150 mm f/8 mirror (25 mm Pyrex)
Polishing and figuring log
2010/10/23
Grinding has started.2011/02/14
Polishing has started.2011/09/04
Final polishing and figuring.
Polishing at first with Cerium Oxide 1670, diluted 1:8 by volume with distilled water. Later on dillution was increased to 1:40.Ronchi test images are taken with 33.5 lines / cm (85 lines / inch) grating at first, 79 lines / cm (200 lines / inch) later.
Foucault and Ronchi images are extrafocal unless otherwise indicated. Knife edge in Foucault is on the left hand side.
Status: HUGE central hill, oblate spheroid, TDE. It's a horrible mess, the result of too many experiments with polishing techniques and materials.
2011/09/10
Full diameter lap, Gugolz #64.Pressed with 10 lb, 30 min with mesh, 30 min without mesh.
Attempting to reduce TDE.
45 minutes MoT, W stroke, 1/4 diameter stroke.
76.6 F
no waltzing
pressure: both hands, center of mirror
Status: still very large central hill, oblate spheroid, huge TDE.
2011/09/12
1/3 diameter lap, Gugolz #64, first time used.Pressed with 1 lb, 15 min with mesh.
Attempting to reduce central hill.
5 min ToT, W stroke, all over the central hill.
74.7 F
Status: Huh, no change.
2011/09/13
1/3 diameter lap, Gugolz #64.Pressed with 1 lb, 15 min with mesh, 15 min without.
5 min ToT, W stroke, all over the central hill. Somewhat better contact this time.
75.4 F
Status: still no change. Needs more than 5 min.
2011/09/17
Full diameter lap, Gugolz #64.Pressed with 10 lb, 30 min with mesh, 30 min without.
60 min ToT, W stroke, 1/4 diameter, slow strokes, low pressure.
75.7 F
Just curious what happens when doing ToT
Status: still no change (TDE and central bulge), looks like ToT by itself won't change the figure.
2011/09/19
full diameter lap, Gugolz #64.Pressed with 10 lb, 30 min with mesh, 30 min without.
10 min ToT, W stroke, 1/3 diameter, just to get contact.
10 min ToT, W stroke, edge of tool pressed on center of mirror, per Texereau - reducing the central bulge.
74.7 F
Trying to reduce the central bulge.
Status: the bulge seems to decrease a small amount, but it's still pretty large. Now there's some yucky roughness in the center (will have to smooth it out later).
Next session: more of the same, maybe 15 or 20 min?
2011/09/24
Full diameter lap, Gugolz #64.Pressed with 10 lb, 30 min with mesh, 30 min without.
10 min ToT, W stroke, 1/3 diameter, just to get contact.
20 min ToT, W stroke, edge of tool pressed on center of mirror, per Texereau - reducing the central bulge.
74.5 F
Trying to reduce the central bulge
Status: Bulge has decreased, but still present. Some roughness in the center.
Next session: same, 30 ... 40 min, maybe 60.
2011/09/25
Full diameter lap, Gugolz #64.Lap was freshly trimmed and re-channeled.
Pressed with 10 lb, 30 min with mesh, 30 min without.
10 min ToT, W stroke, 1/3 diameter, just to get contact.
50 min ToT, W stroke, edge of tool pressed on center of mirror, per Texereau - reducing the central bulge.
72.3 F
Trying to reduce the central bulge
Status: central bulge definitely smaller, but not as small as expected. There appears to be a small hole in the center of the bulge. Less roughness than expected.
Next session: Maybe I should try the sub-diameter tool again.
2011/10/01
1/3 diameter lap, Gugolz #64.Pressed with 1 lb, 60 min with mesh. Now the mesh leaves nice clear mini-channels.
30 min ToT, W stroke over the central bulge, while simultaneously and slowly rotating both mirror and tool. Took a long time to conform. Was a little bit grabby all the way to the end. Required a bit of pressure to stabilize.
75.0 F
Trying to reduce the central bulge.
Status: Bulge diminished pretty good, but still very prominent. Its peak was flattened. Not much roughness, good job.
Next session: more of the same. Maybe lots more.
2011/10/02
1/3 diameter lap, Gugolz #64.Edge of lap was trimmed at 45 deg.
Pressed with 1 lb, 70 min with mesh.
50 min ToT, W stroke over the central bulge, while simultaneously and slowly rotating both mirror and tool. Used less Cerox. Much better contact now. I should always trim the edge at 45 deg.
73.2 F
Trying to reduce central bulge.
Status: Bulge has become a central raised ring. Its height has diminished. It's getting close to the base. Roughness is visible in the central area. Lots of progress.
Next session: Subdiameter lap, chordal stroke over the ring.
2011/10/08
1/3 diameter lap, Gugolz #64.Surface of lap was very soft, being kept with turpentine in a box for a week. Then spent 60 minutes under fan to disperse some turpentine.
Pressed with 0.5 lb, 5 min with mesh.
30 min ToT, chordal stroke over central ring, while simultaneously and slowly rotating both mirror and tool.
Trying to reduce ring.
73.2 F
Status:
Next session: I should try to smooth the entire surface. Maybe 1/3 W strokes with full lap.
==================
Full diameter lap, Gugolz #64.Pressed with 10 lb, 30 min with mesh, 30 min without.
60 min MoT, W stroke, 1/3 diameter. First 30 min normal hand pressure. Next 15 min only mirror weight pressure (holding mirror by the edge with gloves). Last 15 min mirror weight pressure and slow strokes.
Trying to smooth the entire surface.
73.2 F
Status: Central bump reduced. Central raised ring reduced. Very smooth surface, sleeks almost gone; excellent job - low pressure slow strokes are the key. Dirt from rinsing the mirror still visible.
Next session: more sub-diameter lap.
From now on working with diluted CerOx. Before was 8:1 water:CerOx, now it's more like 40:1 by volume.
2011/10/15
1/3 diameter lap, Gugolz #64.Pressed with 0.5 lb, 30 minutes with mesh. Then left a couple hours with no weight (not intentionally).
30 min ToT, W stroke, all over the central ring and bump, but insisting on the ring (not much time spent on the bump). Edge of tool sometimes reaching near the edge of mirror. Mirror and tool in slow continuous rotation.
Working on the central defects.
73.0 F
Status: Looks like the central ring and bump are being flattened. At the same time, their radii are growing. They look like they're melting and flowing to the edge. A little roughness, unavoidable with the little lap. No sleeks.
Next session: more of the same.
2011/10/16
1/3 diameter lap, Gugolz #64.Pressed with 0.5 lb, 20 minutes with mesh. Then left 1 hour with no weight (not intentionally).
