# 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 Not completed plans, but guidelines.
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 Optical system: Purpose
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 How do you decide on size? Tradeoff:
obstruction of light entering tube size of 100% illuminated disk at eyepiece

Secondary D L

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
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 / == about .5”

1/2 degree image size vert: diameter hor: f#

Secondary size: example
Secondary = d + (D-d)*L/F An 8”f6 assume L = 7.3” (D/2 + D/ ) sec = (( ) * 7.3 / 48) = 1.57 standard sizes: % illuminated .36” % obstruction ” %

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) Your L will be different.

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

Cell 8” or less: plywood 10” or more: metal or sling or both

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 Hold Secondary Make or buy: you choose (not hard…)
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…)

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) (yes) (10” or bigger)

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