20" mirror grinding log

by Mel Bartels

20" mirror specs:
fine annealed 7740 pyrex,
back ground flat with large shallow bevel at back edge, 20 3/8" dia.,
2 1/8" thick,
50 lbs,
0.25" sagitta (for f/5.0),
fl = 97",
bevel of 1/8" around rim, clear dia. of 20 1/8" with f.r. = 4.8

some comments:

I thought it might prove interesting to amateur telescope makers the following log in grinding a 20" mirror. Because I used a Hindle type grinding machine, the hours spent are longer than if done by hand, also, remedies undertaken to fix the mirror were selected with the machine in mind. For instance, when I accidently gouged the mirror face near the edge when beveling the edge with a drill and grinding bit, it was much easier to stay with 500 grit despite it taking many more hours with the grinding machine, than it would have taken if I had worked the mirror by hand, and gone back to 220 grit.
My first attempt to figure the mirror ended with a turned edge. The second attempt was much more successful.


3/31/93 pregenerated blank ordered from InterMountain Optics in Utah,
4/12/93 received,
4/12/93 full sized 20" grinding tool cast from casting plaster 1 3/4" thick,
4/16/93 1/4xlxl" unglazed ceramic bathroom tiles fiberglassed to front of tool, mirror put on top to help keep shape,
4/17/93 had to strip the tiles from the tool - they did not stick - tool not yet cured or too cold for fiberglass to set or tool not cast properly (too much water?),
4/19/93 new tiles fiberglassed to tool (it's a little warmer) - job turned out very nice,
4/29/93 started machine fine grinding with 200 grit, 1/3W strokes, MOT (mirror on top) - 2.5 hrs,
5/l/93 continued as above, generating marks on face of mirror gone, no change in sagitta, tool contact patchy - 8.5 hrs,
5/2/93 as above, still patchy contact - 12 hrs,
5/4/93 opened channels between tiles (which had been filled with fiberglass) using skillsaw with blade to open up the fiberglass between the tiles; ground for 1 hr,
5/5/93 continued grinding with 220 grit, grinding going much faster with channels opened up - 2 hrs,
5/6/93 as above - 4 hrs,
5/7/93 as above - 4 hrs,
5/8/93 as above - 3 hrs,
5/9/93 finished with 220 grit - total time with 220 grit = 37 hrs, almost all tiles now in contact, started 500 grit - 4 hrs,
5/10/93 continuing with 500 grit (changing every 20 min) - 4 hrs, just a few extremely tiny pits near the extreme edge remaining,
5/11/93 tool on top to catch edge of mirror better - 3 hrs,
5/12/93 tool on top - 3 hrs, no pits to be seen at edge, total 500 grit time = 14 hrs,
5/13/93 tried to bevel edge with power drill and bit - bit jumped onto mirror surface and gouged mirror's face near edge - 1 hr,
5/14/93 continued beveling for 2 more hrs, will take a lot of grinding with 500 grit to fix mirror's surface near edge, 3 hrs with 500 grit, using longer strokes to concentrate on edge, beveled edge keeps grit longer on mirrors surface,
5/15/93 500 grit - 8 hrs, long strokes, almost back to f 5.0, edge surface defects almost gone,
5/16/93 500 grit - 8 hrs, 1/3W strokes, some progress on surface gouges at edge, 0.250" sagitta,
5/17/93 500 grit - 6 hrs, 0.251 " sagitta, gouges reduced to roughly 100 grit pits, had to slightly shrink final belt on grinding machine as it was intermittently slipping,
5/18/93 500 grit - 6 hrs, same sagitta, remaining pits about 1/2 reduced,
5/19/93 500 grit - 6 hrs, 0.253" sagitta (101" f.l.), little change in pits,
5/20/93 500 grit - 6 hrs, pits reduced, difficult to see now, no change in sagitta,
5/21/93 500 grit - 6 hrs, one small pit left,
5/22/93 500 grit - 3 hrs, pit gone, sagitta unchanged, total 500 grit = 67 hrs,
5/23/93 9u grit - 3.5 hrs, sagitta = 0.254" (f.l. = 102"), 4 more hrs of 9u grit, sagitta unchanged, surface clean with no scratches or pits, total 9u time = 8 hrs,

breakdown of time for fine grinding: 220 grit 37 hrs, beveling 3 hrs, 500 grit 67 hrs, 9u grit 8 hrs,

