40cm f4.9 mirror

 

40 cm f4.9  Duran(borosilicate) mirror

This one proved to be easier than the porthole!

Date Work
Dec. 2006 406×36 mm borosilicate disc bought from Jilles Eindhoven in Haaksbergen.He had already made a radius of 500 cm in it.Total cost: 325 euro
Dec. 2006 I used a 3 kg steel ring(diameter 15 cm,used in weightlifting) to bring the mirror to a focal length of about 195 cm ( radius 390 cm) .The work was done with straight strokes and W-strokes TOT.I made a grinding tool by casting a a layer of 55 mm plaster on the mirror.Around the mirror I had made a cardboard dam(2 mm thick) and also I put on a water resistant coating of vaseline.After the plaster had dried for one week I glued little  ceramic tiles(19x19x2mm)on the plaster with the help of polyurethane glue( the two-component variety).Distance between the tiles ca 7 mm.
Dec. 2006 Mirror made spherical with the 40 cm tool and Carborundum 80After that work is continued with C120:1/4 W-stroke MOT and TOT.
Jan 5, 2007 C180 : 60 turns TOT and 40 turns MOT  1/3-1/4 W-stroke and straight stroke( also used for next powders)Carborundum suspension is added every two turns. Mirror edge is ground regularly to prevent chipping(first wth rough wetstone, later with 400 stone).
Jan .6 ,2007 C240 : 40 turns TOT and 20 turns MOT.I start with marking big pits near the edge of the mirror
Jan 7, 2007 C320 : 50 turns TOT
Jan 8, 2007 C320 100 turns TOT and 25 turns MOT : pits are gone
Jan 9-11, 2007 C600 : 100 turns TOTC800 : 100 turns TOTNo big pits: edge ground with 400 stone
Jan 12, 2007 15 micron Aluminium oxyde: 52 turns TOT and 18 turns MOT
Jan 13, 2007 9 micron aluminium oxyde:152 turns5 micron : 6 turns( there was only a little bit…)No pits
POLISHING
JAN 15,2007 Polishing was started with an old 35 cm pitchlap, made on a 350×50 mm plaster disc.Channels were made in the pitch with a 7mm stick.Test on Polaris after  45 min TOT: No astigmatism with 6.3 mm eyepiece.
Jan 18,2007 After 2 hrs TOT : big scratch on mirror!!
Jan 28,2007 8 hours polished with 35 cm tool TOTW-stroke with max. 8 cm overhang.Edge of mirror is ground several times with a 400 stone.Reflection test with incandescent wire( old fashioned clear lamp focussed with a lens: reflection weak at center and strong near edge.
 Jan 30 Polishing goes very slow. Therefore I use 10 kg weight on top of the tool. Polishing is now really tiring and I can only do 10 min sessions
Feb.6 Total polishing time now : 15 ¼ hour. Always W-stroke with max. 8 cm overhang
Feb 10 17 hrs polishing .Temporarely stopped because of arm injury
March 11 I continue polishing with a 25 cm pitchlap. 35 cm lap has become very hard.
March 18 24 hrs polishing (also 35 cm lap again, 2 hours)Reflection test: edge now very weak
March 25 W-stroke with 35 cm tool;6 cm overhangRonchitest: spherical with small TDE(turned down edge)

 

CORRECTION

 

Step 1   April 1st,2007

 

I made a 40 cm pitchlap from Gugolz 55+64 .Pitch squares are 3,5×3,5 cm. I ddn’t check the hardness

 

Breaking in the lap: 35 min 1/3-1/4 straight stroke and 1/5 W-stroke with cerium oxyde (TOT)

Ronchi: TDE and weak hill in center

Fouc: no TDE ; diffraction ring around the whole mirror(!?)

 

Step 2

I start the correction with a 2 months old 20 cm pitchlap( with pitch squares tapered to the edge).Pitch has a 10 mm thickness .Polishing with 2 kg weight on the lap.

  1. Straight stroke 1 cm left of Center; 5 cm overhang: 10 min
  2. idem 5 cm left of C;5 cm overhang : 3 min
  3. idem 1 cm left of C; 5 cm overhang : 10 min
  4. idem 5 cm left of C ; 5 cm overhang : 3 min
  5. Large W-stroke (5 cm overhang): 2x 1 turn.

Pitchlaps are coldpressed between the different runs.

