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Newtonian Telescope Making

Newtonian Telescope Making. Mike Lockwood February 18, 2004. What is a reflecting telescope?. 1) Optics – two or more mirrors 2) Structure to keep optics stationary with respect to each other (to high tolerance!!) 3) Mechanism for aiming the whole thing Sounds like a big heavy object

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Newtonian Telescope Making

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  1. Newtonian Telescope Making Mike Lockwood February 18, 2004

  2. What is a reflecting telescope? • 1) Optics – two or more mirrors • 2) Structure to keep optics stationary with respect to each other (to high tolerance!!) • 3) Mechanism for aiming the whole thing • Sounds like a big heavy object • However, it must be portable and move easily, or it will be used infrequently!

  3. Optics - Newtonian • Primary mirror • Focuses light from  at a point • This requires a parabola shape, paraboloid • Focal ratio • Light (from ) comes to focus in this # of mirror diameters • Higher F, longer scope, easier to make • F/4 to F/8 common

  4. Optics - Newtonian • Secondary mirror • Flat, reflects light to eyepiece • In optical path, degrades image slightly Light enters here  

  5. Optics – Make or Buy? • Highest quality • Made (slowly) by experts or amateurs • Good quality • Machine made (quickly) by bigger companies • A 6” mirror is a good first mirror • 12”+ requires practice, patience, time, muscle • Grind with carborundum, aluminum oxide, cerium oxide, jeweler’s rouge (iron oxide)

  6. Making Optics - Grinding • Grinding – “Pushing Glass” • Randomness cancels out in theory to provide a nice spherical shape….. hopefully

  7. “Class with Glass”

  8. Grinding in progress…

  9. Making Optics:Polishing, Figuring • Polish mirror after grinding ~ 1 hr. / inch • Done by polishing on a “pitch lap”, a layer of slowly-flowing resin • Figuring is the process of getting a mirror to a parabola (but usually first to a sphere)

  10. Testing • Focault tester • LED and “Knife-edge” • Precision of 1/1000th inch in knife travel yields 0.5/1,000,000th inch surface accuracy • Cost of $20!!!

  11. Bad Figuring – “Dog Biscuit”Polishing gone wrong

  12. Good figuring – smooth surface

  13. Equatorial Mounts • Required for most photography, but heavy and not really needed for visual astronomy

  14. Building a Portable Newtonian • Old telescopes were heavy, cumbersome • 8” scope, 80 lbs! • Large mirrors have become common, so a new design was needed to accommodate portable amateur scopes

  15. The Classic “Dobsonian”(Dob) • John Dobson – telescope pioneer • Made from readily available, cheap materials • Portable, fast setup • Resembles a cannon or missile launcher (to cops late at night)

  16. Components of a Dobsonian • Common parts, made of Sonotube, plywood • Optics, spider, focuser available

  17. Many Variations

  18. Mirror Cell • Holds mirror, provides adjustment of angle • For smaller mirrors, glue works fine

  19. Mirror Cell – going in

  20. Mirror Cell - installed

  21. The Big Scope Dilemma • How do you build and transport a really big Dobsonian with a tube? • Answer: Buy a Mack truck & trailer • Big tubes = big $$, big weight, flexing, pain in the back • What do I mean by really big?

  22. Okay, this is really big.Yard Scope – 36” F/5 Doug Me

  23. Okay, this is bigger!Steve Swayze, 40” F/5, (12” F/4 finderscope)

  24. The BIGGEST!Erhard Hännsgens – 42” F/4.5, Germany

  25. Future 45”Hugues Laroche – Luxembourg?

  26. Truss Tube Dobsonians • Truss replaces solid tube • Truss maintains alignment between primary mirror (in mirror box) and the secondary mirror and focuser (in diagonal cage) • Rigid, lightweight • Clamp, bolt, screw together • Mirrors thinner than in the past – 1:6 ratio is dead

  27. Components • Mirror box • Holds, protects mirror, can have a finder on top • Attaches to truss tubes • Pivots on mount, ground board • Diagonal cage • Attaches to truss tubes • Holds diagonal mirror, finder (sometimes), focuser • Together, these replace a tube

  28. 32” F/3.5, Frédérick Géa • Minimal diagonal cage and mirror box • Open structure, lightweight construction

  29. What materials, skillsand tools are needed? • Optics, spider, focuser, finder – specialty • The rest: Plywood, formica, bolts, screws, tubes, misc. metal • Skills: Carpentry, common sense • Tools: Circular saw, jigsaw, drill, hacksaw • Nice things to have access to: table saw, drill press, belt sander, router, etc.

  30. 30” F/4 project (Local!) • Mike Conron – Optics, port wine, endless enthusiasm and optimism • Bob Nonnemann – Everything else! • Mike Lockwood – Encouragement, nagging, impatience, and occasional useful advice • Mirror nearly polished, several scope parts done, displayed in public for the first time!! • First light this spring?!?!

  31. 16” F/4 Truss-tube Dobsonian • Costs • Mirror Blank, $500 ($2000+ finished) • Diagonal, holder, spider, $200 • Other parts, (wood, welding), ~$300 • Time • Mirror grinding – 4 months • Construction – 1 month (lots of work!) • Next scope – 17.5” F/4.5

  32. My Telescope Stats • Scopes built (8): • 5.25” F/8.8 (V1), 8” F/6 (sold), 10” F/6.8 • 8” F/3.9, 5.25” F/8.8 (V2), 16” F/4 • Two 5” F/5 (5” prototype, club scope) • Scopes I’ve helped with (13+): • Eleven 5” F/5 scopes • 12” F/6 (Mark Haun) • 30” F/4 (minor role) • Mirrors made or refigured (9): • 8” F/3.9, 5.25” F/8.8, 10” F/6.8, 16” F/4, 6” F/6, two 12.5” F/6, 14.25” F/4.5, 4.25” F/4.5

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