60 CM MIRROR
This one has always been a dream for me. But the prices of the glass!! When I started thinking about a 60 cm, the price of a disk was ca 5000 dollar..
Suddenly, I heard about pyrex glass that was not too expensive. It was available from Newport in California.
April 2004 : The pyrex 610x 45 mm disc arrives from California. It already has a radius of f4.4 . Cost : 980 euro for the disc and 600 euro for transport and taxes.
|April 9,2005||First work is to grind the backside flat. I used a 400x 19 mm float glass disc and carborundum 240|
|May 13, 2005||I made a 48×5 cm plaster disc ,cast on the mirror,that was protected by vaseline.I had made a dam of thick cardboard .After a week drying I glued (polyurethane 2 component glue) small tiles (19x19x2 mm) on the plaster .Distance between tiles was 7 mm.. The disc was protected against moisture with 3 layers of varnish.This grinding tool of course has to be made fit on the mirror. The progress is checked with the help of little marks on the tiles made with a waterproof inkStroke : Mirror below ; small W-stroke ; overhang not more than 10 cmCarborundum 180 and 240 ( C180 and C240)The grit is applied as a suspension in water with a little detergent added . It is squirted on the mirror or the tool with a plastic bottle with small opening. I add some after every round. A walk around the table has 8 positions|
|May 16||C180 finished after 12 rounds . Now C240|
|May 17||Ink marks still around on the edge tiles after 7 rounds|
|May 19||C 240 : ink marks gone after 10 rounds , even near edge|
|May 21||45 rounds 240. Only 4 tiles are not ground.I regularly round off the edge of the mirror and the edge tiles with a 400 grinding stone|
|June 1||Next : C320Done : 100 rounds C320 and 3 rounds with C600 to be able to see and mark some pits on the mirror.The grinding tool is fitting well|
|June 2||4 big pits marked within 2 cm of the edge of the mirror.It is quite a heavy job to carry the mirror around!|
|June 11||Next : C600 : to avoid bigger particles I decant a C600 suspension and use only the upper part. After 200 rounds most marked pits are gone|
|C800 : after 500 rounds I mark some pitsAfter 5 rounds the ink cross on the mirror is already gone.Pits marked after round 59|
|June 27||After 100 rounds C800 and 145 rounds Al-oxyd ( 15 micron microgrit) : all marked pits are gone. I treat the mirrors edge often with a 400 stone|
|June 28||Pity. I discovered a number of scratches.Back to C800. After 40 rounds scratches still visible|
|June 29||I return to C500 and 600 ; 800 too fine for these scratches|
|June 30||After 40 rounds : scratches smaller|
|July 1||After 80 rounds : scratches have turned into a number of small pits|
|July 3||After 140 rounds : scratches are gone. To C800; 40 rounds done|
|July 18||Round 135 : finished. Mirror edge and tool edge(the tiles) rounded with stone 400|
|July 19||On to C1200120 rounds; then on to 20 rounds with Al-oxyd microgrit 9 micron and 22 rounds with 3 micron.No pits or scratches visible.|
|This time, I didn’t cast the pitch on a plaster tool but chose for a plywood tool, made of 5 discs of 18 mm plywood (diameter 48 cm). On top of that were glued 2 layers if 3mm thick board ( of dfferent diameter, so as to mimic a convex surface). 2 rectangular wooden blocks are glued to the side of the thick disc. They function as grips while polishing.Around the disc came a cardboard dam and the molten pitch was cast through a nylon stocking onto the wooden base. After the pitch had cooled enough I made cross channels with a 10 mm thick piece of wood. This way of making a pitch lap I read from Dobson’s methods. Fitting the pitch lap to the mirror took some time. I heated the pitch lap with warm water and pressed it with 10 kg. on the mirror. It took several sessions and I had to trim the pitch a numer of times. Pitch squares are 50 mm wide.|
