Mirror making:

The following information is based on my own, and other Club -members' experience in making, or helping make, test and finish about 50 or so mirrors over the last fiteen years. No guarantee is given that this information will enable you to make a perfect mirror, but is intended simply to share our knowledge of the subject in order to help those who have no experience - and are making a mirror for the first time:

This file will print out on about 12 pages (letter size)

This information updated October 12th 2001

If you have never made a mirror or a telescope before - one of the best things you can do is search for a local Astronomy Club. In many clubs you will find members who have already made telescopes - either from 'kits' which contain all the necessary parts - or 'from scratch' using the least expensive materials and methods. Most telescope makers are quite proud of their efforts and very willing to pass on their experience and knowledge to others. Another (most essential, in my opinion) is the acquisition of a good BOOK on telescope making. Both the above can make life much easier - even for the experienced mirror and telescope builder.

Before we start - a bit about the shape of a reflector-type telescope mirror The primary mirror in a Newtonian type reflecting telescope is a 'concave mirror.' It is of course usually a round mirror and thus is shaped as though it were a 'slice' cut out from a sphere, or a round ball. So that across any diameter of the mirror the curve in its surface is a section of a circle. This is normally the shape that is automatically formed by the process which is described in the following text. (In practice though, this does not give perfect images when parallel light rays coming from very distant objects are reflected to its focus point. So, in practice, we have to shape it so that its cross-section across any diameter becomes a cross-section of a parabola, rather than a cross-section of a circle. This is what is referred to as 'parabolising' the mirror. The reasons for this are explained further in the following text, and the process of 'parabolising' is done after the mirror is ground, polished and tested.)

TO START OFF - a bit about optics

Figure 1 (A) shows light coming from a point source located at the center of curvature of a concave spherical mirror. This would form an image of the source superimposed on the source - as all the rays are reflected back to the source, as they all meet the mirror's surface at a 90 degree angle.

If we moved the point source slightly to one side the image would move slightly to the opposite side and could be seen by placing a screen at that point. Below is shown the same mirror (B) but with parallel light rays coming to the mirror surface from distant objects - stars, planets etc. In this case the light rays meet the mirror surface at differing angles from edge to edge of the mirror. They are bent further inward and form an image at what is termed the FOCUS of the mirror. Once the source of incoming rays is a few miles away - the rays are effectively parallel, and they will meet at the 'focus' of the mirror. This point will be a point halfway between the mirror and the center of curvature of the mirror's surface.

Note:the numbers given in the following are in inches...if you are used to metric numbers 1 inch = 25.4 millimeters, So an 8" mirror would have a 203 mm diameter in metric measurement.

From the foregoing you will see that if a mirror is to have a 'focal-length' of let's say 40 inches - the radius of the curve in the mirror's surface will be 80 inches. Therefore if we want to make a 6 inch mirror and have a 36 inch focus - the "radius of curvature" will be 72 inches. We would refer to this mirror 6" diameter, 36" focal-length as an "F-6" mirror.

This is simply the ratio between the mirror's diameter and the focal-length. So an "F-8 " mirror of 8" diameter would focus at 64 inches ( and would have a radius of curve in the mirror surface of 128 inches.)

O.K . NOW HOW DO WE GRIND THE MIRROR - and how do we figure out how to get the depth of curve that we want?

The details that follow apply equally to any size and any f-ratio mirror --(within the ranges up to 20" and F5 to F10 - if you're planning to make a 30" F-3, you're either nuts! or you've had a ton of previous experience)

We will start with the decision that we are going to make an F-8, 8 inch mirror: This means that our radius of 'c' will be 128 inches. The sectional curve in the mirror is really part of a complete circle, and we can visualise that our cross-section is a small slice from this circle which has a radius of 128"

The depth of this 'sagitta' as it is called is calculated by multiplying the radius of the mirror by itself and dividing the result by twice the radius of 'c'.. that is, in this case 4 x 4/256 = .0625" a mere 62 thousandths of an inch! (It just happens that this is 1/16th of an inch, which is much easier to remember!)

