40cm f4.5 porthole mirror

THE 40 CM PORTHOLE MIRROR

 

In march 2003 I obtained a 405 x24.5 mm float glass disk from St.Helens Specialized Glass .This firm is based near Liverpool. Cost about 80 pounds,of which half was for the shipping to Holland.

This mirror proved to be my greatest challenge! It took a long time to get something useful out of it. But after the success: how easy it was to carry!!

I had the curve generated by J.Eindhoven in Buurse for 41 euro

April 3, 2003

Grinding tool ready. I made it from model plaster ( for sale in bags of 25 kg for 12 euro in 2009) and little ceramic tiles( 19×19 x 2mm). They were glued with epoxy resin onto the plaster with a space of 7 mm between tiles.

 

I start with C240 and tool on top(TOT). The tiles have to fit onto the mirror. I keep track of the process with the help of water insoluble ink marks on the tiles. W-stroke with 5-7 cm overhang

After 10 rounds : ink marks on mirror gone , but those on the tiles still largely present.

April 5

75 rounds with C240 and C180 ; only 10 tiles not reached yet.

C320 TOT

I start to mark (extra large)pits on the mirror ( near the edge), keep track of them and only change to the next( finer ) abrasive) when those pits are gone.

May 6
150 rounds done:

C600 TOT

I bevel the edge of the mirror with a 400 stone

After 50 rounds (8 positions per round or turn around the table) : smooth surface with numerous pits. I mark 4 pits.

Round 97 : scratch found

Round 180 : pits gone but scratch still visible

C800 TOT

May 12, round 45 : scratch much smaller

Round 160   stop

C1200 TOT

June 2. After 50 rounds 2 big scratches visible. Also some bigger pits.

Back to C800

Round 30 : scratches still big

Back to C600 ( suspension decanted after 1 minute , this to leave out the bigger particles)
Round 31 : scratches much smaller

Rond 60 : stop

C800 (decanted)

It took 200 rounds to clear the pits and the scratches.

C1200

100 rounds ; a few larger pits. I leave them

 

POLISHING

I use at first an old 30 cm pitch tool that I have covered with molten  Gugolz 64 pich with the help of a stiff brush: of course only on the pitch squares. Temp. is high in my working room( 25-30 oC)

Polishing table: 30 mm plywood(varnished)  On that thick carpet and a layer of bubble plastic. All to prevent astigmatism in the thin mirror

Polishing method: ½ W stroke TOT slow with decanted ceriumoxyd

After 1 round ( 8 positions) I turn the mirror about one third of a turn in the opposite direction.

August 14

15 min polishing time : it is going bad. Tool just slides over the mirror. Solution : scratch the pitch squares with an exacto knife

August 15

90 min; mirror clear except for the last 4 cm to the edge ( was to be expected ,since the polishing lap is subdiameter).

August 16 : measured F = 185 cm No obvious astigmatism visible in Ronchi and foucault test. I didn’t perform a star test; this proved to be a major mistake!!

Measuring method: Mirror and tester are inclined 15 degrees to prevent buckling of the thin mirror. The mirror rests in a sling against a 9 points(3×3) support
August 18

250 min polishing
I make a 35 cm tool : more reach than the 30 cm

August 26

After a number of daily sessions in my holiday I have now polished for 800 minutes

Sept. 8

Total time 1730 min .Still the reflection test is a failure. I think the pitch is to hard
Reflection test: I focus a 25 W clear bulb with the help of a 50 mm cameralens on the mirror surface.

I “paint”molten Gugolz 55(softer than 64) with a brush on the old squares.

Sept. 13 : 2130 min

Sept. 27   55 hours of polishing!

Ronchi test (always inside focus):

– Large hole in center

– Turned up edge area

Foucault test: leftside bright

First try at correction and first test

 

4x 15 min straight stroke ¼ – 1/3 diameter with 35 cm tool

 

Test : same figure ; bad surface R of central zone= 3801mm

Zone                   r2/2R     (slit and knife on same moving stage)

1                          0 mm

2                         1.9

3                           3,8

4                           4.3

5                           3.7

Turned up edge area!

PART 3 : PARABOLIZING

 

Introduction:

– the test is in general only performed after the mirror has been on the test stand for about 2 hours. The plate glass has a higher thermal expansion than borosilicate glass. Sometimes adaption was speeded up with the help of a fan.

– On tools larger than 10 cm I usually use 2 kg lead weight, just to get even pressure during polishing.

– I very often use a straight stroke left of center. If it says: 3 cm left of C than the center of the tool makes a straight stroke 3 cm left of the center of the mirror.
– my polishing stand has 8 positions per turn around

First work ;

  • new 10 cm tool for the edge
  • new 20 cm tool for parabolizing
  • 35 cm tool : bring up fresh soft pitch on the squares

Step 1

  1. work on high edge:
    1. 10 cm tool : small W-stroke on r=17 to r=19 cm : 2 turns around(8 positions per turn)
    2. circular stroke on edge : 7 turns
  2. 20 cm tool (starfish form : pitch squares narrowing towards the edge)
    W-stroke ; especially on the edge area.

Test: Ronchi : practically no change

Step 2

20 cm tool
a. straight stroke on r=15 to r=19 cm             : 3x 5 min

b . circular stroke r= -1 to r=+ 17 cm             : 5 min
this means that the center of the tool moves from r= -1 to r = +17 cm from the center     of the mirror

Ronchi : edge lower now

Foucault:

Zone 1 2 3 4 5
Measurements 0 1.4 2.8 3.3 3.1
Parabola

 Step 3
Oct. 1, 2003

20 cm tool

  1. Straight stroke on r= 17-19 cm : 3×5 min
  2. W-stroke ; 7 cm overhang ; emphasis on edge area

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.5 2.5 2.9 2.8
Parabola

Edge area too high!

Step 4 : work on high edge!

10 cm tool:
1. straight stroke on edge area: 2×5 min
2. circular stroke on edge: 5 min

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.5 2.3 2.6 2.7
Parabola

There is improvement!

