Although planets and stars reflect and emit vast amounts of light, by the time that light gets to earth and through the atmosphere, it is very weak. A telescope mirror collects this faint light and focuses it on a second mirror that bounces the image into an eyepiece. In this way, the telescope user can get a closer look at the star or planet being observed. Telescope mirrors are precision pieces that require careful shaping to get the best image. Manufactured ones are very expensive, but with enough time it is possible to make your own.
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Things you need
- Glass blanks
- Ceramic shards
- Abrasive grit
Purchase two circular pieces of glass or pyrex (otherwise known as "blanks") that have the diameter you need for your mirror. You can buy them online or from glass stores; some enthusiasts also use old ship portholes.
Calculate the depth you need to dig out from the centre of the blank to make your mirror. This is called the "saggita." The radius of curvature for the saggita must be twice the focal length you want the mirror to have. To calculate the depth of the saggita, you need to consult depth tables that relate focal length to lens width (see Resources). For example, a lens with a 1,000mm focal length and a 150mm lens width must have a curve dug out 1.4mm deep at its centre point.
Prevent chips from developing by giving the edge of the disk a bevel. To do this, grind down the outer edge with a wet sharpening stone held at a 45 degree angle.
Assemble a grinding tool. To grind away the glass by hand, you need something harder. Shards of shattered ceramic tile work well. Glue them down to one side of a second glass blank the same size as the first. Once the glue dries, coat the whole tool with boat varnish or epoxy.
Lay down a piece of thick carpet to support the disk and spray it with a bit of water. This will keep abrasive dust from entering the air.
Sprinkle the blank you wish to dig out into a lens with no. 80 grain abrasive grit (the same as would be attached to no. 80 sandpaper) This is available from most large home improvement stores.
Lay the grinding tool down on the piece of carpet and set the mirror blank on top of it. Press down firmly and move the mirror blank in even, circular strokes to dig out (or "hog out") glass from the centre. The strokes should have a length equal to 1/3 of the mirror blank's diameter; spin the grinding tool around from time to time to keep the grinding even. You will need to re-wet the mirror blank and add more grit whenever the grinding starts to make a loud noise.
Flip your project over so that the glass blank is on the bottom and the tool on top, then press down on the tool and keep grinding with strokes of random direction. Move the tool right up to the edge of the glass blank as you do this, and add grit and water as necessary. You will need to check the depth at the centre of the area you are hogging out on a regular basis. Switching back to having the tool on the bottom allows you to deepen it, while tool-on-top shallows and evens the curve.
Remove all the no. 80 grit by thoroughly washing both disks, the whole work area, your clothes and your skin. Any leftover bits of this grit will damage the mirror as you finely grind it.
Apply a succession of finer grits, and grind again. A good sequence would be to start with no. 120 grit, then move to 220, 400, 800, and finally 1,000. This will take many hours of work. You can see when to change grits by looking at the lens under a magnifying glass; you will see tiny pits in the surface. If those pits are all the same size as the grit you are using, change to the next size down.
Draw a grid on the surface of your mirror blank using a marker. This will let you check that you are grinding evenly; if you are, the grid lines should wear off at the same rate. If they are wearing down faster in one spot, adjust your strokes to compensate.
Mix the finer grits -- anything smaller than no. 400 -- with water at a 1:8 ratio before applying it to the disk. These finer grits will not remove very much glass; they instead smooth the surface. After grinding thoroughly with no. 1,000 grit, check to make sure that the curve is even by drawing another grid, then use your finest grit to grind these marks away.
Clear out any grit from your work area and thoroughly wash everything. Change your clothes as well. Any grit left over will damage the mirror during the polishing process.
Purchase a brick of pitch and some cerium oxide. These may be available from local laboratory suppliers; otherwise, it's a good idea to ask around at local astronomy clubs.
Melt the pitch very slowly in a saucepan over an electric burner set to medium heat. Don't stir too much, as this will leave air bubbles in the pitch. This will ruin the saucepan, so use an old one.
Tape a ring of masking tape around the edge of your grinding tool, with half of the tape protruding past the edge. This will hold the pitch in when you pour it.
Mix some cerium oxide into a jar of water. The colour of the mix should be similar to 2 per cent milk.
Let the melted pitch cool slightly, then pour it into the taped area of the disk, adding between 3/8 and 1/2 inch of pitch in one smooth pour. Strip off the tape once the pitch cools enough to be semi-solid, then apply some of the cerium solution to your mirror blank and press the pitch-covered tool down onto it. This will give the pitch a shape that exactly matches the curve of the mirror, with channels in between the pieces of ceramic on the tool.
Wait for the pitch to harden, then cut a grid of deep, v-shaped channels into the pitch with a razor blade. This will give the cerium a place to flow as you polish. Shape your tool to the lens again by coating it the mirror blank in cerium and pressing the tool hard against it for a few minutes. You can also hot-shape the tool by placing the tool in a bucket of hot water to soften the pitch.
Polish the mirror by applying a few tablespoons of the cerium mixture and then pressing the tool to the mirror blank and moving it with the same strokes as you used for grinding. You will know that polishing is complete when the glass loses its frosty appearance and you can shine a small laser pointer at your blank and see no hazy grey reflection where the laser hits. This will take several hours; apply more cerium as necessary to keep the tool moving smoothly.
Send your mirror off to be aluminised. This is a chemical process that uses electrical current in a vacuum chamber to apply a layer of reflective aluminium to the mirror. It involves expensive equipment and extensive training, so isn't a feasible DIY job. Ask other local astronomers for recommendations for shops that will do it for you. Once the mirror is aluminised it is ready to mount in a telescope.
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