Polishing and Figuring
Thin Telescope Mirrors
Abridged from an article appearing in Telescope Making #12
Provided: Courtesy of Astronomy Magazine
Article's Author: Bob Kestner
Part I - Grinding
For many years, amateur astronomers and telescope makers thought it
essential that glass for telescope primary mirrors be at least one-sixth as thick as its
diameter. In the last decade, however, it has become increasingly common to find
telescopes with primary mirrors very much thinner than the standard 6 to 1 ratio giving
excellent optical performance. In fact, the trend has now gone so far that it is unusual
to hear of an amateur telescope project much over 12" aperture that has a standard
Why has this happened? Is the 6 to 1 ratio a myth perpetrated by opticians of a past
generation to obstruct the building of large amateur telescopes? Is the 6 to 1 ratio
somehow wrong? The answer is no, it is not wrong- it has just been misunderstood. In the
optical industry, 6 to 1 is a good compromise between making the glass so thick that
flexure can almost be ignored, and making the glass thin but spending much more time and
money seeing to it that the glass does not flex and will hold its figure in use.
The avid and somewhat uncritical acceptance of standard - thickness glass in the early
years of telescope making - plus some notable failures with thinner glass - gave the
general impression that telescope primaries much thinner than 6 to 1 were unmanageable.
They are not, but without using the proper techniques, they can be extraordinarily
difficult to figure.
However, with the right techniques, thin mirrors aren't much more difficult to make than
ordinary mirrors, and for the purpose of large amateur telescopes, thin primaries are
advantageous. They are less expensive than thick primaries, are lighter and easier to lift
(more important than one might think at first), add less mass to the telescope, and
equilibrate to temperature changes more rapidly.
The purpose of this article is not to teach you how to make a telescope
mirror if you have never done it before. If you have made mirrors before, it can guide you
in adapting the techniques you already know to the problems peculiar to big, thin mirrors.
But I offer this warning: if you have never made a mirror before, don't start with a
large, thin mirror. It's a bigger job than you realize.
There are many different ways of grinding mirrors. The methods I describe are those I use
when working at home, grinding and polishing entirely by hand. Where I refer to machine
working, it's for your information, and not essential to making a successful mirror by
The techniques described in this article are for mirrors of 16" and larger. Again, I
consider it essential that anyone attempting a mirror of this size on his own must have
made at least 2 mirrors successfully before, unless he or she possesses an extraordinary
aptitude of optics.
Obtaining glass is often a problem. As of this writing, there are no companies supplying
thin mirror blanks to amateurs. Until somebody recognizes that there is money to be made
supplying the glass, telescope makers will be forced to buy rough cut glass from glass
companies not used to selling small quantities over the counter.
It will take some tact and understanding on your part to deal with these places, to
outline what you do need, as well as what you don't need from them. Remember, if you tell
them what it's for (people love telescopes-promise them a look through it), many
businesses will bend over backward to help you. But don't expect them to - they'll
probably lose money on you.
There are still places where you can obtain plate glass portholes. Try surplus stores in
coastal cities. Many 16" and 18" diameter portholes 1" thick are still out
there, ranging in price from $10 to $150 (1981). A reasonable price is whatever you're
willing to pay. The problem is finding one.
Pyrex Sheet Glass
Pyrex sheet glass is available from those glass companies not used to selling to amateurs.
Corning makes Pyrex in many different thicknesses, the thickest now available is
1.625" (Editorial Note:Corning now produces sheets in two thicker forms, i.e.,
1.875" and 2.250"). They sell it to glass companies (Corning distributors) in
large square sheets.
Most companies sell coarse annealed and fine annealed. I recommend fine annealed for
telescopes. I don't know if coarse annealed would suffice because I've never tried it,
It's probably a hit-or-miss thing - most will work and some may fail.
The prices for sheet glass vary quite a bit. The last time I checked (late 1980), pieces
16" diameter by 1.62" thick were about $225, and 24" diameter by 1.62"
thick were about $500.
When you order from a company, specify the diameter(rough round) and thickness. They'll
cut out a square piece on a saw that will yield your diameter. Then they cut off the
corners several times until the piece is round. What you'll get is a piece of glass blank
with 16 to 60 sides with the surfaces only roughly flat - to less than 1/8". Be sure
to insist they cut lots of sides - you don't want just 8.