5/25/93 made polishing tool from stacking 6 layers of 3/4" plywood, used slightly larger than mirror diameter of 21" to help fight TDE, with 1/8" door skin layers of 16" dia and 6" dia on top, simulating convex face, to keep pitch at a constant thickness, waiting for Elmers glue between ply layers to dry before fiberglassing,
5/31/93 covered sides and bottom of wood polishing tool with fiberglass (did not cover top because pitch reacts negatively with fiberglass),
6/4/93 started polishing at l0 pm, cutaway a little too much in the center of the pitch lap, good shine at 2 hrs, haze quite visible with flashlight, figure ok, 4 hrs total polishing,
6/5/93 started polishing at 2pm, re-did main drive belt so that motor will stall before belt slips 12 hrs polishing (total = 16 hrs), haze no longer visible with flashlight at edge, some haze barely visible at center, figure shows high center (slight lack of pitch at center and short strokes are the reasons),
6/6/93 started polishing at 2pm, bearing may be going out on idler shaft of grinding machine, adjusted pitch for slightly heavier polishing at center of mirror, after 5 more hrs of polishing, center polished out (total polishing time 21 hrs), figure more spherical, R.C. = 194 3/8", fl = 97 3/16", 9 hrs polishing, total polishing time 25 hrs, figure: high center with turned edge (alternative way of looking at figure is high 90% zone),

break down of time for polishing: polishing 25 hrs,

6/22/93 started figuring attempt #1 after 2 wk vacation, pitch slumped a bit in the middle of the pitch lap after the 2 wk hiatus, ended up over compensating a bit - leaving more pitch concentrated in center, figured for 3 hrs, at least at this stage, I like the 100 lines per inch grating better than the 200 lines per inch (less diffraction), surface roughly spherical, surface staying very clear of scratches and sleeks,
6/23/93 2 hrs figuring with pitch lap concentrated towards center, low/high zone at 80%,
6/24/93 2 hrs figuring with normal pitch lap and progressively longer, wider strokes, Ronchi bands straighter, slight low/high zone at 70%,
6/25/93 1 hr figuring with longest, widest strokes possible, no change,
6/26/93 1 hr figuring with parabolizing pitch lap, that is, with progressively more pitch cutaway as one moves to edge of tool, long strokes, some parabolization starting to show in outer 50% of mirror,
6/28/93 1.5 hr figuring as above, parab. coming along nicely, approx. 50% parabolized,
6/29/93 1 hr figuring as above, parab. 75%, overall figure matches parab. quite nicely, no turned edge, some slight zonal ringing corresponding to figuring strokes, pleased with how it is coming along,
6/30/93 45 min figuring with extremely long strokes, not quite so wide though, and standard pitch lap with changing of stroke lengths every 5-10 min (it takes an average of 8 min for mirror to rotate fully with respect to tool), surface smooth with no zonal ringing, about 90% parabolized, making steady progress,
7/2/93 30 min figuring with parab lap and long, medium-wide strokes, changing strokes every 2.5-5 min for surface smoothness, roughly 90-100% parab., over polished at 50% (too low) zone,
7/2/93 15 min figuring with 50% zone on pitchlap cutaway, slightly more pitch in center than at edge, 1/2 time 1/3W strokes, 1/2 time long wide strokes, always avoiding 50% zone, 50% zone problem gone, smooth parabolization overall, approx 90% parabolized,
7/3/93 15 min figuring with standard lap using long extra wide strokes, no change in overall parab., 50% zone being hit too much again,
7/3/93 15 min figuring with 50% zone on pitch lap cut away, mirror now fully parab., but 50% zone dug out a bit too much,
7/3/93 15 min figuring with 50% zone cut away, normal 1/3W strokes, Ronchi bands not completely smooth, roughly property parab., though, surface clear of scratches and sleeks,
7/4/93 10 min figuring with newly poured pitch lap (3/4"thick), old pitch worn down and hardened, long wide strokes, surface clear of sleeks and scratches, Ronchi bands a touch less rough, parab. still ok, TDE of 1/4" appearing for first time,
7/4/93 10 min figuring with slightly parab. lap, long wide strokes, smoother, a little too deep at the 50% zone,
7/4/93 10 min figuring with 50% zone cutaway, long wide strokes, 40% zone over parab.,
7/4/93 10 min figuring with 40% zone cutaway, long strokes, not quite so wide, overall parab ok, TDE of 3/8", 20% zone now a little low (diagonal shadow not quite covers this defect), total figuring time 15.5 hrs,

