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

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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 –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: – 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 /57.3 == 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 =.42 + ((8 -.42) * 7.3 / 48) = 1.57 –standard sizes: % illuminated.36” 13% obstruction ” 22%

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 –no stress –no flexure Allow easy collimation Provide for ventilation

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

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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

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19 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…)

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23 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)

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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


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