Next you'll have to contrive a way to grind the blank round and flat. Grinding it round is
not very difficult, especially if your piece has enough sides - it's just a matter of
grinding off the high spots. I use a hand-size piece of tile and #80 grit. Grinding by
hand against the outside of the glass, I've made a 16" with 30 sides presentable in 2
or 3 hours.
The surfaces are not as easy. You must grind both flat - especially the back. This can
take 20 or 30 hours per surface on a 16" if you're working by hand, but the grinding
time may be much less depending on surface quality.
If you're lucky, you may find a company willing to diamond-generate your blank. They can
Blanchard the back flat, edge it round, and generate the radius. This work on a 16"
will run $100 to $200. The problem is finding a company willing to do it. Work of this
sort is usually done at your risk - if your glass breaks, you've lost the blank. Although
glass breaking is uncommon, chips occur more frequently. All these problems may seem
insurmountable, but TM is constantly publishing information about suppliers, so some firms
dealing with amateurs may eventually turn up. Until then, you will have to use your own
resources. Someone possessing the ambition to make a large telescope mirror can probably
overcome these supply problems with a few dozen phone calls and letters. When you succeed,
let TM know about your results so others can gain from your experience.
Thin Pyrex versus Thin Portholes
I was going to skip this section and leave it to your discretion, but the subject can't be
avoided. It's paradoxical that something inferior (i.e., porthole glass) works as well as
something better (i.e., Pyrex), given reasonable circumstances.
Plate glass has a three times greater coefficient of thermal expansion than does Pyrex. It
should, therefore, be worse for telescope mirrors than Pyrex. Yet, when a plate glass
mirror is used in a solid insulated tube with a closed back (such as a big Dobsonian),
after initial equilibration in the evening, temperature changes in the mirror are quite
slow. The telescopes that I've observed with most have plate glass mirrors mounted in
closed-back tubes, and I can testify that the figure change after equilibration during the
night is quite small.
If the back of the mirror is directly exposed to the air, temperature changes in the air
affect the mirror more directly. At times I've been annoyed with the changing figure of a
plate glass mirror under these conditions.
A crucially important fact, however, is that portholes come round and 99% of them have
surfaces flat enough to start grinding the curve right off. With Pyrex, you must start by
making the surface flat, which is a lot of work.
Now the bad news! Plate glass is more difficult to figure than Pyrex, and sometimes the
strain in portholes is not negligible. Figuring plate glass is more time-consuming and
more tedious than figuring Pyrex. After you attack a plate glass mirror with a warm pitch
lap, it's several hours before you can tell anything about the progress you've made
figuring. That means when you're doing the final figuring, you'll wait three hours before
you can test the mirror to see what needs to be done next. Considering the time it takes
to get going again, it can take all day to work the mirror twice. If you plan on making a
porthole mirror, don't let this stop you: just recognize it is plate glass, and allow for
Well, there it is. Pyrex is easier to manage than plate, but it takes much more work to
get a sheet glass blank ready to grind. The choice is yours. These days, I work only
Pyrex, but I can afford to discriminate. I can buy it easily and get all the starting work
done on diamond-grinding machines - something most of you don't have access to. I've also
figured a half-dozen plate mirrors larger than 16" and wouldn't trade the experience
for a million dollars - not to mention the thousands of hours of excellent observing these
mirrors have given in return.
The thickness you choose, if you have a choice, is determined by the
diameter of the mirror and how you plan to mount the mirror in the telescope. For mirrors
under 19", I would not use glass under 1" thick; and if you're buying Pyrex, I'd
use a 1.62" sheet.
For mirrors in the 20" to 25" range, I'd strongly recommend 1.62" as a
minimum thickness, especially if the mount is not going to be mounted in a sling.
Last summer a friend and I made a 25.5" mirror on a 1.37" thick Pyrex mirror.
The curve, f/6, was relatively shallow. We paid strict attention at every step of the way
to prevent flexure, especially when testing the mirror in a vertical position. I remember
wishing that we had had that extra .25" in thickness. On the other hand, when it
turned out well, it seemed good to have a 25" mirror that could be carried with
The 1.62" is just a recommendation on my part, since it allows you a little room for
error especially in a 16" or 18" mirror. For mirrors around 30" in
diameter, working by hand, 1.62" is the minimum thickness that I would consider. If
you plan on working your mirror by machine, you'll run into problems that will increase
the minimum thickness needed even more.