20 min ToT. Several chordal strokes on the left, pressing on the raised ring. One or two strokes on the center to reduce the bump. Several chordal strokes on the right, pressing on the raised ring. Like a complex W stroke, with emphasis on the lateral extremes.
Trying to reduce the defects.
http://www.youtube.com/watch?v=5Sau_7fWq2E
74.8 F
Status: The technique works. The ring and the bump are lower still. As expected, the stroke reduced the ring more than the central bump. But now the work area is rough and uneven.
Next session: Full-size lap for 1 hour to soften the roughness, maybe 1/3 W stroke.
2011/10/22
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh. The lap is a bit hard, the mesh doesn't micro-channel it well.
5 min MoT, 1/3 W stroke for contact. 30 min MoT parabolizing stroke.
Trying to reduce the central bump.
75.4 F
Status: WOW! I dug a giant deep hole in the middle. 30 min was way too long. Well, at least the surface is smooth. :) Yet another lesson learned the hard way - which means I probably won't forget.
Next session: Work on the hole's "shoulder", tapering off to the edge, either with full size lap (MoT with shoulder on edge of lap) or with subdiameter lap.
2011/11/05
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh. The lap is a bit hard, the mesh doesn't micro-channel it well.
60 min ToT, 1/3 W stroke.
Just curious what is the effect of the normal stroke on such a deep central defect.
69 ... 71 F, variable (central heating is on)
Status: Depth of central hole reduced a little. But this technique is too slow in such a case (as I expected). A little roughness, since I worked a little fast and with some pressure.
Next session: Re-normalize to another spherical surface, work on the shoulder of the hole, petering out to the edge (work a more strongly curved sphere, tangent to the center of the hole and the edge of the mirror). Maybe this will take care of the oblateness, too.
2011/11/12
Full diameter lap, Gugolz #64.Pressed with 10 lb, 90 min with mesh. The lap is a bit hard, the mesh doesn't micro-channel it well.
5 min ToT, 1/3 W stroke, just for contact. 20 min ToT, short WWW stroke, starting with edge of tool near center of mirror (a bit outside of it) and going out to the edge. Pressure on the tool's edge, per Texereau.
Trying to rebase to a deeper sphere (tangent to the center and the edge): hole in the middle = doughnut around the middle on a deeper sphere.
70.3 F
Doughnut reduced a tiny bit, maybe.
Next session: This doesn't work fast enough. Time to attack the doughnut with the mini-lap.
2011/11/24
1/3 diameter lap, Gugolz #64.Pressed 30 min with 1 lb, with mesh.
30 min chordal stroke at 1/2 radius, with occasional excursions to the center to avoid making a central bump.
Trying to reduce the doughnut.
69.0 F
Progress! The doughnut is reduced. Some zones remain. Central hole remains, but smaller. The mirror is much more spherical now (was oblate spheroid), but zone-y. Not much roughness.
Next session: Some smoothing with full size lap and normal stroke.
==================
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh. The lap is hard, the mesh doesn't micro-channel it well.
60 min MoT, 1/3 W stroke. Strong friction after 30 min, somewhat diminished after that.
Trying to smooth out the zones and various defects.
70.0 F
More progress! The doughnut is reduced even more. The central hole is reduced even more. Quite smooth surface despite working with very hard lap.
Next session: No idea.
2011/11/25
1/3 diameter lap, Gugolz #64.Pressed 2 hr 20 min (not intentional) with 1 lb, with mesh. This time it made micro-channels.
10 min ToT, short chordal stroke close to the edge over the doughnut, blending to the center every several strokes. Turntable in constant rotation. Good contact.
69.4 F
A little progress. Spherical outer zone, central hole, more sharply defined. A raised lip around the central hole.
Next session: No idea. More of the same, I guess.
2011/11/26
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh. The lap is hard, the mesh doesn't micro-channel it well.
60 min ToT, 1/4 W stroke. Reasonable contact, reasonably smooth strokes.
Trying to blend in all the zones and ridges a little.
70.0 F
Central hole less deep. Raised lip around it less prominent. There's an overall reduction in TDE from the first ronchigrams at the top of this log.
Next session: more with subdiameter lap.
Replaced bottle of CerOx. Still diluted, maybe 40:1 - a few mm of CerOx in a 300 ml bottle, then add 200 ml distilled water. The fluid looks milky, but a bit more watery than regular milk.
2011/12/03
1/3 diameter lap, Gugolz #64.Pressed 40 min with 1 lb, with mesh. Moderately deep micro-channels.
15 min chordal stroke on the ridge around center, going sometimes to the edge, other times stopping short.
Trying to get rid of the ridge around the center and blend the whole thing into a single sphere.
70.0 F, variable, heating is on.
Sweet progress. :) It's close to an overall sphere now, for the first time. Some rings around the center. The TDE has decreased a lot, too, compared to the first sessions.
Next session: Full lap, normal stroke. Focus on reducing the rings and blending a sphere across the whole surface. Maybe try and reduce TDE too (short stroke). Maybe use only full lap from now on, if all goes well.
2011/12/04
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh. Followed idea by Mike Lockwood, did not separate mirror and tool between pressing and polishing, just moved the mirror/tool stack on the turntable (before, I was separating them, washing both with water, drying them out under the muffin fan). By not separating them, contact is supposed to be better.
1 hour ToT, W stroke 1/3 D. Slow strokes, almost no vertical pressure. Yes, contact did improve, was near perfect right from the beginning.
Trying to smooth out the zones.
70.5 F
More progress. The two raised rings around the center are reduced. TDE is reduced, now it's hard to see in the ronchigram; I'll have to do Foucault from now on to estimate TDE. Foucault reveals there is actually some roughness (please ignore camera noise, the image contrast was highly increased to show the rings more clearly, which made the noise a lot worse).
Next session: Rings, TDE and roughness - the cure for all is the same, more slow gentle polishing with full lap. I should try MoT, to work the center more strongly.
2011/12/10
Full diameter lap, Gugolz #64.Pressed with 10 lb, 90 min with mesh.
1 hour MoT, W stroke 1/3 D. Slow strokes, almost no vertical pressure. A tiny bit skippy/grabby (surprising) but overall rather smooth. Maybe the lap is getting too hard.
Trying to smooth out the zones.
65 ... 70 F (heating is on)
Awesome! For the first time the 6-stripe Ronchi looks nearly perfect (except the TDE). The two raised donuts around the center are visible only in the near-focus Foucault - pretty faint. Overall smooth surface.
Next session: More like this, maybe ToT. Maybe I should start reducing the TDE with short strokes; or maybe not.
