In June 2007 I had finished a pyrex mirror with a focal ratio of 4.9.
The mirror had a thickness of 35 mm. Probably Jilles Eindhoven from Buurse (NL) had made it from an 80 mm thick disk.
The mirror ended up quite good: 1/20 labda on the wavefront( Foucault test) . The images were good
In 2022 I decided to rework it to a focal ratio of 4.0 . That gave me the possibility of observing while seated.
July , 2022
- The first job is to increase the sagitta from 4.66 mm ( over a diameter of 38 cm; the size of my measuring device) to about 5.6 mm.
For that I used a plaster tool with embedded big steel nuts .Diameter 16 cm. It took some days with radial strokes ( center over center) to deepen it and W-strokes to make it more spherical. I used Carborundum 36 for this rough work.
I worked on a heavy wooden stool with a 4 cm thick granite tile on it. This had a diameter of 60 cm. This was also used for my 55 cm mirror. I had 8 positions around the stool - When the sagitta had become 5.52 mm I made a plaster tool of 45 mm thickness and 406 mm diameter.
That needed 6 kg powdered plaster, 4 liters of water and some dishwashing liquid. I made a cardboard dam for around the mirror and smeared cardboard and the frontside of the mirror with vaseline. - After a week I glued hexagonal tiles of about 2 cm diameter and 6 mm thickness with polyurethane glue (2-component) on the plaster tool. The stroke with the plaster tool was a Tool on Top (TOT) stroke : 1/3 to 1/4 diameter W-stroke and center over center stroke ( 1/4 to 1/5 diameter). Now and the I used the 16 cm tool again to increase the sagitta again, if it had diminished.
I used C80 in stead of C36. By using ink marks on the tiles I could see if the plaster tile tool was in good contact with the mirror. That took some time!
I sprinkled the C80 ( a dinner spoon ful) on the mirror, added water with a spraybottle and smeared it with my hands to a suspension. After a few minutes of strokes the C80 was mostly broken up to a sludge and I cleaned the mirror with a sponge and renewed the charge. - Against a strong light and using a magnifying glass I checked for large pits. After 32 wets I changed to C120. Sagitta was 5.6mm.The stroke was the same as with C80 ; I needed 10 wets with the 16 cn tool and 15 wets with the 40 cm tool
August , 2022 - C220 : 50 wets to get rid of larger pits
- C 400 : 25 wets
- C600 : 17 wets
- 25 micron Microgrit: 25 wets
- 15 micron Microgrit : 30 wets
- 9 micron : 90 minutes
- 5 micron : 60 min
With 15, 9 and 5 microns I didn’t lift off the grinding tool after every wet: too much risk of scratches: after every 2-4 turns with 8 positions I added the microgrit suspension with a squirting bottle.
September,2022
PITCHLAP
The base of the pitchlap was a plywood disk of 54 mm thickness and a diameter of 406 mm( 100 %)
On it were glued 2 hardboard disks of 3 mm thickness, one with a diameter of 25 cm and upon that one a disk of 10 cm diameter.The purpose was to make the bottom less flat and to give the pitch a more even thickness.
Around the disk I taped a cardboard ring to hold the pitch inside.
The pitch was cast on top (pitch 28 from Stathis Kafalis in Munchen) . and left to cool a bit.Then I pressed a wooden plank ( wetted with ceriumoxyd suspension) in a number of places in the pitch.I wanted a regular pattern of 4-5 cm squares. You have to repeat the pressing a couple of times.
While the pitch was stiil workable, I pressed the wet ( water and some dishwashing liquid)mirror on it and slided it around. In this way you can obtain a faster fitting of the pitchlap.
POLISHING
Polishing is a heavy job. Of course I gave the pitchlap a cold press with transparent paper first
You can easier see wheter it fits After that with soapy water alone.
The stroke was in general a straight stroke with mirror on top . Not more than 6 cm overhang and a few cm side movement now and then. I made scratches in the pitch to ease the polishing. Without the scratching I could hardly move the mirror!
