Astronomy Telescopes Optics Accessories

Grinding your Telescope Mirror

Home
Telescope Mirror Cells
Mirror Cell Quote Page
Personal 32" Telescope
14 Foot Palomar type Dome
Grinding your Telescope Mirror
Telescope Cradles and Holders
Telescope Finders and Rings
Telescope Vane Spiders
Telescope Mount Types
CCD Image Science Method
Telescope Casting Parts
Joe's SNe Spectroscopy
Contact Us
Our Location is in the High Desert of California, USA

There are many steps in the fabrication of you precision telescope mirror.  The steps take time and patients, but can be preformed by just about anyone willing to try.
 
Below are details of the mirror fabrication process.
 
Choose a mirror diameter size that you think you can handle.  A good starting scope would be a 6" mirror.  Once you acutally finish all the steps and see the 6" actually make images of the sky you can then push your ability to larger mirrors.

Mirror selection is the first step:
 
Here are some pictures of glass mirror blanks. The one on the left of the picture is a 12"F3 Celestron type blank.  These are used in the Celesctron and maybe Meade Cassigrainian type telescopes.  These are molded pre-curved and the back is webbed for light weighting. The mirror is mounted from the central hole.
 
You can grind and polish and figure a mirror with a hole in the middle. Usually the central hole will have a badly turned down edge, but you cover this with the center mount ring.
 
The other blanks are standard thickness 1 to 10. In other words if the blank is 10" in diameter it should be atleast 1" in thickness.  Ratios of 1 to 8 will give a thicker mirror substrate.
 
Glass blanks are standard flat on both sides. You will want to grind one side flat to fit you mirror cell mount.  The mirror should have a good bevel on it also to keep chipping from occuring during grinding.
 
You can make metal mirrors. But metal will tarnish much faster than the aluminizing and over coating on a glass mirror.

Various Glass Pyrex Mirror blanks 12" to 4"
glassmirror.jpg
Most Glass mirrors come with flat faces. You can special order pre-curved glass.

Methods to measure your mirror curvature surface.
measureglass.jpg
Glass and Diagonal Mirrors can be ground flat just as well as curved.

Mirror rough grinding the Radius curve is the next step:
 
Glass mirror grinding will take the longest time.  If you mirror come flat on the side you want to make the curve then you realize you have to grind out or hog out all that glass to generate the spherical bowl curve.
 
Some mirrors can be purchased precurved to some focal lenght you would like to build. Remmeber the Focal Length = the Radius of Curvature / 2.  The radius of curvature is used to grind and test the mirror.  The radius of curvature is the point infront of the curved mirror that will reflect light exactly back onto itself.  So if you put a fake star light at the radius of curvature you will find the image reflected back to the radius of curvature.
 
If and when you finish the mirror and it is all ready to try on a star you will find that parallel light coming from near infiity will reflect off the mirror curved surface now and flow to the focus point of the mirror. The focus point is 1/2 the radius of curvature of the center of the mirror.
 
example:  and mirror with a radius of 100" will have a focal length of 50". It does not matter what the diameter of the mirror might be!, the Mt. Willson 100" diameter mirror has a prime focal lenght of 400".  Thus, when they were testing the 100" mirror they went to the radius of curvature, or 800" away from the curved mirror surface.  That is a long way to see a foucalt test.

Home Made Hand Grinding Gear-Head-Motor Table Top
tubspinmir.jpg
Place grinder tool on wood holder, which mounts on spindle of the Motor Head. Turns 10 RPM

Auto Grind Machine has one motor with 2 axises
grindmachine.jpg
Using pre-curved steel tools, you can grind many mirrors the same Radius, or polish, or figure

Keeping the Mirror and Tool edges beveled so they don't chip:
 
You must keep your mirror and tool edges from chipping. Chips will occur when you least expect it, and cause chip fragments to fall on to the grinding or polishing surface. This will cause very bad scratches. Keep a good 1/8" bevel at all times during rough grinding. When you reach grit about 220 you can bevel the mirror edge one last time to about 1/16". You will not have to bevel again since the finer grits will not remove enough mirror surface to cause the edge to become sharpe again.