Choosing the Diameter
I recommend that you start your big, thin mirror career with a 16". By keeping your
ambitions relatively modest, your chances of success are much higher than with mirrors
over 20". Even though my current efforts are going into a telescope twice as big as a
16", my observing friends and I agree that we could live the rest of our lives
happily observing with a 16" without the slightest regret.
In choosing a blank size, you should think of the outer 1/4" of your mirror as lost
to a turned edge. You won't be alone in this - most big mirrors have a clear aperture at
least 1/2" less than the blank diameter. If you find a 16.5" porthole, think of
it as a finished 16" mirror. If you're buying sheet glass, add 1/2" to the
diameter of the finished mirror you want.
Choosing the f/ratio
After the aperture, you must choose the f/ratio. The most important factor
from the standpoint of an optician is that a longer focus mirror is easier to make, since
it departs less from a sphere. From the observer's standpoint, a long focal length makes a
big telescope too big! Unless you have an army of people to help you set it up, you should
not let the focal length get out of hand. Don't minimize the problems of simply using a
telescope longer than ten or twelve feet - you'll spend a lot more time clambering up and
down the ladder than you ever dreamed.
At the lower bound of useful focal ratios, coma and eyepiece aberrations limit your
mirror's performance. Furthermore, short focus mirrors - are very difficult to figure, and
as a further practical consideration, remove valuable thickness from your glass.
My recommendation is to play fairly conservative, and choose a focal ratio between f/5 and
f/6 depending on your circumstances and desires. For most observers, f/5 may be the best
In addition to finding a blank for the mirror, you will need another disk
for a grinding tool. It may be either a solid glass tool or a plaster tool covered with
hard ceramic tiles - once again, your taste, energy, and previous experience will dictate
what you want to try.
The tool need not be as large as the mirror. You can use a tool 75% of the mirror diameter
without much difficulty. Tools smaller than 75% have less tendency to produce a spherical
curve during fine grinding. Although a 16" tool works well with a 16" mirror,
for larger mirrors I would recommend a smaller tool simply because full size tools are
heavy and awkward.
Usually, what is available dictates what tool is used. Take care, however, not to use a
tool that's too thin. If the tool is too thin, it will bend while you are grinding,
presenting two problems. First, astigmatism in the mirror will not completely grind out
because the tool will conform to the astigmatic contour. Second, the weight of your hands
on the back of the tool will force the center of the tool to grind harder on the center of
the mirror, generating a curve deeper (i.e., more parabolic) than spherical curve.
Usually, this is not a severe problem, but you'll find the mirror requires extra polishing
to polish the center.
A 16" tool with an edge 1/2" thick is approaching a thickness where troubles
start. A friend of mine recently used a 16.5" glass tool slightly thicker than this
safely. However, I once used a 20" glass tool 5/8" thick at the edge, and got a
bad case of astigmatism. We ended up pitch blocking a 16" diameter porthole 3/4"
thick to the back of it, and then it worked well. While 3/4" would be a reasonable
minimum for a tool this size, with either plaster or aluminum tools, a thicker minimum
should be chosen.
There are two main types of grinding tools - solid and segmented. Solid tools are usually
glass and segmented tools are usually plaster faced with ceramic tiles.
A solid glass tool can be a disk cut from sheet glass, as you are used to, or can be made
by laminating thin plate glass disks together. You can buy circular plate glass from
retail window glass companies. You may have trouble finding a piece thick enough to serve
as your tool face for rough grinding - you do not want to grind through it into the next
piece. Although it has been done, it increases the chances of scratching in fine grinding.
For grinding the flat surface on the back of the blank, just glue on a new piece of glass
if you grind through the first. Aquarium cement is readily available and sticks to glass.
For mirrors with the curve already generated, or for those wishing to try
a segmented tool, a ceramic tile grinder is the answer. Tile tools are made by blocking
ceramic tiles onto a support. Such tools can be made flat or curved to fit a generated
blank. The tile support can be plaster, aluminum or glass. Epoxy or hard pitch is used to
glue the tiles into place. The greatest problem with pitch is that the tiles may fall off
in rough grinding.
A plaster support is made by wrapping a metal or cardboard dam around the mirror a couple
of inches high. Smear soap over the surface of the glass to keep the plaster from
sticking, then pour on the plaster. If the mirror has a curve, this method will give a
mating curve on the plaster.
I highly recommend Kerr Dental plaster, Vel-Mix Stone Pink. This stuff hardens like stone!
