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Amateur Telescope Making (UPDATED 9/23/18) - 20" f/4 Nearly Complete

_Augustus_

By _Augustus_
in Science & Spaceflight

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_Augustus_

_Augustus_

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53 minutes ago, munlander1 said:

What's the difference?

A Chief uses a tilted primary mirror to avoid having a central obstruction entirely, and then uses off-the-shelf lenses to correct for the aberrations. Also known as a Jones-Herschelian. I've met the inventor of the design.

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Ty Tan Tu

Ty Tan Tu

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I once found a new 5" mirror on Astromart. An odd size to be sure, but it fit perfectly into the end of #10 cans, like large coffee cans. I soldered several cans together. I bought a beat up 4.5" telescope just for the focuser and secondary mirror with adjustable 45 degree holder (it was cheaper than buying those new, and I was very broke at the time). I used bicycle spokes for the spider, which the local bicycle repair shop gave me for free after I explained why I wanted what they considered trash. The bicycle spokes were a touch of genius. They have those little hex nuts that make collimation a lot easier.

I was going to add light baffling, but decided that the ribbed nature of the cans with flat black paint would be good enough. The primary mirror has a long focal length, but works quite well, especially for planets in Portland OR's light polluted skies. 

I want to grind my own mirror. The Rose City Astronomers (RCA) has a weekend shop where I can do that with expert advise. Does anyone have a recommendation on where to buy a couple of glass blanks and grinding grit?

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  • 2 weeks later...
_Augustus_

_Augustus_

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I wanted to try out the motions so I built a rocker box for my Edmund 6" f/8 and put it on the f/4's base. It doesn't make it to the zenith but it should work.

f4I8T2l.jpg

UPDATE: Motions are pretty smooth. Saturn, Moon, and DSOs were pretty nice with this scope. Still have to finish the f/4.

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_Augustus_

_Augustus_

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The 8" SCT is going to be out of here on Monday; as soon as that's done I can order everything - I already have the money from it but my dad won't let me spend it until I ship it out.

I'm going to be getting:

  • Destiny curved spider (already have the secondary)
  • Orion single-speed Crayford focuser
  • 38 pounds of plaster

And of course I'll have some $ left over for aluminizing.

You might be asking why I'm getting 38 lbs of plaster. The answer to that is basically because ordering in small quantities is a ripoff. For the amount needed for an 8" tool, you're paying about $2.50 per pound. If I buy 38 lbs, I'm paying about $1.42 per pound, and I don't have to order more for my next mirror.

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