45 cm f/4.9 thin mirror
A mirror of 45 cm diameter and only 2,5 cm thickness is something I would really like to posses. True, there is the danger of astigmatism, but think of the ease of handling such a large, thin mirror! And, then again , I had aquired a lot of experience with the 40 cm x2,4 cm float glass mirror.
The disc was bought from Stathis Kafalis in Munchen in 2006 for about 250 euro.
A REMARK :THE FIGURES COMING FROM FIGURE XP are missing at the moment . DURING CORRECTION I LOOKED AT THE HIGH ZONES DISPLAYED BY FIGURE XP AND WORKED AT THEM.
Part 1 : grinding
A lot of work
Disc was hollowed out with 15 cm steel ring ,straight strokes and carborundum 80.
After reaching the desired depth I made a plaster tool, 45 x 5 cm and glued 19x19x2 mm porcelain tiles on it with PU glue. I made a drawing first on the plaster tool to serve as a guide in placing the tiles.
I ground the plaster tool on the mirror with 1/3 – ¼ W-strokes and straight strokes until the tool fitted.
You can find that out by making marks of permanent ink on the tiles and grinding until all marks have worn away.
After C80 came C120, C180, C320, C 600 ,C800 and C1200
Part 2 : polishing
I made a 35 cm polishing tool from a 35 x5 cm plaster tool that I had cast on a 35 cm mirror (protected by grease) . Otherwise I would have made a plaster tool of 45 cm.
The plaster is held by a dam of cardboard. It takes a week for the plaster to dry properly. I coated the plaster with two layers of varnish. Just before casting on the pitch I smeared the plaster with a thin layer of turpentine , just to facilitate the adherence.
I cast the pitch (quite hard) on the plaster and with the help of a wooden stick I made cross channels in the pitch. Of course you have to wait some time for the pitch to harden.This business takes some practice. Afterwards came the process of hot pressing (with soapy ceriumoxyd suspension) and trimming the 50×50 mm pitch squares
Support of the mirror:
Flat table: on it a 30 mm thick plywood holder where the mirror is placed in. It can be moved a bit to prevent constraining. Between the mirror and the wood is a 9 mm thick hard foamrubber support ( the kind used for sleeping mats). During polishing the mirror is turned about 120 degrees very often. This to prevent astigmatism.
The mirror is held by a little wooden keg during polishing otherwise it moves to much. But the wood is a bit compressible, so the danger of astigmarism is not great .
Small W , mirror below.
After 3 h of polishing ; reflection test of mirror surface ; done with a clear 25 W bulb focused with a 50 mmf2 lens. Reflection everywhere very strong.
6.5 hrs of polishing : reflection test: center of miror weak; edge very strong.
Nov. 11 : 10 hrs of polishing. Polishing now with the expensive Ceri-Hex polishing powder ( is even grained). Polishing is going very heavy, so I think it is effective.
I smeared new pitch on the old squares with a brush. They are now a bit thicker.
It costs some time to fit the pitch lap again.W-stroke with mirror below and overhang left to right and up and down not more than 3 cm.
34 hrs : a few scratches are visible. I leave them
Pitch very hard now. Reflection test : still quite visible in the last 5 cm to the edge. I grind the edge with a 400 stone
New 35 cm pitch lap from Gugolz 55 and 64.
After fitting and one hour polishing : pitch lap works very bad despite warm and cold pressing.I discover again that I have to scratch the pitch squares to get an even polishing action
I polished 5 periods of 15 minutes . In between cold pressing with a 5 kg weight.
Last 15 minutes didn’t go too well, because the minichannels in the pitch ( made by scratching) closed up.
Polished 2 x15 min.
Ronchi test with glass ronchi ruling from Edmund Scientific ( this was a bit expensive!).
Light source round with a 3 mm diameter.
– weak hill in the center
– turned up edge area
– Turned down edge( TDE)
The Foucault test wasn’t a success because of air turbulence.
The whole test apparatus and the mirror stand aren’t standing vertical but are leaning 10 degrees from the vertical. I read this method somewhere in Telescope Making Journal. It should prevent bending of the mirror. I support the thin mirror with a sling and a 9-points support (3×3) in the back.
Ronchitest : the lines move somewhat in position when I move back and forth through the focus . Could this be astigmatism!?I continue with polishing and work on the same time at the star test contraption that I need to test the figure of the mirrors.
5 x 15 minutes with the 35 cm lap. W-stroke with 8 cm overhang.
Jan. 2007 : Star tester ready: no astigmatism with 358x magnification.
Reflection test: extremely weak at edge.
Part 3 : Correction
Pitch lap : 25 cm diameter ( on a plaster tool) .Gugolz 55; pitch squares cut smaller near the edge.
W-stroke with max. 6 cm overhang: 2x 10 minutes with light pressure.
