This document summarizes a study on making parabolic primary mirrors for Newtonian telescopes using local resources. Two 15cm glass disks were ground into spherical then parabolic shapes using silicon carbide, aluminum oxide and pitch as abrasives. Testing showed the mirrors met quality standards, with a wavefront error under 1/4 wavelength and Airy disk radii between -1 and 1, indicating minimal aberrations. The technique was concluded to be effective for making precise optics for amateur astronomy telescopes locally with minimal cost. Future work involves expanding to larger mirrors, implementing silvering and computerized tracking mounts.

1. PRELIMINARY RESULTS ON THE PARABOLIC PRIMARY
MIRROR MAKING STUDY FOR THE APPLICATION IN
NEWTONIAN TELESCOPE
Dhanushka Subath Amaradasa
Research Assistant
Department of Physics
University of Ruhuna
Sri Lanka
by
H.D.S Amaradasa, S.S.Abeywickrama, E.M.ranatunga, G.D.K.Mahanama
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2. OUTLINE
1. Introduction
2. Objective
3. Materials
4. Procedure
5. Testing
6. Results
7. Conclusion
8. Future Directions
2

3. 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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4.  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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5. • 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. 6

7. PROCEDURE
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1
• Rough grinding
2
•Fine grinding
3
•Polishing

8. 8
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 repeated
decreasing the grit size.
1.Rough grinding

9. 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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10. Overall grinding process
Grit 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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11. 3.Polishing
making 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.

12. • 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.
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13. 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

14. • Final testing
1. Ronchi test, for uniformity of the Spherical
surface
2. Focult test for uniformity of the Parabolic
surface
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15. Figure-4
Shadowgram of Focult test
Figure-5
Shadowgram of Ronchi test
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16. 16

17. Zonal mask
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18. Zone # Zone % Zone
Centre
Zone
Edge
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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19. 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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20. Wavelengths–Edge zone radii
characteristics
-0.2500
0.0000
0.2500
0.000 1.000 2.000 3.000 4.000
Wavelengths(500nm)
Edge zone radii
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21. • 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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22. 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
Airydiskradii
Average radii of zones
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23. • 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

24. • 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

25. 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 Tushar
Purohit

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Thanks
Amazing night sky ,Milky Way photographed at Matara ,Sri Lanka
THANK YOU