Now we have decided how deep the curve in our mirror will be - we can move on to the really messy stuff - rough grinding:

I assume that you are starting with a mirror-kit, from one of the suppliers of kits and materials:

Some suppliers will send you a 'pyrex' mirror disc along with a ceramic disc which will be used as the tool, some still supply two 'pyrex' discs and you should pick the better one to become the mirror.

An important note here: The disc - from whatever source it comes should be of even thickness all the way around. There should not be any significant difference in thickness (less than perhaps 1/2 millimeter - ( 10 thousandths of an inch)) Also the back surface should be quite flat and even - some discs are cut from sheets of glass and may have either noticable corrugations or other unevenness. These discs should be ground flat before starting to grind the mirror.

Note the two discs are both flat discs to start with. It is the type of offset stroke which produces the curvature in both the mirror disc and the tool disc. The mirror disc is used on top of the tool disc. This produces a concave curvature in the mirror, and the tool disc becomes convex. If the correct strokes are used then they both develop an almost perfect 'spherical' curvature.

Note also that you should bevel both edges of the mirror and the tool at a 45 degree angle - or sharp edges will develop during grinding - and a sharp edge will 'splinter' very easily if it happens to hit the edge of the tool or any other hard surface. Use a sharpening stone or a fine sanding wheel and be sure to do this by grinding the edge in a direction away from the surface of the disc.

We have cut expense a couple of times - by buying a kit containing two pyrex 'blanks' ( along with the necessary grit etc.,) and using both pyrex blanks to make two mirrors. To do this we have bought 1/4" or 3/8" thick glass discs from a local glass shop - cut the same size as the mirror - glued these to 3/4" plywood discs (with epoxy glue - be sure to spread the glue evenly right to the edges of the surface of the ply-disc) Bevel the edge of the glass and then paint the plywood base to seal it - so that grit cannot hide in any cracks. If necessary repaint the edge between grit sizes. Make absolutely sure that the disc is evenly glued to the glass, and that it cannot flex due to spaces between the glass and the plywood backing disc.

As the depth of curve in the average 6 or 8 inch mirror is usually less than 1/10", these tools work fine. (the plate-glass wears down a little more than the pyrex but not enough to cause a problem) The kit also usually contains enough grit etc., to make at least two mirrors. The other expense-cutter has been to buy a few port-hole glass discs. 9" or 10" diameter from marine suppliers. About five or six of our members have successfully made mirrors with these. Again using a thinner glass disc on plywood as the tool for grinding them

You now need to find a place to work - where the temperature is as even as possible - and also the area should be dust-free. You also need a sturdy and stable small bench - preferably one which you can walk around. You also need a bucket of water or a nearby sink in which to wash the discs, and a few small sponges 3 or 4 inches square for wiping things off. We have frequently bought a packet of cellulose sponges - about 4 x 8" and cut them in half.

During the grinding and polishing process - if you have any concern about 'dust' or breathing problems - I would recommend wearing a mask, or tying a large handkerchief around your mouth and nose. In the finer stages of grinding and polishing microscopic bits of the powders, or polish and glass particles could be inhaled as they are present in the air around your bench, especially as the 'wets' start to dry out.

In the rough grinding process - we place the tool disc on the bench - and then apply a small amount of the abrasive grit (number 80 grit) with enough water to form a creamy paste spread around the surface - we place the mirror disc on top; then swirl the discs around gently to evenly spread the mixture. See below for details on applying the grit. summary

Click here for a summary of the procedure:

Then we off-set the mirror about half its diameter - but not enough that it will tip over the edge of the tool as we move it backwards and forwards across a 'chord' of the tool. We use pressure as we stroke the mirror disc across a chord of the tool - making about 10 or 12 double strokes. -- but not counting exactly the same number of strokes each time. Then we turn the mirror disc about a quarter turn (on average - but not exactly a quarter turn - we are relying on the law of averages to even out the grinding process) We also step around the bench a little in one direction (left or right - but once you decide which - you must always go the same way) and we stroke across a different chord of the mirror - and, as we have stepped around a little way we are also stroking across a different chord of the tool. You should try to make about eight or ten small steps around as you do this - so that you gradually work your way around the lower disc. This ensures that the discs are ground out evenly and ensure that the surface becomes spherical. In this way as we keep grinding and turning around the mirror we will gradually cover the complete surfaces of both so that they will become ground evenly. (See diagram below)Don't forget to turn after every 10 or 12 strokes !

