From wpk@saao.ac.zaFri Jul 17 22:46:12 1998
Date: Fri, 22 May 1998 12:14:32 +0200 (GMT+0200)
From: Willie Koorts <wpk@saao.ac.za>
To: Callum Potter <CallumP@selectst.com>
Subject: RE: split ring equatorial mount


On Fri, 22 May 1998, Callum Potter wrote:

> Thanks Willie,
> 
> your emails, these have been really useful.

Glad to be able to help.

> I was trying to keep the size of my ring down, my mirror box is about
> 14" square (for a 10" mirror") so i need a split width of about 14.5" or
> 15". I was a bit worried about the width from the end of the split to
> the ring. I think i should be a lot more generous in my calculations (in
> fact i was trying to cut a cost corner, as i have some suitable wood,
> but probably not wide enough for the size of ring i need, looks like
> another sheet of 3/4" ply on its way!).

I just realised that the 4" I quoted as the distance from the spilt's end to
the ring is brobably more like 3" - if I remember, I will check over the
weekend.  I just made mine this size because (like you) that was the size of
wood I had avaliable. 

I probably also went a bit overboard with not wanting to add a metal ring
which would give it more strength - I then had great ideas of doing
astro-photography but before I can do that I will have to fix a more serious
problem - the drive chain is not geared down enough and the steps from the
stepping motor is visible in a high power eyepiece when looking near the
equator and the ratio needs to be reduced more - this is OK for visual work. 

You probably also realise that the telescope does not need to be at the 
exact centre of the ring - mine is actually a bit lower than the centre 
(that's probably why I needed a fair amount of weight to balance it). If 
you put it slightly higher than centre, you could get away with a smaller 
ring and still have enough material left between the end of the split and 
the edge of the ring. If you can somehow work out how to accomplish 
getting the exact position above the centre to get the balance right at 
the same time, you would not need any counterweights at the top of the 
ring!

Either way, I would recommend you also make the telescope a good fit into the
split and provide teflon pads pushing on the sides of the DEC rings - this
just gives confidence that the ring is not forced out of round. 

> Clear skies, and congratulations on the cricket yesterday.

Thanks,  somehow our team just battle to bring it off in the finals.

Cheers
Willie

                       Mr. W.P. Koorts ( wpk@saao.ac.za )    
                     South African Astronomical Observatory 
                   PO Box 9   Observatory  7935  South Africa
                 Tel. (27) (21) 47 0025  Fax. (27) (21) 47 3639
               World   Wide   Web  ( Work ) http://www.saao.ac.za
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