 

Ronchi: spherical with light TDE

Step 3   april 2

 

20 cm tool (tapered) with 2 kg weight.

a. straight stroke 1 cm left of C and 5 cm overhang  : 10 min

  1. idem 5 cm left of C and 5 cm overhang : 3 min
  2. idem 1 cm left of C and 5 cm overhang : 10 min
  3. idem 5 cm left of C and 5 cm overhang : 3 min
  4. large W-stroke 5 cm overhang 3x 1 turn

 

Ronchi : lines start to bulge; R= 3890 mm

 

Step 4

20 cm tapered lap with 2 kg

a.Straight stroke 1cm left of C and 5 cm overhang: 7 min
b. idem 3 cm left of C and 5 cm overhang : 3 min
c. id 5 cm left ov C and 5 cm overhang : 2 min

I repeated this series once

 

Ronchi: lines bulging more but a kink is coming at r= 10 cm!

 

Step 5

 

20 cm tapered lap with 2 kg weight.

  1. Straight stroke 1 cm left of C and 5 cm overhang : 7 min
  2. idem 3 cm left of C and 5 cm overhang : 3 min
  3. idem 5 cm left of C and 5 cm overhang : 3 min
  4. Large W-stroke 2×1 turn

I repeated this series twice.

Ronchi:lines bulging more;weak kink at r=15 cm

Step 6

20 cm tapered lap with 2 kg weight

 

  1. Straight stroke 1 cm left of C ;5 cm overhang : 7 min
  2. idem 3 cm left of C and 5 cm overhang : 3 min
  3. idem 5 cm left of C and 5 cm overhang : 3 min
  4. idem 7,5 cm left of C and 5 cm overhang : 2 min

I repeated this series once

Ronchi: looks good!

Foucault test with 7 zone mask

 

zone 1 2 3 4 5 6 7
Measurements 0 0 0 0,43 0,95 1,38 1,59
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

 

Step 7

  1. Work more on center:

15 cm tapered lap (gugolz 55) with 2 kg weight:: straight stroke 1 cm left of C and 3-4 cm overhang:   2x 7 min

  1. 20 cm tapered lap with 2 kg weight
  2. straight stroke 1 cm left of C and 5 cm overhang : 6 min
  3. idem 3 cm left of C and 5 cm overhang :3 min
  4. idem 5 cm left of C and 5 cm overhang : 3 min
  5. idem 7,5 cm left of C and 5 cm overhang : 2 min
  6. Large W-stroke with 5 cm overhang : 2×1 turn

Ronchi: center a bit flat

Fouc:

zone 1 2 3 4 5 6 7
Measurements 0 0,18 0,44 0,84 1,35 2,0 2,35
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Too much difference between 5 and 6; outer zones polished too little.

 

Step 8

  1. Increase correction

15 cm tapered lap with 2 kg
a. straight stroke 1 cm left of C and 4 cm overhang:2×7 min

b. straight stroke 5 cm left of C and 4 cm overhang: 2×3 min

c. Smoothing the figure: 40 cm MOT ½ W-stroke overhang  left and right 3 cm  : 3x 1 turn with pressing in between.

Ronchi: OK, but central area a bit flat,it seems.(however : look at Fouc. Test!)

zone 1 2 3 4 5 6 7
Measurements 0 0,37 0,76 1,26 1,61 2,12 2,57
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Better! Large lap helps!

Step 9

  1. Increase correction

15 cm tapered lap with 2 kg
a. straight stroke 1 cm left of C and 4 cm overhang : 4x 5 min
b. idem 4 cm left of C and 4 cm overhang : 4x 2 min

2. Smoothing
20 cm tapered lap with 2 kg : large W-stroke 7×1 turn

Ronchi : there seems to be a TDE which I can’t see in the Foucault test.

zone 1 2 3 4 5 6 7
Measurements 0 0,50 0,91 1,42 1,91 2,54 3,14
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Edge zone gets lower..

Step 10

  1. Increase correction
    15 cm tapered lap with 2 kg
    a. straight stroke 1 cm left of C and 4 cm overhang : 5 min
    b. idem 5 cm left of C and 4 cm overhang : 2 min

I repeat  these sessions 3 times
2. Smoothing : 40 cm tool MOT ½ W-stroke with 5 cm overhang left and right: 4x 1 turn

zone 1 2 3 4 5 6 7
Measurements 0 0,69 1,03 1,54 2,14 2,75 3,33
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Too much correction between 1 and 2

Step 11  

a.Increase correction, but now with 20 cm tapered lap with 2 kg

  1. Straight stroke 1 cm left of C and 5 cm overhang: 5 min

2.   idem 5 cm left of C and 5 cm overhang             : 2 min

I repeated thes sessions 2 times

  1. Straight stroke 2,5 cm left oc C and 5 cm overhang: 5 min
  2. idem 5 cm left of C and 5 cm overhang:    : 2 min

I repeated this once
5.  20 cm lap( with 2 kg) large W-stroke with 5 cm overhang : 5×1 turn
6. Smoothing with 40 cm tool: MOT ½ W-stroke(5 cm overhang left and right 2×1 turn
7. 40 cm tool TOT ¼ straight stroke 1 turn

 

zone 1 2 3 4 5 6 7
Measurements 0 0,38 0,64 1,27 1,62 2,32 2,94
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Correction lowered by large tool!