After 25 hours the edge still failed the laser reflection test.
The pitch is now very hard. I added extra pitch, softened with linseed oil on the pitch tiles with a brush.
Ronchi test: high center.
Foucault test : diffraction ring everywhere; more pronounced on the right side.
To get rid of the high center: I continue with MOT and longer strokes : 1/3 straight stroke and 1/3 W-stroke
Also TOT 1/3 -1/4 straight stroke
October 16 : 30 hrs and reflection of laser on edge mirror not visible in dark room.
January 2023
CORRECTION
The temperature in the attic is so low (14-16 deg cent) that I made a soft pitch : pitch 23 degrees from Stathis Kafalis, further softened by the addition of linseed oil .I can easily press a fingenail into the pitch
Test :
Ronchi : spherical; weak TDE?, weak hill in centrer
Foucault: diff ring around mirror: on the right side a bit brighter. The knife comes from the left into the light beam.
During the correction I use the same heavy stool as during the grinding and the polishing stages.
8 positions per turn around the stool
Step 1 ; Jan 10, 2023
20 cm tool with 1.5 kg weight ; slightly tapered ; 1% Ceriumoxyd suspension. Pitch 10 mm thick
Stroke a : center of tool 1 cm left of center mirror ; edge of tool 5 cm overhang over mirror edge: 7 min
Stroke b: 3 cm left and 5 cm overhang: 5 min
Stroke c: 5 cm left and 5 cm overhang: 3 min
Stroke d: elliptical stroke with center of tool between 1 cm inside mirror center and ca 3 cm inside mirror edge
3 min
a ,b,c and d are repeated 3 times
Test: Ronchi: straight lines and weak TDE
Step 2, Jan 11
20 cm slightly tapered tool with 1.5 kg weight
1% CeO2
strokes a, b and c: same as step 1
stroke d: Large W-stroke with 5 cm overhand left and right and up and down : 4 times
Test : Ronchi: lines are bulging a bit; correction starting in real.
Step 3
Now I use a 15 cm pitch tool from the same pitch. 10 mm thick .Extra weight : 0.5 kg
a. straight stroke 1 cm left of center; 5 cm overhang : 7 min
b. id 3 cm from center : 5 min
c. id 5 cm from C : 3 min
d. elliptcal stroke from -1 cm to 3 cm from edge : 3 min
a, b,c and d repeated once
I made a 11-zone Couder mask and took several measurements .The outer zones are the most difficult( and important!) so I repeated them a couple of times.
Zone Measurement Parabola
11 1.85 mm 5.51 mm
10 1.48 4.93
9 1.11 4.40
8 0.91 3.86
7 0.71 3.29
6 0.65 2.70
5 0.43 2.14
4 0.28 1.57
3 0.12 1.01
2 0 0.44
1 0 0
The center is still flat, but the edge zones have too much difference (relatively) !
The measurements are entered in the XP-program of the Foucault test to obtain the current surface
of the mirror.
Step 4
20 cm lap +1.5 kg
a. Straight stroke 1 cm from C : 7 min
b. id 3 cm from C : 3 min
c. id 5 cm from C : 3 min
d. id 7 cm from C : 3 min
e. Elliptical stroke from -1 to + 17 cm , without weight: 3 min
a , b, c, d and e are repeated 2 times
Test:
Zone Measurement Parabola
11 2.36 5.51 mm
10 1.91 4.93
9 1.62 4.40
8 1.37 3.86
7 1.12 3.29
6 0.93 2.70
5 0.71 2.14
4 0.52 1.57
3 0.33 1.01
2 0.26 0.44
1 0 0
Difference 10-11 becoming too large
Step 5
I have to do less action on edge area
a. 15 cm lap + 0.5 kg : W-stroke within 1 cm from edge of mirror : 4 turns
b. id elliptical stroke (without weight) from -1 cm to +17 cm ( 3 cm from edge) .That is: the center of the lap moves between those radii. : 3 min
Zone Measurement Parabola
11 2.66 5.51 mm
10 2.17 4.93
9 1.84 4.40
8 1.55 3.86
7 1.27 3.29
6 1.03 2.70
5 0.73 2.14
4 0.56 1.57
3 0.42 1.01
2 0.24 0.44
1 0 0
It is not going well with the outer zones
Further ideas:
