Notes on building a newtonian
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Notes on building a Newtonian. Dave Siegrist ATMoB (nt1u at hotmail dot com). 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!. Goals for today. I will highlight

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Notes on building a newtonian

Notes on building a Newtonian

Dave Siegrist ATMoB

(nt1u at hotmail dot com)


Newtonian telescope notes

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!


Goals for today

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!


Optics

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.


Secondary issues

Secondary Issues

  • How do you decide on size?

  • Tradeoff:

    • obstruction of light entering tube

    • size of 100% illuminated disk at eyepiece


Secondary

Secondary

D

L


Notes on building a newtonian

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


Fully illuminated image d

Fully Illuminated Image: d

  • d = F * angle /57.3

    • F focal length angle: .5 degree

  • Image size example:

    • 8”f7 will have a focal length = 8 * 7 = 56”

    • size = (F * angle)/57.3

    • = 56 * .5 /57.3 == about .5”


1 2 degree image size vert diameter hor f

1/2 degree image sizevert: diameter hor: f#


Secondary size example

Secondary size: example

  • Secondary = d + (D-d)*L/F

  • An 8”f6

    • 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%


Notes on building a newtonian

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.


Mechanics

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


Optical tube assembly ota

Optical Tube Assembly (OTA

  • Sturdy and light (1/2” plywood/sonotube)

  • diameter: > 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


Mirror cell

Mirror cell

  • Hold mirror in place

    • no stress

    • no flexure

  • Allow easy collimation

  • Provide for ventilation


Notes on building a newtonian

Cell

  • 8” or less: plywood

  • 10” or more: metal or sling or both


Side bearings

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 = 1.1”sq

      • farther apart pads: stiffer the motion


Spider

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…)


Focuser

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)


Dobsonian mount

“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


Ground board

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


References

References

  • Build Your Own Telescope Berry, R

  • All about Telescopes Brown, S

  • The Dobsonian Telescope Kriege/Berry

  • How to make a Telescope Texareau


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