(Call Kerr Co. at 313-946-7800 and ask them for the name of a distributor in your area.
They have 400 distributors nationwide. It costs $12 for 25 pounds). Add water to it and
stir until it is slightly thicker than cream.
Almost any glazed ceramic tile will do for the grinding surface. They should be around
1" to 2" square or round. Attach them with epoxy. Epoxy can be bought in cans at
the hardware stores. It helps if you can buy an epoxy softening agent, making the epoxy
To epoxy the tiles to the base, start by protecting the mirror surface. Smear a release
agent on your mirror so that excess epoxy will not stick. (Buy a release agent with the
epoxy - but I suspect grease or soap would work, too.) Place the tiles in the desired
pattern in mirror. Space them no less than 1/4" apart. Pick them up one by one, and
spread a thick coat of epoxy on them. Set them back in place epoxy side up. It is a good
idea to get a friend to help you, because you must coat the last tiles with epoxy before
the first ones set. After they're all coated, carefully set the support onto the tiles and
let it harden.
You can see that it is important that the curve of the tool match that if the mirror, and
also that the tiles are uniform in thickness. If you are using plaster, be sure it is
sealed well. It would probably be a good idea to coat the face of the plaster with a thin
layer of epoxy to help the tiles stick.
This is a very general discussion of making tools. Experimentation with the materials and
use of your smarts to keep you out of trouble.
My favorite method for working mirrors by hand is on top of a 55 gallon
drum with 300 pounds of sandbags in it. You can buy drums at salvage yards, and sandbags
can be brought at nurseries.
One reasonable substitute for a barrel is a sturdy counter - remember, you don't have to
walk around your glass. Another is so obvious it's usually overlooked: I used to grind and
polish on my knees with the mirror on the floor. This proved successful, but the trouble
is learning to walk again after 1/2 hour of polishing.
It is ideal to have two working area --one for grinding and another one for fine grinding
and polishing. For reasons of cleanliness and temperature uniformity, polishing is better
done indoors. Grinding with abrasives larger than 30 micron is a messy and, depending on
your climate, may be better done indoors. Grinding with abrasives larger than 30 microns
is messy and, depending on your climate, may be better done outdoors.
Grinding the Back Flat
The first step in generating the optical surface is to grind both front
and back flat. When I say "flat", I mean "regular" more than flat. The
back can be a few thousandths of an inch convex or (preferably) concave, but it must be
free from astigmatism, what opticians call "cylinder". But flat is best. Even if
the back has been Blanchard ground, you will still need to grind it smooth with 220.
Grinding the flats can be done with a glass or tile grinder. The best way to do it is to
grind two blanks, one with the other. This way you get two flattened for the work of one.
The front, that is the side the curve is to be ground on, needs to be regular to a much
lesser degree. However, if grossly high and low areas are present near the edge, by the
time you grind in your curve, you're still likely to be missing contact near the edge, and
it will cost you a lot of work to bring the surface down to meet them. Remember: On the
face side, you're concerned with low areas near the edge that might be left unground after
the curve is ground.
To grind, place the mirror face up on something soft like an old piece of deep shag carpet
so the glass won't rock. (The carpet will ruined.) Wet it down, add the appropriate amount
of #60 abrasive, add a few pounds to the top of the tool if you wish, and grind like mad.
When it stops making lots of noise, splash it off and start it again.
After three or four wets, wash it off and look at the surface of the mirror. The high
spots will be ground and the low spots will still look unground. This will give you some
idea of what you're up against. Look at your tool as well and look how it started. Keep
track of the convexity and concavity of both surfaces with a straightedge.
If your mirror starts going convex, concentrate grinding on the center with short strokes.
If the convexity persists, grind with the mirror on top for a while. It's not a difficult
thing to manage. John Dobson says, "Rough grinding is a caveman job; do it like a
caveman. Eat well, sleep well, and work like hell."
A few isolated low spots on the back can be tolerated it they are small. What absolutely
cannot be tolerated is a cylindrical curve on the back, or a curve in one axis and a flat
in the other. This will cause the mirror to bend during grinding and polishing, and you
will happily polish away until you discover that the mirror has astigmatism. When you're
satisfied with the flatness of the back, fine grind it through 120 and 220. When grinding
with the fine abrasives, low areas will have coarser pits on them and you'll have no
trouble seeing them. If the back has been Blanchard ground, it should not take long to
grind it flat with 220, just a warning; Blanchard grinding may give a perfectly plane
surface - but grind it for planeness anyway. You're better safe than sorry.