Beveling your Glass Mirror to reduce chipping
beveling.jpg
Grinding can cause a sharp edge which can chip a mirror. Use hand knife sharpening tool to bevel

Hand grinding  or hand polishing or hand figuring is all possible. There are many types of machines and hand tooling to help you fabricate your mirror. Here are three devices, the Barrel, the pipe stand, and the hand grinder home made machine.

The 50 lbs barrel filled with sand or water
barrelhands.jpg
Hand pushing is ok. Just walk around the barrel and push and pull and grind and grind. Is that ME!!

The PIPE stool bolted to the concrete flooring
stoolpolilsh.jpg
Using a 3" or 4" pipe and floor mounts, bolt to cement. Attach wood work piece. Walk around !!

The home build hand grinder with motor
handgrinder.jpg
A motor head with slow gear ratio is mounted on a stand so you can grind or polish

Fine grinding through many grits will give your mirror surface the smooth looking fine luster ready for polishing:

Here is a table of grinding grit pit sizes. Most glass mirrors come flat-flat. The top and bottom of the glass is flatish.  The edges are rounded. You do not have to have a round mirror, any shape of the mirror, square, triangle, broken will not effect the star image. The star is round, the image is round. The mirror is only a light bucket to collect more photons of light than your eyeball can collect.
 
60 grit is like buckshoot... never use it on small mirrors.
80 grit is usually your best hogging startup grit.
120 grit is next. You must grind out the 80 grit pits using 120
        until all the pits are 120 grit. Then proceed to the next grit.
220 grit is usually next. This is a fine powder. This will leave 
        your mirror surface with a nice luster.
240 grit can be used next.
320 grit can be used next.  This will leave a reflective luster. If
        you get scratches at this grit go back one grit.
400 grit is getting pretty fine.
600 grit is very fine and you can stop here and polish out from
       600 grit if something happens to your grinding tool. It will
       take 100's of hours to polish out from 600 grit.
800 grit is very fine and the small the grit you can grind to now
        will shorten the polish hours needed.
1000 grit is like talcum poweder, and scratching your mirror by
         any dust or other contamination is very easy at this stage.
1200  grit is almost polilshing compound particle size. If you get throught the 1200 grit with out a scratch you are lucky and ready for a good poilsh.

Checking for pit sizes using a magnifier. When the pits are uniformally the same size in the glass surface you can proceed to the next grit:

Final mirror preparation for the next polishing steps.  Save clean soap pop top plastic bottles to put grits and polishing agents into:

Making the tar pitch lap to use in polishing the mirror pits out of the mirror surface.  A semi-soft polishing tool is made from melting roofers pitch tar with some hardener pitch blends. The steps in preparing the pitch lap for your mirror are shown in the next series of photos.

Polishing the mirror pits into a glassy reflective surface is the longest time period you will work on the mirror.  Rough polishing can be done with Cerium Oxide or Red Rouge.

Performing a quick surface polish can be done on the grinding machine.  This technique must be done for only a short time, less than 1 hour probably.  If you do this quick polish too long or incorrectly you mirror may suffer a permanent polish warp that you can not polish out. If this happens you can just return to the last grinding grit and regrind.

Pre-polish is done and you are ready to do the slow smooth polishing of the mirror surface. Polishing is done to make the pits melt and flow into a smooth clear reflection surface. This surface must be held reasonably spherical in Radius over the whole mirror during polishing. Later you will polish a figure into the mirror surface.

Polish takes about 3 hour per surface square inch on the mirror surface. You can not polish too much. The glass will melt and flow as you polish, filling in the last ground pits. You must polish until there is no visible pits under a 10x magnifier anywhere on the mirror surface.  The edge of the mirror is the last area that polishes out. So, keep polishing with the 1/4 or less stroke until the very edge of the mirror is free of pits. If you do not polish the pits out of the mirror surface then when you aluminize the mirror you see a frosted non reflecting surface that just will not work in astronomy.