1. Smoothing with the 40 cm lap is a lot of work; the pitchlap is too hard and has to be made thicker.
2.Work on low outer zone with small lap
3. Try this first: leave outer zone and correct the inner area first.
Step 6 Jan 15, 2023
15 cm lap + 0.5 kg
a. straight stroke 1 cm from C with edge lap untill 3 cm from edge mirror :5 min
b. id 3 cm from C : 3 min
c. id 5 cm from C : 2 min
d. id 1 cm from C with edge untill 5 cm from edge mirror : 5 min
e. id 3 cm from C : 3 min
f. id 5 cm from C
Test
TDE makes reading zone 11 quite difficult…
Zone Measurement Parabola
11 3.28 5.51 mm
10 2.85 4.93
9 2.56 4.40
8 2.26 3.86
7 1.97 3.29
6 1.69 2.70
5 1.43 2.14
4 1.12 1.57
3 0.82 1.01
2 0.39 0.44
1 0 0
Zones 4-10 are not enough polished .
Zone 11 is saved for the moment!!
Step 7
I use techniques that are coming from Carl Zambuto from the US
He uses the idea of the sweet spot ( the active spot) of a lap. That is on 80 % of the radius.
I use a 20 cm lap of which the sweet spot must not go beyond zone 9 ( 180 mm radius).
That means the edge of the 20 cm lap goes to the edge of the mirror
a. W-stroke 20 cm tool+ 0.5 kg to the edge : 3x 1 turn
b. W-stroke with 15 cm tool + 0.5 kg to 2 cm from edge mirror : 1 turn
c. id to 4 cm : 1 turn
d. id to 6 cm : 1 turn
Test:
Zone Measurement Parabola
11 3.36 5.51 mm
10 2.93 4.93
9 2.52 4.40
8 2.22 3.86
7 1.91 3.29
6 1.71 2.70
5 1.38 2.14
4 1.03 1.57
3 0.63 1.01
2 0.37 0.44
1 0 0
S= 0.055
Hardly any change. Only a bit more correction
M-L-plot quite irregular
Step 8. Jan 18, 2023
15 cm tool + 0.5 kg
a. Straight stroke 1 cm from C and edge tool to edge mirror :5 min
b. id 3 cm from C : 3 min
c. id 5 cm from C : 2 min
d. id 1 cm from C and edge tool to 2 cm within edge mirror : 3 min
e. id 3 cm from C : 2 min
f. id 5 cm from C : 2 min
g. elliptical stroke with center tool from 1 cm behind C and to 3cm within edge 3 min
Test:
Zone Measurement Parabola
11 3.74 5.51 mm
10 3.25 4.93
9 2.86 4.40
8 2.59 3.86
7 2.31 3.29
6 1.95 2.70
5 1.63 2.14
4 1.27 1.57
3 0.87 1.01
2 0.47 0.44
1 0 0
68 % correction ; S=0.051
ML-plot less irregular; correction zones 9-3 too low
A 68 % parabola shows too lttle correction in the area within R = 150 mm
Step 9
15 cm tool + 0.5 kg
“Sweet spot” of tool has to work within r= 150 mm : edge tool works to 165 mm.
But I plan to use a number of different strokes because I want to prevent zones
Under the mirror lies a plastic sheet with a number of circles with different diameters. They are a help for figuring.
a. edge tool to r=180 mm
1. straight stroke 2 cm from C : 4 min
2. id 4 cm : 3 min
3. id 6 cm: 3 min
b. Edge tool to r=160 mm
1. straight stroke 2 cm from C : 4 min
2. id 4 cm from C : 3 min
3. id 6 cm from C : 3 min
c. Edge tool to r=200 mm
1. Straight stroke 2 cm from C : 3 min
2. id 4 cm from C: 2 min
3. id 6 cm from C : 2 min
d.