gave up figuring to fix turned edge and start again,

7/6/93 1 hr of short strokes on normal pitch lap to try and fix TDE, no real change,
7/7/93 1 hr as above, parab much less, TDE no change or worse,
7/8/93 1 hr of 1/3W strokes, parab=90%, TDE less noticeable, but too much correction now at edge and not enough in center of mirror, also, a central high spot of 4" dia. continues to persist (probably due to pressure of grinding machine's articulation arm pressing down on the mirror's back center), a few short little sleeks appearing,
7/10/93 45 min wide strokes, no big change,
7/12/93 1 1/2 hr short strokes on normal lap, outer several inches zone badly over parab now, as rest of miror zones' R.C shrink,
7/12/93 1 1/2 hr as above, parab. of mirror being gradually reduced with edge trailing,
7/13/93 4 hr 45 min, normal strokes, slightly longer at for last 45 min, total figuring time = 27 hrs, closer to spherical, outer 2 inches badly parab, high center (or low 80% zone), no sleeks or scratches,
7/14/93 1 1/2 hr of varying strokes, now just 1 inch outer zone turned,
7/14/93 1 1/2 hr as above, TDE of 3/4 inch extent,
7/14/93 1 hr as above, no change,
7/15/93 3 hr with short strokes, TDE still 3/4 inch wide, but degree of TDE is less,
7/15/93 2 hr as above, TDE less, perhaps 3/8" wide and just slightly hooked,
7/16/93 3 1/2 hr as above, TDE 1/4" wide with very slight hooking, TDE worse as mirror is reaching equilibrium,
7/16/93 1 hr of slightly longer, wider than 1/3W strokes, TDE barely seen, central hill, procedure to prepare lap/mirror for polishing: hot water poured over lap (or left standing over lap using duct tape wrapped around lap's edge) until lap is softened (lap left sitting out in sunshine will also soften pitch), several drops of dishwashing detergent are poured over lap and mirror is worked on lap until lap is shaped, cold water from hose poured over lap to stabilize shape, mirror and lap are cleaned,

using Gugolz #55 pitch with cerium oxide - Lensmax B from Optical Manufacturers International,

7/17/93 3 hrs of long and short strokes, 1/4" TDE, central hill, no change from before, f.l. reduced by 3/16" to 97", 28 hrs spent returning to sphere and reducing TDE, total figuring time 43 1/2 hrs,

second parabolizing attempt:

