ATM NOTES on Making an 8" Mirror

by Michael Thompson-Renzi


Notes 9/5/92

Started with two 8" Pyrex blanks.

Chipped mirror face during first grinding session.

Had to use mirror as tool.

Center of Tool had circular rise abot 3" in diameter

Tried recommended 1/4" plate glass to grind down rise. The method did not seem to work.

With #60 carbo, tried grinding mirror with powerful chordal strokes. Good results.

After 3 hours of grinding, tool is getting convex and mirror has 1/32 inch sagitta.

Made template for measuring depth.

To Be Done...

Need to build stand and set up other tests.

Need to go up to #80 carbo now that curve is progressing.

Mirror Dimensions

The mirror will have the following dimensions:

Diameter of blank = 8"

Focal Length (f.l.) = 56"

Sagitta (depth) (S) = 1/14" or .0714"

Focal Ratio (F) = F/7

RC (radius of curvature) = 112"

Radius (r) = 4"




Sagitta (depth) = r2 /4F

where r is radius, F is focal length and D is diameter of measuring unit.

4*4/4F = 16/4*56 = 224 = 16/224 = 0.07



· Sagitta = approx 1/14"

· Template fits

· Have gone back and forth between #60 and #80 carbo to make tool and blank fit.

· Tool had annoying depression in center left over from rise (ironically).

· With strong chordal and W strokes, I've worn down the tool so that virtually all of the depression is gone.


· Tested RC and focal length with light reflection.

· Last night, it was about 114" inches (RC).

· Tonight, it was about 108 - 109".

· Will run tool on top with #80 for a few wets, then make another test.

· I believe that after this, I am ready for fine grinding, starting with #120.

To be done:

· Clean off tool and entire work area.

· Put away #60 and #80, leaving no trace of them in work area.

· Use #120 to make tool and blank fit better.


· Completed about 12 wets with #120.

· Tool and mirror look good.

· Light test showed RC at around 1-9", so I did more grinding with tool on top.

· Later, the test showed RC at around 112-113" - a good value, 112" being just right.

· Probably will do about ten more wets with #120 before going to 220.

· Note: Berry's book says you should do about 40 wets with #120; whereas Howard's book says you should do only about 10-12, or roughly an hour.

· Basically, play it safe by erring on side of more wets.

· Expect to be grinding with 220 by Friday.



· Completed #120

· Radius of Curvature is about 111".

· Started #220 today

· After two wets (10 min. ea.) the pits from #120 are starting to show.

· Went for beer run.



· Started #320.

· Good size pits from #220 hanging around


· Completed #320

· Most pits are small and uniform.

· Cleaned work area, prepared for #500.

· RC is close to 112", near perfect.

· Tool and mirror fit is excellent. No binding at all.

· Mirror has velvety texture.


· Started with #500 carbo.

· Surface looks good.

· Spent Saturday/Sunday looking over Foucault Tester designs. Went to D. Donnelly's house. Great garage, lots of tools and building materials. Dick has a precise carriage for knife edge; it can be used with micrometer and should make the Tester work well. May be able to fit my micrometer with carriage; if not, will have to make a micrometer with knobs and tightly knurled screw.

· Have to build Foucault Tester ASAP.


· Built Foucault Tester using microwave spectrometer carriage, 6" dowel, angle bracket, aluminum baking tins and plywood. May need some improvement as things progress.

· Continued with #500. Surface looked great except for one intractable pit. It was a deep pit, perhaps from a piece of #220. I tried half a dozen wets and that pit was the only one remaining, a blight on the face of an otherwise perfect surface.

· To remove the pit, I backtracked to #320. Completed 8 wets as of this writing. Pit is shrinking, but stil there. I'm going back for another 2 or 3 wets, even though it is 11:20pm. I must get rid of that pit before proceeding back to #500 and beyond!

10/5/92: 11/21/92 09:46 PM

· Did 18 or more wets with #320. Pit is getting smaller, but it is a nasty bugger. Probably need six more wets to eliminate it, then, on to #500 again. Whew! Exhausted.


· During lunch, came home and did about 4 wets with #320. No more #320 left! But that gnarly pit is gone! It took as much work to get rid of that pit as it did to grind out the central depression with #s 60 and 80.


· Cleaned up tool. Recleated. Old cleats disintegrated due to pressed wood decomposition. Used three dowel slices. They're hard and solid. Tool looks good.

· Cleaned up work area. Then did five wets with #500. Baby soft feel is back. No discernable pits. Looking good. Plan on doing at least five more before going to #12 micron.

· Need to get tin for lighthouse on Foucault Tester. I hate the aluminum baking tins I now use. They're too flimsy.

10/11/1992 Sunday

This morning I completed the 5 micron aluminum oxide. The 12 micron went well, no hitches. The 5 micron went well also. The surface looks good; when dry, I can read 10 point type showing through the other side of the blank.

Foucault Tester.