Ronchi: weak hill in center and low edge area
Foucault : 7-zone mask ; 1 series of measurements. Later on I take 3-4 sets of measurements and average the numbers.
7 + 0.19mm
This looks more like an ellipsoid!
a. 25 cm lap ; straight stroke; 2 cm offset(from center) and 5 cm overhang.
I nearly always use a 2kg weight on the polishing lap. During polishing I walk around the polishing stand. 10 turns
b. Trying to get a sphere with the 35 cm lap : W-stroke 8 cm overhang ; 5 x 1 round( cold pressing in between with 5 kg during 5 minutes). I scratch the pitch squares with a sharp knife.
Ronchi : still a hill in the center
7 + 0.14
Too irregular to try to make a parabola
25 cm lap : deepen the central area
a. W-stroke with 6 cm overhang : 9 x 1 turn
b. Straight stroke 0-5 cm left of C and 5-8 cm overhang : 25 minutes.
Ronchi : no visible change
25 cm Pitch lap : warm press and make mini-channels
with a knife.
Straight stroke 0-5 cm left of C and 5-8 cm overhang : 2x 20 minutes
Lines now more straight; weak hill in center.
Fouc.: no TDE ; diffraction ring visible on the whole circumference.
25 cm lap (with a mild starfish pattern: pitch squares a little narrowing towards the edge) straight stroke 0-5 cm left of C and ma. 8cm overhang : 25 min.
35 cm lap : Smoothing : W-stroke with 8 cm overhang : 1 turn (8 positions)
Test : Ronchi : a bit better than in step 4. But the ronchi pattern shows the tell-tale hooks of a TDE, that is invisible in the Foucault test.
25 cm lap (see step 5)
I have studied the Zambuto mirror making files (Yahoo group) and try to do a procedure that is followed in some Zambuto mirrors.
Offset in cm Overhang Time
1 6-7 9
3 6-7 4
4 6-7 2
I repeated this series 4 times.
It was difficult ( say impossible) to keep to that offset.
Test : Ronchi : slight hill in the center and a bit of bending in the edge area towards the center. No edge hooks.
25 cm lap (see step 5)
I try the Zambuto-group advice:
Vary the offset, diminish the overhang. It is advisable to put the correction first in the outer zones because it is later on quite easy to extend the correction to the inner zones
Diminish the central hill: 19 minutes straight stroke with 0 cm offset and 6 cm overhang.
Zambuto group method:
The times vary. Mostly it is more than the given time because I want to finish the turn around the table.
Conclusion from Ronchi test: it is not going well. I think the pitch lap is too large for a f/4.9 mirror. The central area stays too flat
March 1 ,2007
I made 3 pitch laps on glass disks : Gugolz 55 pitch with a bit of linseed oil to soften the pitch. It is rarely warmer than 18 degrees in my working room.
Sizes : 10-15-20 cm
20 cm lap with narrowing squares towards the edge(tapered lap)
Deepen the central area but also work more on the outer zones to prevent a long edge.
This series was doen twice
Ronchi : no visible change
Foucault: zone r2/2R (mm) Light source and knife on one moving stage
6 + 1,22
5 + 0,35
1 +1,1 !! Obvious hill in C
March 2, 2007
Deepen a cenral area of 30 cm diameter. Use a 15 cm lap( narrowed pitch squares towards the edge)
Straight stroke 1 cm offset and 3 cm overhang : 15 min
Circular stroke between -1 and + 19 cm from the center : 15 min
This circular stroke is meant to polish more slowly and with less :” ploughing” !
The center of the lap moves from 1 cm under the mirror center to 19 cm above the center.
Ronchi : central hill gone
|r2/R 0||0||0||0,13||0,6||1,23||2,06 mm|
Parabola : 0 – 1,13 – 1,84 – 2,69 – 3,54 – 4,34 – 5,12
The edge is beginning to get long. Large difference between zone 6 and 7
Step 10 Correcting the long edge…
a. circular stroke with small lap on edge area: this may cause a zone!
b. tangential stroke with small lap on edge area: might be better
c. Work with larger lap.
20 cm starfish patterned lap
tangential stroke on r=18 cm with a stroke to the center every three strokes :30 min.
W-stroke 5 cm overhang : 5 rounds
The difference in the outer zones becomes less. And the mirror is quite flat within a diameter of 30 cm
20 cm starfish lap : deepen the center
a. straight stroke 1 cm offset till 2 cm from edge (edge of pitch lap doesn’t reach the edge of the mirror)
b. W-stroke 5 cm overhang : 1 round
a and b are repeated four times
The jump in the outer zones becomes too large again!