Grinding the back flat is just the first step in preventing astigmatism.
The next step is the proper mirror support. Fine grind and polish the mirror face up. A
thin mirror bends over the edge of the tool if it's worked on top, so the problem reduces
to supporting the mirror from its underside. The best way I know is to support it on a
piece of deep shag carpet. The carpet will support the mirror and will allow you to rotate
the mirror with no trouble. The carpet goes between your mirror and your work surface
which, by the way, must also be flat. (Plywood or particle board is flat enough.)
The key to the success of the techniques is to rotate the mirror on the carpet frequently
while working. This prevents non-uniformities in the support working into the mirror and
showing up as astigmatism. Every time you complete one turn around the barrel, then rotate
the mirror almost constantly in the direction opposite the direction you rotate the tool.
As simple as it sounds, this method is quite effective.
For machine working, it's not so easy because the mirror usually can't be rotated with
respect to its support. This is why you don't want to make mirrors to be worked on
machines too thin. For 16" to 18" mirrors, 1.62" thick is the thinness I
There are several ways to support the mirror on a grinding and polishing machine
turntable, but first make sure the turntable itself is rigid. It must not bend. If your
machine lacks a sturdy turntable, one way to make one is to cast it out of Kerr plaster
4" to 5" thick.
One way to support the mirror is to block the mirror right to the base
with pitch. This is somewhat risky because the pitch can deform the mirror. A more
manageable way is to pour pitch on the base 3/8" thick. Groove it like a pitch lap
with grooves about 1/2" wide. Cover it with a single pieces of paper, and set the
mirror on the paper and tape it down. Be careful not to tape it too tight. Let the mirror
sit on this lap for 24 hours. The back of the mirror should press the pitch to its exact
shape. Each time you take the mirror off for testing, put it back in the same orientation
and let it sit again for a few hours. You must not let the grooves in the lap under the
mirror press together. When they start closing in, regroove them. Although this method is
not without its troubles, it is widely used in the optical industry.
Another method is an 18-point flotation system. I've never used this
method for support while working a mirror, but there are those who swear by it. The
problem seems to be preventing the mirror from rocking under the lateral forces of
grinding and polishing.
Once you have the back of the mirror ground flat through 220 and the face
reasonably flat, you're ready to start roughing the curve into the mirror. I recommend #60
carborundum for rough grinding. By the way, as in all optical work, be sure that you
always maintain bevel on the edge of the mirror throughout grinding.
If your mirror is light enough to grind on top, grind much the same way
you would a smaller mirror. Your goal is to grind the curve from the middle out, timing it
so that you reach the desired focal length at about the time the curve reaches the edge of
Start grinding by concentrating the center of the mirror on the edge of
the tool. Use a long "W" stroke, occasionally stroking the center of the mirror
out to 2" to 3" from the edge of the tool, progressing around the circumference
of the tool rotating the mirror.
For mirrors ground face up, a small tool is desirable - 75% of the mirror
diameter or smaller. Rough grind the curve by concentrating on the center.
As grinding progresses, you must monitor the focal length of your curve.
If the Sun is available, splash water on the mirror and focus its image on a piece of
cardboard and measure the distance to it.
Alternatively, you can keep track of the sagitta of the curve. Calculate
the sagitta of your curve. Find something that can act as a gauge is a drill bit). Check
the progress of rough grinding by slipping the gauge under a straightedge placed across
Rough grinding usually produces a grossly hyperbolic curve. Toward the end
of rough grinding, start using shorter strokes to produce a smoother, more spherical
Shorter strokes also move the curve toward the edge.
It is not difficult to manage grinding so that the curve meets the edge at
about the same time it reaches the desired focal length with a reasonably spherical form.
Placing the mirror face up and grinding with a moderate "W"
stroke will also move the curve toward the edge, and tends to produce a more spherical
surface. Leaving the mirror grossly hyperbolic will make fine grinding difficult.
We've all heard stories about fine grinding taking hundreds of hours. If
done correctly, however, fine grinding takes a fraction of this time, and in fact, is one
of the more manageable jobs in making a large mirror. Depending on your circumstances,
about 2 hours of continuous work at each grade is usually sufficient.