As a try: from now on I use rouge(ferric oxide) as a polishing agent

Step 5

Work on central hole and the high edge

20 cm tool

  1. straight stroke on r=10-12 cm : 2×5 min
  2. straight stroke on r=16-19 cm : 2x 5 min
  3. circular stroke r= -2 to +17 cm : 5 min

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.3 2.2 2.6 2.55
Parabola

No success…

Step 6

Back to Cerium oxyd . Rouge makes such a mess…

20 cm tool

  1. Narrow W-stroke on r=8 to 16 cm 10 turns
  2. W-stroke with 8 cm overhang : 3 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.1 1.7 2.1 2.1
Parabola

 Step 7

40 cm tool TOT

Straight stroke ½ – 1/3 diam. 2x 1 turn
Ronchi: no improvement

Step 8

15 cm  starfish tool

  1. W- stroke from r=6 to r= 16 cm : 2 turns
  2. circular stroke on edge area : 5 turns
  3. circular stroke r= -1 to + 17 cm : 5 turns

Ronchi: definitely better

Step 9   Oct. 4

Work : edge area and increase correction

15 cm tool
1. Narrow W on r=15-17 cm : 10 turns
2.    straight stroke on r=3-5 cm : 10 turns
3. circular stroke r= -1 to + 18 cm :5 turns

test : no improvement

Step 10

1. 40 cm tool ( new one!) : improve mirror surface
1/3- ¼ Straight + narrow W : 3x 15 min; slow and no extra pressure
2.  20 cm starfish tool 1) straight stroke r=1-3 cm 10 turns

3. 20 cm tool  W-stroke mostly on the edge ; 5 cm overhang: 5×1 turn

Ronchi : TDE ; edge area too flat
Fouc.

Zone 1 2 3 4 5
Measurements 0 0.6 1.2 1.5 1.7
Parabola

Edge area better now !

Step 11

20 cm starfish tool

  1. straight stroke 3-5 cm left of C :                           30 turns

2.  W-stroke with center between r=10 and 18 cm     10 turns

Ronchi : quite regular

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.43 0.78 1.22 1.59
Parabola

 

Step 12

Correction hasn’t increased because of the use of counteracting strokes. But the edge is better.

20 cm starfish tool

  1. straight stroke 2-5 cm left of C                          2x 10 turns
  2. W-stroke   5 cm overhang :                                   3×1 turn

Ronchi : regular

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.66 1.10 1.56 2.05
Parabola

It appears to go well

Step 13

20 cm starfish tool

– straight stroke 2-5 cm left of C : 3x 10 turns

– W-stroke   5 cm overhang :   4x 1 turn

Ronchi : regular ; small TDE

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.47 0.87 1.67 2.22
Parabola

Step 14

Deepen the cente
1. 15 cm tool Straight stroke on r=1-3 cm left of C ; 3 cm overhang   : 2x 10 turns
2.circular stroke r=-1 to r= + 15 cm : 10 turns

Test: worse; flat center; only 0.05 difference between zone 1 and 2! Surface a bit spotty, despite the thin Cerium oxyd suspension.

Step 15

15 cm tool

  1. straight stroke r= 0 cm : 5 turns
  2. W-stroke   5x 1 turn ; emphasis on center

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.90 1.31 2.0 2.69
Parabola

A  central  hole  is coming

Step 16

20 cm tool
W-stroke ; less emphasis on center ; 5 cm overhang : 5x 1 turn

Ronchi : flat center

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.2 0.6 1.4 2.1
Parabola

In deed flat center
Correction is reduced by the work on the outer zones.

Step 17

15 cm starfish tool, now again with rouge (ahum)

  1. straight through center :               3 turns
  2. W-stroke between -3 and + 3 cm : 3x 1 turn
  3. elliptical stroke r=-1 to r= + 16 cm

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.59 1.03 1.73 2.32
Parabola

 Step 18

15 cm tool

  1. Straight stroke 1-4 cm left of C   : 2×10 turns
  2. W-stroke from -5 to + 5cm and 2 cm overhang :   4×1 turn

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.89 1.45 2.16 3.0
Parabola

Step 19

Same work as with 18

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.90 1.59 2.45 3.16
Parabola

 Step 20

15 cm starfish tool with rouge

  1. straight stroke 1-5 cm left of C   4×10 turns
  2. W-stroke 3 cm overhang             4 x 1 turn
  3. circular stroke   15-18 cm            5 min
  4. elliptical stroke -1 to 15 cm 5 min

Fouc.( 2 meas.)

Zone 1 2 3 4 5
Measurements 0 0.93 1.71 2.52 3.48
Parabola

Edge is going down

Step 21

15 cm tool
a. Straight stroke on r=15-19 cm 10 turns

b . idem   0-4 cm                             10 turns

  1. same as a
  2. same as b
  3. same as b.
  4. w-stroke 5 cm overhang          3x 1 turn
  5. elliptical stroke -1 to 17 cm 10 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.6 1.5 2.0 3.0
Parabola

This is bad… Long edge

Step 22

20 cm starfish tool

  1. straight stroke 0-3 cm left of C   ; 6 cm overhang       :4x 10 turns
  2. W-stroke ; doesn’t polish well : stop

test : strong TDE

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.5 1.4 2.1 3.0
Parabola

 Step 23

15 cm tool : add pitch to the squares with a brush

  1. straight stroke 0-1 cm : 20 turns
  2. W-stroke : mostly on center ; 5 cm overhang     5 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.99 1.84 2.44 3.36
Parabola

The figure is better now

Step 24

15 cm tool

Try for w-stroke alone: what happens?

5 cm overhang; most work on center   : 9x 1 turn (cold press in between)

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.41 2.21 2.85 3.80
Parabola

That was a mistake ! TDE and hole in center .Too much work on center

Step 25

20 cm 8-point star shaped tool
W-stroke   5 cm overhang :  7x 1 turn

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.26 1.64 2.32 3.48
Parabola

 

Step 26

20 cm tool

  1. try to deepen central area

Narrow W-stroke r= 5 to r=10 cm   10 turns
2.     Normal W-stroke 3 cm overhang   : 4 x 1 turn

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.6 0.9 1.65 2.75
Parabola

The edge is too long

Step 27 and 28

  1. 10 cm tool straight stoke r=15-17 cm ; every 3 strokes a stroke to the center ( to blend) : 5 turns
  2. 15 cm starfish tool
    0-1 cm straight stroke 5 turns

1-5 cm straight   3x 10 turns
3.    15 cm tool elliptical stroke -1 to + 17 cm 20 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.0 1.8 2.6 3.8
Parabola

The difference between zone 4 and 5 is too large

Step 29
10 cm tool

  1. circular stroke on r=16-19 cm : 30 turns
  2. elliptical stroke -1 to + 18 cm : 30 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.0 1.7 2.3 3.7
Parabola

 

No improvement!

I will try a smaller tool on the edge!

Step 30

  1. 7 cm tool (star-shaped)

Circular stroke on edge ; max . 7 mm overhang     : 20 turns
2.    smoothing : 15 cm tool (5- points star) : elliptical stroke -1 to + 18 cm : 2x 10 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.67 1.41 2.03 3.07
Parabola

This looks better

Step 31

  1. 7 cm tool circular stroke on edge 5 min
  2. Increase correction 15 cm tool straight stroke 0-5 cm left of C 3×10 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.1 1.9 2.7 4.0
Parabola

Alas !!

