47cm f3.9 mirror

 

47 cm f3.9 mirror

 

The suprax disk measuring 480×40 mm was purchased in 1998 and converted to a f5.1 mirror in 2004. After some years I decided to convert it to a mirror with shorter focal length, just to be able to observe without standing on a stepladder.

June 5-11   2009
With the help of a steel ring (15 cm in diameter ) and 1.2 kg of carborundum 80 I deepened the mirror. The focal length    I could follow by measuring the sagitta or using the sun on the wet mirror.

I ground with the mirror face up and W- and straight strokes. The mirror didn’t become spherical in this way.

June 12 :

A 480×60 mm disc of plaster of paris was cast. After a week drying it was twice coated with varnish to make it moisture proof. Then small tiles (19×19 x 4 mm) tiles were glued with polyurethane glue to the disc, using a rectangular pattern with 6mm between the tiles.

 

July 2009 Working with C120 and mirror below and mirror below alternately.

Check for sfericity : waterproof ink cross on mirror is gone after 10 turns around the grinding table (8 positions) ,using a quarter to one third W-stroke.

August 1st :

After 50 turns with mirror above and 50 rounds mirror below, change to C180

August 7

after some 10 turns with C180 three conspicuous pits were marked on the rim with waterproof ink.It took 40 rounds mirror above and 75 rounds with the mirror below to get rid of the pits.

New pit marked and change to C320

92 turns with C320 (only mirror below) pit still present.

I ground the mirrors edge again with a C400 stone and marked 4 new pits

Sept. 6th After 200 turns with C320 the pits are gone. One third to one quarter W-stroke.

C600

Sept 19 ,2009

After 200 turns with the mirror below I marked 3 pits.

Sept. 23 : After 150 turns with C800 2 pits are gone. Small scratch found. I ground the edge of the mirror again with a 400 stone and did another 50 turns with C800. All with the mirror below.

Sept. 28   C1200 : 120 turns . Time to start polishing.

 

POLISHING

First I cast a plaster of paris disc of 400 mm diameter and 50 mm thickness on the mirror.I used a cardboard ring to get this diameter. After a week drying the plaster was coated with 2 layers of varnish.

I built a paper dam around the tool and cast a layer of pitch(Gugolz 55) on it(edge thickness 10 mm) and ,after some cooling ( you have to monitor that!) I put in grooves by pressing a 7 mm wide wooden stick(coated with ceriumoxyde slurry) in the pitch in a cross pattern. This has to be repeated a number of times.The resulting pitch squares were 40 to 50 mm in diameter.It would have been smarter to push up the cooling pitch a bit to the center. This can be done with soapy fingers! ( tip from Mel Bartels) After that you can press channels and fit the miror.

After cooling I laid the pitchlap on the mirror with weights on it. Slowly the pitch adapted to the form of the mirror. Of course you can accelerate the process by warming the pitch. I put ceriumoxyde slurry on the mirror first.

October 5th 2009

45 minutes of polishing with the mirror below and a one quarter straight and a one quarter W-stroke. Polishing isn’t going well ( grab and slip) because the pitch is too hard at this low working temperature in the attic of the house.

November 1st.

After 6 hours of polishing a deep scratch appeared suddenly. Because the polishing work is terribly hard I leave it and don’t go back to grinding. Most of the times I make scratches in the pitch squares to assure a smoother glide over the mirror. Without the scratches in the pitch polishing goes quite jerkily.

February 10, 2010

After 37,5 hours of polishing with a pitch lap that is by now quite hard the reflection of a concentrated light is still visible in the outer 5 cm of the mirror. I coated the squares with softer pitch, made biy adding turpentine and linseed oil tot the pitch Gugolz 55.

February 28th , 2010

40 hours of polishing. The pitch is quite soft and I have to renew the scratches in the pitch every 30-45 minutes.

March 7, 2010

After 45 hours of polishing : reflection in outer cm of mirror weak but visible.

May 2010: Finished after 75 hours of polishing

 

CORRECTING

Ronchi test within Focus: spherical with weak TDE and a weak hill in the center.The main correcting lap will be a 20 cm lap with tapered pitch squares, thus growing smaller towards the edge.