Elliptical stroke till 5 cm from edge : 3 min
Zone Measurement Parabola
11 3.99 mm 5.51 mm
10 3.51 4.93
9 3.21 4.40
8 2.86 3.86
7 2.54 3.29
6 2.27 2.70
5 1.86 2.14
4 1.36 1.57
3 0.83 1.01
2 0.40 0.44
1 0 0
ML plot quite irregular
Zones 6 -10 too low
P-V 1.84 labda
Step 10
I will increase the correction with a larger tool
20 cm tool; somewhat pointed, with a weight of 0.5 kg on top.The sweet spot must reach all zones including nr 9 ( not 10 and 11)
The sweet spot must reach all zones including nr 9 ( not 10 and 11)
1. Straight stroke 1 cm from C : edge tool to edge mirror : 5 min
2. idem 3 cm from C : 3 min
3. idem 5 cm from C : 2 min
4. repeat this series 1x
5. Smoothing: 20 cm tool + 0.5 kg : large W-stroke with 5 cm overhang: 1 turn( 8 positions) .
In the meantime I made a 25 cm tool with soft pitch. Pitch 23 ( from Stathis Kafalis in Munchen) softened further with some linseed oil. In my working spac the temperature is only 12 to 15 degrees..
The tool is 10 mm thick and is”worked in “with cerium oxyd suspension on another mirror, a 35 cm f4.5 plate ( not used anymore).
Zone Measurement Parabola
11 4.32 5.51 mm
10 3.83 4.93
9 3.46 4.40
8 3.14 3.86
7 2.81 3.29
6 2.44 2.70
5 1.93 2.14
4 1.44 1.57
3 0.92 1.01
2 0.46 0.44
1 0 0
P-V = 1.48 labda S=0.09
Step 11
Increase correction
a. 20 cm tapered tool + 1.5 kg
1. Straight stroke 1cm from C; edge tool untill edge mirror : 3 min
2. Idem 3 from C; edge tool untill 1.5 cm past edge mirror : 5 min
3. idem 5 cm from C; edge tool untill 1 cm inside edge mirror :3 min
b. Repeat a, but with 2-4-6 cm and 3-5-3 min
c. repeat a, but with 0-3-5 cm and 3-5-3 min.
I do it like this to lessen the chance on zones.
d. Smoothing: 25 cm soft tool: large W-stroke with 6 cm overhang: 2 turns( 2×3 min)
Zone Measurement Parabola
11 4.52 mm 5.51 mm
10 4.04 4.93
9 3.67 4.40
8 3.28 3.86
7 2.85 3.29
6 2.39 2.70
5 1.88 2.14
4 1.34 1.57
3 0.86 1.01
2 0.44 0.44
1 0 0
ML-plot more regular
P-V = 1.06 labda
S=0.10
Step 12
Increase correction with 15 cm tool + 0.5 kg
1. Straight stroke 2 cm from C; edge tool to edge mirror : 3 min
2. idem 4 cm from C; edge tool to 1.5 cm past edge mirror : 5 min
3. idem 6 cm from C ; edge tool to 1 cm inside edge mirror : 3 min
4. Repeat this series once
5. Repeat 1-2-3, but now 1-3-5 cm from C and again 3-5-3 min
6. 25 cm tool + 1.5 kg large W-stroke with 6 cm overhang : 2 turns(6 min)
Zone Measurement Parabola
11 4.90 5.51 mm
10 4.34 4.93
9 3.89 4.40
8 3.45 3.86
7 2.99 3.29
6 2.51 2.70
5 1.94 2.14
4 1.34 1.57
3 0.86 1.01
2 0.56 0.44
1 0 0
Difference 10/9,9/8,7/6,3/2 too small; 2/1 too large
Correction: 89 % ; S=0.20
For a 89% parabola ( you can try this in Figure XP when you change the conic constant from -1 to -0.89) the Strehl ratio is 0.96. Taht means that the figure is quite regular.
