25 cm f4.5 mirror

25 cm f4.5 mirror

This mirror will be used in a grab and go dobson.
I have already a 255 mm x42 mm pyrex disk that has a f3 curve in it
The sagitta has to be brought back from 4.7 mm to 3.2 mm over 240 mm
I had made a 40x 255 mm plaster tool  with porcelain tiles of 25 mm diameter

Jan 5, 2018
I grind TOT with the center of the tool over the edge area of the mirror. Powder =C80

6 strokes per position and there are 8 postions aroungd the grinding stand.
The sagitta will decrease in this way. This stroke is alternated with W-strokes and straight strokes to keep the mirror at least a bit spherical.
It takes some hours to reach the desired sagitta.

Jan 8, 2018
The tiles on the plaster tool have become quite thin; therefore I glue a second batch of tiles on the first ones ( with epoxy glue).
After some wets with C80 to get the tool fitting I clean the grinding stand,the tool and the mirror and start with C220 powder

TOT 1/3 narrow W-stroke
Wet 10: ink cross disappeared on tiles and mirror.
Sagitta =3.32 mm  over 24 cm.
F will be 108 cm
Now a wider W-stroke to decrease sagitta and increase F

After 10 new wets: sag = 3.15 mm
Now normal narrow W-strokes again
I mark some large pits and grind on till they are gone
Total nr of wets:40

C400
Jan 29 ,2018
Wet 15 (3 rounds per wet) : I  marked some large pits
Wet 25 : pits are gone

Feb 4
  25 micron Microgrit
10 wets : no obvious pits

15 micron Microgrit
40 wets( 3-4 rounds per wet)

Feb 22
9 micron Microgrit
90 minutes grinding
I start with  powder suspension and grind for 20-25 min with a 1/4 W-stroke TOT.
Every few minutes new suspension is added with a squirt bottle
After 20-25 min I clean tool and mirror and start anew

5 micron Microgrit
35 min  : I add regularly new suspension  but I don’t take off the tool during those 35 min. I want to diminish the risk of scratching the mirror.

POLISHING

March 1 ,2018
Polishing is done MOT , 1/3 straight and W-stroke (narrow)
I use Ceri-Hex polishing powder.Pitch 28 deg from Stathis

March 7, 2018 After 13 hr `15 min the mirror shows no reflection of the 1 W laserbeam in a dark room. Of course I test the edge the most

CORRECTION

Ronchi test: oblate spheroid  and weak TDE

I will make soft pitchlaps with diameters of 26, 10 and 7 cm
Bottom of the lap: plywood disk of 36, 18 and 18 mm
There is also an old 20 cm pitchlap that I will use now and then

All laps are of course worked in on the mirror . I use very thin Cerium oxyde suspension(1:100). The surface of thepitchlaps is made rough with a steel brush

Hardness pitch: 10 mm  at 20 deg C in 5 min.
I use the Texerau pitch tester.

Step 1  april 2019  T=18 deg
MOT 1/3 straight stroke and narrow W-stroke 15 min
No test

Step 2
MOT 1/2 W-stroke 15 min

1/2 stroke means the overhang of the mirror over the tool is 1/2  mirror radius forward and 1/2 mirror radius backwards.
In this case  1/2 x1/2 x 25 cm forward and the same amount backward
I roughen up the pitch squares when necessary
Test:Ronchi: TDE and hill in center.
According to Foucaulttest there is no TDE!!

The diffraction ring is equally bright on both sides!

Step 3
MOT 1/2 W-stroke wide 2x 10 min
In between 7 min cold press with 3 kg
Test: Ronchi lines more straight

Step 4
Pitch squares now tapered
MOT 2/3 narrow  W-stroke  15 min
Test: ronchi lines are billowing <F; no TDE

Step 5
Normal pitchlap
MOT 2/3 W; that is:6 cm overhang left and right: 11 min
Tst: Fouc.
-hole in center
– edge area flat
So I worked to much on the center!

I use a 7 zone mask for the Foucault test measurements

Zone                        1             2           3                   4                   5                  6           7
Measurements (mm) 0         1.08!!       1.70          2.09              2.22           2.27        2.27
Parabola   (mm)        0          0.49        0.96           1.42             1.88            2.34        2.81

Step 6
– in the central 10 cm of the lap the pitch is partly scratched away to lessen the action here
– outer pitch squares are tapered

Straight stroke 2/3   10 min
Test:
Zone                           1           2              3           4             5              6               7
Measurements           0          1.30         2.07       2.47         2.63       2.63          2.63
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81

This looks baaaadd!
Back to a more spherical figure:

Step 7
1/3 straight stroke and narrow W-stroke   : 2x 20 min

Zone                           1           2              3           4             5              6               7
Measurements           0          0.54        1.03        1.19         1.52        1.60          1.85
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
At least the large central hole has disappeared.
Ronchi: flat outer area very obvious

Step 8
Improve figure : I USE 1:100 CERI-HEX SUSPENSION. Very thin!
a. MOT 1/3 narrow W-stroke  15 min
b. increase correction: MOT tapered lap 4/5 str stroke 2 rounds

Zone                           1           2              3           4             5              6               7
Measurements           0          0.69         1.12       1.53         1.89        2.18          2.33
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81

Step 9
– central 10 cm of the pitch partly scratched away
– tapered tool
a. 1/3 narrow W-stroke 2 rounds  (smoothing)
b. 4/5 str stroke    1 round ( increase correction)

