GRINDING
January 2000
The pyrex disc (205x 20mm) is hollowed out to approx. f4.5 by J.Eindhoven in Haaksbergen.
I cast a 4 cm thick plaster disk on it and after 1 week drying I glued 19x19x2 mm porcelain tiles on it with epoxy resin. Distance between the tiles ca 7 mm. Overhanging tiles at the edge were nibbled smaller with a pair of pliers.
Start with C320 powder
It took 85 minutes MOT(mirror on top) and TOT(tool on top) to remove all the obvious pits( I marked them on the backside of the mirror).I used 1/3 and ¼ W-strokes and straight strokes.
C600 powder
60 minutes TOT and MOT.I bevelled the edge again with a fine grinding stone( grade 400, to prevent chipping)
C800
90 minutes MOT and TOT
1/3 -1/4 W stroke and straight stroke.
C1200
1/3-1/4 W and straight strokes
I was busy for 95 minutes.In the end I could see 12-print through the mirror .
POLISHING
February 2000
I made a pitchlap from old, hard pitch. Squares of 3 cm.
I polished most of the time in sessions of 30 minutes, MOT and TOT. Polishing agent: cerium oxyde suspension in water with some drops of soap solution.
To test the surface I used a clear 25 W lamp and a focussing lens. In that way I could focus the bright filament on the glass surface.
180 min: reflection still strong
330 min: reflection much weaker
560 min : no reflection visible( with magnifying glass)on edge
CORRECTION first try
Nov 11, 2000
Ronchi test: spherical with TDE (turned down edge)
I made a soft correcting lap from old hard pitch and some turpentine
Step 1 MOT
1/3 W stroke very slow
Result: spherical with TDE
Step 2 MOT
½ W-stroke : 5x 1 turn (pressing with 2 kg weight for several minutes between the turns)
Ronchi : straight lines
Step 3 MOT
8x 1 turn 2/3 W-stroke
Ronchi:lines start to bulge. Flat center,though
Foucault test (5 zone Couder mask)
Zone Measurements Parabola
5 0,7 2,43 mm
3 0,3 1,28
1 0 0
Step 4 MOT
- ¾ W-stroke more on center 3×1 turn
- 2/3 W-stroke 2×1 turn
Ronchi:bulging center lines, edge more flat
Step 5 MOT
½ W-stroke very narrow 3x 1 turn
No change
Step 6 MOT
10 minutes 1/3 narrow W-stroke
Ronchi : parabola
Fouc zone Meas. Parabola
1 0 0
2 0,8 0,70
3 1,9 1,28
4 2,3 1,83
5 2,8 2,43
Step 7 Decrease correction MOT
Central area of pitchlap(ca 5 cm diamter) is scratched away.Less work on center!
Narrow 1/3 W-stroke 7 min.
Ronchi : still hole in center
Fouc.
Zone Meas. Parabola
1 0 0
2 0,9 mm 0,7mm
3 1,4 1,28
4 1,9 1,83
5 2,2 2,43
Step 8 MOT
Increase correction and get rid of central hole
Central part of pitch scratched away
- 1/3 W-stroke (narrow) : 3 turns around (8 W-strokes per turn around)
- ½ W-stroke wide 2x 1 turn around
Ronchi : looks better now
Fouc. Zone meas. Parabola
1 0 0
2 0,7 0,7
3 1,0 1,28
4 1,4 1,83
5 1,8 2,43
Step 9 MOT
a. 1/3 W-stroke with pitch scratched away in central 5 cm : 4 turns around.
- Renewed warm pressing to get all the pitch into full contact again.
Then: 2/3 W-stroke to increase correction 2x 1 turn
Ronchi: looks OK
Fouc.
Zone meas. Parabola
1 0 0
2 0,63 0,7
3 1,1 1,28
4 1,7 1,83
5 2,3 2,43
-After this , I used the Ross test which is described in Telescope Making Journal. I found undercorrection with the Ronchi lines in the test.
I made a new pitchlap which I “initiated”by doing 3 turns with 1/3 W MOT.In hindsight this was no good idea: the pitchlap should be saturated with polishing agent to get an even polish
Step 10 MOT
Increase correction: 2/3 W-stroke 2 x ½ turn (cold pressing in between)
Still undercorrected
Step 11
Same as step 11: still undercorrected
Step 12
MOT 2/3 W-stroke 4x ½ turn
Ross test: ronchi lines straight
Startest feb 13, 2001: star image astigmatic at 88x. The astigmatism rotates with the main mirror.
