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Telescope Making

Telescope Making. Mike Lockwood March 25, 2002. What is a Telescope?. 1) Optics 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. .

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Telescope Making

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  1. Telescope Making Mike Lockwood March 25, 2002

  2. What is a Telescope? • 1) Optics • 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.

  3. Optics - Newtonian • Primary mirror • Focuses light from  at a point • This requires a parabola shape, paraboloid

  4. Optics - Newtonian • Secondary mirror • Flat, reflects light to eyepiece

  5. Optics – Make or Buy? • Highest quality • Hand made by experts or amateurs • Good quality • Machine made by big companies • A 6” – 8” mirror is a good first mirror, 12” or larger requires skill and practice • Grind with carborundum, aluminum oxide, cerium oxide, jeweler’s rouge

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

  7. Making Optics:Polishing, Figuring • Polish mirror after grinding ~ 1 hr. / inch • Figuring is the process of getting a mirror to paraboloid shape (but first to a sphere) • Done by polishing on a “pitch lap”

  8. Testing • Focault tester • “Knife-edge” • Precision of 1/1000th inch in knife travel yields 1/1,000,000th inch surface accuracy

  9. Bad Figuring – “Dog Biscuit” – Mirror making gone wrong

  10. Building a Portable Newtonian • Old telescopes were heavy, cumbersome • Large mirrors have become common, so a new design was needed to accommodate portable amateur scopes

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

  12. The Classic “Dob” • Made from readily available, cheap materials • Portable • Resembles a cannon (to cops late at night)

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

  14. Many Variations – location?

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

  16. Mirror Cell – going in

  17. Mirror Cell - installed

  18. The “Light Bucket” Dilemma • How do you build and transport a really big Dobsonian with a tube? • Answer: Buy a Mack truck • Big tubes = big $$, big weight, flexing • What do I mean by really big?

  19. Okay, this is really big.

  20. Truss Tube Dobsonians • Truss maintains alignment between primary mirror (in mirror box) and the secondary mirror and focuser (in diagonal cage) • Rigid, lightweight • Clamp, bolt, screw together • Make them the same length, and they’re interchangeable

  21. Components • Mirror box • Holds, protects mirror, can have a finder on top • Attaches to truss tubes • Diagonal cage • Attaches to truss tubes • Holds diagonal mirror, finder (sometimes), focuser

  22. 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

  23. Astrofest – Telescope Showcase:

  24. My Scope: Details • 10”, F / 6.8 • Built in 1992 • Cost: $250 • Hours: ~ 200 • Weight: ~ 80 lbs • Height: At zenith, ~ 6’ 4” • Building it

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