Polishing the mirror surface requires a clean area. Any dust, dirt, sand, old grits, finger nail dirt, dandroff from you head, can and will scratch your mirror surface. If you should become a victum of the scratch, you genearlly just go back to the last fine grinding grit and regrind.  If the scratches are very bad and dig  into the mirror deeply, you may have to go back to a grit size that is the depth of the scratch inorder to grind it totally out.  Some small scratches or sleeks can be tollerated if this is your first mirror. You need to learn the fabrication process and perfection is not necessary.

Polishing techniques require some forthought.  When you place your fingers around the edge of the back of the mirror so you can push it to-and-frow, you will find that your fingers heat up the back of the mirror.  Your palm resting on the back of the mirror also heats up the glass.  Now, pyrex mirrors have a low expansion to heat, but not that low.  Be warned that the heat from your fingers and hands can cause the mirror front surface to expand.  If you then polish off the high spots and let the mirror cool, you end up with low spots.  So, you must learn to polish for short periods of time, or isolate your fingers and hands from touching the glass by wearing soft cloth gloves.  The gloves must not contaminate the polishing liquid or surfaces, else you get scratches.

During and all through the polishing steps you need to utilize  a folcault star tester.  You will need the folcault testing device to learn to measure the Raius of the mirror, and to learn to view the actual curved surface for smoothness.  Testing can take some time to learn.  You have to point the fake star at the mirror from the Radius of Curvature of the mirror. You then utilize a sharp knife edge, like a razor blade edge, to find the point of light returned to the Radius of Curvature of the mirror.  You will learn to see that the mirror surface stays close to a spherical surface while polishing.

Building the folcault tester is a necessary thing to do.  Go to the library and get several of the HOW TO MAKE A TELECOPE books.  I will list a few here at the end of the page.
 
Testers can be simple flashlights and a pin-hole, or a bright LED source shining though a small pin-hole.  A fake star must be created that is smaller than the resolution of the mirror for best results.  But a small 50 thousandths diameter pin-hole is not easy to make cleanly with hand tools.  The folcault tester requires some mechinal layout, scales and practice to use in testing the mirror surface for defects.
 
There are other testing methods. One is the RONCHII test. This material can be purchased from Edmund Scientific Corporation (www.edmundoptics.com).  You will need two pieces of the RONCHI test material. One to put in front of an illuminated frost glass as the projector.  The other you will look through with your eye.  You aling the vertical lines so they cause a morie pattern interference. When placed at the Radius of Curvature of the reflecting mirror, you can produce the test patterns needed to see if your mirror is spherical, smooth, or irregular.

You must learn to interpret the Foulcault testing shadows seen on the mirror surface.  Remember you are creating a fake star and pointing that star so the mirror will reflect it back to an image.
 
The image is viewed by your eyeball.  Just infront of the eyeball you place the Knife-Edge-Razorblade or the Ronchi screen.  As you move your eye and the knife-edge toward or away from the mirror you will view shadows crossing the mirror.
 
Your knife-edge must be very very close to the returning light where it focuses this fake star image.  We mean very close is like thousandths of an inch close to the place where the mirror is making the fake star image.  This point is called the Radius-of-Curvature of the mirror. It is the point where light form the fake star shines out to the mirror and the mirror polished curved surface reflects the 4% of the light back toward the fake star.  You have to tilt the mirror every so slightly so you can get your eyeball and knife-edge (K-E) in a position so you can view the shadow relief of the mirror surface.

Here is a series of photos take with the KE inside the Radus-of-Curvature (RC), then moved toward the eyeball to get  closer to the RC. then moved closer tell the whole mirror seems to be reflecting the star back into your eye.  If you move the KE forward and backwards you will see that different shadows appear.  If the mirror were perfectly spherical, one Radius-of-Curvature over the whole surface then you would not see any shadows. The mirror surface will be fully illuminted and then blink out at the exact RC.
 
When you get your mirror finished you can take it out to a star in the sky and perform the same KE test at your telescope eyepiece.  The light from the far away star should cause the mirror to have only one focus, where the whole mirror blinks out at one instant when the KE is at the focus of the telescope.  If your mirror has any polishing defects like turned edge (up or down) or incorrectly figured Parabola curve, you will see shadows when using the KE test on a real star in the sky.