Step 32

15 cm star shaped tool

  1. straight stroke 0-5 cm left of C 3×10 turns
  2. elliptical stroke -1 to 18 cm 2x 10 turns
  3. straight stroke on edge area 10-15 cm with 5 cm overhang : 5 turns

Test : little change

Step 33

  1. 7 cm tool : circular stroke on edge : 30 turns
  2. 15 cm tool straight stroke 0-5 cm 3x 10 turns
  3. 20 cm tool :W-stroke   3-4 cm overhang : 5x 1 turn

Test : TDE obvious

Step 34

  1. 15 cm star shaped tool

Straight stroke on r=6-18 cm : with pressure: 8 min
2. W-stroke with 2 cm overhang   : 4 turns

Diff. Zone 4-5 still 1.4-1.5

Step 35

  1. 7 cm tool : tangential stroke on zone 4 (r=17 cm) ; also side strokes to blend
  2. 15 cm tool w-stroke   : 2 turns

Test : diff 4-5   1.2-1.3 mm : also large zone as a result of the digging with the 7 cm tool

I find out the width of the TDE with the help of cardboard masks

Inside diam mask 390 mm : the diffraction ring around the mirror is very even. So the TDE is less than 0,5 cm wide. Without mask: diffraction ring only on the right side

Step 36

I want to get the mirror spherical and start again.

  1. 40 cm tool ¼-1/3 narrow w-stroke 3 x20 min

Terrible zone!

Step 37

20 cm starfish tool

-W-stroke esp. on 14-15 cm with 5 cm overhang : 20 min

– 15 cm star tool : W-stroke , esp. on edge area     : `18 min

Test : mirror is practically spherical, with small TDE

Step 38

40 cm tool slow stroke : narrow W and straight stroke: 4×15 min : in between 10 min press.

Test : spherical; weak hole in C; slightly turned up edge

 

CORRECTION:   SECOND TRY

Mistakes made during first try:

    1. too little correction on the edge zones, with the result that they became flat.
    2. too soft pitch: digs into the edge zone?

 

Step 1

20 cm tool

  1. Straight stroke ( St.St) 1-5 cm left of C 5 cm overhang 3 x `10 turns
  2. St.St through center     : 10 turns
    Ronchi:

Step 2

20 cm starfish tool:

  1. St St 0-5 cm 2×10 turns
  2. W-stroke on r=1-15 cm   2x 5 turns

Ronchi: lines billowing out

Step 3

  1. 15 cm tool (5pts star shaped)

St St 1-5 cm left of C ;3-5 cm overhang 3x 10 turns

St St r=16-18 cm 5 cm overhang   : 2x 10 turns

  1. 10 cm tool   (star shaped) smoothing : elliptical stroke r=-1 to r= 15 cm 23 min

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.3 0.8 1.3 1.3
Parabola

Flat edge area

Step 4

15 cm star tool : increase correction.

  1. St St 1-5 cm left of C   2x 10 turns
  2. W-stroke ; less pressure on 70 % zone (to avoid decreasing the correction again) : 5x 1 turn

Step 5

20 cm 6-point startool

I try to improve the figure with the large W-stroke .Max 5 cm overhang .Less pressure on 70 % zone : 5 x 1 turn

Ronchi: much more regular: the obvious zone has disappeared.

Step 6

20 cm starshaped tool

  1. St St 0-5 cm 10 turns
  2. W-stroke   2+ 3 turns   5 cm overhang

Ronchi : regular figure

Fouc.

Zone 1 2 3 4 5
Measurements 0 0,25 0.60 0.90 1.15
Parabola

 

Step 7   Oct. 22   2003

20 cm star shaped tool

Try to increase correction with a combination of straight strokes and W-strokes

  1. st st   0-5 cm : 2x 10 turns
  2. W-stroke (regular)   2+ 3 turns max 5 cm overhang   :   20 min

Fouc.

Zone 1 2 3 4 5
Measurements 0 0 0 0.7 1.1
Parabola

It is going wrong again. Pitch tool too large!?

Step 8

15 cm tool(5pts star)

  1. St St 0 cm 5 turns
  2. St St 1-5 cm 3x 10 turns
  3. W-stroke 3-5 cm overhang   1 turn

W-stroke with the 20 cm tool :2 turns

Ronchi: regular figure

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.23 0.45 1.1 1.5
Parabola

This is better !

Step 9

15 cm new starfish tool

  1. St st 0cm             5 turns
  2. St st 1-5 cm    3x 10 turns
  3. 20 cm star tool : w-stroke   3 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.37 0.83 1.55 2.08
Parabola

 Step 10

I made a new 20 cm tool : a 5 points star ,deeply cut in, just to follow the deep curve in the mirror better.

  1. 10 cm tool St St 0-1 cm 10 turns
  2. idem     1-5 cm 3×10 turns
  3. W-stroke with 20 cm :     3x 1 turn

Test : zone 1 and 2 the same!?

Step 11

15 cm starfish lap

  1. St St    0 cm 10 turns
  2. idem   1-5 cm 2x 10 turns

test : stil a flat center!!

It seems the pitch is too hard. Temperature in room low( 16 deg.Cent)

Step 12

Pitch tool warmed just before using it.

  1. 10 cm tool St St 0-3 cm 10 turns
  2. 15 cm tool   id                   10 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 0.2 1.1 2.1 2.8
Parabola

Step 13

Work on center!
a. 10 cm tool : small W-stroke ; edges tool between -10 and + 10 cm
b. Smoothing :

– circular stroke on edge:               10 turns

– elliptical stroke   -1 to + 17 cm   10 turns

Ronchi : better!

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.26 2.31 3.19 4.02
Parabola

Quite a change !

Step 14

Work with rouge again

15 cm starfish tool

  1. St St 0-5 cm                      10 turns
  2. W-stroke   5 cm overhang :                2×1 turn
  3. circular stroke on edge:                     10 turns
  4. elliptical stroke   -1 to + 17 cm 15 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.39 2.38 3.26 4.10
Parabola

High edge and hole in center

Step 15 Rouge

a. 15 cm tool
St St 4-5 cm left of C 5 cmm overhang : 10 turns
b.  20 cm 5pts star tool : w-stroke light pressure 5 cm overhang   1 turn

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.1 2.1 3.0 3.8
Parabola

Worse again ! Might be from the large W-stroke

Step 16

15 cm starfish tool

  1. St St   0 cm 5 turns
  2. idem 1-5 cm 2×10 turns
  3. W-stroke inside mirror                     1 turn
  4. 10 cm tool circular stroke on edge : 5 turns
  5. 15 cm : elliptical stroke -1 to + 17 cm 10 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.1 2.1 3.2 4.1
Parabola

 

Step 17

In crease correction

  1. 15 cm starfish tool StSt 0-5 cm 3x 10 turns
  2. W-stroke                                                     2x 1 turn
  3. Elliptical stroke -1 to + 17 cm 15 turns

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.0 2.2 3.0 4.0
Parabola

Slight TDE visible in Foucault test

Step 18

Increase correction: more pressure on polishing laps

  1. 15 cm StSt 0-5 cm 3x 10 turns
  2. 20 cm startool( 5pts) W-stroke   5 cm overhang                     1 turn
  3. 15 cm tool : W-stroke on left side of mirror : 5 turns

Ronchi: a zone in the lines at r=14-15 cm is visible

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.0 2.1 3.0 4.0
Parabola

Slight TDE

Step 19

Increase correction : I use CeO2 again

10 cm starfish tool

  1. St St 0-3 cm                                2x 10 turns

15 cm tool:

  1. circular stroke edge                     15 turns
  2. elliptical stroke  -1 to + 18 cm 15 turns

I use a new 15 cm tool : pitch test(Texereau) 6mm at  20 oC

Ronchi : same as 18
Fouc.

