Presentation on theme: "Notes on building a Newtonian"— Presentation transcript:
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 opticsBuy a kit, or do it all from scratchBuy some parts or build it allBuild 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 workYou can build a telescope!
4 Optics Optical system: Purpose primary, secondary, and eyepiecesPurposegather lightbring it to a focal planeAssume: 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 tubesize of 100% illuminated disk at eyepiece
7 Size Guiding principles illumination can drop to 70% with no noticeable visual affectwant the 100% image to cover at least 1/2 degree.Secondary Size = d + ((D - d) * L / F)d fully illuminated field D diameter of mirrorL distance to focus F focal length
8 Fully Illuminated Image: d d = F * angle /57.3F focal length angle: .5 degreeImage size example:8”f7 will have a focal length = 8 * 7 = 56”size = (F * angle)/57.3= 56 * .5 / == about .5”
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/pulledStays where pointed (vertical or horizontal)Vibrations quickly dampenMirror cools quicklyEasy to carry, set up, tear down
13 Optical Tube Assembly (OTA Sturdy and light (1/2” plywood/sonotube)diameter: > mirror diameter + FL/100minimum 1” space all aroundLength:mirror end: leave enough space for celleyepiece end: 1 diameter of mirror beyondcan always cut it shorter later!Holes: do focuser 1st, then spider, then try cell without attachingBalancing
14 Mirror cell Hold mirror in place Allow easy collimation no stressno flexureAllow easy collimationProvide for ventilation
15 Cell8” or less: plywood10” 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 formicaride on teflon: size = weight load / 15ex: 70lb. Scope, 4 pads size = (70/15)/4 = 1.1”sqfarther 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’ armsmust not vibrate (no single arm spiders)easy access for collimationMake or buy: you choose (not hard…)
23 Focuser Make or buy: your choice Types: Size: helical: can be low profile, can be home madecrayford: can also be home made, w/o machinerack and pinion: purchased, stay away from plasticSize:.96 (no) (yes) (10” or bigger)