2011/12/11
Full diameter lap, Gugolz #64.Pressed with 10 lb, 80 min with mesh.
1 hour ToT, W stroke 1/3 D. Slow strokes, almost no vertical pressure. Good contact.
Trying to smooth out the zones.
70.2 F at first, down to 69.4 F at the end
The donuts are gone. There's a very faint residual central hole, very hard to see in Foucault (!) but somewhat visible in a 1-bar Ronchi. Impossible to tell if the surface is rough, there were too many air currents and I don't have a tube. Some residual TDE; it's getting better, the diffraction ring in Foucault is picking up a little bit of brightness on the opposite side.
Next session: Before I focus on eliminating TDE, I'm curious if I can make the central hole disappear with normal W strokes. Let's try 30 min MoT.
2011/12/14
Full diameter lap, Gugolz #64.Pressed with 10 lb, 80 min with mesh.
1 hour MoT, W stroke 1/3 D. Slow strokes, almost no vertical pressure. Rather inconstant contact, sometimes good, but often skipping/grabbing. Perhaps the lap is getting too hard. Contact seems okay with ToT, but somewhat poor with MoT.
Trying to smooth out the center.
70.2 F ... 68.8 F
The very slight central hole is probably a bit worse. Looks like it's surrounded by a slightly raised ring. The defect is very slight, barely visible in Foucault. Very smooth surface, good job, pat on the back. Very narrow TDE still persists, barely visible in Ronchi, clearly visible in Foucault.
Next session: Okay, so it's time to attack TDE. Switch to ToT. Perhaps rebuild the lap some time soon, using fresh and softer pitch.
2011/12/17
Full diameter lap, Gugolz #64.Pressed with 10 lb, 75 min with mesh.
50 min ToT, W stroke 1/4 D. Slow strokes, almost no vertical pressure. Okay contact.
Trying to reduce TDE.
70.1 F ... 69.2 F
Hard to say if TDE has decreased since last time. It probably did, but can't tell from the images. There's still a hole in the middle, perhaps a ring around it. Hard to tell if they got better.
Next session: More 1/4 strokes to reduce TDE? This will take a while until it's fixed.
2011/12/18
Full diameter lap, Gugolz #64.Pressed with 10 lb, 90 min with mesh.
60 min ToT, W stroke 1/4 D. Slow strokes, almost no vertical pressure. Contact sketchy at first, better later on.
Trying to reduce TDE.
71.2 F ... 70.0 F
Looks like TDE is decreasing, I'm starting to see the diffraction ring in Foucault on the left side. The central defect appears to be getting worse (but I don't see it in Ronchi, so it's actually pretty shallow, it's nothing compared to the gigantic defects at the beginning of the sphericization). But there seems to be a flat raised zone, narrow, near the edge, due to the rotation of the tool and mirror at the end of each group of strokes. Rotating the tool concentric with the mirror has imprinted the tool's shape into the mirror. I'll have to rotate it de-centered, like I did before.
Next session: More like this. Rotate the tool de-centered.
2011/12/21
Full diameter lap, Gugolz #64.Pressed with 10 lb, 85 min with mesh.
60 min ToT, W stroke 1/4 D. Slow strokes, almost no vertical pressure. Now I'm rotating the tool eccentric at the end of each W. Very good contact all the time. Huh.
Trying to reduce TDE.
71.4 F ... 67.1 F
Not much apparent progress. The raised/flat lip at the edge due to concentric tool rotation seems to have decreased a little. The central defects (central hole, raised ring) appear a bit worse. No sign of progress for TDE.
Next session: More of the same. Re-channel the tool, maybe this will fix the central hole. Wash the mirror more thoroughly, it was dirty this time; pass a single layer of wet paper towel across the whole surface.
2011/12/22
Full diameter lap, Gugolz #64.Pressed with 10 lb, 150 min with mesh.
60 min ToT, W stroke 1/4 D. Slow strokes, almost no vertical pressure. Eccentric rotations. Awesome contact, smooth, deep, buttery.
Trying to reduce TDE.
71.1 F ... 67.6 F
The central hole is getting worse, now I see it in Ronchi. A bit of oblate spheroid, too? The lip defect due to concentric rotation seems to get a lot better. It's possible that I'm getting close to fixing the TDE, I see a bit of glimmer on the dark half of the edge.
Next session: No idea. I don't want the center to go bad, but I must fix TDE. Maybe a bit of warm pressing? Maybe re-channel the tool yet again? MoT? Long cold pressing, remove the mesh at midpoint?
2011/12/26
Full diameter lap, Gugolz #64.Pressed with 10 lb, 90 min with mesh, 60 min without mesh.
60 min MoT, W stroke 1/4 D. Slow strokes, almost no vertical pressure. Eccentric rotations. Decent contact.
Trying to reduce TDE.
71.8 F ... 70.0 F
Central hole and raised ring around it - probably getting better. TDE still persists. The lip defect seems to disappear slowly. Very dirty mirror; either distilled water is impure, or the compressed air can is bad.
Next session: No idea. MoT, I guess.
2011/12/27
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 70 min without mesh.
60 min MoT, W stroke 1/4 D. Slow strokes, almost no vertical pressure. Eccentric rotations. So-so contact.
Trying to reduce TDE.
70.0 F ... 67.3 F
Hard to say if the central defects are getting better. The outer 1/4 is oblate spheroid. The flat lip does seem to get better. If there's any progress with TDE, it's invisible.
Next session: Same.
2011/12/29
Full diameter lap, Gugolz #64, old and very hard.Pressed with 10 lb, 240 min without mesh.
30 min MoT, W stroke 1/4 D. Slow strokes, almost no vertical pressure. Eccentric rotations. Bizarre contact, smooth when pushing, skippy when pulling.
Trying to reduce TDE.
69.4 F
The surface is basically the same: a central narrow hole, a fat slightly raised donut around it, the outer stripe too concave. The flat lip due to concentric rotation is reduced a lot. A little bit of bright sharp edge is appearing on the left, which may indicate that TDE is decreasing; it's not very bright and not very sharp yet, and it's not continuous all around the mirror, which means TDE is definitely still present. Overall the surface is smooth; tendency towards oblate spheroid (I guess because of MoT). I can't believe how dirty it is, I need to keep track of various distilled water brands, and avoid the bad ones.
Next session: Same.
2011/12/30
Full diameter lap, Gugolz #64, old and very hard.Pressed with 10 lb, 6 h 35 min without mesh.
