigmatic surface if you don't rotate enough and this can cause sticky places.

       When you get down to the last and finest grit size, make a good check for an even pit size with the grazing of the light on the mirror. When the light looks the same over the entire surface, then go over the mirror with a fine tooth comb and make sure that there are no large pits in the surface as they will definitely show up in the polished mirror.

       I often make the last wet last about 3 times or even more than normal as the grinding still continues and you're just making smaller pits as you go along. Make sure that you don't get the tool and mirror stuck together as you do this.

      If you've kept with me to this point, you will now have a piece of glass with a nice shallow fuzzy or frosty hole in it that is of a spheroid nature. It is now time to put a polish on that surface and then make it a very particular shape that will reflect the light from a distant star to a nice point. We'll do this in this page for polishing and figuring your mirror as you will be wanting to refer to that page alone after this. Good Luck in finishing your mirror.

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