Zone 1 2 3 4 5
Measurements 0 0.9 2.1 3.2 4.1
Parabola          

 Step 20

Work at center again

  1. 10 cm tool narrow w-stroke -10 to +10 cm 2 turns
  2. circ. Str. Edge                                                       10 turns
  3. ellipt. Str. -1 to + 18 cm 10 turns

Ronchi : zone at r=10 cm

Fouc.

Zone 1 2 3 4 5
Measurements 0 1.5 2.7 3.6 4.6
Parabola

A bit of overcorrection . Measured R : 3790 mm

Step 21

20 cm star formed tool   with ROUGE

W-stroke   light pressure, less on Center:    1 turn

Foucault with 7-zone mask

Zone 1 2 3 4 5 6 7
Measurements 0 0.69 1.92 2.63 3.29 4.07 4.76
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure evaluation : large bump on 55 % ; peak to valley labda/3       Strehl 0.78

—————————————————–

Step22

10 cm starfish tool ROUGE

Circular stroke on 55% 6 turns ;

Light pressure

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.82 1.98 2.70 3.40 4.16 4.86
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure   : labda/3   S=0.78

———————————————————————————————————–

Step 23

  1. Try to increase difference 1-2

10 cm starfish tool   St St 0 cm 2 turns
2.     decrease correction:

–   40 cm tool 2 x 1 turn 1/3 W stroke

–   making figure 8 strokes : 4×8 ( 4 turns with 8 positions)

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 1.0 2.3 3.0 3.6 4.3 5.0
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Worse!

Step 24

Decrease correction

40 cm tool ¼ W stroke : 2 cm left and right only   : 3x 1 turn. Lot of pressing between turns

Test : diff. 1-7 still 5 mm

Step 25

40 cm tool ROUGE

Same as 24 : 6x 1 turn : between turns 5 min cold press with 10kg

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.83 2.01 2.71 3.29 3.94 4.72
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure:                                           labda/ 4.37   Strehl   0.851

Step 26

40 cm tool
Same as 24 : 3x 1 turn

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.93 2.11 2.70 3.29 3.91 4.61
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Labda /3.23   S= 0.736

Figure : high zones on 50 % and edge

Step 27

Try to polish 50 % zone
10 cm tool circular stroke on 50 % zone   5 turns

  1. elliptical stroke -1 to + 17 cm 10 turns

Test :diff. 1-7 ca 4.7 and diff 1-3 ca 2.2

Step 28 : work on 50 % zone
10 cm tool : circular stroke on r=10 cm : 15 turns (5.5 min)

Test after 2 ¾ hours : ronchi : zone somewhere at r=10 cm

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.76 1.85 2.58 3.13 3.75 4.56
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Labda/4.67   S=0.844

Figure:   still a high on 50% and edge

Step 29

New soft 10cm tool ( starfish pattern)

  1. circular stroke on 50 % zone                           : 5 turns
  2. StSt 0-3 cm                                      3 turns
  3. Elliptical stroke -1 to + 18 cm 5 turns

Foucault

Zone 1 2 3 4 5 6 7
measurements 0 0.81 1.87 2.59 3.18 3.73 4.56
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

labda/4.0   S=0.816

Star test: undercorrection!

Step 30   Jan 4, 2004

10 cm tool

  1. St St on r=10 cm :                                 2 turns
  2. StSt on r=18 cm                                     2 turns
  3. StSt   0 cm    4 turns
  4. elliptical stroke -1 to 18 cm 5 turns

Foucault( 3 meas.):

Zone 1 2 3 4 5 6 7
Measurements 0 0.78 2.01 2.75 3.25 4.04 4.70
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Labda/ 3.5
Step 31

Edge lower and more work on 50 % zone

10 cm tool

  1. circular stroke on edge; light pressure:                               10 turns
  2. elliptical stroke -1 to + 18 cm 10 turns
  3. circ. Str. On 50 % zone                                                        10 turns

Foucault(2-3 meas. After 3 hours waiting)

Zone 1 2 3 4 5 6 7
Measurements 0 0.80 1.82 2.46 3.10 3.74 4.65
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Labda/7.5   S=0.922

Test in telescope: DISASTER!!                    ASTIGMATISM OBVIOUS AT 300X

Cause : backside mirror is warped. Forgotten to grind it against a flat disc!

March 27, 2004 The work on this mirror was suspended to be able to finish a borosilicate 480 mm x 40 mm f5 mirror. That one proved to be far easier

I have a flat 400 mm diam float glass disc(19 mm thick) and I am going to grind the curved backside of the failed mirror against it with C320  and a small W-stroke

 

  1. GRINDING STAGE   THIRD TRY

 

This mirror will be a “practice mirror”for the 60cm f4.3. I plan to use a 75 % grinding tool and a polishing tool smaller than 40 cm

Grinding tool: 300 x 60 mm plaster disc with the 19x19x2 mm tiles.

This was cast on the mirror with the help of a cardboard dam inside a 300 mm (inside diameter) plywood form.Grinding the mirror starts with C240 : after 60 turns the ink marks on the tiles have disappeared.

October 3, 2004

C600 W-stroke TOT : 90 turns

Oct. 14

Al oxyd (Microgrit)  15 micron 45 turns

Oct 23

Al-oxyd 9 microns   125 turns

3+ 5 micron (rest parties)   25 turns

  1. POLISHING

35 cm tool (plaster tool happened to ly around) with a f 4.5 curve present

60 min polishing and a test in the telescope.

But, the focal distance has shortened by 73 mm as a result of the second grinding stage.

I have to shorten the truss tubes of the telescope

Result : 5 mm eyepiece (350x) : no astigmatism (18 pts cell)

Polishing is completed in 22 hours with Ceri-Hex polishing powder from Pieplow-Brandt

Ronchi: elliptical with some zones .No TDE

 3. CORRECTION

Pitch laps : 15 and 20 cm starfish formed. Gugolz 64 softened with turpentine. Texereau test 8 mm /20 degr. cent. in 5 min.