Fine grinding rather naturally divides into two stages: before #220 and
after #220. Before 220, you're getting the radius and a smooth curve. After 220, you'll
need to pay attention to preventing astigmatism. I also switch from my rough grinding area
to my polishing area at 220.
The fine grinding compound you start with depends on the condition of your
mirror and fit of your tool. If you rough ground your mirror with 60 grit, you should
start fine grinding with 120 grit before going on to 220. If you have been fortunate
enough to have the curve generated, you can start with 220 providing your tool has a
reasonable fit to your curve. If you are working with a machine, you can use 30 micron
For abrasives smaller than 120, I prefer aluminum oxide to carborundum
because it has less tendency to cause large pits in the fine-ground surface. There are
several sequences of fine abrasives you can take, but as long as you are careful to have
completely ground away the last abrasive pits before going on, you'll be all-right.
More than anything else, your sequence depends on the quality of the
abrasive you use. Most abrasives you buy have a few larger abrasive particles in them. For
example, although #320 is mostly 320, there will be some grains closer to 220 size in it,
plus some much smaller stuff. There is nothing wrong with this - but you must compensate
for it by not taking a big step between abrasive sizes.
On the other hand, if you can get some really high quality abrasive like
Microgrit(, (made by Micro Abrasive Corp) you're in luck. 30 micron Microgrit( really is
30 micron. There are no 32 micron grains and no 28 micron grains - just 30. As a result,
you can spread out the size you use. For example, I use #220 followed by 30 micron, 12
micron, and 3 micron.
When using more conventional abrasives, such as those available from
Edmund, the sequence should be 220, 320, 400, 600, E305. There is not too much difference
- but the 3 micron is finer than the E305.
It is difficult for me to even guess how much abrasive you'll need. I've
never kept track of it. Depending how you happen to use it, the amount can be quite small
or large. If you clean the glass after each wet and start by sprinkling new abrasive on
the wet glass, you may use as little as a tablespoon of the finer line abrasives for each
grade. If you use the abrasive may not be enough. For 120 and 220 abrasives, the amount
varies mostly with the amount of work you need to do. On the average, I've found 2 to 3
cups of 120 and 1 cup of 220 will do for me.
Getting a Sphere
Even the most careful finishing up job with 60 grit will leave the curve
somewhat hyperbolic. After 60 grit, your main interest, besides removing the pits, is
getting the mirror spherical. At this point, the support of the mirror is not at all
critical, but using a piece of carpet under the mirror will allow you to rotate it easily.
Start by sprinkling the abrasive on the face of the wet mirror, as you did
with smaller mirrors - not too much and not too little. Place the tool down edge first,
and start grinding. Walk around the barrel, rotating the tool as you go, and also rotating
the mirror every time around.
A solid tool traps abrasive in the center of the mirror and resists
grinding the mirror spherical. When you first start a new and smaller abrasive, this shows
up as a tendency to form a large bubble in the center, especially in the larger abrasives.
Remedy this by using short "W" strokes and stirring the abrasive under the
mirror every minute of so until the bubble is gone. To stir up the abrasive, just run the
center of the tool clockwise around on the 50% zone of the mirror once or twice while you
spin the tool clockwise. You will need to do this often all through fine grinding.
Because a segmented tool does not trap abrasive, a spherical curve will
come a good deal more rapidly. A short "W" stroke serves well here, too.
When the wet comes to the end, stop grinding, separate the mirror and the
tool, and more water and grit, then continue. If you have a tile tool, you can slop some
water and grit on from the side without separating the mirror and tool, but don't drag the
tool off on the edge of the mirror - this can roll off the edge. Instead, pull the tool
three-fourths of the way off, then lift the tool up. In the finer abrasives this is
difficult. A solid tool won't want to separate, and lifting will tend to pull the tiles
off a tile tool - so take it easy.
As you probably suspected, you're done with 120 when the surface looks
evenly ground and no 60 grit pits remain. Don't be fooled by some of the larger pits that
120 can make itself. These will come out in 220.
Going on to 220
As you begin 220, start to pay a little more attention to preventing
astigmatism especially if the tool is thin. Be sure to rotate the mirror on a regular
basis, and be sure you're grinding it on something soft. Also, keep mixing the abrasive
between the mirror and the tool. In most respects, 220 is about the same as 120, except
that this is your last chance to remove any large pits and the surface must be spherical
when you finish 220.