|Sept. 6||Start of the polishing. The work is heavy!!. Mirror below . W-stroke with 3-5 cm overhang. It is not going well. Tomorrow I will sratch the pitch squares with a sharp knife to ease the polishing|
|Sept. 7||Polishing is going much better. 45 min ( 3 periods of 15 minutes)|
|Sept. 11||Total time : 1h45m. Today 4x 15 minutes. When the pitchlap catches, polishing is very heavy work, despite the handles on the pitch lap. The pitch squares have to be scratched often.|
|Sept. 20||Total time : 7 hours . It is going well because the temperature is not so high.Reflection test( 25 W clear bulb focussed with 50 mmf2 lens on the mirror) : weak till r=25 cm. Edge area still has a quite strong reflection.|
|Sept. 29||Total time 12 hour ; 2x 15 min. a day is already quite a job! Afraid of arm injury|
|Oct. 5||Total time 15 hours ; reflection in last 5 cm still obvious.|
|Oct. 21||Total time 22 hours. Reflection at edge very weak|
|25 hours of polishing : stop|
CORRECTING THE MIRROR
I use the Figure XP program for evaluating the mirror figure
Jan 19, 2006 : I made a 25 cm pitch lap from Gugolz 55 pitch with some turpentine to soften it. I didn’t check out the hardness
- Fit the pitch lap on a 25/5.5 mirror (to get a curve in the pitch))
- Work ceriumoxyd polishing powder into the pitch
- Cut a star form with 8 points in the pitch
- Fit onto the 60 cm and trim the pitch squares.
Step 1 jan. 21 ,2006
I rounded off the mirrors edge with the help of a 400 stone.Start of correction.
1a . 25 cm 8 points starlap with 2kg weight on it
Straight stroke 0-1 cm to the right of the center with 5-7 cm passover: 20 turns ( 8 positions per turn).
5 min cold press after every ten rounds.
1b . Large W with 5-7 cm overhang 2x 1 turn with cold press after every turn around
First 10 min warm press with 5 kg, followed by 1 hour coldpress with 2 kg
a. Straight stroke 25 cm lap 0-3 cm from center 3x 10 rounds with 5-7 cm overhang
b. Large W-stroke with 5-7 cm passover 2 rounds with pressing in between
Test : ronchi : sferical with strong TDE ( Turned down edge) . Foucault diffraction ring much brighter on the right side. Also polishing scratches and blotchy surface
3a Increase correction
25 cm lap 30 rounds Straight stroke 0-5 cm ; 7-10 cm overhang
3b Large W-stroke, mostly on edge of mirror; 10 cm overhang
Test: TDE ca 5 cm wide
zone 7 : 2.2 mm
Zone 6 : 0,65 mm
Zone 5 : 0 mm
Zone 1 : 0 mm
4a . First try to get rid of TDE
- old 10 cm pointed lap( squares are made narrower near the edge) circular stroke on outer zones. Now and then an elliptical stroke to the center
- circular stroke 2-6 cm from edge: 5 turns around
- Dito 2-25 cm from edge: 1 turn around
Total time: 30 min.
4b. Different approach: work with 25 cm eight points starlap (with 2 kg)
- Tangential stroke 4-8 cm from edge : 30 min
- Straight stroke 0-5 cm from Center to increase correction: 2x 15 min with 2 kg
- Large W with 5-7 cm overhang : 3 turns around.
Ronchi: TDE seems to have lessened.Surface blotchy.Pitch too hard?
7 : 1.9 mm
6 : 1. 3 mm
5 : 1.2 mm
1 : 0 mm Getting better!
Step 5 .
jan 25 – 2006
25 cm pitchlap (8 points star) : 10 min in lukewarm water and then 30 min pressing with 5 kg
- tangential stroke 4-8 cm from edge with 2 kg : 15 min
- Straight stroke 0-5 cm from center with 2 kg : 2x 15 min
- large W stroke max 8 cm passover
Test: difficult to perform: the light passes through several rooms . Measurements are impossible if the house is warming up.
Jan. 26 ,2006
- Straight stroke with 25 cm lap +2kg ; 0-5 cm from c and 5-8 cm overhang : 3x 15 min (pressing in between, which is always done during long polishing sessions).