60 min MoT, W stroke 1/4 D. Slow strokes, almost no vertical pressure. Eccentric rotations. Excellent contact, deep, smooth and oily; even a hard lap can have good contact if pressed long enough. Lots and lots of drag, probably because I'm not using the mesh.
Trying to reduce TDE.
70.0 F
The ring around the edge is still not perfect, more shiny and sharp on the right side. The figure is deteriorating, the central hole is wider and deeper, the donut is bigger, the outer stripe is more steep. The flat lip due to concentric rotation is almost gone. Some wide roughness, I'm guessing due to drag.
Next session: Maybe ToT this time around? Anyway, I need to press with mesh.
2012/01/02
Full diameter lap, Gugolz #64, old and very hard.Pressed with 10 lb, 5 h 10 min with mesh, 2 h 10 min without mesh. Very clear and regular microchannels imprinted in the lap.
55 min ToT, W stroke 1/4 D. Slow strokes, almost no vertical pressure. Eccentric rotations, pulling the tool a bit up (but keeping contact) when rotating it before rotating the turntable. Okay contact.
Trying to reduce TDE.
73.0 F - 70.7 F
Not much progress. TDE still present. Figure still imperfect. Some large-scale roughness, maybe improved from last time. The bright ring in the images does not match the visual impression: it's closer to continuous visually, and sharper and brighter on the left side, but still not perfect. The flat lip due to bad rotation technique seems gone. Mirror still dirty - maybe it's the paper towels? I need to keep track of the types of paper towels and distilled water I use.
Next session: I should re-channel and trim the lap. Maybe I need to rebuild it? Try and don't wipe the mirror off with paper towels at all.
2012/01/03
Full diameter lap, Gugolz #64, old and very hard.Pressed with 10 lb, 2 h 55 min without mesh. Microchannels still visible from last time.
30 min ToT, W stroke 1/4 D. Slow strokes, almost no vertical pressure. Eccentric rotations, pulling the tool a bit up (but keeping contact) when rotating it before rotating the turntable. Good contact.
Trying to reduce TDE.
70.3 F - 69.6 F
Next session: Keep doing this. Trim the lap, and re-channel it. Maybe try CoC strokes instead of W.
Re-channeled and trimmed the lap.
2012/01/04
Full diameter lap, Gugolz #64, old and very hard.Pressed with 10 lb, 2 h 40 min without mesh.
60 min ToT, W stroke 1/4 D.
Trying to reduce TDE.
68.4 F
2012/01/05
Full diameter lap, Gugolz #64, old and very hard.Pressed with 10 lb, 2 h 10 min without mesh.
30 min ToT, W stroke 1/4 D.
Trying to reduce TDE.
67.1 F
2012/01/07
Full diameter lap, Gugolz #64, old and very hard.Pressed with 10 lb, 5 h with mesh, 4 h 40 min without mesh.
35 min MoT, W stroke 1/4 D.
Trying to reduce TDE.
69.3 F
2012/01/08
Full diameter lap, Gugolz #64, old and very hard.Pressed with 10 lb, 5 h 20 min without mesh.
30 min ToT, W stroke 1/4 D.
Trying to reduce TDE.
67.5 F
Poured a new lap, Gugolz #64:
Re-filled the CerOx bottle with 1670 diluted 1:40.
Using 200 lines/inch Ronchi screen from now on. The figure will be much amplified.
The goal now is to obtain a sphere.
2012/01/21
Full diameter lap, Gugolz #64.Pressed with 10 lb, 30 min with mesh, 30 min without mesh.
65 min MoT, W stroke 1/3 D. Loose contact at first, became very good towards the end.
Trying to break in the lap.
69.1 F - 67.7 F
The figure is overall oblate (undercorrected). A wide central hill, not too tall; perhaps the peak is flattened a little. Smooth surface overall, no roughness, good job.
Next session: Attack the central hill.
Put a drop of turpentine in the lap's box until tomorrow, to prevent drying out.
2012/01/22
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 60 min without mesh.
MoT. 15 min W stroke 1/3. 5 min center-on-edge short W strokes to fix central bulge, per Texereau.
67.5 F
No visible changes.
==========================
Full diameter lap, Gugolz #64.
Pressed with 10 lb, 30 min with mesh, 30 min without mesh.
MoT. 15 min W stroke 1/3. 10 min center-on-edge short W strokes to fix central bulge, per Texereau.
69.3 - 68.5 F
Hill maybe decreased a little. Small central hole developing.
==========================
Full diameter lap, Gugolz #64.
Pressed with 10 lb, 30 min with mesh, 30 min without mesh.
MoT. 25 min W stroke 1/3. 15 min center-on-edge short W strokes to fix central bulge, per Texereau; this time center not quite all the way to the edge, to avoid increasing central hole.
70.9 - 69.4 F
Compared to yesterday, the hill is wider (it's as if correction pushes the defect down and towards the edge), perhaps less tall. There's a hole at the top. Hill + hole = donut. The figure is very smooth, surprising after all the tough correcting strokes and negligent (fast) polishing.
Next session: Reduce the donut.
Put a drop of turpentine in the lap's box until tomorrow, to prevent drying out.
2012/01/23
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 30 min without mesh.
MoT. 20 min W stroke 1/3. 10 min donut-on-edge short W strokes to fix central bulge, per Texereau. Good contact but drag was a bit bizarre; it seemed as if the surface of the lap was soft, probably due to the turpentine drop.
69.6 - 68.7 F
Donut reduced in height. Central hole increased. Some large scale irregularities that may not be due entirely to air turbulence during testing.
Next session: More of the same? Also, re-channel the lap, make deep channels.
No turpentine drop this time.
Re-channeled the lap, deep channels.
2012/01/28
Full diameter lap, Gugolz #64.Pressed with 10 lb, 80 min with mesh, 65 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 10 min W stroke 1/3. 10 min donut-on-edge short W strokes to fix central defect, per Texereau. Wet paper band around the lap; rotated mirror and lap during an additional stroke; per Ingalls. Excellent contact from the get-go.
69.5 F
All major defects have decreased markedly. The donut is almost flattened. The steep outer band is reduced. The mirror is now close to an oblate spheroid, with only a few ripples.
Next session: full lap, normal stroke, hoping to smooth the residual defects and remove the undercorrection.
===========================
Full diameter lap, Gugolz #64.
Pressed with 10 lb, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 30 min W stroke 1/3. Wet paper band around the lap; rotated mirror and lap during an additional stroke; per Ingalls. Very good contact, smooth strokes indicating closing in on the sphere. Just a tiny bit of grabbing at the end.
Trying to smooth the surface.
