Hello, Straightdopers; AMAPAC here with a question about Dobsonian optics.
I’m building a Dobsonian telescope, and I can’t seem to figure out where to put the eyepiece focuser.
I’ve got an 8" f/6 mirror with 1200 focal length–my plan is to put the eyepiece focuser so that the focal point of the mirror comes up from the secondary and ends up about 1 mm outside the outer wall of the tube.
Will that work?
Thanks…AMAPAC
I’m too out of practice on this to be able to answer your question directly, but I can tell you that you’ll probably have better luck looking for information about Newtonian telescopes. Dobsonian isn’t the kind of optics, just the mount.
Solid tube or truss tube?
This site has this pic which gives the formula for a solid tube.
Secondary mirror is; focal length - radius of tube - focuser height +1".
Focuser hole is; focal length + distance between the surface of the mirror and the end of the tube - radius of tube - focuser height +1".
(Though, I think, the +1" really should be half the travel of the focuser.)
Basically you want the focal point to be at the mid point of the focuser travel.
Too short, your screwed and have to make new holes. Too long, you can add an extension tube between the focuser and the eyepiece.
If there’s one book you really want for home built Dobs it’s, The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes by David Kriege and Richard Berry (Amazon link). IMHO the book for making truss Dobs.
If your making a truss Dob I can type out the instructions the book gives, they’re nothing like the ones for tubes, more art than science.
CMC +fnord!
My truss Dob; 12.5 ASM black vitrified ceramic primary (1/15[sup]th[/sup] wave - f/5.6 - 1,810mm fl) in a Novak heavy duty mirror cell, Protostar 2.1" secondary, Starlight “Feather Touch” 2" focuser (w 2" travel), Tom Mittler custom built Baltic birch mirror/rocker box and secondary cage.
I think that’s a bit too close to the tube. You’re going to have to get your eyepieces to within a focal length or so of that position in order to focus. With short fl eyepieces, that’ll probably force you to run the focusing tube down into the main mirror’s lightpath.
Try digging around under ‘Newtonian’ here for better info:
Thanks, Dopers! And thanks for the kind offer, Crowmanyclouds, but no need; I’m doing a solid tube.
You didn’t provide the tube OD, so it is not possible to answer your question.
This application will let you play with various options:
http://www.dalekeller.net/ATM/newtonians/newtsoft/newtsoft.htm
I think the best location is dictated by several things.
Of course, you have to be able to put all your eyepieces, cameras, and other image plane devices at the image plane with room to spare on both sides.
You want the minimum possible secondary mirror center obstruction diameter, which means you want the image plane as close as possible to the telescope centerline so that the cone of light required to illuminate the image just barely fits into the secondary. This is slightly complicated by the problem that you don’t actually want the secondary centered on the axis of the eyepiece, because the cone of light’s intersection with the secondary mirror’s surface extends further toward the primary than toward the sky, because you’re slicing that cone at a 45° angle. In longer Newtonians this is often safe to ignore but if “Dobsonian” implies a fast f/ it’s more of a problem.
You also don’t want the mirror end of the focuser to vignette the converging cone of light. It is at the extended end of its travel range that the focuser can most easily cause this problem.
You also certainly want to avoid having any part of the eyepiece or focuser extend into the main optical path. Bear in mind that if you want to cover, say, 2° at low power, the path of light hits the periphery of the primary and gets wider at a 2° radial angle (it’s a 4° cone). If you want to be more particular about it, recognize that at the low power extreme your exit pupil may be limiting your path, rather than your primary, so there is a low power regime in which the exit pupil limited path is the one you don’t want to penetrate, and a high power regime in which it’s the primary diameter limited path. At high power the cone angle for the main path is tiny. Finally, consider that an exit pupil limited path is going to be wiggling around because your head isn’t perfectly still. Having said all this about paths, it doesn’t matter if these things extend into the tube per se, because your tube is (or better be) outside your intended optical path. Your 1 mm distance from the tube OD to the image plane sounds small to me, but then we don’t know how much extra radius you have in the tube.