 

Step 1

Correction stroke with 20 cm tool (squares of 3-4 cm )

 

  1.    W-stroke 3-7 cm overhang       5 turns
  2.   St St 1-3 cm 5 cm overhang 10 turns
    3. W-stroke   3-7 cm overhang 3 turns

test : elliptical form(oblate spheroid)

Step 2

20 cm tool         CeO2 ( the less expensive form)

  1. St St 0-3 cm 3x 10 turns
  2. W-stroke 5-7 cm over 5 turns

Test : elliptical

Step 3 
Nov. 22 , 2004

20 cm tool

  1. StSt 0-5 cm 4×10 turns
  2. W-stroke 3-8 cm over 3 turns

Test : more sferical

Step 4

20 cm tool

  1. St.St 0-5 cm 4×10 turns
  2. W-stroke 2 turns

Test:

Ronchi: spherical with hill in center

Step 5

  1. 15 cm tool 5 pts star

StSt 0-5 cm            :  5×10 turns
2.    20 cm tool :W-stroke 3 turns

test : same as 4

Test in telescope : no astigmatism at 350x

Step 6
1. 15 cm starfish tool
St St 0-5 cm                      20 min
2.  20 cm tool   W-stroke : 1 turn

test : spherical with slight bulging of the ronchi lines

Step 7

  1. 15 cm tool
    St St   0-5 cm                                                     25 min
  2. 20 cm tool
    W-stroke 3-7 cm over                                        5 turns

Test : ronchi Parabola starts

Step 8

  1. 15 cm tool St St 0-5 cm 20 min
  2. 20 cm tool   W-stroke 3-7 cm over                     7 turns

Step 9

15 cm starfish tool
1.  StSt 0-5 cm 22 min
2.  W-stroke 5 turns

Ronchi lines keep on bulging

Step 10  
Nov. 26     2004

  1. 15 cm starfish tool StSt   0-5 cm 20 min
  2. 20 cm starfish tool W-stroke 3-7 cm overhang   5 turns

Test : surface a bit spotty

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.05 0.35 0.5 1.0 1.6 1.85
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Flat center

Step 11
Cer-Hex polishing powder(more even)

  1. 15 cm starfish tool   StSt 0 cm 10 turns (4min 40 sec)
  2. idem 0-5 cm 30 min
  3. elliptcal stroke -1 to + 18 cm 10 turns
  4. idem   W-stroke 1 turn
  5. now 20 cm tool because 15 cm is too hard
    W-stroke   3-7 cm over                       5 turns

Ronchi : regular parabola

Foucault:

Zone 1 4 7
Measurements 0 1.3 2.9
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 Step 12  
work more on center

15 cm tool
1.  StSt   0 cm 3-7 cm over                      2×10 turns
2.       id   1-5 cm 3-7 cm over                     20 min
3.  20 cm tool   W-stroke   3-7 cm over             5 turns

Ronchi looks OK except for flat center

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0 0.6 1.2 1.7 2.5 3.2
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

What to do about the flat center?

Step 13

15 cm tool

  1. StSt 0-5 cm 5×10 turns
  2. circular stroke edge area         20 turns
  3. elliptical stroke     -2 to + 15 cm 20 turns

Ronchi: OK

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0 1.65 3.65
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Flat center .It’s time for the 10 cm tool

Step 14

10 cm starfish tool

  1. W-stroke inside 20 cm diameter of mirror                         2 turns
    StSt 1-2 cm                                                                    3 turns
  2. 20 cm starfish tool w-stroke 5-6 cm over                           3 turns

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.7 1.3 1.7 2.4 3.0 3.8
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66
  • problems : edge goes down

Step 15

  1. 10 cm tool(starfish)

St St 0-1 cm                                                                    5 turns
2.    15 cm tool   W-stroke within mirror diam.               3 turns
3.      15 cm tool : ellipt. Stroke -1 to + 18 cm                  10 turns

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.65 1.5 2.1 2.65 3.4 4.2
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 

Step 16

First : pitchlaps 10 and 20 cm have to be warmed to 25-30 deg Cent

  1. 10 cm tool StSt 0-1 cm                                                              10 turns
  2. 10 cm tool narrow W-stroke within 20 cm diam.                         2 turns
  3. 20 cm tool W-stroke                                                                      3 turns

I have tried the Zirconiumoxyd polishing powder of willmann-Bell , but it acts much jerky, it seems, than the mor3e expensive Cerio-hex polishing agent.

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.6 1.3 1.9 2.6 3.4 4.2
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Step 17

  1. 10 cm tool : small w-stroke on central area        4 turns
  2. 15 cm tool StSt 0-5 cm 5 cm over                           10 turns
  3. 10 cm tool ellipt. Stroke -1 to +18 cm            2×10 turns

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 1.0 2.4 3.1 3.8 4.5 5.4
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Edge is going down. Correction too great

Step 18

Work at hole in center:

  1. 20 cm tool Tangential stroke (= straight stroke) on r=12-14 cm 2-4 cm over     5 turns
  2. 20 cm tool W-stroke for smoothing 5-7 cm over (less on C) 3 turns

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 1.35 2.45 2.9 3.6 4.4 5.35
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

No success

Step 19

Work at low edge!

  1. 10 cm tool tangential stroke at r=16-17 cm , every three strokes a stroke to the center                                                           10 min
  2. 20 cm tool : W-stroke between r=8 and 15 cm to leave out the center         4 turns

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.9 2.0 2.4 2.9 3.7 4.35
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Better!

 

Step 20

Try to increase correction without enlarging the central hole

15 cm tool

stSt   4-6 cm   5-7 cm over                               10 turns

Ronchi : zone at r=10 cm

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 1.02 1.88 2.39 2.91 3.61 4.39
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Same profile as after step 19

 

Step 21  
Nov. 3
1.    15 cm tool St St 3-8 cm 5 cm over 10 turns
2.           15 cm tool W-stroke   5 cm over 3 turns
3.        15 cm tool ellipt. Stroke -1 to + 18 cm 5 turns

Ronchi : kink at about  50%

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.65 1.85 2.30 2.91 3.48 4.35
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Flat center ;little change compared with 20

Step 22

  1. 10 cm starfish tool StSt 0-1 cm 10 turns
  2. 15 cm starfish tool StSt 1-3 cm 10 turns
  3. 15 cm starfish tool ellipt. Stroke -1 to +18 cm 10 turns

Test : worse! Flat center : nr 2 was a wrong idea!