During the initial grinding we are going to be applying fairly heavy pressure between these discs. (The grit works best at cutting the glass under strong pressure) So we need 3 small wooden 'cleats' less than the height of the disc - fastened to the bench with screws. This will prevent the lower disc from moving around. The cleats should be positioned close to the disc, at 3 points close to but not quite touching it, so that we can turn it as necessary.

For an 8" disc we would apply about a half to one teaspoonful of #80 grit and sprinkle it around the surface. (NOTE: for 4" 5" or 6" discs we would recommend using #120 grit for the rough grinding -this of course works less quickly, but you need a certain minimum number of strokes in the rough-grinding process to ensure that the 'spherical figure' is obtained) Then apply a small amount of water and spread the mix with our finger to make a reasonably good mix of water and grit. If the mix is too dry, then the discs will tend to bind and be hard to move - if so - we add a little more water and try again. If it is too wet the grit will wash out over the edges of the tool . With a little practice you will soon get to know the right amount to use. When grinding - if the mixture is good you will be able to tell by the loud grating noise as you rub the discs together.

If you have to work on a bench which does not allow you to walk around - then to work around the mirror evenly you need to turn both the mirror disc and the tool disc about a quarter turn (but not exactly a quarter turn each time) so that you will grind across a different chord of each. You will in effect "walk around the tool" by turning it - but you will make your strokes across the bench in the same direction all the time. When you turn the discs - try to do this methodically. Center them first, then turn both discs together, now turn the top disc only. This way you will have turned each of them about a quarter turn. Then you off-set the top one and continue with the next set of strokes. (Remember to vary this 'quarter turn' - as we want this turning to average out so that both discs become ground out evenly across their whole surfaces.)

We continue with the off-set stroke constantly working our way around the mirror . The mix dries out after just a few minutes of work - ( 2 to 4 minutes) and the loud grinding sound dies away. The discs get harder to move and tend to bind together. As the grit mixture dries out, we stop - clean off both discs and add a fresh mix of grit and water. This few minutes of work, after which we clean and recharge the mix, is referred to as a "Wet".

Depending on the size of the mirror and the depth which we need in the mirror's curvature, after an hour or two we need to check the depth. The easiest way to do this is with a good straight-edge, a steel rule or other such item laid across the mirror from edge to edge and across the centre. Using a feeler gauge or small drill bits of known size we can measure the clearence between the rule and surface at the center of the mirror. If we are not yet close, we continue with the grinding until our measurement is reasonably close to the depth we require. Once we reach this point we can stop the rough-grinding and begin to consider the next steps.

FINISHING ROUGH GRINDING - Now that our depth is about right we need to change to a different type of stroke. We also now reduce the pressure used previously using only the weight of the mirror disc.

The stroke which we will use now - and for all of the smoothing and polishing of the mirror, is going to be a stroke in which the discs will be approximately centered and the pressure used is just enough to keep good contact between the tool and mirror.

As you can see from the drawing - the stroke is more over the center of the mirror and moves only about 1and 1/4 inches to each side (for an 8" disc) and the vertical stroke is about 2 and 3/4 inches. The center of the mirror therefore, describes a letter "W" shape.

When pushing the disc back and forth across the mirror - be sure that you don't 'rock' or turn the disc during or at the end of a stroke. This can be a common mistake, the disc between your hands should remain oriented in the direction in which you are moving it.

You must not allow it to twist between your hands as it goes back and forth - whatever direction you may be pushing it. Remember to turn both discs frequently as before.

You still keep turning the mirror (between each series of back and forth 'W' strokes) when using this new W - stroke ..its just that you should avoid twisting the disc when doing the back and forth stroke.

After a number of strokes you then turn the mirror a little as you walk around the bench so that you keep stroking across different diameters as you apply the strokes (about 10 or twelve double strokes at a time.)