Work on the low edge and increase the correction
low edge: 7 cm diam. Star lap circular stroke on r=20 -21 cm : 10 min.
low edge: 15 cm lap : tangential stroke on r=20 cm with astroke veering off to the center every 3 strokes. I do this to prevent zones : 4x 5 min
Increase correction : 15 cm lap Straight stroke with 0-1 cm offset and 5 cm overhang : 4x 5 min
The figure of the mirror is becoming better!
Step 13 March 4, 2007
Work on center : 15 cm starfish patterned lap : small W-stroke on center: 5 cm movement left and right and up and down : 10 min
Increase correction: 20 cm lap : straight stroke 0-1 cm offset and 6 cm overhang : 2x 5 min
W-stroke (large) 2×1 round ( 8 positions) 5 cm overhang
Test : R=4303 mm
zone 1-2 : 45 % correction
2-3 125% !!!
4-5 90 %
5-6 75 %
6-7 90 %
From now on I use the (expensive) even-grained polishing powder Cer-Hex.
However: later on I found out that decanting had been necessary: I had many polishing scratches on this and other mirrors!
For now: polish the center, increase correction and make the figure smoother
15 cm tapered lap : W-stroke on central area; only 2,5 cm movement to all sides: 10 min
20 cm tapered lap: straight stroke with 0-1 cm offset and 6 cm overhang : 6 min
35 cm lap: large W-stroke (8 pos.) 1 turn with 8 cm overhang
Test : Fouc.
Test : Fouc.
Evaluation : too irregular and edge too high,center better!
Try to polish zones 3 and 4
March 6, 2007
a. 15 cm lap: (tapered) : straight stroke on r=13 cm with 5-6 cm overhang : 8 min
b. id: straight stroke offset 1 cm with 5 cm overhang 4 min
c. smoothing : 35 cm lap W-stroke 8 cm overhang 2×1 turn
Polish zone 6-7 and increase correction:
10 cm tapered lap : circular stroke on r=18 to r=21 cm; every 3 strokes a stroke to the center to prevent zones : 10 min
20 cm lap : straight stroke with 1-5 cm offset: 10 min
35 cm lap : W-stroke : 2×1 turn
One can change the ROC of the mirror in the Figure XP program. Then the necessary correction steps may change.
In this case, if I shorten the ROC a large central hill appears, which is easier to correct than other problems.
Work on edge and the large central hill.
15 cm starfish lap : tangential stroke on r=20 cm; every 3 strokes a stroke to the center
20 cm lap (starfish) : W-stroke within r=17.5 cm: edge of the tool stays within a diameter of 35 cm
35 cm lap: W-stroke 8 cm overhang 1 turn
Step 18 march 8, 2007
Work on zone 2-5
10 cm lap : circular stroke on r=150 mm( work between r=100 and r=200 mm). Every 3-4 circular movements I make an elliptical movement over he center( a bit past it) : 2x 10 min.
35 cm lap : W-stroke 6-8 cm overhang : 1 turn
Test with new mask :8 –zone.
Parabola: 0 – 0,746 – 1,472 – 2,191 – 2,919 – 3,621 – 4,319 – 4,971
Next plan: work on radius 100-200
10 cm lap: circular stroke on r=150 mm : 2x 10 min
35 cm lap: W-stroke 1 turn
Test : almost no change
20 cm tapered lap: tangential stroke on r=150-160 mm(every 3 strokes one stroke to the center) : 10 min
35 cm lap : W-stroke 8 cm overhang 2x 1 turn.
Test : problem in edge area : difference between zone 7 and 8 is 1.05 mm (much too large). It seems the edge area is neglected.
Work on the edge problem
7 cm lap : circular stroke on r=20-21 cm : 10 min
15 cm tapered lap : tangential stroke on r=20 cm ( with side strokes: see earlier): 4x 5 min
35 cm lap W-stroke 2×1 turn
Test : difference zone 7 and 8 : 0,8-0,85 mm
Repeat step 21 : no improvement
Improve the edge
10 cm lap : tangential stroke on r=20 cm (with side strokes; see earlier) : 4x 5 min
25 cm lap : W-stroke with 6 cm overhang : 3×1 turn (the 35 lap was to hard)
Difference between zone 7 and 8 still 0.8-0.85 mm
Comment: I consider the mirror a failure. It is possible the edge bends too much because the foamrubber under the mirror is a bit too small. I replace the foamrubber by a new and larger piece and try to improve the edge
New 7 cm lap: tangential stroke on r=21 cm (with side strokes,see earlier) : 15 min
20 cm tapered lap
Tangential stroke on r=16-18 cm : 5 min
W-stroke 5 cm ovehang : 1 turn
Session b is repeated three times
It is obvious that the outer zones are wrong.
I am going back to a sphere….
