1. Notes on building a Newtonian
Dave Siegrist ATMoB
(nt1u at hotmail dot com)

2. 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!

3. 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!

4. 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.

5. Secondary Issues How do you decide on size? Tradeoff:
obstruction of light entering tube
size of 100% illuminated disk at eyepiece

6. Secondary D L

7. 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

8. 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”

9. 1/2 degree image size vert: diameter hor: f#

10. 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
” %

11. 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.

12. 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

13. 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

14. Mirror cell Hold mirror in place Allow easy collimation
no stress
no flexure
Allow easy collimation
Provide for ventilation

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

17. 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

19. 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…)

23. Focuser Make or buy: your choice Types: Size:
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)

26. “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

27. 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

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