To control pits, follow the progress of the ground surface with a loupe
magnifier. If you've had your curve generated, watch the generator marks closely. They can
fade into the ground surface and seem to be hidden - and you will not know it until there
when you polish.
You should remember that 220 can leave some isolated pits a bit larger
than the normal 220 pit size. However, you can tell these pits from leftover 120 pits
because they will not stay in the same place after a spell of grinding. It is not uncommon
for an ATM to grind extra hours trying to remove these pits when, in fact, they are caused
by the abrasive and will be removed by the next abrasive. But don't ever let this be an
excuse for falling to remove all the pits from the previous abrasive.
After a couple of hours of 220, you should be ready for the light test and
320 or 30 micron grit.
The light test is a very good test for the evenness of your ground
surface. This test is described in detail in many telescope making books, and is very easy
Place a light several feet behind your face-up mirror. Stand back from the mirror a couple
of feet and lower your until the angle between the light, the mirror, and your eye is so
large that the surface takes on a shine. The finer-ground the surface, the less extreme
Study the surface; It should stay evenly lit from edge to edge as you move
from side to side. If the center of the mirror is hazy, it has not be fully ground by your
latest abrasive. That also suggests that the mirror's curve is not spherical - so continue
grinding until it is. This test will only give information on the overall smoothness of
the ground surface, and is not a test for isolated pits.
Keeping the edge properly beveled becomes more important as fine grinding
progresses. After finishing 220, I like to fine grinding the bevel through 12 micron grit.
This helps prevent scratches at the edge in the finer stages of fine grinding and
polishing. To fine-grind the bevel, obtain a small piece of sheet metal or brass 2"
to 3" square. Tape the sheet to a block of wood the same size, leaving the face of
the sheet to a block of wood the same size, leaving the face of the sheet exposed. Start
grinding the bevel with 220 grit, rounding if off. When the bevel is round with 220, you
can continue to line grind the same way through the finer abrasives, or wait and grind the
bevel with smaller abrasives as you use them on the mirror.
Meanwhile, here is a good time to get a very accurate reading of your
focal length. The 220 fine-ground and spherical surface, when wet, reflects an image of
the Sun that can be accurately focused and measured.
The Fine Side of Fine Grinding
#320 and 30 micro aluminum oxide are about the same size, and they are
your next abrasive. From now on, the precautions described in the "Preventing
Astigmatism" section must be applied; cleanliness is also an important factor. Use
the normal precautions, wash up thoroughly so you don't carry grit in your clothes, and
keep your work area clean.
I prefer to suspend fine abrasives in water, although sprinkling very
small amounts of abrasive on, then smearing it around with your wet hand is acceptable.
For suspending abrasives, I use 1 tablespoon of abrasive to 1/2 cup of water. This goes a
long way when using a solid tool. More abrasive mixture may be needed with a segmented
In the finer stages, you should grind at a slow, steady pace being very
careful not be let the surfaces get dry. If they show a tendency to slick, or if it's hard
to push the tool center over center, it's a sign the mirror surface is not spherical. Keep
the abrasive stirred and use a shorter stroke for a while to remedy the situation.
If you're grinding with a solid tool, be careful not to let the tool and
the mirror stick together. If they do get stuck tight, you must act quickly. Splash water
on the exposed mirror, then place a 2x4 on the edge of the tool and hit it hard with
another 2x4. Be careful the tool does not fly off and land on the floor. Do not use a
hammer. I've seen it done: if you slip, you will break the mirror. If they refuse to
separate, soak them in warm water.
Sticking is caused by vacuum between the tool and the mirror. Segmented
tools will not stick unless you leave the tool and the mirror together until the water
Continue to keep track of pits as before. Apply the light test often - it
is effective from here on out, so use it. Continue as described through your finest
abrasive. The smaller the abrasive gets, the more danger of getting a scratch from the
tool, You must be increasingly careful. In these finer stages the light test can be done
without a light, because room light will be sufficient. With your eye at the correct
angle, you'll see an evenly illuminated mirror surface. A note of caution: With the finest
abrasives, rubbing the mirror with your hand will give it an extra shine in that spot and
will reflect more light in the light test.
When your final abrasive is complete, your mirror is ready to polish.
Don't discard the grinding tool or put it to other uses; it may be required again if you
find that you have astigmatism in the mirror after your first polishing period. Part two
of this article will discuss pitch laps, polishing, figuring, testing, and most important
for big mirrors, test interpretation.