Step 12 : deepen central area
-15 cm tapered lap with 2 kg

  1. Straight stroke 1 cm left of C and 4 cm overhang : 5 min
  2. idem 5 cm left of C and 4 cm overhang : 2 min

I repeated these sessions 5 times
3. same lap: large W-stroke 4 cm overhang : 3x 1 turn
4. 40 cm tool TOT 1/3 W-stroke (3 cm overhang left and right) 1 turn

zone 1 2 3 4 5 6 7
Measurements 0 0,66 1,14 1,79 2,36 2,99 3,69
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Difference 6-7 already a  bit large !

Step 13

Try to avoid outer zones: correction within zone 6

  1. 15 cm tapered lap with 2 kg
    straight stroke with edge of tool not further than r = 17 cm
  2. 2 cm left of C : 5 min
  3. 5 cm left of C : 2 min

I repeated these sessions 2x

4. 40 cm tool : smoothing 1/3 W-stroke narrow : 1 turn

zone 1 2 3 4 5 6 7
Measurements 0 0,46 1,15 1,90 2,53 3,20 3,80
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

 Step 14

Deepen central area

  1. 15 cm tapered lap with 2 kg

Straight stroke 2 cm left of C and 4 cm overhang:  5 min

Idem 5 cm left of C and 4 cm overhang        :  2 min

I repeated these sessions 2x

2. Smoothing with 40 cm lap TOT 1/3 W-stroke narrow: 2×1 turn

zone 1 2 3 4 5 6 7
Measurements 0 0,35 1,02 1,86 2,49 3,16 3,80
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Little change yet..

Step 15

I have to deepen the center

  1. 15 cm tapered lap: W-stroke within r=15 cm light pressure: 10 min
  2. elliptical stroke with 10 cm lap from r=-1 cm to r=+17 cm : light pressure 5 min. The middle of the tool passes from r=-1 to r=+17 cm

Ronchi: this was too much! Hole in center

zone 1 2 3 4 5 6 7
Measurements 0 0,82 1,52 2,52 3,16 3,87 4,48
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Step 16

Try to improve with 40 cm lap(not tapered) with 2 kg

1/3 W-stroke narrow: 2x  1turn

zone 1 2 3 4 5 6 7
Measurements 0 1,0 1,7 2,6 3,1 3,8 4,43
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Step 17

Work very carefully now

  1. Circular stroke with 10 cm tapered lap on r=10 cm.Light pressure 5 min
  2. Work on high edge:40 cm tol : ½ W-stroke 1 turn

Ronchi: kink at r=10 cm

zone 1 2 3 4 5 6 7
Measurements 0 0,82 1,65 2,39 3,09 3,71 4,31
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Lambda/4.9 on wavefront Strehl =0,91

Step 18

  1. work on zone near r=10 cm

10 cm tapered lap : circular stroke on r=10 cm with light pressure: 4 min
2. work on high edge: 40 cm lap MOT ½ W-stroke narrow 1 turn

zone 1 2 3 4 5 6 7
Measurements 0 0,58 1,37 2,19 2,84 3,52 4,15
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Lambda/7.7 on wavefront  S=0,964

Step 19

40 cm tool : MOT 4/5 W-stroke left and right 5 cm overhang: 1 turn

zone 1 2 3 4 5 6 7
Measurements 0 0,66 1,47 2,19 2,85 3,47 4,20
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Lambda/5.8 wavefront error; S=0,92

Step 20

40 cm tool : same as step 19

Total correction 4,0 mm: undercorrection!

Step 21

Increase correction

  1. 15 cm tapered lap with 2 kg

straight stroke 2 cm left of C and 4 cm overhang:   5 min

idem 5 cm left of C and 4 cm overhang            :   2 min

2. Smoothing with 40 cm tool 2/3 W-stroke narrow : 1 turn

Ronchi: looks very good

Foucault: 3-4 measurements

zone 1 2 3 4 5 6 7
Measurements 0 0,71 1,31 2,23 2,82 3,65 4,29
parabola 0 0,63 1,37 2,10 2,83 3,58 4,28

Lambda /22 wavefront error  S=0,995

Startest   May 30 ,2007 6,3 mm ortho: no astigmatism. Ronchilines straight

Good performer with expensive 8 cm secondary