If you can't walk around the bench you need to turn both the tool and mirror (in opposite directions) approximately 30 to 40 degrees of turn. This ensures that your smoothing action smooths the whole surface of the mirror evenly as you work.

For any size of mirror the sideways motion should be no more than a quarter of the mirror's diameter (this is the total sideways motion - about an eighth of the mirror's diameter on each side, about 1 inch overhang on each side for an 8 inch mirror) - and the back and forth motion should be about a quarter to a third of its diameter in total. This can and should vary a little bit, but not by much. Again the law of averages works in our favour. ( When lenses and mirrors are made by machine, this variation has to be deliberately introduced or 'cyclic' effects will cause errors in the final surface figure.)

For the final part of our rough grinding we will grind the mirror for perhaps a couple of dozen or more 'wets' using this 'normal stroke'. Also we no longer apply weight to the mirror - we let the weight of the mirror apply the only pressure needed. Now we simply guide the mirror during our strokes. If the depth in the center starts to get too deep - we simply reverse the tool and mirror - and continue with the tool grinding on top of the mirror. This reduces the depth of curve in the mirror. It is also one of the reasons that we don't have to be too precise in attaining our depth exactly to a thousandth of an inch during rough grinding. As we start to use the finer grit sizes to smooth out the surface we can adjust the depth more exactly.

After perhaps another hour or so using the centered stroke we can check that our depth is close to what we want, and, if so we can call a end to the rough grinding process.

*Note - If the depth is more than the depth required, you can reduce the depth by doing a number of 'wets' with the tool on top. If it is not yet deep enough, you continue with the mirror on top.

At this point - if you have done everything right - you now have a rough-ground mirror of very nearly the correct depth of curvature. The surface figure should be almost perfectly spherical and you can look forward to smoothing it out and polishing it.

Now you have reached the most IMPORTANT part of the whole process:

The bench, mirror, tool and bucket (if used) and anything else that has come into contact with the #80 grit must be thoroughly cleaned. You need to wash your hands - including scrubbing your fingernails, to be sure that not even a single grain of the #80 grit remains. The cleats which held the tool on the bench should preferably be discarded - along with the sponge , and new cleats and a new sponge used for the next grit size (which will probably be # 120.)

This 'thorough clean-up' must be done when each session with any grit size is completed. If for example, you were to smooth out the mirror and get down to using a #400 grit size, and one single grain of 120 or 80 grit 'got into the works' you would wind up with a very obvious 'scratch' - which you will either have to live with, or go back two or three grit sizes and start over, to remove it.! In any case you would have to stop - and clean everything up again before you could continue. So you might as well do it the first time and avoid such disasters.


The grit sizes supplied with most kits are #80, #120, #220, #320, #500, then 12, or 15-micron, 5, or 8-micron. The last of these are often Aluminum Oxide rather than the Carborundun or Emery grits. The numbered grits are finer as the numbers increase and the micron sizes are finer as the micron-size decreases.

These should be kept in separate containers and , if stored together - should always be kept with the finer grits above the coarser ones if they are ever stacked one above the other.

SMOOTH GRINDING - or smoothing as we will call it from here on, is quite similar to the final stage of the rough-grind. Here again we do not use any more pressure other than the weight of the mirror disc or tool.The tool is again set on the desk with the mirror disc on top - a mix of water and the next size grit (usually the #120, is applied and using the 'Normal' W-stroke we do a series of 'wets' (for perhaps about an hour and a half. )
During this time do about 15 minutes with the mirror on top of the tool. Then, for the next 15 minutes - change positions of the discs - putting the mirror on the bench, and the tool on top. Continue to use the same stroke and keep interchanging the position of the mirror and tool at 15 minute intervals. This will ensure that the curvature remains the same. Then we clean off the mirror, dry it off and examine the surface carefully under a good light - with a hand magnifier if available, to see if the surface looks evenly pitted.