SO THIS IS A FAILURE…
Start again with a much thicker pyrex disk
Grinding and polishing the same as with the previous disk.Astigmatism wasn’t visible at 140x ( in the beginning of polishing). Polishing took 10,5 hours.
CORRECTION : nearly always done with mirror on top(MOT)
First try
I made a soft pitchlap from Gugolz 55 with some 64
The pitch squares were made smaller going from center to edge( tapered pitchlap)
Polishing agent : cerium oxyde
Step 1
MOT 1/3-1/4 W-stroke : slow with little pressure : 3x 15 min
Ronchi : weak bulge
Fouc: good surface ( no dog biscuit)
F=837 mm
Step 2 MOT
2/3 W-stroke 5x 1 turn around
Ronchi: big central bulge: no correction in outer zones
Step 3 MOT
a.1/3 W-stroke narrow 5 turns
b.½ W-stroke 2×1 turn
Ronchi: better, outer zones starting to parabolize.
Step 4 MOT
2/3 W-stroke narrow 2×1 turn
Test: outer zones not enough correction( big central bulge in Ronchi and parallel outer lines)
Step 5 MOT
½ W-stroke narrow 5x 1 turn
Test : about the same as step 4
Step 6
Scratch away central pitch area((ca 3 cm)
MOT ½ W-stroke 3 turns
Fouc
Zone meas.
1 0
2 0,24
3 0,80
4 1,04
5 1,17 mm
Step 7
½ W-stroke 3×1 turn
Fouc
Zone meas.
1 0
2 0,14
3 0,63
4 0,91
5 1,01
Step 8
Scratch away top layer of the pitch and do a hot press
2/3 W-stroke 2 turns
Test: undercorrection, kink in Ronchi lines near 80%
August 2001
Step 9
2/3 narrow W-stroke 4x ½ turn
Ronchi: outer zones too little correction
Step 10
2/3 W-stroke narrow (to do more work on outer zones) 2x1turn
Ronchi: outer zones too flat
Step 11
I made the pitch squares narrower(more tapered) in the edge area
½ W-stroke 2x 1 turn
Test: a little bit more correction
Step 12
I have to get rid of the flat outer zones!
½ W-stroke very narrow:2×1 turn
Test: kink in Ronchi lines near outer zones weak
Step 13
2/3 W-stroke very narrow: 2×1 turn
Test : about the same
Step 14
2/3W-stroke a bit wider than 13: 2×1 turn
Test : correction increased,kink still present
Step 15
2/3 W-stroke narrow 2x 1 turn
Test(ronchi) : looks OK
Step 16
2/3 W-stroke less narrow 2×1 turn
Test: kink still present
Step 17
½ W-stroke narrow 2x 1 turn
Kink still present.
Step 18
- 1/3 W-stroke narrow 2×1 turn
- 2/3 W-stroke narrow : 2×1 turn
Test: looks better
Step 19
a.1/3 W-stroke narrow 2×1 turn
b. 2/3 W-stroke narrow 2x 1 turn
Test: undercorrection
Step 20
MOT
a. 1/3 W-stroke narrow : 2x 1 turn
b. 2/3 W-stroke 3×1 turn
test : undercorrection ; kink near 70% in Ronchi lines.
Step 21
- 1/3 W-stroke narrow 3 x1 turn
- 2/3 W-stroke narrow 2×1 turn
Test: undercorrection and central area too flat
Step 22
2/3 W-stroke 2x 1 turn
Test : kink near 70% in Ronchi
Fouc.
Zone Meas. Parabola
1 0 0
2 0,35 0,73
3 0,85 1,34
4 1,90 1,92
5 2,57 2,55
2 meas. +/- 0,05
Step 23
Central area has too little correction
a. 1/3 W-stroke 5 turns
b.2/3 W-stroke wide 2x 1 turn
Zone Meas. Parabola
1 0 0
2 0,50 0,73
3 1,28 1,34
4 1,98 1,92
5 2,77 2,55
Step 24
- Decrease correction 1/3 W-stroke 10 turns
- Parabolizing stroke : 2/3 W- wide 1 turn
Fouc:
zone meas. Parabola
1 0 0
2 0,6 0,73
3 1,2 1,34
4 2,0 1,92
5 2,8 2,55
Step 25
Decrease correction
TOT ¼ W-stroke narrow 10 turns
Test
zone meas. Parabola
1 0 0
2 0,50 0,73
3 1,10 1,34
4 1,90 1,92
5 2,56 2,55
Step 26
- 10cm pitchlap with starfish shape : straight stroke 0,5-1 cm left of center 3 turns
- MOT 20 cm lap 1/3 W-stroke 6 turns
Fouc.
