45cm f4.9 thin mirror

45 cm f/4.9 thin mirror

Introduction:

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 .

Stroke:

Small W , mirror below.

Oct. 24,2006

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.

Oct. 29

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.

Nov. 14

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.

Nov. 30

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

Dec. 8

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

Dec. 9

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.

Dec. 12

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

Step 1

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.

Zone       r2/2R

7             + 0.19mm

6                0

5                0.13

4                0.47

3                1.26

2                 1.89

1                 2.15

This looks  more like an ellipsoid!

Step 2

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

Fouc.

Zone

7          + 0.14

6           0.10

5        0

4        0.61

3         1.23

2          1.70

1          2.0

Too irregular to try to make a parabola

Step 3

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

Step 4

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

Ronchi:

Lines now more straight; weak hill in center.

Fouc.:  no TDE ; diffraction ring  visible on the whole circumference.

Step 5

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.

Step 6

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.

Step 7

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:

Offset (cm) Overhang(cm) Time(min)
1,5 6cm 9
4 4
5,5 2
0,5 6 5
1,5 3
4,5 2
5 5 2
1,5 5
2,5 3
6 5 2
1 5
3 3

 

Offset (cm) Overhang(cm) Tijd(min)
5 4,5 2
2 5
2,5 3
6 4,2 2
1 5
3 3
5 3,8 2
1 5
3 3
5 3,2 2
1 5
2,5 3
4 2

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

Step 8

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.

 

Offset(cm) Overhang(cm) Time (min)
1 5 6
3 5 5
5 5 3

This series was doen twice

Ronchi : no visible change

Foucault:       zone                  r2/2R (mm)  Light source and knife on one moving stage

7                        +1,84

6                        + 1,22

5                        + 0,35

4                          0

3                           0

2                           0

1                          +1,1  !!  Obvious hill in C

March 2, 2007

Step 9

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

Fouc.

Zone 1 2 3 4 5 6 7
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…

Possibillities:

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.

Try first:

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
T

Zone 1 2 3 4 5 6 7
         0 0,4 0,4 0,4 1,0 1,5 2,1 mm

The difference in the outer zones becomes less. And the mirror is quite flat within a diameter of 30 cm

Step 11

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

test:

Zone 1 2 3 4 5 6 7
+0,1 0 0,35 0,50 1,10 1,70 2,50 mm

The jump in the outer zones becomes too large again!

Step 12

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

Zone 1 2 3 4 5 6 7
0 0,1 0,8 1,45 2,0 2,5 3,1 mm

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 3 4 5 6 7
Measurements 0 0,54 1,44 2,14 2,91 3,52 4,21
Parabola

Milles-Lacroix :
zone 1-2   : 45 % correction

2-3             125% !!!

3-4              80%

4-5              90 %

5-6             75 %

6-7             90 %

Step 14

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.

Zone 1 2 3 4 5 6 7
Measurements 0 1,14 2,10 2,80 3,56 4,09 4,85
Parabola

Milliers-Lacroix:

zone 1-2 2-3 3-4 4-5 5-6 6-7
Correction 101 135 82 89 66 98%

Evaluation : too irregular and edge too high,center better!

Try to polish zones 3 and 4

 

Step 15  
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
Step 16

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
Test:

Zone 1 2 3 4 5 6 7
Measurements 0 1,02 1,88 2,47 3,11 3,64 4,56
Parabola

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.

Step 17

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
Test:

Zone 1 2 3 4 5 6 7
Measurements 0 0,76 1,64 2,71 3,60 4,36 5,16
Parabola

 

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.

 

Zone 1 2 3 4 5 6 7 8
Measurements 0 0,80 1,52 2,35 3,07 3,89 4,60 5,38 mm
Parabola

Parabola: 0 – 0,746 – 1,472 – 2,191 – 2,919 – 3,621 – 4,319 – 4,971

 

Step 19

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
Step 20

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.

Step 21

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

Step 22

Repeat step 21 :  no improvement

Step 23

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

Step 24

Low 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

Test:

zone 1 2 3 4 5 6 7 8
Measurements 0 0,75 1,46 1,96 2,41 2,65 2,27 3,13
Parabola

It is obvious that the outer zones are wrong.

I am going back to a sphere….

Step 25

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

Step 28

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

Step 29 
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

Step 30

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

Step 31

 

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.

Step 32

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!

Step 33

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

Step 34

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

 

test Foucault:

Zone 1 2 3 4 5 6 7 8
Measurements 0 -0,27 -0,44 0,55 0,46 0,91 1,47 1,80
Parabola

The surface is quite irregeular!

Step 35

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:

Zone 1 2 3 4 5 6 7 8
Measurements 0 -0,49 -0,50 -0,14 0,53 1,18 1,90 2,26
Parabola

A bit successful ,at least. However: difference 6-7 to great

Step 36

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

Zone 1 2 3 4 5 6 7 8
Measurements 0 0 0,56 1,60 2,59 3,49 4,08 4,49
parabola

Step 37

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)

 

Zone 1 2 3 4 5 6 7 8
Metingen 0 0,80 1,69 2,41 3,28 4,16 4,99 5,62
Parabola

 

Strongly overcorrected!!

 

Step 38

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

Step 39

40 cm tool :

Warm press and w-stroke TOT with 10 cm overhang : 2 x ½ turn .Pressing in between.

Test ( 2 meas.)

Zone 1 2 3 4 5 6 7 8
Metingen 0 0,62 1,54 2,19 2,84 3,70 4,49 5,10
Parabola

This is better. But there is still overcorrection.

Step 40

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.)

Zone 1 2 3 4 5 6 7 8
Metingen 0 0,71 1,53 2,15 2,81 3,65 4,49 5,18
parabola

Still overcorrection

 

Step 41

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)