If there are a number of larger pits scattered around the surface, continue with more wets with the 120 grit, (or the next finer grit if making a smaller mirror - such as a 4 to 6 inch size, where you may start the grinding with 120 grit.) until the surface appears to be smooth and even, and the pits in the surface are all the same size. Pay special attention to the outer edge of the mirror - it seems always to be the last part to smooth out. Check that the pits are the same size all over and that the pits in the glass are all of the same, finer size than those of the previous grit that was used. ( You need to check this smoothness after each size of grit used. )

Once the surface is properly smoothed, take 'time-out' to check the center of curvature and the resulting focal length. This can be done a few different ways: Firstly if you have a sunny day (always the optimist) you can take it outside, and dip it into a bucket of water to wet the surface. Then you reflect the image of the Sun's disc onto a white card or similar 'screen' and adjust the distance to get the best focussed image of the sun. This will directly give you a measurement of the Focal-length! You simply have to measure the distance between the mirror and Sun's image on the screen. You probably need someone to hold the screen and do the measuring for you.

A 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 a very bright reflection which can damage one's eyes.

Another method is to take a good flashlight, and some type of white card to make a vertical screen. Set the flashlight and card next to each other on a table. Then, again using a bucket of water or a spray-bottle wet the mirror surface and from about the distance you expect the radius of curvature to be, try to reflect an image of the flashlight back on to the screen. Move back and forth to get the best focussed image on the screen, if necessary wetting the mirror again

Try to keep the flashlight at the edge of the card and focus the image as close to that edge as possible, so that the distance between the reflected image at the edge of the card is within a couple of inches only to the side of the flashlight.You are looking for an image of the front end of the flashlight., which should be placed with its face level with the card. The distance between the card and the mirror will be the measure of the Radius of Curvature, and half that distance will be the Focal-length.

If you are not close enough to the required radius, you can then continue using the 120 grit and either reduce the curve (using the tool on top) or you can deepen the curve (using the mirror on top) depending on whether your focus measured too long, or too short.

This 'optical' way of measuring should allow you to attain the desired focus to within an inch or so. In any case it is more accurate than any mechanical means of measuring the depth of curve of the mirror disc. The focus can still be 'trimmed ' even during the finer stages of fine-grinding if you want to be within a fraction of an inch of the true 64 inch focus which this particular mirror is supposed to have.

If you are happy with the measured radius. You can now repeat the 'cleanup' process again. Remove the mirror and tool, the cleats and throw away the new sponge which you used for the 120 grit. This will be a standard procedure every time you finish with one size of grit and start on the next finer size.

Now you are ready to start again - but this time with the next finer grit size (220) You will continue this way, until you finally reach the finest grit size. When you finish with the 220 size grit, re-check your focus and use it with either the tool on top (to increase the focal-length) or with the mirror on top (to reduce the focal length) After this step - you continue with the finer grits - changing the position of the tool and mirror every fifteen minutes or so, during all the next finer grit sizes to preserve the curvature of the mirror's surface.

The time required to remove the pits left by the previous grit will vary, usually becoming less time needed as the grit sizes decrease. For the 220 grit you may need less than a couple of hours work, and as the grit size reduces the time spent will become even less. The best guide though - is to examine the surface very carefully - to be sure that all the deeper pits from the previous grit are ground out. It is better to do a little too much than not enough. Above all else - be sure to really clean up perfectly after each grit session is completed, before starting the next one.

When you start to use the finer grit sizes - such as the 500, and especially the 12 micron or 5 micron, you must mix the grit and water carefully and make sure that the mix is evenly spread and does not contain 'clumps' of powder which can act as a much larger size grit and produce 'scratches' After applying and spreading the mix, place the tool or mirror in position and move it around carefully, this should 'spread' the mix evenly before you continue with the normal stroke.You should not hear any 'scratchy' sound nor should you feel the discs trying to bind together.

Once you have finished with the finest grit size, and are sure that the surface is evenly smooth over the mirror - you have reached the point at which the next step is POLISHING - "Oh Boy !! "

And - by the way - CONGRATULATIONS !

POLISHING the Mirror:

To polish the mirror - we must build a new type of tool - using the existing tool-disc. But now the tool has to be covered with a number of pitch squares, evenly distributed over its surface. The pitch squares need to be placed slightly off-center, across both diameters of the tool. If they are precisely centered they can lead to 'periodic' patterns on the final polished surface. Between Each row of pitch squares is a channel - about 3/16" wide to allow water and polish to circulate, and to allow for spreading when pressed on the mirror.