Zone Meas. parabola
1 0 0
2 0,47 0,73
3 1,10 1,34
4 1,83 1,92
5 2,56 2,55 little change!
Step 27
a. 10 cm starlap(see 25) straight stroke 1cm left of Center :5 turns
b. MOT 20 cm lap 1/3 W-stroke 5 turns
fouc.
Zone meas. Parabola
1 0 0
2 0,5 0,73 no improvement
3 1,15 1,34
4 1,8 1,92
5 2,6 2,55
Step 28
a.2/3-3/4 W-stroke very wide 2×1 turn: deepen center
- TOT 1/4W-stroke 10 turns
Test: hole in center!
1 0
2 1
3 1,8
4 2,4
5` 3,2
Step 29
1/3 W-stroke with pitch scratched away in central area: 6 turns
Zone Meas.
1 0
2 1,0
3 1,6
4 2,3
5 3,1
Step 30
MOT ¼ W-stroke 8turns
Ronchi : hole in center and uneven lines
Step 31
MOT 1/5-1/4 W-stroke 10 turns
Zone meas.
1 0
2 1,1
3 1,6
4 2,0
5 2,7 Ronchi: hole in center
Step 32
I scratched away the central 5 cm of pitch
1/3 W-stroke 5 turns
Ronchi: hole in center still clear
Step 33
- Small pitchlap (diam 4 cm) W-stroke on area outside central hole : 3 turns.I want to blend the hole in the general curve.
- 20 cm lap MOT (central area scratched away)
1/3 W-stroke 3 turns
½ W-stroke 2 turns
Test : looks somewhat better
Step 34
Repeat of step 32
20 cm tool with central pitch scratched away
Test:
Zone meas.
1 0
2 0,25
3 0,45
4 1,0
5 1,7
Now we have a flat center and a long edge
Step 35 : On the way back tot a sphere for this is a disaster!
TOT 1/5-1/3 straight stroke 2x 10 min
1 0
2 0
3 0
4 0,2
5 0,5 mm
Step 36
- TOT 1/3-1/5 straight stroke 10 min
- MOT improve the surface: 30 min 1/3 W-stroke with light pressure and a diluted ceriom oxyde suspension.
CORRECTION : SECOND TRY
New pitchlap,made from Gugolz 55.
Pitch squares are now less tapered towards the edge
August 28,2001
First, the pitchlap will be saturated with cerium oxyd : 15 min MOT 1/3 W-stroke(slow)
Test : spherical; surface a bit spotty.
Step 1
MOT 1/3 W-stroke, slow with dilute Cerium oxyd suspension
Test: surface better; weak hill in center
Step 2
MOT ½ W-stroke 3×1 turn. Pitchlap slightly tapered towards the edge.
Test
1 -0,1
3 0
5 0 mm Hill in center
Step 3
MOT 2/3 W=-stroke 2x 1 turn
Test : no change
Step 4
MOT 2/3 W-stroke 2x 1turn
Test
1 0
3 0,09
5 0,13 mm
Step 5
2/3 W-stroke 4×1 turn
Test
1 0
3 0,56
5 0,76
Crests too far outside : more work on edge with narrower stroke
Step 6
2/3 narrow W-stroke 2×1 turn
Test
1 0
3 0,52
5 0,91 Better!
Step 7
2/3 W-stroke 2×1 turn; more on center
1 0
3 0,56
5 0,95
Step 8
2/3 W-stroke narrow 3x 1 turn
1 0
3 0,70
5 1,21
Step 9 : more work on edge
2/3 W-stroke narrow: 2×1 turn
1 0
3 0,6
5 1,2 Crests have moved out . OK!
The crests are the shadows visible on the mirror at 50 % travel of the knife edge of the Foucault tester . They should be at about 50%
Step 10
2/3 W-stroke 3×1 turn
1 0
3 0,77
5 1,47 crests OK
Step 11
2/3 W-stroke 3×1 turn
1 0
3 0,90
5 1,72 crests OK
Step 12
2/3 W-stroke 3×1 turn
1 0
3 0,95
5 1,96 crests too far out
Step 13
2/3 W-stroke wide 2×1 turn
1 0
3 1,20
5 2,26 crests OK
Step 14
2/3 W-stroke normal(not narrow and not wide) 2x 1turn
1 0
3 1,23
5 2,32 crests OK
Step 15
2/3 W-stroke normal 2x 1 turn
1 0
3 1,21
5 2,45 Crests moving out
Step 16
1/3 W-stroke wide
1 0
3 1,27
5 2,55
Sept 2,2001
New meas.