- Large W-stroke with 2 kg and 5-8 cm overhang : 5 turns around.
test : Ronchi : looks OK
Fouc. 1 measurement: First with a 7-zone mask
I trim the old 25 cm pitchlap. The squares are cut in 4. Smaller pitch squares make movement easier!
- 25 cm starlap : straight stroke 0-5 cm from C and 5-8 cm overhang : 50 min
- Large W-stroke concentrating on outer zones ; 5-10 cm overhang : 4 turns around
Test: difficult because of the “clouds”in the lightpath!
Ronchi pattern looks regular
Jan. 28, 2006
First, I made a softer 25 cm starlap
- Straight stroke with 2 kg 0-5 cm from C and 5-8 cm overhang
- The same 30 min
- Large W-stroke + 2 kg with 5-8 cm overhang 3 x 2 turns around
Test : difficult because of the “clouds”in the lightpath
Jan. 31, 2006
25 cm starlap soft
a. Straight stroke with 2kg 0-5 cm from C and 5-8 cm overhang 10 mi
b. Straight stroke with 2 kg 2-5 cm from C and 5-8 cm overhang 15 min
c. Same as b, for 20 min.
d. Large W-stroke with 5-10 cm overhang :5 x 1 turn
I scratched the pitch squares to ensure an easier polishing action.
Ronchi : obvious bend in the lines at r=15 cm
Zone between 3 en 4
Hole in C
Now : how to get rid of the hole in the center and the bend at 15 cm?
For polishing I use Ceri-Hex from now on .This is more uniform but quite expensive stuff!
25 cm soft pitchlap(8 points star)
- Straight stroke 10-15 cm from C : 20 min
- Straight stroke 5-10 cm from C : 30 min
- large W-stroke 10x 1 turn around
Always with 2kg and 8-10 cm overhang. Polishing is only easy when I scratch the pitch squares.
Ronchi : looks regular
Fouc. 2 sets of measurements (30min in between)
Febr. 6, 2006
25 cm starlap 2 kg
- straight though center C : 10 min
- straight stroke 3-5 cm from C 2x 15 min
5- 10 cm overhang
3. straight stroke 10-15 cm from C 10 min
4. straight stroke 1-5 cm from C 15 min
5. large W-stroke 6×1 turn aound
First I turned down the central heating, closed the windows and waited for 3 hours!
Now the atmosphere was stable and I could make a good set of measurements.
Ronchi : regular
Fouc. 2-4 measurements d averaging
Central area is too flat: deepen with 15 cm pointed lap.
- 15 cm lap + 2 kg straight stroke 1-3 cm from C and 5 cm overhang: 15 min
- 25 cm lap + 2 kg straight stroke 0-5 cm from C and 5-10 cm overhang
- 25 cm lap +2 kg large W-stroke 5-10 cm overhang : 5×1 turn around
- 15 cm lap : same as a
- 25 cm lap +2kg : straight stroke 0-15 cm from C and 5-10 cm overhang
It is also necessary to work on the outer zones!
It looks like the edge zones are going flat!
2 aims : increase correction and correcting long outer zones.
- 15 cm pointed lap + 2kg : straight stroke 0-3 cm from C : 20 min
- 25 cm starlap( already a bit thin!) + 2kg straight stroke on r=20-22 cm : 10 turns around
- 25 cm lap + 2 kg: straight stroke 0-5 cm from C and 8-10 cm overhang : 15 min
Ronchi : bend in lines at r=20 cm ; also low edge
Febr. 8, 2006
Aim: improve edge area and keep the amount of correction
- 15 cm starlap : Straight stroke on r=25 cm 10 turns around
- 15 cm sterlap : circular stroke r=25 to r=28 cm 15 min
- 15 cm sterlap : elliptical stroke r=-1 tot r=28 cm 20 min
Average of 2-5 measurements
Strehl = 0,755 labda/3.5 wavefront error
Figure: 0,1R means : 10 % of the radius from the center of the mirror
Febr. 11, 2006
Plan : 1. correcting area r=27-30 cm
- correcting central area to r=10 cm
I use the old pitchlaps
- 10 cm lap circular stroke on r= 28-29 cm 10 turns around
- 15 cm lap +2 kg : narrow W-stroke between 5 cm left and 5 cm right of center:5 cm overhang : 5 min
- 25 cm lap large W- stroke 5-10 cm overhang 2×1 turn around
Ronchi : TDE?