There are many ways of making the lap - One way is to use a specially designed 'mat' obtainable from kit makers, this is placed on the tool and the pitch poured over the surface. When it has set, the mat is peeled off leaving a series of channels between the squares. In some designs the 'squares' are either circular or perhaps hexagonal. In other cases a layer of pitch is poured, allowed to set, and then channels are cut very carefully (with an old saw)

The consistency of the pitch is important. Pitch normally supplied is quite hard and extremely brittle - I usually mix in a little linseed oil (a small bottle from artists supplies can last for a long time) I use perhaps about 5 milliliters for each pound of pitch, then also I add about half a cubic inch of beeswax (about 1/2 ounce.) These two together soften the pitch and also make it less brittle. Before making up a batch of strips - I pour a small amount into a mold - about 1" by 2" and about 1/4" deep. Allow it to set and cool to room temperature. It snaps in two when bent quickly, but will bend if gentle pressure is slowly applied with both hands. The pitch should not be too soft, but must be soft enough to 'flow' when polishing. See additional notes below

Our preference has always been to apply the squares individually. We cast a number of pitch strips - about 3/4" wide and perhaps 8 or 10 inches long. This is done by making a wooden mold, with thin wood strips, about 3/8" thick, to form a mold with 6 or 8 long channels into which the molten pitch is poured. The wood strips and the base of the mold are first given a good coat of varnish, and on the top of the base a layer of aluminum foil is placed. The varnish prevents the pitch from 'glueing' itself to the base and the varnished strips are lightly nailed on top with the nails sticking up so they can easily be removed. When set and completely cooled the wood strips are removed, and then the strips of pitch are cut into squares. This will provide about 70 or more squares of pitch 3/4" square and about 1/4" thick. This is done starting with the outer strips, by twisting them sharply after the nails are removed, and the pitch is quite cold.

If need be we put the mold in the fridge to really cool it. The strips of pitch can be peeled off the foil- covered base plate quite easily. The finished strips are cut using a hot knife. With the right amount of heating the knife will cut 5 or 6 squares, before having to re-heat it. The resulting squares are stuck to the tool's surface. Before doing this a very light thin layer of pure turpentine is applied to its surface. This ensures good adhesion of the squares. The squares are heated with a match or small torch until the bottom side is just beginning to melt - and then quickly applied to the tool's surface.

As shown in the diagram above the columns of squares should be applied so they are off-set slightly from the center of the tool disc. A pattern can be drawn on the tool first with a pencil to act as a guide.

The pitch usually supplied is quite hard, and may need softening by adding a little bit of turpentine, and possibly a small amount of beeswax, the wax is usually supplied as part of the kit. Some suppliers will provide an already softened pitch. You do not want the pitch to be too soft, it should appear hard - but strong pressure for 20 or 30 seconds with your finger should make a definite impression on a square. You can pour a bit of your melted pitch - before you fill the mold, and after it is completely cooled - a few hours later, test it and adjust your mix if you think it is too hard - or too soft.

Once the pitch squares are cut and applied to the tool's surface - the outer squares must be trimmed off. (You can do this with a chisel or an old knife) A sharp tap with a light hammer or even your hand will shear off the pitch squares efficiently. Trim them to just inside the edge of the disc.

A few additional notes:

The pitch should be heated slowly and not made too hot or it will lose some of the natural solvents it contains. A fairly thick creamy consistency is best - fluid enough to pour easily but not too 'runny' Be sure to have the mold ready before the pitch is melted and ready to pour. The sooner it is poured and cooled the less solvents will be lost.

The last time I made a pitch lap - I cast a 'block' of pitch, This was made a little softer than the formula above (I had a deeper mirror to polish) I added about 1 ounce of beeswax and about 8 to 9 milliliters of linseed oil to 1 lb. of pitch. The 'block' I cast was about 10" long by 4-1/2" wide and 5/16" thick. This was heated in a tub of water at 95 degrees F. for about 5 minutes. It was then possible to cut 3 or 4 strips 3/4" wide x 4-1/2" across before re-warming the block again to cut more strips.