Ronchi : kink at about 70 %
Zone Meas. parabola
1 0 0
2 0,31 0,73
3 1,25 1,34
4 2,02 1,92
5 2,49 2,55
I should have measured more zones before! Now there is a flat center..
Step 17
a. TOT W-stroke with edge of tool on zones 2-3 2 turns
b. TOT ½ straight stroke to diminish high edge
1 0
2 0,30
3 1,18
4 1,93
5 2,36
no improvement
Step 18
Have to work on center and edge
MOT
a. ¾ W-stroke wide 1 turn
b. 2/3 W-stroke narrow 1 turn
1 0
2 0,49
3 1,31
4 2,08
5 2,63
center deeper now
Step 19
a .MOT ¾ W-stroke wide 1 turn
b. MOT 1/3 W-stroke 3x 1 turn : smoothing
1 0
2 0,44
3 1,13
4 2,12
5 2,74
No good!
Step 20
- 10 cm starlap : deepen center : straight stroke 1 cm next to center : 6 turns
- Smoothing and bringing back correction : TOT ¼-1/3 W-stroke narrow 10 turns
Zone meas. Parabola
1 0 0
2 0,4 0,73
3 1,0 1,34
4 1,6 1,92
5 2,2 2,55
Step 21
10 cm starlap straight stropke 0,5-1 cm left of Center : 5 turns
Difference zone 1- zone 2 = 0,3 : No good
Step 22
2/3 W-stroke 2x 1 turn
Test : kink near 70% in Ronchi
Fouc.
Zone Meas. Parabola
1 0 0
2 0,35 0,73
3 0,85 1,34
4 1,90 1,92
5 2,57 2,55
2 meas. +/- 0,05
Step 23
Central area has too little correction
a. 1/3 W-stroke 5 turns
b .2/3 W-stroke wide 2x 1 turn
Zone Meas. Parabola
1 0 0
2 0,50 0,73
3 1,28 1,34
4 1,98 1,92
5 2,77 2,55
Step 24
- Decrease correction 1/3 W-stroke 10 turns
- Parabolizing stroke : 2/3 W- wide 1 turn
Fouc:
zone meas. Parabola
1 0 0
2 0,6 0,73
3 1,2 1,34
4 2,0 1,92
5 2,8 2,55
Step 25
Decrease correction
TOT ¼ W-stroke narrow 10 turns
Test
zone meas. Parabola
1 0 0
2 0,50 0,73
3 1,10 1,34
4 1,90 1,92
5 2,56 2,55
Step 26
- 10cm pitchlap with starfish shape : straight stroke 0,5-1 cm left of center 3 turns
- MOT 20 cm lap 1/3 W-stroke 6 turns
Fouc.
Zone Meas. parabola
1 0 0
2 0,47 0,73
3 1,10 1,34
4 1,83 1,92
5 2,56 2,55 little change!
Step 27
a.10 cm starlap(see 25) straight stroke 1cm left of Center :5 turns
b. MOT 20 cm lap 1/3 W-stroke 5 turns
Fouc.
Zone meas. Parabola
1 0 0
2 0,5 0,73 no improvement
3 1,15 1,34
4 1,8 1,92
5 2,6 2,55
Step 28
a. 2/3-3/4 W-stroke very wide 2×1 turn: deepen center
b. TOT 1/4W-stroke 10 turns
Test: hole in center!
Zone Meas.
1 0
2 1
3 1,8
4 2,4
5` 3,2
Step 29
1/3 W-stroke with pitch scratched away in central area: 6 turns
zone meas
1 0
2 1,o
3 1,6
4 2,3
5 3,1
Step 30
MOT ¼ W-stroke 8turns
Ronchi : hole in center and uneven lines
Step 31
MOT 1/5-1/4 W-stroke 10 turns
Zone meas.
1 0
2 1,1
3 1,6
4 2,0
5 2,7 Ronchi: hole in center
Step 32
I scratched away the central 5 cm of pitch
1/3 W-stroke 5 turns
Ronchi: hole in center still clear
Step 33
- Small pitchlap (diam 4 cm) W-stroke on area outside central hole : 3 turns.I want to blend the hole in the general curve.
- 20 cm lap MOT (central area scratched away)
1/3 W-stroke 3 turns
½ W-stroke 2 turns
Test : looks somewhat better
Step 34
Repeat of step 32
20 cm tool with central pitch scratched away
Test:
1 0
2 0,25
3 0,45
4 1,0
5 1,7
Now we have a flat center and a long edge
Step 35 : On the way back tot a sphere. This is a disaster!