March 26 ,2011

I use a code for the strokes I use. In all strokes the edge of the polishing lap goes 3-5 cm over the edge(left –right and up-down).

Ss-1 means straight stroke with the center of the lap 1 cm left of the centre of the mirror .Sometimes a weight is put on the lap.

Step 1
a.  20 cm lap Ss- 0-1 cm 2 kg 5 min.
b. Large W-stroke   1 turn -8 positions approx 3,5 min.

a  and b  are repeated 8 times

Ronchitest : straight with lines bending inwards a little bit at the edge.

 

Step 2

First : trim the polisher and make scratches in the squares. Otherwise the polisher won’t work properly

20 cm tool

  1. Ss 3 cm -2 kg     5min
  2. W-stroke 2kg       5min
  3. Ss 1 cm   2 kg     5 min
  4. W-stroke 2 kg   5 min
  5. Ss 2 cm                 5 min
  6. W-stroke 2kg     5 min
  7. Ss 2 cm 2 kg         5 min
  8. W-stroke 2 kg     5 min
  9. Ss 1 cm 2 kg         5 min
  10. W-stroke 2kg       5 min
  11. Ss 2 cm   2 kg       5 min
  12. Ss 3 cm   2 kg       5 min
  13. W-stroke 2 kg       5 min
  14. W stroke 2 kg       5 min

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 0,25 0,37 0,71 0,70 0,84 1,12
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

Step 3

July 25th 2011

Tool warm pressed and pitch squares scratched.Pitch is now 3,5 months old and quite hard.

  1. 20 cm tool 2kg Ss 1 cm     5 min.              (5 times)
  2. 20 cm tool W-stroke 2 kg   5 min         (7 times)
  3. Ss 2-3 cm                             5 min
  4. Ss 1-2 cm                              5 min

During the polishing it was necessary to use a warm press for the pitchlap to improve its action. As always, the pitch is scratched to make the movement less jerkily.

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 0,37 0,52 0,96 1,29 1,64 1,92
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

The figure is greatly improved!

Step 4

20 cm soft tool. Temp. in attic 25 degrees cent.

  1. Ss 1-2 cm 5 min
  2. W-stroke   1 turn = 3,5 min
  3. Ss 2-3 cm 5 min
  4. W-stroke         1 turn
  5. Ss 4-5 cm 5 min
  6. W-stroke 1 turn
  7. Ss 1-2 cm 5min
  8. W-stroke 1 turn
  9. Ss 2-3 cm 5 min
  10. W-stroke 1 turn
  11. Ss 1-2 vcm 5 min
  12. W-stroke 1 turn
  13. Ss 4cm 5 min
  14. W-stroke 1 turn

Ronchi : lines are bowed regularly . Possibly small TDE

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 0.28 0.60 1.06 1.57 2.04 2.66
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

– Middle part a bit flat

– outer zone too long.

 

Step 5

New pitchlap 20 cm with thicker pitch

Plan: increase correction and work on zone inside outer zone (attack TDE)

  1. Ss 1 cm  6x 5 min
  2. Ss 20-22 cm   10 rounds = ca 5 min
  3. Smoothing with W-stroke 3 turns
  4. W-stroke with emphasis on center and edge 1 turn

Ronchi: regular(?) parabola with possibly light TDE

 

 

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 0.56 0.98 1.52 1.91 2.47 2.93
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

 

Step 6

Increase correction with 20 cm tool

  1. Ss 1-2 cm 5×5 min
  2. Smoothing with W-stroke 5 rounds (ca17 min)

Test : outer zone worse and flat center

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 0.21 1.0 1.42 1.94 2.39 3.10
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

 

Step 7

Plan:           – increase correction by deepening center with smaller lap

-Work on edge area

– smoothing

 

  1. 15 cm tool with pitch squares of 3 cm (narrowing towards the edge)

Ss 1 cm          4×5 min

  1. 20 cm lap W-stroke 4 turns (ca 12 min)

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 0.58 1.43 1.96 2.57 3.18 3.78
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

Step 8

Using Figure XP program it is advisable to work on a large center area.

  1. 15 cm tool Ss 0 cm             5 min
  2. 15 cm tool Ss 1-2 cm       2×5 min
  3. 15 cm tool Ss 4cm        2×5 MIN
  4. 20 cm tool Ss 5-6 cm       5 min
  5. 20 cm tool W-stroke 3 turns ( ca 12 min)

Test : figure has become more irregular!

