30 cm f4.4 mirror
Sept 3, 2016
I bought a 300x 25 mm borofloat disk from Stathis Kafalis in Munchen-Germany
Cost e 132 + e 13 for grinding the backside flat with C320
Wanted focal length : about 135 cm
Sagitta has to be 0,417 cm
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ROUGH GRINDING
First I use a 15 cm diameter steel ring that is used in weightlifting to excavate the disk
I use a straight stroke 0-1 cm left of center and a 5 cm overhang. Now and then I use a W-stroke to prevent a hole in the center of the glass disk.
On the disk I place a weight of about 5 kg to increase the pressure.
The grinding material is Carborundum 80
I didn’t count the hours this time but after a week of intermitten work I stopped at a sagitta of 4,0 mm.
Now I make a plaster tool ( thickness 4 cm) on the mirror that I have coated with vaseline.
The plaster tool was left in the open air on newspapers to dry. That took about a week.
I glued little ceramic tiles on the plaster tool (19x19x2 mm) with the help of two component PU-glue.
This plaster tool is the definitive tool and it has to be worked in on the mirror.Of course the mirror is not actually spherical after the frequent central stroke.
I use the tool above with a narrow W-stroke to make the mirror spherical. I mark every tile with a waterproof ink mark to see if all the tiles will be touching the mirror. That takes a number of strokes!
I made a sagittameter with the help of a measuring clock: see photo
The sagitta was lowered again to 3,75 mm over a diameter of 292 mm .
I excavated again, now with a 9 cm diameter steel weight and a straight stroke 0-1 cm from center. Also Iused a W-stroke with the 15 cm steel ring. Overhang 5 cm.
After reaching a sagitta of 3.88 mm and resulting F = 1373 mm I changed to C180. The mirror had become spherical.
I wanted to do some time with MOT (mirror on top) to shorten the F.
FINE GRINDING
I made a dam with masking tape(very low) aound the tool ,threw in an amount of C180 ,added water and started swirling and grinding for 30 min.
F had become 1358 mm OK
TOT is easier and I did that fort he rest of the time,till all big pits were removed.
Pits were monitored with a strong backlight( behind the mirror ) and a magnifying glass.
Total time with C180 : 2h30min
C320 (I had a lot of that stuff ,and not too much C400,the one I wanted to use)
60 min TOT narrow W-stroke
C400
Total time 120 min with TOT and narrow ¼-1/5 W-stroke
C800
Total time : 60 min TOT 1/4/1/5 narrow W-stroke
9 micron microgrit
Total time : 130 min TOT ¼-1/5 narrow W-stroke
5 micron microgrit
Total time : 40 min narrow W-stroke ¼-1/5 W
POLISHING
I used an old 26 cm pitchlap (6 months old) but after 30 min I found it too hard.
I poured fresh pitch on the old lap and made 7 mm wide grooves with a wooden lat.
Polishing went on,TOT and a narrow W-stroke with about 3-4 cm overhang.
It took about 16 hours before the reflection of a red laser pointer on the edge area of the mirror was almost invisible in a dark room.
CORRECTION
I used pitch 28 degrees from Stathis Kafalis in Munchen and I softened it with linseed oil and turpentine. I made pitchlaps of 10, 15 and 30 cm on plywood bases. The 30 cm disc had a 15 cm diameter and 3 mm thick extra layer of hardboard to make the pitchlayer more even.
The new pitchlaps were broken in during the first part of the correction
The pitch was divided in 4 cm squares.
Ronchi test: hill in center and a bit low outer zones
19 dec 2016
1/3 R (straight stroke) TOT 15 min
1/3 W –stroke 5 cm wide 15 min
During the straight strokes I walk around the polishing table. W-strokes are done in 1 position. The table has 8 positions; so one turn around the table gives 8 W-strokes. I give the pitchlap a turn of 20-40 degrees after one walk around the table. This to lessen the chance of astigmatism. No change in Ronchi
19-12-16
MOT 2/3 W 2x 10 min No change
19-12
MOT 4/5 W-stroke with slightly tapered pitch
3x 10 min; 12-15 min pressing with 2kg between the sessions Lines get a bit more straight in Ronchi test.Still a hill present.Pitch is a bit hard at those low temp( 15-16 deg)
However: Foucaultzone mask test shows an irregular figure with a hill in the center
20-12
I give more attention tot he center:
¾ W-stroke (wide!) 3x 10 min with pressing in between( 10-15 min 2 kg). T= 16 deg
I made a 7 –zone Coudermask from cardboard Zonetest:
Zone reading
1 0
4 0.5
7 1.55
This looks better!
