From wpk@saao.ac.zaFri Jul 17 22:45:16 1998 Date: Thu, 21 May 1998 14:09:31 +0200 (GMT+0200) From: Willie Koorts To: Callum Potter Subject: Re: split ring equatorial mount On Wed, 20 May 1998, Callum Potter wrote: > just had a look at your split ring equatorial mount web pages - really > nice. Hi Callum, thanks for the nice words and also for the questions - it is always good to know what others wants to know to improve the site. I had a similar question before and am planning to expand the page by adding dimesions and the details of drive chain with ratios, drive rates, etc. Your questions: ~~~~~~~~~~~~~~ > 1. What diameter is the ring and how big is the split ? Ring diameter = 893mm - 35.16", split width is such to accomodate the telescope box which is 324mm (12.75") wide with about 6mm (1/4") clearance on both sides, giving an overall width of about 336mm (13.25"). Where the DEC rings go, a wider cutout was made to fit. The length of the split takes it to within (from memory) about 4" from the ring's edge. Although this seems thin, it does not gets deformed under the telescope's weight because the telescope box is a good fit inside the split cutout. > 2. How thick is the ring ? I used two layers of 1/2" ply giving 1" (25mm) thickness > 3. How did you make the ring accurately round ? The ring was first cut out roughly using a bandsaw. Before the "split" (nice word Callum) was cut out, I drilled a hole in the ring's centre to fit a nail which was hammered close to the one end of a wooden workbench. A drilling machine loaded with a sanding disk was clamped, just touching the ring's outer edge (using G-clamps, etc.). The ring was then rotated by hand sanding the edge down till smooth - adjusting the drilling machine closer as needed. > 4. Is the ring edged with anything, like aluminium strip ? (from the > photo it looks like bare wood ?). Correct, just the bare wood. I thought of edging it with a metal strip but thought this would defeat the object of having it so close to round using the sanding method. I consider still sealing the edge with a sanding sealer or similar to keep out the damp. I don't know what this will do to the grip of the drive wheel - probably OK since the tel's weight helps here. > 5. At the top of the ring there are a couple of round objects either > side of the split - are these counterweights ? or what ? Right again. The way I determined the weight needed was to point the telescope to make a 90 degree angle with the plane of the ring, ie pointing at the pole (DEC -90 for the southern hemisphere, +90 in the north) with the drive roller disengaged. If one now rotates the ring (RA movement), the ring would tend to return with the non-split side down having more weight. By adding weights to the ring's top until you can make it stop anywhere without it wanting to return to the bottom, it is balanced. The lead used for the above was then moulded onto round disks and screwed on. > 6. How rigid is the ring assembly after bolting it together ? Surprisingly rigid. I think a lot of the strength comes from the 45 degree bracket which works in a sort of "trying to pick yourself up on your bootlaces" priciple - as the telescope's weight is trying to bend the bracket thus wanting to pull the ring assembly closed, the telescope weight tends to try and open it up. Hope this makes sence! > Thanks for your patience. It's a pleasure, thanks for the questions and feel free to ask more. Happy building Regards Willie Willie Koorts wpk@saao.ac.za Cape Town, Observatory 33d 56' 03"S 18d 28' 36"E GMT + 2h Wellington, South Africa 33d 38' 56"S 19d 00' 52"E GMT + 2h For - Amateur Astronomy - Telescope Making - Satellite Tracking - Visit .... http://www.saao.ac.za/~wpk/