Foucault test( 2 meas.)

Zone 1 2 3 4 5 6 7
Knife edge 0 0.56 1.51 1.97 2.61 3.10 3.82
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

 

Step 9

Increase correction

a.New 10 cm tool with tapered pitch squares.   Ss 1 cm     2×5 min

b .15 cm tool Ss 2 cm          2×5 min
c.   15cm     tool Ss  2 cm 5 min
d.   20  cm tool   W-stroke 4 turns ca 12 min

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 0.99 1.76 2.47 2.98 3.65 4.30
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

 

Step 10

The outer zones have to be worked on, not only the inner zones.

  1. 10 cm tool Ss 2 cm 5 min
  2. 15 cm tool Ss 5 cm 5 min
  3. 15 cm tool Ss 8 cm 5 min
  4. 15 cm tool 2 kg W-stroke   8 turns

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 0.59 1.49 2.06 2.67 3.30 4.08
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

Correction has decreased but difference 4-5 is better now.

Step 11

Increase correction

  1. 10 cm tool       Ss 1 cm 2×5 min
  2. 15 cm tool       Ss 2 cm 2×5 min
  3. 15 cm tool       Ss 5 cm 2x 5 min
  4. 20 cm tool   W-stroke 5 turns.
  5. 10 cm tool circular stroke on outer zones   8 min
  6. 10 cm tool elliptical stroke edge to center   2 min.

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 1.04 2.14 2.87 3.54 4.25 5.06
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

Step 12

Increase correction with laps larger than 10 cm

  1. 15 cm tool Ss   2 cm 2×5 min
  2. 15 cm tool   Ss 6 cm   5 min
  3. 15 cm tool   Ss 9 cm           5 min
  4. 20 cm tool   W-stroke 5 turns
  5. 10 cm tool   circular stroke on outer zone   7 min
  6. 10cm tool ellipotical stroke edge-center 3 min

Foucault test : decreas of correction because I worked to much on the outer zones

Zone 1 2 3 4 5 6 7
Knife edge 0 0.68 1.72 2.44 3.08 3.82 4.70
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

 

 

Step 13 

august 14,2011

Increase correction; 10 cm tool needed again

  1. 10 cm tool   Ss 1cm 5 min
  2. 10 cm tool Ss 2cm 5 min
  3. 15 cm tool Ss 2 cm 5 min
  4. 15 cm tool Ss 4 cm 5 min
  5. 20 cm tool Ss 3 cm 5min
  6. 20 cm tool W-stroke 5 turns

Foucault test ( 2 meas.)

Zone 1 2 3 4 5 6 7
Knife edge 0 0.98 2.04 2.85 3.57 4.35 5.35
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

Step 14

Increase correction

  1. 10 cm tool Ss 1 cm 5min
  2. 15 cm tool Ss 1 cm 5 min
  3. 15 cm tool Ss 3 cm 5 min
  4. 15 cm tool Ss 6 cm 5 min
  5. 20 cm tool W-stroke 5 rounds
  6. 20 cm tool elliptical stroke edge-center 3 min

Foucault test : hole in center!

Zone 1 2 3 4 5 6 7
Knife edge 0 1.30 ! 2.25 3.02 3.76 4.48 5.45
Parabola 0 1,28 2,35 3,42 4,49 5,54 6,55mm

 

Edge too flat but center already deep enough. Needs more correction in zones 4-5-6 ; difference between zone 2 and 6 must be larger.

Try to work on area between r= 100 and r  = 200 mm

Step 15

  1. 15 cm tool : short straight stroke       centered on r= 100 mm with a side stroke towards the edge every three strokes.   7 min
  2. 20 cm tool : W-stroke 3 turns
  3. 15 cm tool : W-stroke 3 turns

Foucault test ( new mask!!) 2 meas. (as always, sometimes 3 meas.)