Afterwards the strips were cut into squares (3/4" x 3/4") in both cases I did this with a cold knife, without the pitch breaking. This seemed to me to save a lot of time. The squares were then fixed to the tool in the normal way: before applying the squares the tool was coated with a very thin layer of turpentine, then the squares were heated, one by one, with a candle flame untill the bottom of the square was 'wet' and then they were applied to the tool and gently pressed to ensure good adhesion.

Following this the whole tool complete with the new pitch squares in place was heated in the 95 deg. water for a few minutes. Then with a layer of Aluminum foil placed on top of the mirror, the warmed tool (or lap) was pressed firmly on top until the squares were all evenly pressed to the shape of the mirror - this needs to be done carefully - so that the squares don't get compressed too much and run together. It is better to do it two or three times - applying more gentle pressure each time until they appear properly pressed. Wetting the back of the tool also allows one to see the squares underneath, so that the degree of pressing of the squares can be seen.

When you are sure that things are o.k. lift off the tool and gently peel off the Aluminum foil. The pattern on the foil may also give a good indication that the tool is correctly pressed. After this 'cold pressing' can be done. (see below)

Before the next step - a word about polish.In most mirror-kits the polish supplied is Cerium Oxide. This is a very fine pinkish colored powder which polishes faster than the Optical Rouge which was used in the past.

Rouge although the best of all polishes is extremely messy and stains everything it contacts. Very few people use it today. An alternative, which is somewhere between the polishing speed of rouge and Cerium Oxide is a product called Zirconium Oxide. It is available from many suppliers of kits and is a clean white powder - and is the polish we recommend.

When the lap is finished it needs to be 'pressed' against the mirror. To do this - take a quantity of polish and water and apply it to the mirror's surface. Then the lap is placed and on top, and swished around to ensure that the polish and water is covering both surfaces of the mirror and tool. Now center the two discs and apply a weight - about 10 to 15 lbs for an 8" mirror.

Allow them to press for maybe 20 minutes to perhaps an hour or more, depending on the hardness of the pitch. During this time you must check that the polish does not dry out - this is very important - or they may stick together as if you had glued them. Every ten or fifteen minutes you must remove the weight and move the mirror around to ensure that it has not stuck. If necessary - add more water and polish. Then continue the pressing. Sometimes the mirror disc or the lap may need warming if the pitch is too hard. This can be done by immersing one or the other first into tepid water and carefully stirring in hotter water until the temperature is about 90 to 95 degrees F.

Once the pressing is complete, check that the squares have not closed together too much and that the channels between them are clear and at least an1/8th inch wide. If the edge squares have pressed out and overlap the edge of the tool, cut them back again so that they do not overhang the edge. Press the mirror again for ten minutes after this - and then you can start to polish.

Note : During the polishing - if you are not able to do at least an hour of polishing continuously, don't start! You will do better to lay the tool aside, and come back to it when you do have the time.

NOW FOR THE BAD NEWS: To completely polish an 8" mirror requires some 12 to 15 hours of polishing time! If possible you need to be able to do 2 or 3 hours work at a time. During polishing, as the tool and mirror reach a 'working temperature' and bed down into complete contact with each other, the polishing becomes much more effective and the whole surface of the mirror polishes most evenly. So never do less than at least an hour at a time. Here we are talking of the actual polishing-time on the mirror, not counting the initial 20 minutes to half hour needed to again press the discs together before starting, nor the time spent cleaning up afterwards.

When you finish a session of polishing - simply clean off the mirror and put it aside. Wipe the tool (lap) gently and set it aside separately also. Never leave them together on the bench, one on top of the other - or again they may stick together as if you'd glued them. This is also the reason that you must press them again before starting another session. The pitch, seemingly solid, is really an extremely viscous fluid! and will gradually settle when left face-up on the bench between sessions. If you are a few days between each session of polishing, you may have to press them for at least a half-hour. You will be able to tell, when you start polishing - if they slip or tend to stick - you may need to press some more

At the start of polishing - after the discs have been pressed - you apply a mix of water and polish, not much polish is needed, and enough water to make a thin 'milky' mix spread across the surface of the tool or mirror. Polishing - like all the previous 'wets' is done with the NORMAL "W" stroke, but, unlike the 'wets' used in smoothing, this is a continuous process. As the polish starts to dry out - (after perhaps 3 to 5 minutes) you simply add a little more water and polish, swish the discs around and then continue polishing. You still need to rotate the discs about a quarter-turn every 15 or 20 minutes to ensure that the mirror is evenly polished. Apart from that the work is a continuous polishing, stopping now and then to add more polish and water, and to turn the discs.