Measured R= 2658 mm
( zone 1)
20-12
Pitch squares are tapered a bit more
I now use Ceri-Hex ceriumoxyde polishing powder.It polishes smoother than the stuff from MTN-Giethoorn
MOT ¾ W-stroke with emphasis on the edge of the tool
Zone Reading Parabola
1 0 0
4 1.52 1.42
7 2.69 3.13
My measurements are the average of 2 readings
20-12
¾ W- stroke; narrow!
I want more action on the edge
1 turn around ( 8 W-strokes)
Zone Reading parab
1 0 0
2 0.22 0.20
3 0.88 0.81
4 1.54 1.42
5 1.98 2.02
6 2.27 2.60
7 2.67 3.13
21-12
Try to improve outer zones
MOT 4/5 straight stroke; 3 strokes per position; 3 turns around No change
21-12
I scratched most pitch off within the 70 % zone. I want more correction on the outer zones.
4/5 straight stroke; 2 turns with 4 strokes per position
Zone reading
1 0
2 0.11
3 0.50
4 1.29
5 1.86
6 2.23
7 2.61
No improvement
23-2-17
Normal tapered pitchlap again
4/5 W-stroke MOT 1 turn
4/5 Straight stroke 2 turns ( 4 str/pos)
Zone reading parabola
1 0 0
2 0.19 0.20
3 0.75 0.81
4 1.50 1.42
5 2.10 2.02
6 2.52 2.60
7 3.11 3.13
The readings are the average of 3 sets
This looks good : 1/11 labda PV and s= 0.97
8 june 2017
HOWEVER!!
After using a 9 zone couder mask and improving the tester I found out the mirror was definitely not good
Now use the tester in the slitless firm and I can read the zones better .
Zone reading parabola
1 0 0
2 0.24 0.54
3 0.44 1.0
4 0.94 1.44
5 1.47 1.89
6 1.96 2.33
7 2.33 2.78
8 2.62 3.22
9 2.98 3.67
1/3.6 Labda P-V S=0.863
Figure XP :high zones at center, r=100 mm and edge. By changing the ROC a bit in the program I can make the outer zones flatter and the center starts to bulge. Then I can have an easier correction to do.
9-6
10 cm strongly tapered pitchlap
W-stroke within r =100 mm (lap stays within that radius)
3 turns with 0.5 kg on the lap
Readings : middle area too shallow
15-6 10 cm lap
Circular stroke with lap centered on r= -2 cm to r = + 12 cm. Light pressure
Readings : even worse!
15-6 I have to do someting about that shallow center!
7 cm pitchlap straight stroke 1 cm left of center. Edge of lap goes to edge of mirror.
5 turns
Now both too little correction in outer zones and in center.
This looks like a disaster
16-6
I am planning to bring back the correction with the full size lap
MOT 1/3 W-stroke 20 min
Ronchi:
<F : curved lines
>F flat center!
18-6
30 cm lap ¼ straight stroke 20 min
Zone readings parabola
1 0 0
5 0.85 1.89
7 1.48 2.78
8 1.85 3.22
9 2.17 3.67
18-6
I go on with the correction stroke
30 cm lap 4/5 W-stroke wide
Pitch a bit scratched away in the outer zones to diminish correction there
2×1 turn Total corr: 3.34 ( too small)
1 aug 17
30 cm lap ¾ W-stroke narrow 1 turn
Zone readings parabola
1 0 0
2 0.29 0.54
3 0.87 1.01
4 1.29 1.44
5 1.83 1.89
6 2.13 2.33
7 2.58 2.78
8 2.85 3.22
9 3.31 3.67
¼ labda P-V s=0.873
1-8
Again work on center area
30 cm lap MOT 4/5 W-stroke wide
Pitchlap is a bit tapered
Ronchi and Fouc:
Bit TDE; zones inside edge a bit too flat
2-8
30 cm lap MOT
4/5 straight stoke 2 turns ; 5 strokes per position
Zone readings parabola
1 0 0
2 0.24 0.54
3 0.87 1.0
4 1.40 1.44
5 1.89 1.89
6 2.35 2.33
7 2.75 2.78
8 3.12 3.22
9 3.51 3.67
¼.8 Labda PV S=0.871
Best figure( -0.075 mm offset) high region from center to r =75 mm
4-8
1. 10 cm starshaped pitchlap
Straight stroke 1 cm left of C and 3 cm overhang : 3 min 30 sec
2. 10 cm starshaped lap circular stroke on r = 75 mm ,with every 5 sec a stroke to C,while I am walking around the table
3. 30 cm tool MOT 3/4 W stroke narrow 2 turns (16 strokes)
Total corr: 3.61
Too little difference between zone 1 and 2
4-8
7 cm starshaped lap:
W-stroke within r = 7 cm
Time ?