7/17/93 started parab attempt #2, 1 hr of parab strokes on parab lap, parab just beginning to show,
7/17/93 1 hr as above, outer zones need more parab,
7/18/93 45 min long strokes on parab lap, no change,
7/19/93 1 hr long strokes on normal lap, edge still needs parab, overall parab roughly 100%,
7/20/93 45 min long strokes on lap cut away slightly at 50% zone, little change at edge, 50% too deep, 1 sleek near edge,
7/21/93 1 hr of 15 min of long strokes and 45 min of short strokes on lap with 60% zone cutaway, mirror better, 50% zone a little too deep, need more parab (approx 80% parab) especially at edge, TDE not exceeding 1/4"
7/22/93 1 hr as above, overall parab shape better but a little undercorrected, low zone a couple inches from center, TDE not exceeding 1/4", a couple of minor sleeks,
7/24/93 1/2 hr parab by hand over standard pitch lap, hard work, mirror shows obvious differences, center is now smooth, overall parab about 85%, need more parab at 60% zone, TDE still 1/4" wide, but stronger, couple of short sleeks near edge,
7/24/93 20 min of long strokes by hand, TDE slightly less, roughly 100% parab, but too deep at 40% zone,
7/24/93 15 min of 1/3W strokes by hand over lap cut away at 40%, zone fixed but need more parab towards edge,
7/25/93 20 min of 1/3W strokes by hand over standard pitch lap, Ronchi bands smoother, TDE of 1/4", overall parab about 85%,
7/26/93 20 min of 1/2W strokes by hand over standard pitch lap, bands very nicely and smoothly curved, 100% parab, no noticeable TDE, no big deviation can be seen from computer generated Ronchi band ray tracings,

Ronchi test looks good enough, not quite perfect, but ready for star testing final figuring:

8/l/93 first star test of mirror with 9mm Nagler (275X), using test ring with A-frame serrurier truss propped up at Polaris, no astigmatism except occasional tiny amount at right angles to mount that does not vary with mirror rotation - could be mirror folding over in half or diagonal figure problem or most likely diagonal cooling problem, mirror figure very close to indistinguishable from perfect with very slightly overcorrected inner zones, very slightly undercorrected middle-outer zones, no noticeable TDE,
8/3/93 5 min of 1/3W strokes on standard lap, Ronchi test: center maybe a little flatter, slightest bit of over parab at 30% zone, star test: no TDE, center maybe a touch better,
8/3/93 5 min of 1/2W strokes on lap with a bit cut away from 30% zone and a touch cut from center, star test: using 9mm Nagler and 2.5X Barlow (~700x), correction closer but went too far and now slightly overcorrected, very slight TDE, Ronchi test: beautifully curved bands all the way to the edge, need to bring back the correction just a tiny bit,
8/5/93 5 min of 1/3W strokes on lap with central 60% area partially cut away, Ronchi test: central area gives impression of looking slightly flatter, smooth curves, star test: correction looks perfect up through 275x, at 700x looks very slightly undercorrected (about 1/2 the amount of the former overcorrection), no noticeable TDE,
8/5/93 2.5 min of long strokes centering mirror over lap's edge - using standard pitch lap, star test: using 4.7mm Meade Ultra Wide Angle (525x), initially looks excellent, after much examination and letting mirror cool thoroughly, mirror shows extremely slight undercorrection, Ronchi test looks really smooth,
8/6/93 1.5 min of strokes centering mirror's center over pitch lap's edge, Ronchi test: smoothly curved bands with no TDE, star test: looks indistinguishable from perfect at 525x, a couple of sleeks noticed, figuring time this time around 10 hrs 4 min in 18 figuring steps (2 hrs 4 min of that by hand), total figuring time 53 hrs 4 min, total time spent = 233 hr 4 min, 3+ months spent working on mirror, 4+ months since order of mirror,

9-pt mirror support with two edge supports at 45 deg each from vertical showed no astigmatism or deformations in the star test, but article in Sky and Telescope shows that 9-pt gives a small wavefront error, so going with 18-pt support,

the thicker 2 1/8" mirror was a little easier to do than 1 5/8" thick large mirrors up to 30", going to hand figuring at end produced a very smooth surface,

8/7/93 spent a few seconds with lap and cerium oxide to clean mirror face with, used grinding stone to remove almost all of the cerium oxide that had dried to edge of mirror near surface,

mirror finished!