25 cm f5.5 thin mirror
- The borosilicate disc of 25 cm diameter and 2,5 cm thickness was bought from Stathis Kafalis in Munchen and a curve belonging to a radius of about 140 cm was generated by Jilles Eindhoven in Haaksbergen.
- The backside of the disc was ground flat on a 40 cm flat glass porthole with carborundum 240,using W-strokes
- I made a grinding tool out of a plaster disc of 5 cm thickness(cast on the mirror,with a cardboard dam around) . I glued ceramic tiles(19x19x2 mm) in a regular pattern on it, with a separation of ca 7 mm. For that I used 2-component polyurethane glue.
GRINDING
I worked on a heavy wooden stand (5 cm thick plywood i.e. a number of layers,glued together with polyurethane wood glue). Every turn around the table had 8 positions.Tool and mirror were turned quite often to prevent working too much in one position, what might result in astigmatism!
April 23, 2005
I used 1/3 W and straight stroke and C240 to get the tile tool fitting (tool on top). After that: 30 rounds with the same strokes with C400 .
April 24, 2005
C600 : 100 turns TOT (tool on top) ; I just went on with the grinding until a number of marked pits had disappeared.
May ,2005
C800 : 3-5 turns per wet : did about 80 rounds with the 1/3 W and straight strokes,
C1200 : 62 turns: surface OK
POLISHING
August 2005
First I used a 20 cm pitchlap that happened to lie around to do the first polishing.
After 3 ¼ hours the reflection of a strong lamp was weak in the central 16 cm but outside that area very strong!
I made a 25 cm pitchlap on a precurved plaster tool of 5 cm thickness. This disc was cast on the mirror.
First 1.5 hour MOT (mirror on top) after that only TOT.
After 131/4 hr polishing (total time) with cerium oxide suspension the edge still shows a weak reflection.
Total time: forgotten to write down….
CORRECTION
| WORK | RESULT |
| 1. 25 CM tool with 3×3 cm pitch squaresBefore every session the mirror is put on the pitchlap with cerium oxide suspension (with a bit dishwashing agent) and cold pressed for at least 30 min (2 kg weight)
MOT 1/3 W stroke : 10 min ½ W : 2x 10 min
|
Radius = 283.5 cmRonchi : weak bulge
No TDE (turned down edge) |
This zirconiumoxyde should be gentler than ceriumoxyde. I did a cold press of several minutes between the turns ( 8 different positions per turn) I had a lot of that stuff but the polishing went jerky |
Ronchi: large central hole (5 cm diameter)! |
| 3. I scratched the central pitchtiles a bit deeper to get less action there.MOT 1/3 W stroke 15 min | Central hole still present |
| 4. MOT 1/3-1/2 W narrow | Ronchi :Hole less deep. Parabola becoming visible |
I use a cardboard zone mask and import the measured values in Figure XP program, found on Internet.
Step 5
I scratched away a little smaller central area of the pitch
MOT 1/3-1/2 W (narrow) : 10 min
Ronchi: small central hole, makes Foucault meas. difficult in that place.
| Zone meas. | 0 | 0,1 | 0,46 | 1,26 | 1,90 mm |
| Parabola | 0 | 0,67 | 1,24 | 1,81 | 2,36 |
Step 6
Pitch squares are made narrower ,going from center to edge( tapered pitch lap)
MOT 1/3 W and straight stroke
Ronchil : still small hole in center.
| Zone meas. | 0 | 0.26 | 0.42 | 0.94 | 1.40 |
| Parabola | 0 | 0,67 | 1,24 | 1,81 | 2,36 |
Step 7
MOT 2/3 W 10 min
Jerky movements. The pitch tiles are smooth now, and according to my experience polishing never goes well then.
Ronchi : small hole in center
| Zone meas. | ? | 0 | 0.12 | 0.7 | 1.4 mm |
| Parabola | 0 | 0,67 | 1,24 | 1,81 | 2,36 |
No good. Return to sphere .Done by TOT and 20 min ¼ narrow W and straight stroke.
SECOND TRY
Step 1
Use Zirconium oxyde polishing powder( have a lot of it).
MOT :2/3 W 4 x 2 turns around. Pressing after 2 turns .8 positions per turn around
Result: light bulge in Ronchi-lines
Step 2
MOT 2/3 W 6x 1 turn around
Result: same; surface a bit blotchy
Step 3
I scratched the pitchtiles and use Ceri-Hex ceriumoxyde suspension in stead of ZrO
Zirconium oxyd doesn’t work well
MOT 2/3 W 5 x1 turn around
Result:
| Meas. | 0 | 0.15 | 0.62 | 1.05 | 1.40 mm |
| Parabola | 0 | 0.67 | 1.24 | 1.81 | 2.36 mm |
Center too flat
Step 4
More work on center: make outer pitch squares narrower and make the W-stroke farther from the center.
MOT 2/3 W 4x 1 turn around
Result:
| Meas. | 0 | 0.29 | 0.91 | 1.44 | 1.91mm |
| Parabola | 0 | 0.67 | 1.24 | 1.81 | 2.36 |
Center too flat
Step 5
a.TOT :work with edge of the tool on central area(diam 6 cm) with W-stroke.2 turns around
b. MOT : ½ W stroke : 1 turn around
Result:
| Meas. | 0 | 0.17 | 0.84 | 1.43 | 1.87mm |
| Parabola | 0 | 0.67 | 1.24 | 1.81 | 2.36 |
No success
Step 6
MOT : ¾ – 4/5 W 2 x 1 turn around; the stroke is concentrated more on the edge of the tool
Result ( average of 3 meas.)
| Meas. | 0 | 0.58 | 1.48 | 2.14 | 2.53 |
| Parabola | 0 | 0.67 | 1.24 | 1.81 | 2.36 |
This was too much! Correction too great. PV 1/3,5 lambda
Step 7
Try to decrease correction
MOT :1/3 narrow W-stroke 2 turns around.
Result:
| Meas. | 0 | 0.71 | 1.47 | 2.03 | 2.61 |
| Parabola | 0 | 0.67 | 1.24 | 1.81 | 2.36 |
No success.
Step 8
TOT : 1/3 W stroke 1 turn around
Result:
Average of 2 meas.
| Meas. | 0 | 0.65 | 1.43 | 1.80 | 2.37 |
| Parabola | 0 | 0.67 | 1.24 | 1.81 | 2.36 |
P-V 1/13 lambda Strehl =0,978 ; no TDE Finished for me.
Measured Radius central zone: 2824 mm