Fouc.: Average of 3 measurements
Strehl = 0,404 labda/ 2.1 Worked too much on C
Work more on edge
- 15 cm lap Tangential stroke on r=27 cm 5 turns around
- 15 cm lap Straight stroke on r=10-15 cm 5 turns around
- 25 cm starlap : large W-stroke concentrated on edge and center 4x 1 turn around
Ronchi : edge area flat and shallow hole in C
Febr. 15 , 2006
Work on edge
- 15 cm pointed lap 2 kg : tang .stroke on r=27 cm ;every 3 strokes one stroke to the center to get a bit of correction in that direction 10 min
- 15 cm lap + 2 kg W- stroke r=6 tot r=12 cm 4 turns around
- 25 cm starlap large W- stroke 3x 1 turn around
10 zone masker made
Average of 2 measurements
I made new pitchlaps from Gugolz ,softened with linseed oil and turpentine.
Pressure test (Texereau) 1kg weight and 1 mm2 surface of pin :4 mm depth in 5 min.
Work on r= 13,5 en r= 28 cm
First try with new 35 cm lap:
- very narrow W-stroke on r=27-29 cm (edge of tool on that area) : 6 turns around
- Narrow W-stroke on r=9-18 cm (edge of tool) : 5 turns around
- Large W-stroke ( smoothing) +2kg : 2 turns around with 8-10 cm overhang
I didn’t scrath the pitch squares, with the result that polishing didn’t go well
Test : zone 9-10 still 1,3mm difference!!
Difference 1-10 = 7,7 mm . Diff . 1-2 = 1,3 mm .There seems to be a TDE
Work on TDE
With earlier mirrors I had great trouble combating this problem ; after using a small tool I had to go back to spherical because of the zoniness.
- 6 cm pointed lap: short tangential stroke on r=27 cm ;every 3 strokes a stroke to the center to prevent zones.. Light pressure 10 min
- 20 cm pointed lap +2 kg tangential stroke on r=25 cm ; every 3 strokes a stroke to the center ; 10 min
- 35 cm lap large W-stroke with 8-10 cm overhang : 3x 1 turn around
Ronchi : TDE en bend at r=15 cm
Fouc. : Total correction is 7.8 mm and diff 9-10 is 1.4 mm
Work on edge with several tools
- work with thumb on r=28 cm 5 turns around
- 15 cm pointed lap : circular stroke on r=27-28 cm with light pressure . 30 min Cold press every 5 minutes
- 15 cm pointed lap circular stroke on r= 15 cm 20 min with light pressure
- 15 cm pointed lap elliptical stroke on r=10 to r=28 cm
Ronchi: – small TDE
- Bend at r =16 cm
- Hole in C
Fouc. TDE less ! 2-4 measurements and averaging
S= 0,306 labda/2
Febr. 27, 2006
Work on edge and center
- 10 cm lap: circular stroke on r=27-28 cm ; light pressure: 15 min ;cold press every 5 min:
- 20 cm tool +2kg : W-stroke on r=9 to r=15 cm; 5-10 cm overhang: 15 min
- 20 cm tool : elliptical stroke on r=10 to r=25 cm ; light pressure : 15 min
Ronchi : TDE visible ; irregular surface in central area
Fouc. Average of 2-4 measurements
Hole in center gone; TDE still around
Febr. 28, 2006
- Edge: 15 cm tool circular stroke with light pressure on edge : 15 min
- central area: 20 cm tool+ 2kg W-stroke within r=12 cm left and right ; 5-8 cm overhang : 15 min
- Edge: 15 cm tool : straight stroke on edge area; every 3 stroke a stroke to the center: 5 min
Ronchi : TDE + bend at r =15 cm
Fouc. Average of 2-4 measurements
|Percent corr.of the zone||88||122||94||73||68||80||80||76||118|
It’s worse,alas! S= 0,01 en labda/1
March 1 ,2006
I am a bit in trouble. Edge no good and correction too small
- 20 cm pointed lap +2 kg : large W-stroke with 5-8 cm overhang.More pressure on edge and center : 3×14 min.