Zone 1 2 3 4 5 6 7
Knife edge 0 0.74 1.84 2.60 3.37 4.18 5.22
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

Work is needed on the zones within zone 7

Step 16

15 cm tool:

  1. Ss 2 cm 5 min
  2. circular stroke edge area (10-30 mm from edge) 5min
  3. elliptical stroke edge to -2 cm (2 cm beyond center) 3 min
  4. circular stroke edge area 5 min
  5. like c 3min

Foucault test ( new mask!!)

Zone 1 2 3 4 5 6 7
Knife edge 0 1.35 2.40 3.23 3.97 4.78 5.78
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

 

Step 17

Work on zones 3-4-5

a.    5 cm tool Ss 14 cm         5 min

b.15cm too circular stroke on r=16 cm  5 min

Foucault test ( new mask!!)   Worse! Correction decreased by working on outer zones

Zone 1 2 3 4 5 6 7
Knife edge 0 1.35 2.14 2.93 3.60 4.51 5.45
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

Step 18

  1. 15 cm tool: W-stroke going from r=10 cm to r=23 cm   3x 5 min.
  2. 15 cm tool elliptical stroke from r= -3 cm to r= + 22 cm. 5 min

 

 

Foucault test ( new mask!!)

Zone 1 2 3 4 5 6 7
Knife edge 0 1.0 1.93 2.48 3.08 3.91 4.91
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

Correction reduced and edge worse!

Step 19

Increase correction

  1. 15 cm tool Ss 4 cm 5 min
  2. 15 cm tool Ss 6cm 5 min
  3. 20 cm tool Ss 6 cm 2×5 min
  4. 20 cm tool W-stroke 8 rounds

Foucault test ( new mask!!)

Zone 1 2 3 4 5 6 7
Knife edge 0 1.02 1.86 2.35 3.07 3.64 4.74
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

Edge even worse! W-stroke has decreased correction

Step 20

10 cm tool

  1. Ss 54 cm 8 min
  2. Ss 6 cm 6 min
  3. Ss 8 cm 4 min
  4. Ss 10 cm 3 min

20 cm tool

Elliptical stroke edge-center             5 min

Test : bad

Step 21

Aim : attack TDE and increase correction

  1. 20 cm tool : small W-stroke (2 cm wide ) with edge of tool on area inside edge
  2. 20 cm tool Ss 1 cm    5 min
  3. 15 cm tool Ss 2-3 cm            5 min
  4. 15 cm tool Ss6-7 cm  5 min
  5. 30 cm tool W-stroke 1 turn ( smoothing)

Foucault test ( new mask!!)

Zone 1 2 3 4 5 6 7
Knife edge 0 0.11 !!! 1.02 1.68 2.45 3.27 4.50
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

Edge worse and flat center!

Step 22

–          20 cm tool has flattened center

–          TDE quite obvious.

  1. Attack TDE with rectangular lap of 2×5 cm (pitch on wood). Straight stroke with little pressure on area 0.5 cm from edge 5 min. Lap is pushed lengthwise.
  2. 10 cm tool : circular stroke on edge area       5 min
  3. 10 cm lap : elliptical stroke edge-center to increase correction again because working on the edge decreases correction.      5 min
  4. 10 cm tool: Ss 2 cm    5 min

Foucault test ( new mask!!)

Zone 1 2 3 4 5 6 7
Knife edge 0 -0.50!!! 0.57 1.55 2.46 3.48 4.25
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

 

TDE has been reduced!! Center too high.

Step 23

15 cm tool

  1. Ss 0 cm 5 min
  2. Ss 3 cm 5 min
  3. elliptical stroke edge-center 5 min

 

Foucault test ( third mask)

Zone 1 2 3 4 5 6 7
Knife edge 0 0.61 1.63 2.83 3.74 4.83 5.50
Parabola 0 0.61 1.74 3,14 4,43 5,63 6,73mm

Outer area too flat

 

Step 24

  1. 15 cm tool W-stroke between r= 4 cm and r=22 cm (center of tool) : 4 turns(12 min)
  2. 15 cm tool Ss 6 cm    5 min
  3. 30 cm tool W-stroke 1 turn
  4. Foucault test ( new mask!!)
Zone 1 2 3 4 5 6 7
Knife edge 0 0 !!! 1.43 2.53 3.37 4.47 5.35
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

Center too flat

Step 25

  1. 10 cm tool Ss 0-1 cm 5 min
  2. 15 cm tool Ss 0-2 cm 5 min
  3. 15 cm tool Ss 3-4 cm 5 min
  4. 30 cm tool W-stroke 1 turn

Foucault test ( new mask!!)