Redesigned lighthouse, using empty olive oil can. Many thanks to Messrs. Torado and Imbao of Portugal for making splendid olive oil cans. And the olive oil ain't bad either. It's in a decanter and I even used some on the chicken last night. I made a slit in the can with two razor blades, roughly measuring it to about .020 inches. I'll make more adjustments to it as testing becomes a critical step. Right now, have to make the pitch lap.


Time for the pitch lap.

I read over the material on making a pitch lap. The most reliable advice seems to be the instructions that were provided with the mirror kit. I may wait till to morrow to begin, although I have all the ingredients for it, including the hotplate.


Last week I made the pitchlap. It took me three tries. The first time the lap was too soft. The second time the lap was too hard. The third time, after doing the 1" long, 1/8" wide rod test, the pitch was just right and the lap was good.

I made the channels, did some preliminary polishing, and tweaked the lap more to make it even more right.

Needless to say, the pitchlap is the bete noir of the whole process.


I hate pitch laps! I have re-channeled the lap several times. Hot presses make the channels close, yet they must remain open to permit free flow of the polishing agent. The pitch gets on everything, on my feet, and therefore the bathroom floor, the shower stall, my hair, hands, and even my bed. Re-channelling is messy. It sucks. But, the mirror has a polish, and it's improving with every session.

Foucault Testers

I hate Foucault Testers, but I hate pitch laps more. When I first conducted a test it was on the living room floor. I didn't know what to make of what I saw. The next night, in a more comfortable setting upstairs, I saw the same thing, didn't know what it was, and scoured all my books for methods of interpreting mirror shadows and so forth. Later on, I realized that I was seeing: turned edge. The mirror was illuminated for about 2/3 the diameter, surrounded by a dark ring. Tonight, I polished with the lap on top using the W stroke. Tests showed improvement. The turned edge was going away. After two polishing sessions like this, the channels closed up again. I've just finished re-channeling - can't get the damn pitch completely off my hands - it's even on the mouse.

· Need more polishing with lap on top. Need to do hot presses only if absolutely necessary. Definitely need to keep up cold presses between sessions.


· After many polishings, the mirror appears to be a sphere, a very good sphere. I have proceeded along with polishing, half-sure that I was doing things correctly, and half-doubtful that I was going in the right direction.

· I have had many battles royal with the Foucault Tester, and trying to find those elusive gray shadows.

· At first, I just could not see what I was "supposed" to see - the shadows, that is. But now I know where to position my eye. I press the side of my nose against the olive can and hold my head there. I can move up or down, and see the mirror gray over. Then I look for the shadows. Lack of dark and light patches tells me the mirror is still a sphere.

· It's in the stand right now, and my stomach hurts.

Ronchi gratings

· Dick Donnelly gave me some Ronchi gratings. When I used them, my mirror seemed to fit the paraboloid pattern, but not entirely. I have to use them more, holding them steadier than I have been.

11/792 12:30am

· Just did a more accurate Ronchi test on my mirror. The ronchi test showed the opposite lines for those of a paraboloid! It seems I have a turned up edge in a major way.


· Cold press for 5 minutes


· Started a 2 minute 1/3 stroke


· Found out the "backwards" ronchi test was due to being outside radius of curvature! When inside radius of curvature, appearance adheres to paraboloid shape!

· Went outside with mirror - reflection of moon looked pretty impressive. Also saw Sirius in mirror and also Rigel.

· Did more Foucault testing. Things are falling into place. I saw strange things in the mirror, wavy clouds drifting across its face, like heat, but more interesting than heat. Strange looking. Seems to be highly magnified view of the glass, appearing almost liquid. Is this due to my eyes? or is this the magnified view I've seen in the books?

· Cold pressing right now.

· Going to do one more figure stroke for about three minutes, then one more Foucault test and one more ronchi test. Might as well do it while I'm in the mood. Got a feeling mirror may be finished after this!

· Also, finally saw knife edge cutting through light outside radius of curvature. It moved from right to left, just as they say it should. I moved knife edge in and met both halves together to exact RC point. I zeroed the micrometer. If necessary, I think I could continue with zone testing now that I've learned how to use this damn Foucault tester, but I may quit now, based on suggestion in WB instructions, which is, if you think you're close to a paraboloid, quit, build rest of scope and come back later if it turns out not to be that good.

· 2:22am

· Did another polishing. I could be finished!

· The ronchi tests look great. The Foucault tester shows me a doughnut with something close to a paraboloid on the left side. One more day!


Did more testing. This time, the ronchi test indicatetes that the mirror is a sphere! Need to do more steady testing next with with ronchi, to determine if I have a turned edge. It seems as the the thermal settling of the glass has reshaped it into a sphere again.

11/21/92 09:46 PM

· Based on ronchi tests, I did a cold press to prepare for a polishing session. I will do a long W stroke to figure the paraboloid. May have to do more than I initially thought. I think I have a turned edge, but I don't know if it is a bad thing at this point.

