Dob base Alt housing

[dannat]

I downloaded some Dob Base plans from SF sidewalk astronomers & the Alt-bearing holding wooden section of the base was not circular - but triangular??

i was wondering how well this would work - would make my cutting a whole lot easier

does anyone have a triangular part on their dob base? or have seen it

[Wavytone]

OK scratch that - photo of the design please showing the piece you're asking about.

The SF Sidewalk Astronomers plan I remember was a bunch of rectangles cut from a sheet of chipboard, no triangles...

[dannat]

i wasn't talking about the base plate but the side support which holds the OTA, the pictures of the design had triangular supports for the tube, not circular ones

[astroron]

Picture or website please:)

[nightstalker]

It will work fine Dan.. Your dob alt bearing will run smoothly on the teflon points you mount it on .. dosn't matter if its a triangle or just suspended in space .. if those four small points of contact are all equal and spaced accordingly (angle wise) through your alt axis it can't not work.


The SWA site is great in that the info is structured around limited tools and experiance in regard to making your own scope .. triangles
(Straight edges ) are easy to cut , the mounting points are no differant to a nice curve cut with a router, dosn't look as pretty ..just easier to do if you have a hand saw and a little determination :)

[mercedes_sl1970]

As Nightstalker says, should work fine.

Andrew

[dannat]

thanks.. here's a pic - i like the idea of 2 pieces, one stuck inside the other

[Wavytone]

Aah OK...

You mean the "cradle boards" in
http://pages.sbcglobal.net/raycash/manualv2.pdf

This is a very old design for dobs.

The point of the cradle boards is that they allow some adjustment to square up the alt axis so the scope will swing cleanly in the rocker box without scraping. In an era when using hand tools only and fairly rough and inaccurate assembly was common, so the rocker box might be distinctly asymmetric, some means of adjustment is necessary.

If, on the other hand, you know how to make the rocker box parts accurately and identical to better than 1mm, AND ensure that when you assemble it, the rocker box does not have a twist in it, there is no need for adjustment. Hence when you look at modern dobs, they do not have these parts and instead the alt bearings are set in a circular arc cut straight in the top of the sides of the rocker box. To do this well you will need a decent router, and be reasonably skilled in its use.

BTW the SWA design is the original dob design. The resulting mount is quite heavy compared to what can be achieved if you have a router table and use birch ply (or aircraft ply) which is much stiffer.