Step 13
Increase correction : 20 cm tool + 1 kg; the central area with a diameter of 7 cm is largely scratched away. The purpose of this is to lessen action on the center.
1. Straight stroke 1 cm from C ; edge tool to edge mirror : 4 min
2. idem 3 cm from C : 3 min
3. idem 5 cm from C : 3 min
4. idem 2 cm from C; edge tool to 1.5 cm inside edge mirror : 3 min
5. Like 4 but 4 cm from C :3 min
6. Like 4 but 6 cm from C : 3 min
7. 25 cm tool + 1.5 kg : large W-stroke with 0.5 cm overhang : 1 turn(3 min)
8 .idem but with 6 cm overhang : 1 turn
Zone Measurement Parabola
11 5.05 mm 5.51 mm
10 4.50 4.93
9 4.04 4.40
8 3.56 3.86
7 3.02 3.29
6 2.44 2.70
5 1.89 2.14
4 1.30 1.57
3 0.90 1.01
2 0.51 0.44
1 0 0
Correction too low in zones 3-4 .Center too deep.
Step 14
a. 8 cm tool + 0.5 kg: tangential stroke on r=14 cm There is a high zone in this place; to prevent digging a furrow I make a stroke to the center every two strokes : 5 min.
b. idem with 15 cm tool + 0.5 kg : 3 min
This is done to widen the action. Small tools can dig trenches and their work has to be ” smoothed”
c. 15 cm tool + 0.5 kg : central 6 cm is largely scratched away to get less action on the center
1. Straight stroke 1 cm from C; edge tool to edge mirror : 3 min
2. idem 3 cm from C : 4 min
3. idem 5 cm from C : 3 min
4. Repeat 1-2-3
5. Straight stroke 2 cm from C with edge tool to r = 135 mm : 3 min
6. idem 4 cm from C : 3 min
7. idem 6 cm from C : 3 min
8. 25 cm tool + 1.5 kg : large W-stroke with everywhere 6 cm overhang : 2 turns ( 6 min) :
Zone Measurement Parabola
11 5.31 mm 5.51 mm
10 4.77 4.93
9 4.25 4.40
8 3.76 3.86
7 3.21 3.29
6 2.56 2.70
5 1.97 2.14
4 1.35 1.57
3 0.82 1.01
2 0.42 0.44
1 0 0
Figure has improved
P-V + 1/6 labda S = 0.96
The measurements are the average of 2-4 measurements.
Since it is (for me) difficult to determine the correct locations of the Foucault shadow positions , I am going to test the mirror with the Bath interferometer test.
I do 10-12 igrams in one postion and average the results
Result : -0.95 correction and S= 0.853
Problems: high area 180-200 mm and high center 0-80 mm
Step 15
First work on the high center:
a. 15 cm tool ( tapered) + 0.5 kg : W-stroke inside r=100 mm : 2 turns.
( edge tool to r=100 mm)
b. 25 cm tool + 1.5 kg : W-stroke with 6 cm overhang: 1 turn
Bath test: Correction – 0.96 and S=0.90
So there is some improvement
But there is obviously a high area in the center.
Figure not too regular.
Step 16
- 8 cm tool + 0.5 kg: W-stroke within r= 100mm. Edge tool goes as far as r= 100 mm : 1 turn
- 25 cm tool : W-stroke with 6 cm overhang : 1 turn
Bath test : 10 igrams in 1 postion of mirror
Corr: -0.97 ; S = 0.948
Little change
Step 17
a. 8 cm tool + 0.5 kg :W-stroke within r=100 mm : 2 turns
b. 25 cm tool + 1.5 kg: large W-stroke with 6 cm overhang : 1 turn
Bath test in 3 positions ( always the mean value of 10-12 igrams)
0 degrees: – 0.99 and S= 0.97
90 degrees: -0.98 and S= 0.97
135 degrees – 0.98 and S = 0.97
This doesn’t say anything about astigmatism.
In the telescope I didn’t see it at 500x
Foucault test: S= 0.95
Coating 96% Taurus Optics in Poland