72.4 - 70.9 F
The outer 1/3 is oblate spheroid, and rather smooth. The inner 2/3 are nearly spherical; you can see the base of the donut still remaining, a tiny shallow hole in the middle. Some roughness in the inner 2/3. There's a ghost square network visible, probably the lap pressed some CerOx into the mirror and I didn't clean it up well.
Next session: not sure. I should eliminate the donut completely, I guess.
2012/01/29
Full diameter lap, Gugolz #64.Pressed with 10 lb, 65 min with mesh, 35 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 10 min W stroke 1/3. 3 min donut-on-edge short W strokes to fix central defect, per Texereau. Wet paper band around the lap; rotated mirror and lap during an additional stroke; per Ingalls. Excellent contact from the get-go.
69.5 F
Trying to reduce the donut.
Donut reduced somewhat. Central hole reduced. Outer 1/3 D still oblate. Inner 2/3 a bit rough. The lap channels are statically printed into the mirror - how did THAT happen? Or is it just CeO2 clinging to the glass? I'll have to clean it up a lot better next time.
Next session: More of the same. Get rid of the donut completely.
2012/02/04
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 2 h 50 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 15 min W stroke 1/3. 5 min short radial W stroke (as opposed to short chordal W stroke until now), mirror center near tool's edge, per Ingalls vol. 1 page 191 - same as chordal but reduced risk of digging a hole when reducing the hill.
71.1 F
Trying to reduce the donut.
No visible changes.
==============================
Full diameter lap, Gugolz #64.
Pressed with 10 lb, 50 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 10 min W stroke 1/3. 10 min short radial W stroke (as opposed to short chordal W stroke until now), mirror center near tool's edge, per Ingalls vol. 1 page 191 - same as chordal but reduced risk of digging a hole when reducing the hill.
70.7 F
Trying to reduce the donut.
Hill seems bigger. Still there's a hole at the top. The squares on the tool are printed on the mirror's face somehow. Maybe it's the turpentine drop?
Next session: Long session of normal W strokes to try and reset this nonsense. No turpentine drop from now on.
2012/02/05
Full diameter lap, Gugolz #64.Pressed with 10 lb, 30 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 25 min W stroke 1/3. Wet paper band around the lap; rotated mirror and lap during an additional stroke; per Ingalls. Okay contact.
68.7 F
Trying to reset the whole mess, I wish I had more time this session.
Okay, I'm stuck. Not making any significant progress lately. Either rebuild this lap, or make a 3/4 diameter tool, or both.
Made a 3/4 diameter lap. Orthodontic stone (dental plaster), about 2.5 cm thick, covered in epoxy resin. Gugolz #64 pitch.
Channeled with hot tool. Pressed about 1 hour (?) with mesh, about 1 hour (?) without mesh, 5 lb.
2012/02/26
3/4 diameter lap, Gugolz #64.Pressed with 5 lb, 60 min with mesh, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
ToT. 30 min oval stroke 3:1, 1 cm overhang, per Mark Harry. Wet paper band around the lap; rotated mirror on support every 5 min. Good contact.
70.3 F
Working towards achieving a sphere.
There's still a raised ring in the middle (bump with hole on top), maybe reduced a little. The weird grid imprinted by the big tool is attenuated but not gone. Surface a bit more rough than ideal.
Next session: More of the same.
==============================
3/4 diameter lap, Gugolz #64.
Pressed with 5 lb, 1 h 45 min min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
ToT. 30 min oval stroke 3:1, 1 cm overhang, per Mark Harry. Wet paper band around the lap; rotated mirror on support every 5 min. Very good contact.
72 F, 46% humidity
Working towards achieving a sphere.
Good and bad news. Good news: the ring is getting flattened. Bad news: the surface is rough from the imprinted grid.
Next session: More of the same, don't mess with the big lap right now.
2012/02/27
Pressed the full size lap 2 hours without mesh, thinking to use it to reduce the ring. Bad idea, the ring is getting flattened anyway, and the full size lap may imprint bad stuff again. Don't do it.
Okay, no, it did not imprint anything. Looks like the full size lap is clean, no need to rebuild it. Phew.
==============================
3/4 diameter lap, Gugolz #64.
Pressed with 5 lb, 1 h 5 min min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
ToT. 60 min oval stroke 3:1, 1 cm overhang, per Mark Harry. Wet paper band around the lap; rotated mirror on support every 5 min. Excellent, deep contact, had to add a drop of CerOx and some water to make it more loose.
72...70 F, 42...45% humidity
Working towards achieving a sphere.
Awesome! This is closer to a sphere than ever. To the naked eye it looks even better. Just some zones, center vs. edge. A bit rough, especially on the outside. Tiny amount of leftover TDE.
Next session: Keep working with the 3/4 lap, maybe oval, maybe W or some other stroke? Bring it closer to a sphere and smooth out the roughness.
==============================
3/4 diameter lap, Gugolz #64.
Pressed with 5 lb, 60 min with mesh, 1 h 15 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
ToT. 60 min oval stroke, varying the shape and size of oval. Three ovals before each rotation. First oval very thin, overhang a few mm. Second oval more fat. Third oval wide and tall. Very slow strokes; no vertical pressure, moving the tool from its edge; this is for reducing roughness. Wet paper band around the lap; rotated mirror on support every 5 min. Very good contact, buttery and smooth, had to add CerOx and water a few times to make it more loose.
68 F, 49% humidity
Working towards achieving a sphere.
Wow. Several surprising and interesting developments. It's sliding towards a parabola; I guess it's actually not far from the final paraboloidal figure. There's a tiny raised button in the middle. The overall surface is much more smooth, good job. Finally, it looks like I raised the edge pretty close to level! This is the first time I see the edge looking rather decent; there's a sharp, nearly continuous diffraction ring around the mirror. The 3/4 lap worked miracles.
Next session: I've a feeling I could stop now, put the mirror in a telescope, and it would do okay. But no. The parabola is probably not perfect. There's some residual roughness on the outside. There's some residual TDE, maybe, so I could probably raise it a bit more. And I need to learn a stroke that will reliably sphericize a mirror and reset it to a sphere reliably, every single time I need it.
2012/02/28
3/4 diameter lap, Gugolz #64.Pressed with 5 lb, 60 min with mesh, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
ToT. 30 min oval stroke. Went back to equal size ovals, pretty narrow, a few mm overhang, but this time swaying side to side with each oval. Basically like the W stroke, only with oval segments. Very slow strokes; no vertical pressure, moving the tool from its edge; this is for reducing roughness. Wet paper band around the lap; rotated mirror on support every 5 min. Good contact.
70 F, 50% humidity
Working towards achieving a sphere.