Zone                           1           2              3           4             5              6               7
Measurements           0          0.62         1.12       1.57         2.04        2.27          2.59
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
P-V 1/2.9 labda  S=0.789
Fg XP offset -0.239 mm.Outer zones now flat: central hill starts at r=105 mm and rises  to 300 nm at the center

Step 10
If I want to erase that high hill with the big tool(26 cm)  and a 2/3 W-stroke, I am afraid to get a big hole again.
Try this:
a. Taper the outer pitch squares
b. partly scratch away  the central 10 cm of the pitch
c. roughen the pitch with steel brush

4/5 straight stroke  3 strokes per position in stead of 6 :  2 x1 round (cold press between the rounds)

Zone                           1           2              3           4             5              6               7
Measurements           0          0.56         1.07       1.56         1.88        2.13         2.37
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
Outer zones stay behind

Step 11  april 10, 2019
Work on outer zones:
After fitting, tapering and steelbrushing: 10 min cold press with 3 kg
4/5 str. stroke 2 rounds, 4 strokes per position

Zone                           1           2              3           4             5              6               7
Measurements           0          0.69        1.32        1.74         2.14       2.45          2.74
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
P-V 1/3.1 labda  S= 0.776   Hole in center!  Fig XP: high at r=6 cm

Step 12
a. TOT : W-stroke with edge of tool on r=4 to r=8 cm : 2 rounds
b. MOT 4/5 str stroke : 1 round
Test: disaster! diff 1-2 = 1.2 mm

Step 13
100 % lap: scratch away central 90 mm
MOT : 1/3 str stroke 2×1 round. Smoothing
Test: better diff 1-2      0.85 mm

Step 14
a. same as step 13
b. 4/5 str stroke 1 round

Zone                           1           2              3           4             5              6               7
Measurements           0         0.60          1.01       1.52         1.98       2.30          2.69
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81

P-V = 1/5 labda  S= 0.928

Step 14a
MOT
a. 1/3 narrow W-stroke  1 round (smoothing)
b. 4/5 narrow W-stroke 1 round ( increase correction)
Central 80 mm of pitch still partly scratched away
Ronchi: lines in outer zones too straight : too little corr.
Zone 1-7  +2.76 mm

Step 15
a. long coldpress
b. Steelbrush over pitch squares
c. MOT 1/3 narrow W-stroke 2×1 round
d. 4/5 narrow W-stroke 1 round
Lap is tapered

Zone                           1           2              3           4             5              6               7
Measurements           0                                                        2.12        2.42         2.74
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
Outer zones not good!

Step 16
Pitchlap will be tapered a bit  more
MOT 4/5 str stroke 1 round

Zone                           1           2              3           4             5              6               7
Measurements           0          0.71         1,15       1.59         2.07        2.44          2.77
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
Hole in center; too little correction in outer zones
P-V = 1/5.3 labda. S= 0.926

Step 17      April 12, 2019
Try to polish more on outer zones and  less on center
Central 80 mm of pitch partly scratched away
1/2 narrow W- stroke : 2 rounds
Result: no improvement

Step 18
a. MOT  1/3 narrow W-stroke + Str stroke: improve figure in this way(?) : 3 rounds
b. 3/4 str stroke 1 round
Center of lap partly scratched away; pitch squares tapered

Zone                           1           2              3           4             5              6               7
Measurements           0          0.60         1.07       1.67         2.21        2.61         2.98
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
Worse! Overcorrection

Step 19
Bring back correction
a. TOT 1/3 str stroke 4 rounds
b. MOT: 2/3 R 1round
Tapered pitchlap

Zone                           1           2              3           4             5              6               7
Measurements           0          0.65        1.15        1.71         2.27       2.65          3.07
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
Can’t understand this….

Step 20
Decrease correction
MOT 1/3 str stroke and narrow W-stroke : 2×4 rounds
Still overcorrection

Step 21
I made a second 25 cm pitchlap with regular squares ( not tapered)
But first I try to flatten the center with a 20 cm tapered lap TOT
W-stroke with 2 cm overhang ( edge of tool over edge of mirror) : 2x 1 round

Zone                           1           2              3           4             5              6               7
Measurements           0          0.43        0.94        1.45         2.07        2.56          2.97
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
Overcorrection   P-V = 1/3.7 labda
Fig XP High of 75 nm at 90 mm

Step 22
I work in the new 25 cm pitchlap on the 40 cm f4.5 mirror that is still in the make
2 rounds TOT 1/3 str stroke

Zone                           1           2              3           4             5              6               7
Measurements           0          0.45        0.97        1.44         2.05        2.49         2.90
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
P-V = 1/5.4 labda     Fig XP : high of 50 nm at r=90 mm

Step 23
a. TOT with 20 cm tapered lap( only one available now)
W-stroke with edge of lap on area from r=5 to r= 10 cm.  2 rounds
b. MOT 25 cm lap ( not tapered) 1/3 narrow W-stroke 2×1 round
c. 25 cm normal lap 3/4 narrow W-stroke  to keep the correction 1 round

Zone                           1           2              3           4             5              6               7
Measurements           0          0.50        0.97        1.43         1.92        2.37          2.83
Parabola                    0          0.49         0.96       1.42         1.88        2.34          2.81
1/33 labda   S= 0.998
Coating 96%  MCM Sept sorts Noord-Frankrijk  e 260

Beeld heel mooi(vind ik)

Fig XP diagram after step 23