- 20 cm pointed lap : circular stroke with light pressure : r= -3 cm to r= +25 cm( that is : the center of the tool moves between those radii)
Ronchi : TDE + bend at r=15 cm
S= 0,049 en labda/1,18
Conclusion; zones 6,7 and 8 radius too short ; too little correction.
March 2, 2006
Try to work on the zones 6-8
- 20 cm pointed lap + 2 kg : W-stroke to max. r=25 cm(edge of tool goes to r =25 cm) : 35 min
- 10 cm tool : tangential stroke on r=28 cm ; every 3 strokes a stroke to the center: 11 min
- Smoothing with 35 cm tool : large W-stroke with 10 cm overhang :2x 1turn around
Ronchi : no improvement!
TDE present s= 0,318 labda/ 1,165
March 2 , 2006
- Increase correction
20 cm pointed tool + 2 kg : straight stroke 0-5 cm from center and 5-8 cm overhang : 20 min
- Edge: 6 cm tool circular stroke on r =28 cm : 10 min.
- More correction and smoothing: 20 cm tool + 2 kg large W-stroke 5-8 cm overhang : 20 min
- Smoothing: 35 cm tool : large W-stroke 2 x1 turn around
Result : less correction! Caused bij large W-stroke? Surface zoned!
Average of 2 measurements
March 4, 2006
- 15 cm tool + 2kg straight stroke 1-2 cm from Center and 5 cm overhang: 15 min
- 25 cm tool + 2 kg straight stroke 0-5 cm from C : 15 min
- 25 cm + 2kg large W-stroke with 5-8 cm overhang : 15 min.
Measurements: 1 : 0 ; 2 : 0,71 ; 9 : 5,92 ; 10 : 6,86
- 25 cm star shaped tool + 2 kg : straight stroke 0-5 cm from C and 5-8 cm overhang: 15 min
- Same tool: large W-stroke 5-8 cm overhang: 3×15 min.
Measurements: 1 : 0 ; 2 : 0,91 ; 9 : 6,16 10 : 7,21 Still a TDE
- Improve edge: 6 cm tool : circular stroke on r=28 cm : 10 min
- Increase correction: 25 cm tool +2 kg : straight stroke 0-6 cm from C : 2x 15 min
- 25 cm starlap + 2 kg : large W-stroke 5-8 cm overhang 2x 15 min.
Measurements 1: 0 ; 2 : 0,60 ; 9 :5,98 ; 10 : 7,15
TDE and too little correction
a. Work on TDE :15 cm pointed tool : circular stroke on r=27-28 cm : 5x 5 min
b. Increase correction and smoothing: 15 cm tool elliptical stroke from r = – 5 cm to r=27 cm : 5x 5 min
a and b are done in turn
Ronchi : bend at r=12 cm
Measurements : 1 : 0 ; 2 : 1,35 ; 9 : 6,76 ; 10 : 8,06
Edge still too flat!
Again: the TDE!
- 6 cm tool : tangential stroke on r=27-28 cm ;every 3 strokes a blending stroke veering off to the inside of the mirror 15 min.
- 6 cm tool : circular stroke on r=27-28 cm 20 min
- 35 cm tool : large W-stroke for smoothing: 1 turn around
Ronchi : hole in C ; bend at 20-22 cm
Zone in outer region of mirror
- Thumbing on r=25 cm : 5 min
- further improvement of the edge:
a. 10 cm pointed lap tangential stroke on r=25-27 cm; every 3 strokes one stroke veering off aside to prevent a zone :………………………………… 15 min
b. 10 cm tool : circular stroke on r=27-29 cm ………………………15 min.