11/21/92 09:46 PM

· Did two long W strokes, one about 5 minutes, other about 10 minutes.

· Seem to have a turned edge.

· Going to polish with tool on top.

· Foucault Tester is jiggly as a blob of jello.

· Ronchi tests jiggly as hell. Seems to take long time for me to remain still enough to perform the tests. Oh for a shop with an optical bench!

11/21/92 09:46 PM


· Testing apparatus sucks.

· Wiggling, wobbling, awful thing.

· No doughnut

11/21/92 09:46 PM

· Doughnut

· Did about 2 minutes of long W stroke, then cleaned mirror in cold water.

· Set it on rack, tested it, saw paraboloidal doughnut.

· Seems finished. Will check tomorrow, but I think this may be as close as I'm gonna get

· for this mirror.

· Might check it again tomorrow night, but foucault tester is so lousy I don't think I can use micrometer tests.

· Ronchi test showed a paraboloid, but light slit may have been to wide to get clean lines. I widened the other night.

· I hope I'm finished, if the shape changes, it's because of the temperature.

· See Epilogue further down.

The Rest of the Telescope

November 21, 1992


1 7/8" or 1 5/8"

Formula for a diagonal

w = d(D-a) + a


where D = diameter of mirror

f = focal length

a = width of focal plane or width of field lense of eyepiece

d = distance from focal plane to diagonal


Epilogue on Foucault Tester

11/21/92 09:46 PM

· I modified the Foucault tester and the modification made all the difference. What did I do? Well, there were two problems: one, the movement of the carriage from left to right was done with my hand. I would try to inch it left, then right, as needed to manipulate the knife edge, cut into the cone of light, etc. This introduced a great deal of error into the situation, for the human hand is not very good at making precise movements of a few thousanths of an inch. Further, this problem aggrivated problem two: the wobbly stand on which I put my tester.

· To solve problem one I bought a joint connector (forget the exact name) which essentially is two eye bolts connected via a bridge with screw holes at each end. It is the kind of thing used to tighten a rope or wire. One of the eyebolts would attach to a wire which would in turn be attached to the leg of the carriage. Instead of using the other eyebolt for the knob, I bought a wingnut bolt, same size as eyebolt, to use for the knob. I then mounted this on the tester. One leg has an elastic band pulling it to the left; the other leg (right leg) has a wire attached to it and running through the eyebolt. On the far right of the eyebolt joint is the wingnut screw, which when turned left or right, moves the carriage left or right. This allows me to make very small, yet precise movements of the carriage/knife edge along the azmith. I also put a washer between the forward/reverse knob, the one that makes the knife go toward and backward from the mirror.

· I was just down stairs on the floor doing some preliminary testing. It is a bit uncomfortable on the floor, but at least there's no wobble! I positioned the tester such that the knife edge can move inside and outside the radius of curvature. I tried cutting the cone of light outside the RC, at RC, and then inside RC. I can manipulate the shadows such that they no longer bounce all over the place. In short, I CAN PERFORM A COMPLETE TEST ON THE MIRROR!!!!!!!!!

· I saw the upside down bowl, the doughnut, and what appears to be a paraboloid. Me happy!

· I loaded the micrometer at RC to zero. I can now conduct precise tests, measuring up to 250/1000s of an inch, well within the tolerances I need.

Wave Rating

The dimensions of my mirror:

Diameter: 8"

Clear face: 7 13/16"

f/ratio: 7

RC: 112"

Focal length: 56"




Edge Zone








First, figure out what one wave is by taking ideal and subtracting min. val. to get difference.

.145 - .117 = 0.028

Then, muliply difference by 4. Result = 1 wave.

0.028 * 4 = 0.112 -------------> = 1 wave

Readings: .073, .075 ------------> avg. = .072

.117 - .072 = 0.045 difference

45/1 / 45/112 = 0.01

45/112 = 0.4 = 2/5 wave or 4/10

Wave rating of edge zone: 2/5l

Now suppose my reading were .091, then .117 - .091 = 0.026, so...

26/112 = 0.23, 23/100, or better than 1/5 wave - not bad



70% Zone








1 wave = .069 - .052 = 0.017 * 4 = 0.068

Readings: .063, so .069 - .063 = .006.

Then... 6/68 = 0.09 = better than 1/10th wave





November 21, 1992 Power Ratings with Eye Pieces

Formula: Fs / Fe in milimeters

where Fs is the focal length of the scope

and Fe is the focal length of the eye piece.

· Thus, one metere = approx. 39 inches, so 1000/39 = 25.64 milimeters per inch.

· Focal length of scope is 56", so 25.64 * 56 = 1435.84 in milimeters.

· Therefore, Power = 1435.84 / Fe

See table.


Eye Piece



~ 205x

9mm (Barlow)

~ 160x


~ 79x


~ 48x


Note: Need to determine field of view in degrees for each eyepiece. It will make Messier and NGC hunting much easier.