Zone 1 2 3 4 5 6 7
Knife edge 0 0.17 1.38
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

Center too flat

 

Step 26

  1. 10 cm tool W-stroke inside r= 10 cm 4 min
  2. 10 cm tool W-stroke inside r= 12.5 cm 3 min
  3. 10 cm tool elliptical stroke edge-center 5 min

Foucault test ( new mask!!)

Zone 1 2 3 4 5 6 7
Knife edge 0 0!! 1.9
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

 

Step 27

  1. 10 cm tool Ss 2-4 cm 2×5 min
  2. 30 cm tool W-stroke 1 turn

Foucault test ( new mask!!)

Zone 1 2 3 4 5 6 7
Knife edge 0 0!! 1.6
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

Not better; is the 30 cm tool spoiling things?

Step 28

15 cm tool

  1. Ss 1 cm 2x 5 min
  2. elliptical stroke -2 to +22 cm (center of tool)   5 min

 

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 1.17 2.86 3.68 4.52 5.50 6.48
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

This was a bit too much for the center

Figure XP : high zone r = 11 cm and edge

Step 29

  1. 10 cm tool   circular stroke on r=11 cm 5 min
  2. 10 cm tool circular stroke on edge 5 min
  3. elliptical stroke -2 to +23 cm 3 min

Foucault test ( new mask!!)

Zone 1 2 3 4 5 6 7
Knife edge 0 1.70 2.91 3.92 4.70 5.77 6.57
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

Correction too great ; hole in center

Figure XP: 1/1.4 lambda peak-valley Strehl = 0,188

Step 30

Work on r=10 cm

  1. 10 cm tool circular stroke on r=10 cm 3x 5min
  2. 30 cm tool W-stroke 1 turn

Foucault test

Zone 1 2 3 4 5 6 7
Knife edge 0 1.18 2.67 3.46 4.42 5.39 6.43
Parabola 0 0.88 2.02 3,11 4,20 5,30 6,25mm

Figure XP 1/ 2.3 lambda S=0.365

 

Step 31

Try to work on r= 75 to r=150 mm

  1. 10 cm tool circular stroke on r=110 mm 3×5 min
  2. 20 cm tool elliptical stroke r=15 to r=19 cm ( smoothing)

 

Foucault test ( new, 10 zone mask!!)

Zone 1 2 3 4 5 6 7 8 9 10
Knife edge 0 0.54 1.10 2.09 2.77 3.39 3.97 4.80 5.89 6.83
parabola 0 0.312 0.737 1.291 1.927 2.663 3.531 4.564 5.623 6.693

Step 32

  1. 10 cm tool : tangential stroke on r=10-12 cm . Every 2 strokes a stroke veering off to the center.                   3x 5 min
  2. 30 cm tool   2x 1/2 round W-stroke

Foucault test ( new, 10 zone mask!!)

Zone 1 2 3 4 5 6 7 8 9 10
Knife edge 0 0.33 1.32 1.97 2.58 2.99 3.77 4.65 5.85 6.78
parabola 0 0.312 0.737 1.291 1.927 2.663 3.531 4.564 5.623 6.693

 

Surface:   1/1.7 lambda   ; strehl = 0.162

Step 33

Work on r= 75 mm

  1. 10 cm tool : Tangential stroke on r= 75 mm 3x 5 min
  2. 30 cm tool : smoothing : 4x ½ round W-stroke

 

Foucault test ( new, 10 zone mask!!)

Zone 1 2 3 4 5 6 7 8 9 10
Knife edge 0 .11 0.4 1.01 1.70 2.46 3.25 4.27 5.43 6.3
parabola 0 0.312 0.737 1.291 1.927 2.663 3.531 4.564 5.623 6.693

Values from two close measurements

1/7.6 Lambda   Strehl = 0.959

In telescope: no astigmatism at 200 x ; images of star look OK.

Maybe call it finished. Dangerous to go on with the correction.

Startest: no astigmatism at 257x

Coating Galvoptics GB   500 euro (transport costs included)