PRELIMINARY RESULTS ON THE PARABOLIC PRIMARY

MIRROR MAKING STUDY FOR THE APPLICATION IN

NEWTONIAN TELESCOPE

Dhanushka Subath AmaradasaResearch Assistant

Department of PhysicsUniversity of Ruhuna

Sri Lanka

by

H.D.S Amaradasa, S.S.Abeywickrama, E.M.ranatunga, G.D.K.Mahanama

1

OUTLINE

1. Introduction

2. Objective

3. Materials

4. Procedure

5. Testing

6. Results

7. Conclusion

8. Future Directions

2

INTRODUCTION

• Quality and economical Newtonian telescopes are made

by amateur astronomers without requiring expensive

accessories.

• The most important step in telescope making is figuring

a primary mirror.

• Various methods are followed by both professional and

amateur astronomers to grind different sizes of mirrors.

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To make 15 cm diameter and 1.5m focal length parabolic mirror

Using an economical technique.

Utilizing maximum amount of local resources.

Maintaining the figure with precision and standards required for

optical astronomy.

Here we present the preliminary results obtained in the study of

mirror making technique of grinding two identical glass disks using a

series of abrasives

OBJECTIVE

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• Glass disks

– 15 cm diameter and 12mm thickness

– Float glass (Soda Lime Flat glass)

• Abrasives

– Sillicon Carbide(SiC)

– Alumenium Oxside(Al2O3)

• Polishing agent

– Optical pitch(Tar)

– Ferric Oxside(Fe2O3)

MATERIALS

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6

PROCEDURE

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1•Rough grinding

2•Fine grinding

3•Polishing

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1.Rough grinding was initiated

fixing the tool on a horizontal

rotatable surface.

2.Paste of

water and

Silicon Carbide

(SiC) of particle

size 125

microns was

poured on the

tool surface.

3.With mirror on top, the

mirror was moved

outward about 40% of its

diameter over the tool

and Chordal strokes were

taken.4.After twenty Chordal strokes mirror was rotated in 450.

5.After four rounds tool was rotated.

6.Procedure was repeateddecreasing the grit size.

1.Rough grinding

2.Fine grinding

• Fine grinding was initiated with 12 micron

Aluminum Oxide (Al2O3)

• Taking center over center strokes and

continued from 5 and 3 microns

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Overall grinding processGrit size from Microns Grinding hours

1.Rough grinding 125 5

Focal length -3m

80 3

Focal length -2m

40 2

Focal length -1.6m

20 2

2.Fine grinding 12 1

5 0.5

3 0.5

Final focal length. Focal length -1.4m

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3.Polishingmaking the Pitch lap

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1.Paper dam was made around the

Mirror to pour melted pitch.

2.Plastic mesh was placed on mirror to

form groove patterns in the lap surface.

3.Melted pitch was poured on the

mirror and tool was placed over it.

• Mirror was placed at the bottom and Pitch

lap was placed on top of it.

• Using Ferric oxide(Fe2O3) as the polishing

agent Chordal strokes were taken to

polish the spherical surface.

• Afterwards W-Strokes were taken so

spherical surface was polished into a

smooth parabolic surface.12

1. Grinding phase testing.

2. Polishing phase testing.

3. Surface error testing (final tests).

Focult Test

Ronchi Test

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TESTING

Mirror was tested in three main stages

• Final testing

1. Ronchi test, for uniformity of the Spherical

surface

2. Focult test for uniformity of the Parabolic

surface

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Figure-4Shadowgram of Focult test

Figure-5Shadowgram of Ronchi test

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16

Zonal mask

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Zone # Zone % Zone

Centre

ZoneEdge

Knife Edge D1

Knife Edge D2

1 35.4 1.061 1.500 0.793 0.792

2 61.2 1.837 2.121 0.870 0.869

3 79.1 2.372 2.598 0.901 0.900

4 93.5 2.806 3.000 0.908 0.900

DATA REDUCTION METHOD

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RESULTS

• According to Raleigh criteria for a standard

mirror maximum wave front error must not

exceed a 1/4 wave.

• The Wavelengths–edge zone radii

characteristics indicated that the surface of the mirror has a peak value of 1/5.44

wavelengths.

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Wavelengths–Edge zone radii characteristics

-0.2500

0.0000

0.2500

0.000 1.000 2.000 3.000 4.000

Wav

elen

gth

s (5

00

nm

)

Edge zone radii

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• According to Danjon-Couder condition, the

radius of the circle of least aberration should be

comparable with that of the theoretical diffraction

disk.

• All Airy disk radii of the mirror are between -1

and 1 proving the mirror can converge light

in focal point minimizing transverse or

longitudinal aberrations

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Airy disk radii-Average radii of zones

characteristics

-1.0000

0.0000

1.0000

0.000 0.500 1.000 1.500 2.000 2.500 3.000

Air

y d

isk

rad

ii

Average radii of zones

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• All Airy disk radii of the mirror are between -1 and 1 proving the mirror can

converge light in focal point minimizing transverse or longitudinal

aberrations.

• Wave-front error has shown that the highest peak in the mirror surface is

less than the quarter wave confirming the mirror is meeting the conditions of

Raleigh criteria and Danjon-Couder condition.

• Ronchi test has shown that the curvature of the mirror is evenly distributed

along the radius.

• Based on these results it can be concluded that the technique utilized in this

research is effective and reliable in producing precise optics for

Astronomical telescopes.

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CONCLUSION

• Grinding can be done using sand or Emery powder.

• We have successfully completed and tested 10 inch F/10

mirror and method can be implemented to make up to 1m

mirror.

• Research grade telescopes can be made locally.

• Silvering is done by chemical dipostion.

• Computerized telescope autotracking GOTO type

telescope.

• Arranging programs and camps to teach the method of

telescope making to school commmunity.

• Contribution to Astrobilogy.

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FUTURE DIRECTIONS

References

• Malacara, Daniel.Optical Shop Testing. s.l. : A John Wiley & Sons, Inc., 2007. ISBN: 978-0-471-48404-2 .

• Shroeder, D.J.Astronomcal Optics. London : Acadamic Press.Inc, 1987. ISBN 0-12-629805.

• Upton, John D. www.atm-workshop.com. The Dykem test. [Online] [Cited: 04 13, 2015.] http://www.atm-workshop.com/dykem-test.html.

• Taylor, H.Denis. The Adjustment and Testing of Telescope Objectives. Newcastle : Pearson & Co, 1946.

• Vandewttering, Mark T.Telescope basics. 2001.

• Berry, Richard. Build Your Own Telescope. Wiscosin : Scribdner, 1985.

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Funding-Faculty of Science, University of Ruhuna,HETC Project - QIG Grant

Collaborators- I.U.C.C.A Pune India,Mr Aravind Paranjpie and Mr TusharPurohit

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Thanks

Amazing night sky ,Milky Way photographed at Matara ,Sri Lanka

THANK YOU