Not good, this is making the correction deeper. Bit of roughness on the outside. Hint of button in the center.
Next session: Maybe I should try the full size lap again.
2012/02/29
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
ToT. 55 min narrow W stroke 1/3 D. Wet paper band around the lap; rotated mirror on support every 5 min. Bad contact, seemed to grab when lap and mirror were coaxial. Kinda skipping a bit everywhere. Had to add water and CerOx all the time.
72...68 F, 53...54% humidity
Working towards achieving a sphere.
Correction was reduced, but it's not a sphere. Not much low-scale roughness. Maybe some large-scale roughness, perhaps not as bad as the picture indicates (air currents were bad). Button in the middle.
Next session: Full lap, MoT?
2012/03/01
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 25 min narrow W stroke 1/3 D. Wet paper band around the lap. Good contact.
68 F, 54% humidity
Working towards achieving a sphere.
Much better! This is very nearly a perfect sphere. Some hints of parabolic correction are visible with the camera (but are invisible to the naked eye - very subtle). If it was any closer to a sphere this test could not have detected any correction. Good edge. Not all roughness in the image is on the mirror - the air currents were bad; I guess it's fairly smooth, but at this level all irregularities are very visible.
Next session: I could start parabolizing now. But I am tempted to just do more of the same, hopefully reducing roughness even more, maybe sphericizing it even better. I need to wait anyway for the Mobsby grid to arrive.
2012/03/02
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 35 min narrow W stroke 1/3 D. Worked very slowly, no pressure. Wet paper band around the lap. Good contact.
68 F, 52% humidity
Working towards achieving a sphere.
(bad image, not fully illuminated)
Sweet smooth sphere, Batman! All zones are nearly gone. Maybe a hint of oblateness on the outside? Maybe, not sure. There's so little roughness, it's hard to tell whether it's real, or projected by air turbulence. If this was a reference sphere for an optics lab, I would probably not stop here; but as a basis for a parabola, it's very good.
Next session: I really need to stop delaying and begin parabolization. There's no excuse now (waiting for the Mobsby grid is not an excuse, the Foucault test is good enough - and better in some ways).
Parabolization
2012/03/04
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 10 min narrow W stroke 1/3 D. Worked very slowly, no pressure. Wet paper band around the lap. Good contact.
MoT. W stroke 4/5 D. 72 strokes (6 strokes, 12 directions), very slow, 1 turn of the mirror. Sketchy contact.
73 F, 47% humidity
Next session: Twice the number of parabolizing strokes.
2012/03/05
3/4 diameter lap, Gugolz #64.Pressed with 5 lb, 60 min with mesh, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
ToT. 15 min wide W stroke 3/4 D. Worked very slowly, no pressure. Wet paper band around the lap. Good contact.
68 F, 57% humidity
Next session: Big lap, classic parabolizing strokes (144).
2012/03/06
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 25 min W stroke 1/3 D. Worked very slowly, no pressure. Wet paper band around the lap. Good contact.
MoT. Wide W stroke, 4/5 D, 144 strokes: 6 strokes * 12 directions * 2 rounds. Okay contact, at the end grabbing when concentric.
66 F, 49% humidity
Looks like I dug a hole in the center, but the outside remained too flat (spherical). It's undercorrected and uneven (center more corrected than edge), but if this was a commercial mirror, it would be good to go now - it's lambda/5. But it's not good enough for me.
Next session: I need to spread it out towards the edge. Perhaps use the same number of strokes; it's probably not enough, but as of now I just want to see how it spreads out. Work 30 min the W stroke 1/3 D, for good contact, before the parabolizing stroke.
2012/03/07
Full diameter lap, Gugolz #64.Pressed with 10 lb, 30 min with mesh, 70 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 30 min W stroke 1/3 D. Worked very slowly, no pressure. Wet paper band around the lap. Good contact. Did not verify if this reset the mirror back to a sphere, but I assume it did.
MoT. Wide W stroke, 4/5 D. Working more on the center: stroke 1 at the edge, stroke 2 half-way, strokes 3 and 4 nearly center-over-center, stroke 5 half-way, stroke 6 at the edge. 144 strokes total: 6 strokes * 12 directions * 2 rounds. Good contact, at the end grabbing when concentric.
70 F, 43% humidity
The good news is that it's now a lambda/15 mirror, 0.987 Strehl - for many purposes an excellent catopter, far better than many commercial mirrors. The number of parabolizing strokes seems just right. The bad news is that it's not the way I want it; while very accurate, it still shows a shoulder around a central hole that is steeper than I'd like. In other words, the center is a bit hyperbolic (strongly corrected), while the outside is a bit elliptic (weakly corrected). I want a smooth shape all around.
Next session: For some reason, I dig a central hole more quickly than I manage to touch the outside. Perhaps I should try a parabolizing zig-zag even more compressed towards the center, almost like a very long W stroke. Or do a few strokes with the 3/4 lap on the surface as-is? (but perhaps that one would deepen the center too much) Or go with the full lap once-around, a few strokes, polishing off the shoulder with its edge.
2012/03/08
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 30 min W stroke 1/3 D. Worked very slowly, no pressure. Wet paper band around the lap. Good contact. Did not verify if this reset the mirror back to a sphere, but I assume it did.
MoT. Wide W stroke, 3/4 D. Working even more on the center: stroke 1 towards the edge, stroke 2 near center, strokes 3 and 4 at center, stroke 5 near center, stroke 6 towards the edge. 144 strokes total: 6 strokes * 12 directions * 2 rounds. Good contact.
68 F, 49...52% humidity
So, I reduced the amplitude of the parabolizing stroke, and gathered it even more at the center. It's still digging a hole in the center and leaves the outside spherical (or possibly even oblate). Not good. Maybe the 30 min W stroke 1/3 D doesn't sphericize it? At least it's smooth, no obvious large defects other than imperfect correction.
Next session: Not sure. Perhaps don't try to parabolize it, just reset to a sphere and then check it's really a sphere.
2012/03/09
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 60 min without mesh. Did not part the mirror and the lap when transferring the stack to the polishing table, per Mike Lockwood.
MoT. 30 min W stroke 1/3 D. Worked very slowly, no pressure. Wet paper band around the lap. Very good contact.
68 F, 47% humidity
Trying to reset to a sphere.
Well, that explains it. The normal W strokes with full lap don't reset it; I was not working from a sphere when trying to parabolize it. There's still a hole in the middle, the outside is spherical or maybe a tiny bit oblate (raised towards the hole).
Next session: Try W strokes with the 3/4 D lap.