About every hour of polishing - you can interchange the mirror and tool positions - this helps them polish evenly from the center to the edge. This is similar to the effect of deepening or lessening the depth during the grinding process. It has little or no effect on the curvature - but does allow a more even polish.

At the start of polishing, you may feel that the mirror (assuming that you start with the mirror on top) feels 'sticky' and not very smooth. If this happens, try pressing again for another 15 minutes and try again. If it still feels 'sticky' then persist for a few minutes, stroking fairly slowly, and applying a bit of pressure on the center of the mirror until they begin to feel smoother. Then continue without pressure letting the mirror's weight and the weight of your hands be the only force applied between tool and mirror. Also try to move the mirror in its w-stroke without letting your fingers hang over the edge of the mirror. Believe it or not! this can cause local areas of the mirror disc to become heated and 'expand' enough to cause local wear around the outer edge of the mirror during polishing. (Remember that when finished the surface of the mirror should be accurately ground and polished to within a few millionths of an inch - or better!)

Don't skimp on the polishing - even after a few hour's work the mirror will start to look polished - but don't go by first appearances. It takes a lot of hours to properly polish the mirror, and you cannot really polish it too much.

After about an hour of polishing, take a look at the pitch squares on the tool. If they are beginning to flatten out and spread a little, this is a good indication that the pitch is about the right hardness. If they are flattening out too much then you must trim them to ensure that the channels between them do not disappear. If they have not deformed at all, your pitch may be too hard. They should also appear evenly impregnated with the polish - a sign that they are making the proper contact with the mirror's surface. If some squares are shiny and don't have any sign of polish, then you should press the tool again.

If things get too messy during polishing, you can rinse off the 'muddy' mix of polish and water, from both the tool and the mirror. Just rinse - don't wipe them with anything - let them dry out a bit. Re-press for a few minutes and then continue.

When completely polished you won't be able to see any surface characteristics even under a strong magnifier ( It is possible that you may see a number of 'scattered' pits around the surface, probably left over from the fine-grinding stages. However, if there are not too many of them, they will make very little difference to the performance, and they cannot be removed by polishing.

A final word about accuracy - The median wavelength of visible light is about 22 millionths of an inch. Thus a quarter-wavelength, which is given as the required accuracy for the images reflected from a good mirror - is a mere 5.5 millionths of an inch. However, the surface of the mirror has to be better than that by a factor of two.

This is because a ray of light reflected from a mirror is reflected at the same 'angle-of-incidence' but in the opposite direction to its approach angle. Therfore any errors on the mirror's surface will double upon reflection. So now the required accuracy for a 'good' mirror means that it must be accurate to a little under 3 millionths of an inch ! or - to an eighth wavelength.

Don't let this scare you ! If you have done everything right , the grinding and smoothing and polishing stroke tends to form a mirror with this kind of surface accuracy naturally. Surprising that a hand-process can produce such a result !!! But the most accurately made mirrors are all hand-finished ! See my comment below about testing.

Once you are sure that you have done enough hours of polishing ( or done so many that you used up all the polish supplied with the kit- which is probably enough to do more than one, and maybe two!) then you can give yourself a good pat on the back, sit back awhile and savour the feeling of accomplishment!!

Now that you've celebrated a while - and are getting the urge to have it coated and put it into a Telescope - !! A bit of testing is necessary to ensure that it really did come out right!! Recall that at the start of this text I mentioned that a 'pefect mirror' should have a 'parabolic' cross-section. So some testing is needed to ensure that you have ground and polished your mirror accurately and to something very close to the right shape.

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