3. Increase correction : 20 cm pointed lap + 2 kg narrow W-stroke between r =5 en r= -5 cm and 5 cm overhang …………………………………30 min.
4. Smoothing with 35 cm tool+ 2kg : large W-stroke 5-8 cm overhang 2 turns around
Measurements: 8 o clock in the morning in closed house and no heating: Foucault image stable!!
Ronchi : hole in center and bend at r =22 cm
Correction too large : 30 min was too long! Whole outer area irregular
I apply the radial stroke with the 35 cm tool to blend the zones.
Lap is placed in the center and moved to the edge; 10 cm overhang.
Move it back to the center and repeat in a different direction. 16 different positions in one turn around.
Total time 3x 5 min with pressing in between.
Ronchi : bend at r=22 cm
Correction strongly decreased(was to be expected with the large tool)Hole in center gone
Zone still present
- Increase correction : 20 cm pointed lap +2 kg : narrow W-stroke between r=- 6 and r=+ 6 cm ; 5 cm overhang…………..10 min
- 35 cm tool : radial stroke (see step 32) ………………..5 min
Ronchi : bend at r=22 cm
Fouc. 3-4 measurements “ spread” 0,05 mm
Edge is “ rising”
Increase correction; less work on edge.
a. 25 cm starlap + 2 kg narrow W-stroke between r=-6 and r=+6 cm; 4 cm overhang : 10 min
b. 35 cm radial stroke with 4 cm overhang : 5 min
Fouc. 3-4 measurements :
Too much correction and edge has risen
S= 0,472 en labda/1.9
35 cm lap + 2 kg : large W-stroke with 5-8 cm overhang: 2x ½ turn around(pressing in between)
4 measurements at 16 o’clock ( after 4 hours stabilizing)
S= 0,889 labda/5,48
Work at 60-80 % radius
15 cm pointed lap (8 mm thick):
a.circular stroke at r=18-24cm;light pressure 5min
b. elliptical stroke from r= -1 to r=+20 cm; light pressure : 5 min
How the measurements are done:
- mirror on the stand in the bathroom; doors and windows closed
- 4 measurements
- Mirror is turned one quarter
- After 40 min again 2 measurements
S= 0,831 labda/3.9
In the Figure XP program one can choose the radius of curvature to make the correction of the figure easier.
If the edge is less turned up ( 20 nm in stead of 70 nm) ,than more work has to be done on r= 90 to r=240 mm. I am going to try that.
- 20 cm tool tangential stroke on r=150 mm; every 3 strokes one stroke veering off to the edge……………………….10 min
- 20 cm pointed lap+ 2 kg : elliptical stroke from r= 10 to r=28 cm…………10 min
test : average of 4 measurements
S = 0,936 labda/7.2
The worst of the measurements still delivers : s=0,931 and labda/6.7 on the wavefront
May 24, 2007
10 cm tapered tool : work on the high zones in the figure
- Tangential stroke on r=240 mm: every 3 strokes a stroke veering off to the edge; light pressure ; ………………………………………………………5 min
- Elliptical stroke r=240 to r=270 m …………………………3 min
- Circular stroke r= 100 mm……………………………………3 min
- Elliptical stroke r= 50 to r=270 mm…………………………4 min
Test : average of 4-5 measurements
Undercorrected as a result of working on the outer zones
S= 0,914 en labda/4.8
Work on the high zones
- 15 cm tool straight stroke on r=100 mm with 5 cm overhang; light pressure ….6 min
- elliptical stroke r=0 to r=230 mm………………………………………………3 min
S= 0,989 labda/ 14 PV
June 2, 2007 02.45
Startest : mirror in sling in 9 points cell
5 mm eyepiece : no astigmatism. Ronchi lines straight
Aluminium + QUARTZ LAYER : Galvoptics Basidon Essex (GB) : juli 2007
Price : 120 pounds only!