2012/03/10
3/4 diameter lap, Gugolz #64.Pressed with 5 lb, 60 min with mesh, 60 min without mesh.
ToT. 30 min W stroke, a few mm overhang. Worked very slowly, no pressure. Wet paper band around the lap. Very good contact.
70 F, 51% humidity
Trying to reset to a sphere.
==============================
3/4 diameter lap, Gugolz #64.
Pressed with 5 lb, 60 min with mesh, 1 h 35 min without mesh.
ToT. 30 min W stroke, a few mm overhang. Worked very slowly, no pressure. Wet paper band around the lap. Very good contact.
68 F, 57% humidity
Trying to reset to a sphere.
The outer zone is being corrected towards a parabola. The central hole is still big.
Next session: Time to use the full lap and rebase to a new sphere, the ToT local edge pressure method per Texereau.
Full lap and 3/4 lap were rechanneled.
2012/03/11
1/3 diameter lap, Gugolz #64.Pressed with 1 lb, 60 min with mesh, 2 h 45 min without mesh.
ToT. 2 ... 3 min of short strokes on the lip of the central hole, petering out towards the edge. Good contact actually.
70 F, 49% humidity
Trying to rebase to a new sphere.
==============================
1/3 diameter lap, Gugolz #64.
Pressed with 1 lb, 30 min without mesh.
ToT. 10 min of short strokes on the lip of the central hole, petering out towards the edge. Good contact actually.
70 F, 49% humidity
Trying to rebase to a new sphere. Strong medicine.
Next session: Smoothing with the big lap.
==============================
Full diameter lap, Gugolz #64.
Pressed with 10 lb, 60 min with mesh, 60 min without mesh.
MoT. 60 min W strokes 1/3 D. Good contact.
68 F, 55% humidity
Zones are more or less gone, some traces remain. The mirror is overall a bit oblate. A bit lumpy, but it's small amplitude (Foucault image is very close to focus). Not bad, considering the brutal treatment to remove the hole.
Next session: I think it's actually ready for parabolizing again. Either full lap, 30 min W strokes, then parabolizing strokes; or the 3/4 lap 30...60 min. The lumps should be greately reduced or eliminated, and correction should appear instead.
2012/03/12
3/4 diameter lap, Gugolz #64.Pressed with 5 lb, 30 min with mesh, 60 min without mesh.
ToT. 60 min large W stroke, until edge of tool goes over center of mirror. Worked very slowly, no pressure. Wet paper band around the lap. Good contact.
68 F, 53...56% humidity
Trying to parabolize.
No parabolization took place. This is different from oval stroke. Mirror is actually somewhat close to a sphere. Outside sliding towards a parabola. Central bump, very slight. Edge might be a bit raised actually. Probably reduced lumpiness overall, compared to last time.
Next session: full lap, parabolizing stroke.
2012/03/13
Full diameter lap, Gugolz #64.Pressed with 10 lb, 70 min with mesh, 1 h 35 min without mesh.
MoT. 20 min W stroke 1/3 D. Worked very slowly, no pressure. Good contact.
MoT. 72 strokes (6 x 12 x 1) W 3/4 D. Stroke 1 near the edge, stroke 2 near the center, strokes 3 and 4 at the center, 5 near the center, 6 near the edge - all strokes pulled towards the center, to avoid digging a hole again.
68 F, 57% humidity, it's raining outside.
Trying to parabolize.
It's actually not far from a sphere. The outside has a hint of correction. The center has a hint of a bump.
Next session: Parabolizing stroke with full lap again. The question is, how many strokes. Need to think about it.
2012/03/14
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min without mesh. No pressing with mesh, I want it to get into contact quickly. Mirror + lap stack exposed to warm air a few minutes (HVAC went through a cycle). This is to make the lap warmer.
MoT. 10 min W stroke 1/3 D. Worked very slowly, no pressure. Very good contact, considering it was only 10 min.
MoT. 72 strokes (6 x 12 x 1) W 3/4 D. Stroke 1 near the edge, stroke 2 half-way, strokes 3 and 4 very close to the center, 5 half-way, 6 near the edge - strokes pulled a bit towards the center, to avoid digging a hole again.
70 F, 66% humidity, it's raining outside.
Trying to parabolize.
No, it just doesn't work. The outside has a tiny amount of correction. The center is sunk into a hole. At least it's smooth and the edge is very good. It could be that the lap is too hard and pitch is not flowing.
Next session: Try and reset it to a sphere with W strokes 1/3 D. After that try the 3/4 lap, oval strokes. Or classic strokes with full lap at high temperature, to get the pitch flowing. Or use a star lap.
2012/03/15
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 1 h 30 min without mesh.
MoT. 60 min W stroke 1/3 D. Worked very slowly, no pressure. Good contact.
68 F, 66% humidity, it's raining outside.
Trying to reset to a sphere.
Okay, target acquired. It's a reasonable approximation of a sphere. The outside is a tiny bit raised. The inside is very close to a sphere. Trace amounts of lumpiness. Excellent edge. Ah, the wonders of a hard lap.
Next session: Okay, time to start getting results. I purchased an electrical heater for the room, I'll do parabolizing strokes with the full lap at a higher temperature, to get the pitch flowing and avoid digging a hole in the middle again. Maybe 74 F?
2012/03/16
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min without mesh, at 73 F. Working at a higher temperature to allow the pitch to flow.
MoT. 10 min W stroke 1/3 D. Worked very slowly, no pressure. Very good contact, oily. Pitch feels softer.
MoT. 72 strokes 3/4 D (6 strokes * 12 directions * 1 turn). Strokes even between edge and center. Much better contact with the 3/4D strokes at higher temperature.
72 F, 62% humidity.
Trying to parabolize.
Next session: Same thing, more strokes.
2012/03/17
Full diameter lap, Gugolz #64.Pressed with 10 lb, 1 h 30 min without mesh, at 72 F. Working at a higher temperature to allow the pitch to flow.
MoT. 10 min W stroke 1/3 D. Worked very slowly, no pressure. Very good contact, oily. Pitch feels softer.
MoT. 144 strokes 3/4 D (6 strokes * 12 directions * 1 turn). Strokes even between edge and center. Contact was good at first, but then it started dragging a heck of a lot, I had to push the mirror real hard. Not pressing with mesh was the cause, I guess.
73 F, 54% humidity.
Trying to parabolize.
I dug a hole in the center again. Pressing with mesh is not optional, I must do it every single time.
Next session: fix the hole.
==============================
3/4 diameter lap, Gugolz #64.
Pressed with 5 lb, 40 min with mesh, 60 min without mesh.
