Notes on building a Newtonian

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Notes on building a Newtonian

• Dave Siegrist ATMoB
• (nt1u at hotmail dot com)

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Newtonian Telescope Notes

• You can make a telescope!
• Grind or buy the optics
• Buy a kit, or do it all from scratch
• Buy some parts or build it all
• Build it!

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Goals for today

• I will highlight
• both the optics and mechanical issues.
• Not completed plans, but guidelines.
• Give an intuitive feel for why things work
• You can build a telescope!

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Optics

• Optical system
• primary, secondary, and eyepieces
• Purpose
• gather light
• bring it to a focal plane
• Assume you have already decided on a primary
  size, and its focal length.

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Secondary Issues

• How do you decide on size?
• Tradeoff
• obstruction of light entering tube
• size of 100 illuminated disk at eyepiece

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Secondary
D
L
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Size

• Guiding principles
• illumination can drop to 70 with no noticeable
  visual affect
• want the 100 image to cover at least 1/2 degree.
  
• Secondary Size d ((D - d) L / F)
• d fully illuminated field D diameter of
  mirror
• L distance to focus F focal length

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Fully Illuminated Image d

• d F angle /57.3
• F focal length angle .5 degree
• Image size example
• 8f7 will have a focal length 8 7 56
• size (F angle)/57.3
• 56 .5 /57.3 about .5

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1/2 degree image sizevert diameter hor f
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Secondary size example

• Secondary d (D-d)L/F
• An 8f6
• assume L 7.3 (D/2 D/10 2.5)
• sec .42 ((8 - .42) 7.3 / 48) 1.57
• standard sizes
• 1.52 100 illuminated .36 13
  obstruction
• 1.83 .72
  22

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Secondary Mirror sizes (calculated for d .5
degree) Size vertical is mirror diameter
horizontal is f Size is nominal find the
closest size that meets requirements. L
(D/2) (D/10) 2.5 Your L will be different.
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Mechanics

• Move smoothly when pushed/pulled
• Stop moving when not pushed/pulled
• Stays where pointed (vertical or horizontal)
• Vibrations quickly dampen
• Mirror cools quickly
• Easy to carry, set up, tear down

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Optical Tube Assembly (OTA

• Sturdy and light (1/2 plywood/sonotube)
• diameter gt mirror diameter FL/100
• minimum 1 space all around
• Length
• mirror end leave enough space for cell
• eyepiece end 1 diameter of mirror beyond
• can always cut it shorter later!
• Holes do focuser 1st, then spider, then try cell
  without attaching
• Balancing

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Mirror cell

• Hold mirror in place
• no stress
• no flexure
• Allow easy collimation
• Provide for ventilation

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Cell

• 8 or less plywood
• 10 or more metal or sling or both

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Side Bearings

• Bigger is better
• Must be exactly the same
• 3/4 plywood (want stable)
• bearing surface
• bumpy formica
• ride on teflon size weight load / 15
• ex 70lb. Scope, 4 pads size (70/15)/4
  1.1sq
• farther apart pads stiffer the motion

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Spider

• Hold Secondary
• must be adjustable to center (not easily though)
• must have narrow arms
• must not vibrate (no single arm spiders)
• easy access for collimation
• Make or buy you choose (not hard)

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Focuser

• Make or buy your choice
• Types
• helical can be low profile, can be home made
• crayford can also be home made, w/o machine
• rack and pinion purchased, stay away from
  plastic
• Size
• .96 (no) 1.25 (yes) 2.0 (10 or bigger)

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Dobsonian Mount

• The box the OTA sits in, named for John Dobson,
  who is probably here.
• Goals
• light and sturdy (no racking, flex) 3/4 plywood
• low as possible low center of gravity eyepiece
  low

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Ground board

• Azimuth bearing (rotate parallel to ground)
• size diameter as large as bottom of box holding
  OTA
• teflon on bottom facing up, formica facing down
• 3/4 plywood
• feet hockey pucks

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References

• Build Your Own Telescope Berry, R
• All about Telescopes Brown, S
• The Dobsonian Telescope Kriege/Berry
• How to make a Telescope Texareau