40 cm lap . Tool on Top W- stroke with 5 cm overhang : 40 min
Ronchi: the edge is the bad part
Step 26 : work on edge
10 cm tool : tangential stroke on r=18 cm : every 2 strokes a stroke to the center 2×10 min
40 cm tool : W-stroke with 5 cm overhang : 1 turn
Test: turned down edge ; difference between zone 7 and 8 is 0.6 mm; weak hill in the center.
Step 27 : work the edge
7 cm tool : circular stroke on edge area with 1 cm ovehan : 10 min
10 cm tool Circular stroke on edge area with now and then elliptical strokes towards and over the center: 25 min
Test : spherical with turned edge area . Zone diff. 7 and 8 : 0.68 mm
Old 25 cm tool: small w-stroke on area within edge. Lots of pressure: 23 min
Larger w-stroke with 5 cm overhang : 5 turns
Test: no TDE!. Diffraction ring around the mirror. Only the outer zone is too long. Diff 7-8 still 0.65 mm. Central hill
Work at the edge
10 cm tool : tang. stroke on r=18-19 cm. Every 3 strokes a stroke to the center : 3x 10 min.
25 cm tool : straight stroke on r=15 cm: 3x 15 min; alternated with straight stroke on r= 3 cm : 3×5 min. Overhang 5 cm
Test: no improvement. Difference zone 7-8 still 0.6 to 0.7 mm
Try to improve the figure with the help of the big tool
40 cm tool : narrow W-stroke 2 cm left and right and 5 cm overhang.Time ?
Diff. 7-8 is now 0.48-0.50 mm
a. 40 cm tool Narrow W-stroke (see 30) : 4x 10 min
b. Increase correction: 20 cm tapered tool
Straight stroke on r = 2 cm : 5 min
Id on r= 5 cm : 2min
c. 20 cm tool : W-stroke with 6 cm overhang: 15 turns
d. 20 cm tool
Straight stroke on r=2 cm 2×10 min
Id on r=5 cm 2×4 min
Test (Ronchi):Hill in center : edge area less turned down.
20 cm tapered lap
a. Straight stroke on r= 2 cm : 7 min
b. id r=5 cm : 3 min
Repeat a +b eight times!
c. W-stroke with 6 cm overhang : 9 turns
Test : Ronchi: Still a weak hill in the cnter. Edge area better!
I use more concentrated CeO2- suspension from now on because I think that the one I used till now wasn’t effective enough.
20 cm tapered lap:
a. straight stroke on r= 1 cm : 7 min
b. id r= 3 cm : 3 min
c id r=6 cm : 2 min
a+b+c : repeat 9 times
Test Ronchi : center flat : rough surface
Deepen the center and leave the edge
15 cm tapered lap :
a. Straight stroke on r=1 cm ; stay 3 cm within edge: 7 min
b. Straight stroke on r= 3cm: stay 1 cm within edge: 3 min
c. 20 cm starfish tool : W-stroke with 6 cm overhang : 3x 1 turn
The surface is quite irregeular!
15 cm starfish tool : work within zone 5
W-stroke within 36 cm diameter area : 3×10 min
b. 20 cm tool :W-stroke with 5 cm overhang : 4x 1 turn
Test : Foucault:
A bit successful ,at least. However: difference 6-7 to great
Work on central area untl zone 4
15 cm starfish tool : W-stroke within 32 cm diameter: 4x 10 min
15 cm straight stroke on r=5 cm ; overhang 5 cm : 10 min
Test : 2 measurements
Change to the expensive Ceri-Hex polishing powder
10 cm tool : tangential stroke on r=16 cm; every 3 strokes a stroke to and over the edge; 10+5+10 MIN
Circular stroke with 10 cm tool on r=16 cm : 10 min
Idem, but now from r=-10 to + 6 cm (elliptical) : 5 min
Test : foucault (3 measurements)
Work on 70 % zone to lower the correction:
20 cm tapered tool ;short tangential stroke on r=16 cm with side stroke to the center(1 in 3) : 2x 5 min.
Test : still 5.5 mm total correction
40 cm tool :
Warm press and w-stroke TOT with 10 cm overhang : 2 x ½ turn .Pressing in between.
Test ( 2 meas.)
This is better. But there is still overcorrection.
10 cm tapered tool:
a. circular stroke on r=12 to r=17 cm : 5 min with light pressure
b. circular stroke r= -10 to r= +18 cm : 2 turns (elliptical stroke)
Test (2-4 meas.)
10 cm tapered tool: work on outer zones to bring down correction
a. tangential stroke on r=18.5 cm : every 3 strokes a stroke to the center: 5 min.
b. circular stroke on r= 12 to r= 18 cm : 3 min
Strehl = 0.967
Peak to valley : labda/11
Star test: May 30, 2007 with 6 mm eyepiece; support with 9-point cell in sling.: no astigmatism
Rochi pattern on bright star: straight lines.
Aluminized and quartz overcoated by Galvoptics in Basildon (UK)