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– 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: – 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 /57.3 == about .5”

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

5.00 5.50 6.00 6.50 7.00 7.50 8.004.50 0.20 0.22 0.24 0.26 0.27 0.29 0.316.00 0.26 0.29 0.31 0.34 0.37 0.39 0.428.00 0.35 0.38 0.42 0.45 0.49 0.52 0.5610.00 0.44 0.48 0.52 0.57 0.61 0.65 0.7012.50 0.55 0.60 0.65 0.71 0.76 0.82 0.87

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%

5 5.5 6 6.5 7 7.5 84.5 1.19 1.12 1.06 1.01 0.97 0.94 0.926 1.43 1.34 1.28 1.23 1.18 1.15 1.138 1.75 1.65 1.57 1.51 1.47 1.43 1.4110 2.06 1.95 1.87 1.80 1.75 1.71 1.6912 2.46 2.33 2.23 2.16 2.10 2.06 2.04

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

• 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– 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– 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

• 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”

• 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