ToT. Oval strokes 3:1, overhang a few mm. Worked very slowly, no pressure. Very good contact.
70 F, 53% humidity.
Trying to reduce the hole.
The hole was probably reduced somewhat. The outside is showing correction. It's a bit lumpy and shows regular patterns - the result of awkward parabolizing stroke.
Next session: Keep fixing the hole. After that: once I have a sphere, do oval strokes with 3/4 lap.
2012/03/18
Full diameter lap, Gugolz #64.Pressed with 10 lb, 40 min with mesh, 60 min without mesh.
ToT. 45 min W stroke 1/3 D. Worked very slowly, no pressure. Contact a bit skippy at first, improved later, started to degrade before I stopped. Maybe I need to press more with the mesh.
70 F, 49% humidity.
Trying to fix the hole.
The hole did not seem to decrease much; normal strokes don't reduce steep holes like this. The outside does seem a bit less corrected, moving back to a sphere. Overall less lumpiness, very good. The edge is pretty awesome. :)
Next session: Need stronger medicine - use the 1/3 lap. It will create lumps, but it should make the hole go away, and the lumps can be reduced later.
==============================
1/3 diameter lap, Gugolz #64.
Pressed with 1 lb, 1 h 25 min with mesh, 2 h without mesh.
ToT. 10 min chordal stroke over the lip of the central hole, petering out towards outside and towards center, while rotating the mirror underneath. Good contact.
66 F, 48% humidity.
Trying to fix the hole.
Hey, not bad. I made the hole disappear, pretty cool. It's overall a sphere, some lumps in the middle. This is very good for this method.
Next session: Smoothing over with the full lap.
==============================
Full diameter lap, Gugolz #64.
Pressed with 10 lb, 2 h 10 min with mesh, 60 min without mesh.
MoT. 30 min W stroke 1/3 D. Good contact at first, skippy later. Too cold?
68 F, 52% humidity.
Smooting out the lumps.
Yes, it's a lot more smooth now. Quite close to a sphere now, a couple dips, hint of central button.
Next session: 3/4 lap, not to parabolize it yet, but just to exercise it down and up, to shake off the lumps. After that full lap again to bring it out of correction; after that I can start the parabolization attempt with the 3/4 lap.
2012/03/19
3/4 diameter lap, Gugolz #64.Pressed with 5 lb, 60 min with mesh, 1 h 45 min without mesh.
ToT. 60 min oval stroke, a few mm overhang. Medium speed, no pressure. Good contact.
70 F, 49% humidity.
Smoothing.
Hint of correction on the outside. Lumps are reduced.
Next session: full lap.
2012/03/20
Full diameter lap, Gugolz #64.Pressed with 10 lb, 2 h 10 min with mesh, 1 h 45 min without mesh.
MoT. 60 min W stroke 1/3 D. Medium speed, no pressure. Good contact.
70 F, 52% humidity.
Smoothing.
Irregularities are gone! Awesome. A few zones, but that's okay. Hint of oblateness.
Next session: Here we go, parabolization with 3/4 lap.
==============================
3/4 diameter lap, Gugolz #64.
Pressed with 5 lb, 60 min with mesh, 1 h 40 min without mesh.
ToT. 60 min oval strokes, a few mm overhang. Slow speed, no pressure. Very good contact.
70 F, 50% humidity.
Attempting to apply some correction.
Oblate outer zone is gone, now it's nearly spherical. Central button, hint of moat around it. Now that it's spherical and the test is more sensitive, I see some remnants of lumps, very very faint. The surface is very smooth.
Next session: More of the same. I need to apply correction to the outer zone. I'll deal with the center after that.
2012/03/21
3/4 diameter lap, Gugolz #64.Pressed with 5 lb, 60 min with mesh, 2 h 20 min without mesh.
ToT. 45 min oval strokes, 1 cm overhang. Slow speed, no pressure. Very good contact.
68 F, 60% humidity.
Attempting to apply some correction.
Very slowly soaking up the correction. Excellent edge. Very smooth and regular surface. Central button will have to be reduced later.
Next session: More of this. It's probably good to overcorrect a little, then clean up the button, then reduce correction with a W stroke while smoothing the surface at the same time.
2012/03/22
3/4 diameter lap, Gugolz #64.Pressed with 5 lb, 60 min with mesh, 1 h 20 min without mesh.
ToT. 35 min oval strokes, 1 cm overhang. Slow speed, no pressure. Very good contact.
70 F, 52% humidity.
Attempting to apply some correction.
It's a lambda/7.6 right now, almost 2x better than the minimum commercial standard; estimated Strehl factor 0.947; I could stop now. But I don't like the central button, and it's overall undercorrected - best fit conic constant is -0.55 instead of -1.0, which is not enough; it's elliptic instead of parabolic.
Next session: Maybe I should try the big W parabolizing stroke with the full lap. That should reduce the button and maybe put more correction on the surface. Do only a little of it, as it may dig a hole instead.
Rechanneled the full diameter lap.
2012/03/24
Full diameter lap, Gugolz #64.Pressed with 10 lb, 60 min with mesh, 70 min without mesh.
MoT. Once around with edge of tool on center of mirror, short zig-zag strokes to reduce the central button.
MoT. Once around wide long W strokes (like parabolizing) to smooth out the previous strokes.
70 F, 51% humidity.
==============================
Fresh 1:40 CerOx.
==============================Full diameter lap, Gugolz #64.
Pressed at 75 F, with 10 lb, 30 min with mesh, 30 min without mesh.
MoT. 10 min W strokes 1/3 D, enhancing contact, smoothing it out and possibly reducing the existing parabola.
MoT. W parabolizing strokes, 3/4 D. 96 strokes total (8 strokes * 12 groups * 1 turn). Okay contact.
75 F, 50 % humidity.
This is it. A small button persists in the center, but it will be masked off by the secondary mirror anyway, so it won't affect the overall performance (it's the "dead" part of the primary mirror). The rest is a nice parabola - the Foucault test says it's lambda/22 which is very very good (many commercial mirrors are lambda/4). Very smooth surface. Excellent edge, beyond reproach. This mirror is going to perform very well.
Next session: There will be no next session. Eliminating the central button will make no visible difference in the eyepiece in a newtonian reflector (it might make a difference in an instrument without a central obstruction, but that's not the case here). I need to stop now, otherwise I will miss the upcoming two events in a couple months (the partial solar eclipse, and the transit of Venus). I may revisit this mirror next year, after I finish the big telescope, but it will be purely for the sake of perfection.
Time to build the telescope.
P.S.: This is how the telescope looked like during construction:
Here it is in use during an event (the solar transit of Venus in 2012):