Zone reading parabola
1 0 0
2 0.44 0.54
3 0.96 1.0
4 1.48 1.44
5 2.12 1.89
6 2.56 2.33
7 2.98 2.78
8 3.28 3.22
9 3.73 3.67
High hill at r= 90 mm ( offset -0.038 mm)
4-8
1. Tangential stroke on r= 90 mm with frequent strokes to the center : 5 min
2. 30 cm lap weakly tapered ¾ W-stroke narrow 1 turn(8 strokes)
3. 30 cm lap ¾ W wide 1 turn Overcorrection! (3.78 mm)
4-8
30 cm lap MOT 1/3 W-stroke narrow 2 turns(16 strokes) Overcorrection
4-8
30 cm lap
MOT 1/3 straight stroke 3 turns Overcorrection
5-8 30 cm lap (normal,not tapered)
MOT 1/3 Straight and narrow W : 6 turns Overcorrection(but less)
5-8 Same as previous Correct still 3.95 mm
5-8 30 cm lap TOT
¼ straight 3 turns Corr.3.83 mm
8-8 30 cm lap TOT
¼ straight light pressure 3x 1 turn Corr. 3.9!
8-8 30 cm lap TOT ¼ Straight stroke with pressure 3x 2 turns
Zone reading parabola
1 0 0
2 0.47 0.54
3 1.0 1.0
4 1,52 1.44
5 1.99 1.89
6 2.40 2.33
7 2.82 2.78
8 3.24 3.22
9 3.69 3.67
Startest 12-8-17
33 % mask 10 mm NLV + 2x Barlow(266x) Outside focus: bright inner ring
Inside focus: bright outer ring
Outside focus: shadow of secondary comes much faster than inside focus.
There is OVERCORRECTION!
13-8 I want to increase the correction in the outer zones but to decrease the total correction.
a. 30 cm lap tapered 4/5 straight stroke 2x 1 turn.
b. 30 cm lap : scratch pitch partly away from all zones except 70 % and around( bring back correction)
Still overcorrection.
It is especially visible when I compare the jump between zone 2 and zone 9
Mirror : 3.45
Parabola : 3.13
Startest: overcorrection
14-8
Normal pitchlap 30 cm
TOT straight stroke 2,5 cm from C 5 cm overhang: 2 turns ( 2x8x6 strokes) Readings
Zone 2 – zone 9 : 3.26
Still more than parabola (3.13)
15-8
30 cm lap TOT
Straight stroke 2.5 cm next to Center.
1 turn( 8×6 strokes)
Readings :
Outer zones too flat and overcorrection
15-8
30 cm lap:
a. |Tot straight stroke 2,5 cm from C ; 4 turns
b. MOT 4/5 straight stroke with tapered pitchlap : 2 turns
Readings :
2-9 3.21 mm (too much)
15-8
30 cm lap TOT
Straight stroke 3 cm from C; 5 cm overhang
2 turns with light pressure
2-9 diff still too large
16-8
30 cm lap TOT
Narrow W-stroke ( 5 cm overhang and 2 cm sideways) 2 x 2 turns
Readings : 2-9 = 3.30 (too much)
Star test: overcorrection
16-8
Pitchlap is thickened with new pitch and broken in on a 35 cm f4.5 mirror ( an old atigmatic one)
MOT small W-stroke 3 turns Correction still slightly too large
17-8 MOT small W-stroke: 2 turns
2-9 now 3.13 ( OK)
Startest ?
Test in telescope after coating:
Feb 2018 : I see the rille in the Alpine valley completely at 440x!! ( 6 mm radian and 2x Clave Barlow)