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.3 1.5 2.2 2.75 3.4 4.25
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Step 23  
Dec. 1

  1. 10 cm starfish tol 0-1 cm 30 turns
  2. ellipt. Stroke -1 to + 18 cm 10 turns

Ronchi:kink at r = 12 cm

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.6 1.95 2.73 3.23 3.81 4.63
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 

Figure:high zone at 50 %

Step 24 

  1. 15 cm tool: work on 0.35 r to 0.65 r

narrow w-stroke                                                           4 turns
2.  Increase correction
StSt 15 cm tool 0 cm 5 cm over                                   6 turns
3. Smoothing
35 cm tool ¼ W stroke 2 turns

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.74 1.83 2.52 3.05 3.76 4.57
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure:

 

Step 25

Aim : increase correction and improve the edge

  1. 15 cm starfish tool : narrow W stroke on r=14 to r=18 cm 4 turns
  2. idem St st 0-1 cm 20 turns
  3. 35 cm tool ¼ W   2×1 turn

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.8 1.6 2.4 2.9 3.65 4.50
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Ladba/2.5

Figure

 

 

I just can’t get rid of the zone!!

 

Step 26 :

just try to smooth it

20 cm tool W-stroke 5-7 cm over                                         6 turns

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.7 1.54 1.94 2.49 3.34 4.22
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure

 

Step 27

  1. work at low edge.

10 cm tool Tang stroke on r=16-17 cm (with side stroke every 3 strokes)         10 min

2. increase correction with 15 cm tool

StSt 0-3 cm                                                                                                 20 min
3.    ellipt. Stroke -1 to +18 cm        20 turns

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.5 1.2 2.03 2.73 3.35 4.16
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 Step 28

Deepen the center

  1. 10 cm tool StSt 0-3 cm 4 cm over                                          2×10 turns
  2. 20 cm tool W stroke   ; light pressure                                2 turns

test : strong TDE!

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.15 1.0 1.8 2.4 3.05 3.84
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

I can’t believe it…

Step 29       

1.  10 cm tool : narrow W stroke on central area : 3 turns
2.      15 cm tool StSt 0-1 cm                                     15 min

Test : failure; zone 1 and 2 the same; TDE

Plan : back to sphere

Step 30

Work with 35cm tool and w-stroke

Result : sphere with turned edge area. Diff 6-7 = 1.1 mm
Ronchi: straight lines  with strong TDE

Step 30a

  1. 20 cm tool W-stroke within mirror: pressure on right side

20 min
2.    35 cm tool : W-stroke within mirror diameter   15 min
3.      35 cm tool : W-stroke with 5 cm overhang   : 50 min

Step 31

  1. 7 cm tool (star) circular stroke on edge area     1 cm overhang :                   25 min
  2. 10 cm tool (starfish) : circular stroke on edge area: 1-3 cm over ;                 23 min
  3. 35 cm tool W-stroke 6-8 cm overhang

Ronchi: Hill in center and TDE

Foucault:

Zone 5 6 7
Measurements -0.2 0 0.8
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Step 32

I think the circular stroke works too slow

  1. 7 cm tool tangential stroke on r=15 cm ( with side stroke to the center every three strokes) 15 min
  2. 15 cm tool ;W-stroke on left side of mirror ; 3 cm overhang 25 min

Ronchi :   Same as nr. 31

No TDE ! (seen with diffraction ring)

Foucault

Zone 5 6 7
Measurements -0.1 0 0.1
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 

Step 33

Try to improve the figure with the help of the 35 cm tool

W-stroke : 50 min

Ronchi:

Weak ellips

No TDE

 

 

SECOND TRY   FOR THE CORRECTION

 

New pitch laps are made fron Gugolz 55 with linseed oil as a softener.

Hardness test( Texereau) 5 mm /5 min/20 deg. Cent

Plan:

  • correct to parabola with 20 cm 5 points starlap with straight stroke close to C. Also W-strokes to reach the edge. When a flat center occurs: switch to 15 cm 5 points star lap

Step 1         Dec. 6 , 2004

20cm lap

1.   StSt 0-1 cm 3-5cm overhang                                                       20 min
2  By turns: – st st 20 cm tool   0-5 cm 5 cm over                           10 min

– W-stroke                                                                     10 min

All the time I tried to give the 70 % zone as little action as possible

Ronchi: spherical

Step 2

  1. StSt 20 cm tool 0-2 cm 50 min
  2. W-stroke         3-5 cm over 10min

The pitchlap is scratched and given a warm press now and then.

Foucault

Zone 1 2 3 4 5 6 7
Measurements +0.4 0 +0.2 0.9 1.3 1.5 1.7
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Step 3

I now use the 15 cm 5 points star lap

  1. StSt 0-1 cm 3-5 cm over 40 min
  2. W-stroke   3-5 cm over

Much pressure on C, less on edge and still less on 70 % zone         20 min

Ronchi: lines bulging but kink at 50%

Foucault: (2 meas.)

Zone 1 2 3 4 5 6 7
Measurements 0 0.89 1.82 2.30 2.67 3.14 3.34
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Edge area turned up and center too deep.

Step 4

20 cm tool with 2.5 kg

  1. StSt   on r=10 cm 10 min
  2. W-stroke 4-6 cm overhang 20 min

Step 5

Work plan disappeared…

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.89 1.82 2.30 2.67 3.14 3.34
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 

Step 6   dec 8

Improve the edge with the 20 cm tool

  1. stSt   r=10 cm 10 min
  2. W-stroke   3-6 cm overhang 20 min

Foucault(1-2 meas)

Zone 1 2 3 4 5 6 7
Measurements 0 0.65 1.09 1.37 1.83 2.24 2.68
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Because of the work outside the central zones the correction is decreased. But the figure is more regular!

Step 7

  1. Increase correction : 15 cm tool StSt 0-5 cm 30 min
  2. Smoothing             : 20 cm tool   W-stroke 15 min

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0 0.52 0.96 1.37 1.97 2.37
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Flat center, caused by polishing next to the center

Step 8         Dec. 9

 

  1. 15 cm tool T St 0-1 cm 30 min
  2. idem : W-stroke with extra action on C 20 min

Ronchi : the zone is less sharp

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.61 1.34 1.90 2.36 3.0 3.59
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 Step 9

  1. 15 cm tool StSt 0-2 cm                                              15 min
  2. idem W-stroke                                                            10 min

Ronchi:lines bulging; kink not too bad

Foucault( 2 meas.)

