Notes on building a Newtonian Dave Siegrist ATMo. B (nt 1 u 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 • L distance to focus D diameter of mirror F focal length
Fully Illuminated Image: d • d = F * angle /57. 3 – F focal length angle: . 5 degree • Image size example: – 8”f 7 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#
Secondary size: example • Secondary = d + (D-d)*L/F • An 8”f 6 – 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” • 1. 83. 72” 13% obstruction 22%
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 1 st, 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: 70 lb. 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 • All about Telescopes • The Dobsonian Telescope Berry, R Brown, S Kriege/Berry • How to make a Telescope Texareau
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