TOT 1/5-1/3 straight stroke 2x 10 min
1 0
2 0
3 0
4 0,2
5 0,5 mm
Step 36
- TOT 1/3-1/5 straight stroke 10 min
- MOT improve the surface: 30 min 1/3 W-stroke with light pressure and a diluted ceriom oxyde suspension.
CORRECTION : THIRD TRY
New pitchlap,made from Gugolz 55.
Pitch squares are now less taperd towards the edge
August 28,2001
First, the pitchlap is broken in : 15 min MOT 1/3 W-stroke(slow)
Test : spherical; surface a bit spotty.
Step 1
MOT 1/3 W-stroke, slow with dilute Cerium oxyde suspension
Test: surface better; weak hill in center
Step 2
MOT ½ W-stroke 3×1 turn. Pitchlap slightly tapered towards the edge.
Test
1 -0,1
3 0
5 0 mm Hill in center
Step 3
MOT 2/3 W=-stroke 2x 1 turn
Test : no change
Step 4
MOT 2/3 W-stroke 2x 1turn
Test
1 0
3 0,09
5 0,13
Step 5
2/3 W-stroke 4×1 turn
Test
1 0
3 0,56
5 0,76
Crests too far outside : more work on edge with narrower stroke
Step 6
2/3 narrow W-stroke 2×1 turn
Test
1 0
3 0,52
5 0,91 Better!
Step 7
2/3 W-stroke 2×1 turn; more on center
1 0
3 0,56
5 0,95
Step 8
2/3 W-stroke narrow 3x 1 turn
1 0
3 0,70
5 1,21
Step 9 : more work on edge
2/3 W-stroke narrow: 2×1 turn
1 0
3 0,6
5 1,2 Crests have moved out . OK!
The crests are the shadows visible on the mirror at 50 % travel of the knife edge between center and edge . They should be about 50%
Step 10
2/3 W-stroke 3×1 turn
1 0
3 0,77
5 1,47 crests OK
Step 11
2/3 W-stroke 3×1 turn
1 0
3 0,90
5 1,72 crests OK
Step 12
2/3 W-stroke 3×1 turn
1 0
3 0,95
5 1,96 crests too far out
Step 13
2/3 W-stroke wide 2×1 turn
1 0
3 1,20
5 2,26 crests OK
Step 14
2/3 W-stroke normal(not narrow and not wide) 2x 1turn
1 0
3 1,23
5 2,32 crests OK
Step 15
2/3 W-stroke normal 2x 1 turn
1 0
3 1,21
5 2,45 Crests moving out
Step 16
1/3 W-stroke wide
1 0
3 1,27
5 2,55
Sept 2,2001
New meas.
Ronchi : kink at about 70 %
Zone Meas. parabola
1 0 0
2 0,31 0,73
3 1,25 1,34
4 2,02 1,92
5 2,49 2,55
I should have measured more zones before! Now there is a flat center..
Step 17
a.TOT W-stroke with edge of tool on zones 2-3 2 turns
b.TOT ½ straight stroke to diminish high edge
1 0
2 0,30
3 1,18
4 1,93
5 2,36 no improvement
Step 18
Have to work on center and edge
MOT a. ¾ W-stroke wide 1 turn
- 2/3 W-stroke narrow 1 turn
1 0
2 0,49
3 1,31
4 2,08
5 2,63 center deeper now
Step 19
a.MOT ¾ W-stroke wide 1 turn
b. MOT 1/3 W-stroke 3x 1 turn : smoothing
1 0
2 0,44
3 1,13
4 2,12
5 2,74 No good!
Step 20
- 10 cm starlap : deepen center : straight stroke 1 cm next to center : 6 turns
- Smoothing and bringing back correction : TOT ¼-1/3 W-stroke narrow 10 turns
zone meas. Parabola
1 0 0
2 0,4 0,73
3 1,0 1,34
4 1,6 1,92
5 2,2 2,55
Step 21
10 cm starlap straight stropke 0,5-1 cm left of Center : 5 turns
Difference zone 1- zone 2 = 0,3 : No good
Step 22
TOT Narrow W-stroke with edge of tool on center 2 turns
Test : zone-1 – zone-2 =0,5 : better
Step 23
The same as 22
Test : zone1-zone2 = 0,55
Ronchi : smooth lines,maybe a bit scruffy
Step 24
MOT 1/3 W-stroke very light pressure and very thin CeO2 suspension
5 turns MIRROR OK