Zone 1 2 3 4 5 6 7
Measurements 0 0.50 1.55 2.06 2.54 3.21 3.89
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Step 10

  1. Deepen the center

– 10 cm tool StSt 0-1 cm       4-6 cm overhang                                      10 min

– 15 cm tool       id                                                                                   20 min
2.  Improve the edge

20 cm tool   W-stroke   4-7 cm overhang                                                           20 min

Ronchi: kink at 50%

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.80 1.86 2.54 3.15 3.83 4.56
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure:

P-V  = Labda/2

Step 11

Increase correction and smooth the figure

10 cm tool

  1. circular stroke on r=16-18 cm 10 min
  2. ellipt. Stroke r=-1 to r=18 cm 15 min

Ronchi: hole in center

Hole in center caused by  2

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 1.08 2.21 2.95 3.46 4.24 4.94
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure:

 

Step 12

  1. Attack 50 % zone
    10 cm tool circ. Stroke on 50 5                               10 min2.   smoothing :  20 cm tool W-stroke   5-6 cm over                                   3 turns

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.88 1.81 2.49 3.07 3.80 4.50
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure

 

 

 

Step 13 
Dec. 12,2004

Increase correction

15 cm tool

  1. stSt   0cm 5 min
  2. ellipt. Stroke   -2 to + 17 cm 5 min

Ronchi: kink at about 60 %

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 1.03 2.15 2.81 3.47 4.06 4.93
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 Step 14
Dec. 13

  1. work on 50 % zone

10 cm tool   circ. Stroke on 50 %                     10 min
2.  15 cm tool   ellipt. Stroke -1 to +17 cm            5 min

(this was a bad idea: increases correction!)

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.90 2.15 2.95 3.56 4.25 5.10
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure:

 

 

 

Step 15 
decrease correction!

35 cm tool (already quite hard) W-stroke   5-7 cm overhang            2×1 turn

Foucault(4 meas.)

Zone 1 2 3 4 5 6 7
Measurements 0 0.91 1.96 2.57 3.21 3.92 4.85
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure:

P-V  : Lambda/4

Step 16

  1. 10 cm tool :circular stroke on r=18 cm 4 min
  2. idem on r=10 cm 4 min
  3. ellipt. Stroke on r=-1 to + 18 cm 5 min

Ronchi : zone at r=8 cm

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.95 1.92 2.54 3.18 4.0 4.8
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure   labda/4.3

Step 17

  1. 10 cm tool circ. Stroke   r=18 cm 4 min
  2. idem   r=10 cm         4 min
  3. ellipt. Stroke r=-1 to +18 cm 7 min

Foucault(4 meas.)

Zone 1 2 3 4 5 6 7
Measurements 0 1.02 1.96 2.68 3.31 4.02 4.90
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Too much done  work on center

Labda/4.5

Figure:

 

 

Step 18

Work very carefully on the high zones

  1. 6 cm tool : circ. Stroke on r=17-18 cm 10 turns
  2. 10 cm tool ellipt. Stroke r=5 to r=12 cm 10 turns

Test:

  • overcorrection!(diff 1-7 = 5.1 mm)
  • diff 1-2 = 2 mm (too much)
  • diff 6-7 = 1 mm

Step 19

20 cm tool   W-stroke   4-6 cm overhang                                                     2 turns

Ronchi: kink at r = 15 cm

Zone 1 2 3 4 5 6 7
Measurements 0 1.08 1.97 2.65 3.28 4.10 4.91
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Step 20

From now on I use the 10 zone mask

From step 19:

Zone 1 2 3 4 5 6 7 8 9 10
Measur. 0 5.3
Parabola 0 0.46 0.92 1.41 1.95 2.48 3.05 3.52 4.12 4.84

Overcorrected!

Step 21

  1. 20 cm tool: tang.stroke on 70 % zone 5 turns
  2. idem : ellipt. Stroke on r=-1 to + 16 cm 5 turns

 

Zone 1 2 3 4 5 6 7 8 9 10
Measur. 0 2.92 5.19
Parabola 0 0.46 0.92 1.41 1.95 2.48 3.05 3.52 4.12 4.84

 Step 22

Decrease correction with 10 cm tool and circular polishing on the 70 % zone : 10 min light pressure

Zone 1 2 3 4 5 6 7 8 9 10
Measur. 0 0.49 1.38 2.85 5.04
Parabola 0 0.46 0.92 1.41 1.95 2.48 3.05 3.52 4.12 4.84

 Step 23

Decrease correction further

10 cm tool circ. Stroke on 70 %   10 min

Also : 2 min ellipt. Stroke r=-1 to + 18 cm

Zone 1 2 3 4 5 6 7 8 9 10
Measur. 0 0.71 1.57 3.09 5.26
Parabola 0 0.46 0.92 1.41 1.95 2.48 3.05 3.52 4.12 4.84

Hard to believe  :corr. Heas increased!

 Step 24

Further  attempts to lower the correction

  1. 15 cm tool Tangential stroke on 70 % zone 6 turns
  2. 15 cm tool   W-stroke 5 cm overhang 3 turns
Zone 1 2 3 4 5 6 7 8 9 10
Measur. 0 0.79 1.78 3.00 5.34
Parabola 0 0.46 0.92 1.41 1.95 2.48 3.05 3.52 4.12 4.84

Hole in C getting larger .?????

Step 25

10 cm tool   circular stroke on 70 % zone                                        20 min

Zone 1 2 3 4 5 6 7 8 9 10
Measur. 0 0.82 1.94 2.92 5.28
Parabola 0 0.46 0.92 1.41 1.95 2.48 3.05 3.52 4.12 4.84

No success

Step 26

What to do?!

Work with the 10 cm tool too slow?

I try to decrease correction with 20 cm tool

W-stroke with 5-6 cm overhang and less polishing on the central area               6 turns

Zone 1 2 3 4 5 6 7 8 9 10
Measur. 0 0.9 1.94 2.3 2.5 2.5 2.8 3.2 3.85 4.85
Parabola 0 0.46 0.92 1.41 1.95 2.48 3.05 3.52 4.12 4.84

At least the correction has decreased
Figure:   labda/0.93

Step 27

  1. Edge : 6 cm tool circ. Stroke 1 cm overhang                           10 min
  2. Edge area : tang. Stroke with 10 cm tool on r=17 cm (with side strokes to prevent zoning)                                                              10 min
  3. 15  cm   str str on r=6-9 cm                              10 min

Back to 7 zone mask again

Zone 1 2 3 4 5 6 7
Measurements 0 1.2 1.5 1.8 2.2 2.9 3.8
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Low edge area

Hole in center

Flat area outside C

Step 28

Just try something

  1. Edge : 6 cm tool   circ. Stroke 1 cm overhang 10 min
  2. Increase correction zone 2-3 St.St r=9-13 cm with 0-1 cm overhang       10 min
  3. smoothing 15 cm tol ellipt. Stroke r=   8-16 cm
Zone 1 2 3 4 5 6 7
Measurements 0 0 0 0.3 0.9 1.4 2.4
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Edge bad

Hole in center gone

Step 29         Dec. 24

  1. Now try to attack low edge with rectangular pitchlap (3x8cm)

Tangentialstroke on r=17-18 cm                                                             10 min

  1. 6 cm tool smoothing with circular stroke on r=16-19 cm 10 min
  2. Increase correction 15 cm tool StSt 0-2 cm 5-7 cm over                         10 min
  3. 15 cm tool ellipt. Stroke -1 to +16 cm 10 min

Ronchi : disaster: kinks in the lines!! I worked too long with the rectangular tool

 

Zone 1 2 3 4 5 6 7
Measurements 0 -0.6 +0.5
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 

Step 30

Glass has to be removed in the entire outer zones too smooth the figure!

 

  1. 35 cm tool (already quite old…)

W-stroke with edge pressure on r=15-19 cm                                               20 min

  1. W-stroke   4-6 cm overhang 2 turns

 

Mirror is now spherical. TDE present . Diff zone 6-7 = 0.9mm

Step 31

Work at low edge

  1. 6 cm tool tang. Stroke on r=18 cm ( with side stroke to the center every three strokes) 15 min.
  2. 15 cm tool W-stroke to smooth the figure   3-5 cm overhang                 11 min

Test : not better

Zone 1 2 3 4 5 6 7
Measurements 0 +0.3 0 +0.7
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Still a kink in the lines

Step 32 

Try to smooth with 10 cm tool

  1. circular stroke on edge area, max 2 cm overhang 20 min
  2. ellipt stroke with 15 cm tool r=-1 to + 16 cm 30 min

Ronchi: hole in center and TDE

Zone 1 2 3 4 5 6 7
Measurements -1.6 0 -0.1 +0.4
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Better now than 31! But ellipt. stroke has caused hole in Center.

Step 33

Try to improve the edge:

  1. 6 cm tool circ. Stroke on edge area max 1 cm overhang 30 min
  2. 10 cm tool : same stroke 20 min

Ronchi:just a wriggling snake! No good

Zone 1 2 3 4 5 6 7
Measurements 0 1.8 2.0 2.0 1.5 1.3 1.7
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Step 34

I use the old big lap again: 35 cm

W-stroke 5-7 cm over   70 min

Result : spherical with TDE

THIRD TRY FOR THE CORRECTION

Step 1   Jan. 2 2005

 

  1. 20 cm tool (starfish shape)   stst 0-1 cm 2-5 cm overhang : 60 min
  2. 15 cm tool (starfish shape)   W-stroke 20 min

Ronchi: lines start to bulge

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.38 1.07
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 Step 2

15 cm tool

  1. stst 0-5 cm 2-5 cm overhang 30 min
  2. W-stroke 25 min

Ronchi : regular
Foucault(2 meas.)

Zone 1 2 3 4 5 6 7
Measurements 0 0.16 0.51 0.71 1.0 1.57 1.79
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 Step 3

Deepen the central area

  1. 10 cm starfish tool   StSt 0-1 cm 3 cm overhang 10 min
  2. 15 cm tool   StSt 0-5 cm 3-5 cm over 30 min
  3. 15 cm tool   W-stroke 15 min

Ronchi:

It seems a zone is arriving at r=15 cm

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.21 0.84 1.33 1.88 2.38 2.90
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 

Step 4

Clearly the central area has te be polished a bit more

  1. 10 cm tool StSt 0-1 cm max 3 cm overhang                                    15 min
  2. 15 cm tool StSt 0-1 cm 5 cm over 10 min
  3. 15 cm tool W-stroke   5 cm over 15 min

Ronchi:lines billowing; bend at r=12 cm

Foucault(2-3 meas.)

Zone 1 2 3 4 5 6 7
Measurements 0 0.41 1.24 1.90 2.29 2.95 3.52
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

 Step 5
Jan. 5, 2005

Deepen the center and increase correction

New pichlaps are made 10 and 15 cm tools Gugolz 55 +turpentine + linseed oil to soften the pitch)

  1. 10 cm tool StSt 0-1 cm 2-4 cm overhang 20 min
  2. 15 cm tool StSt 0-1 cm 10 min
  3. 15 cm tool W-stroke 5 cm overhang 20 min

Ronchi : center too deep!

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.81 1.60 2.16 2.72 3.44 4.17
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Here it looks OK

Step 6

Try not to work too much on the center!

  1. 15 cm tool StSt 1-5 cm                                                            10 min
  2. idem W-stroke   5 cm overhang 10 MIN

Ronchi: weak kink at r = 12 cm

Foucault (4 measurements)

Zone 1 2 3 4 5 6 7
Measurements 0 0.62 1.49 1.99 2.69 3.47 4.25
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Step 7
Jan.6 2005

Deepen the center

  1. 10cm tool StSt 0-1 cm 3 cm over 15min
  2. 15 cm tool stsst 0-5 cm 3-5 cm over 10 min
  3. 15cm tool W-stroke 5 cm over 10 min

Ronchi : same as step 6

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.98 2.04 2.68 3.25 4.18 4.92
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure:

 

Labda/6   S=0.911
The work is suspended for 7 months

New tests are performed:

Ronchi: parabola with hole in center

Fouucault( 4 measurements)

Zone 1 2 3 4 5 6 7
Measurements 0 1.19 2.19 2.82 3.38 4.19 5.04
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure:

 

 

Labda/3. 7 and S= 0.747

Quite a difference from   jan. 6 !

Now the mirror is turned 90 degrees and measured again(2 meas)

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 1.10 2.22 2.87 3.40 4.32 4.99
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Not too different from the earlier measurements

Step 8

New 25 cm tool (soft)

  1. W-stroke with edge pressure on r= 6 to r=12 cm( the right edge of the tool moves in this region)                                                        2 turns
  2. W-stroke with 5 cm overhang                                          1 turn

Foucault:

Zone 1 2 3 4 5 6 7
Measurements 0 0.83 1.93 2.72 3.32 4.12 4.92
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure : labda/5.95   S=0.919

 Step 9

Plan: zone 4 lower and C deeper

  1. 10 cm tool (soft pitch) Stst through C   3 cm over 2 turns
  2. W-stroke with the edge of the 25 cm tool on r=8 to r=12 cm         1 turn
  3. W-stroke with 25 cm tool     1 turn

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.85 1.77 2.48 3.21 4.10 5,01
Parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

Figure   labda/6.2   S=0.956

After a 3 hour wait another set of 4 measurements:

Foucault

Zone 1 2 3 4 5 6 7
Measurements 0 0.85 1.90 2.42 3.30 4.16 4.97
parabola 0 0.94 1.70 2.44 3.19 3.93 4.66

This is better, but the measured values are always a bit uncertain.

Star test : 7 mm Nagler and 2x Barlow:   no astigmatism

August 1 , 2005   22.30      I put an end to a long journey….

 

The mirror gave many nice observing nights ,for instance  in the Auvergne  and performed OK on the planets