From wpk@saao.ac.zaFri Jul 17 22:41:02 1998
Date: Wed, 15 Apr 1998 11:44:37 +0200 (GMT+0200)
From: Willie Koorts <wpk@saao.ac.za>
To: Anthony Davoli <davoli@localnet.com>
Subject: Re: gearing help


On Sun, 12 Apr 1998, Anthony Davoli wrote:

> My name is Anthony and I am constructing a split-ring 18"telescope very
> similar to the one you have designed.  The design of the telescope is
> going very well.  The problem I am having is understanding gearing for
> the telescope drive.  I am using Sky Probe 1000 positioning system to
> computerize both drives.  What I was wondering is with a 36"ring and a
> 1-2" drive wheel (roller blade wheels) what additional gearing should I
> have to move the 18" telescope.  If you can help in any way it would be
> greatly appreciated.


Hi Anthony 

Nice to hear from someone who finds my site useful.

I will answer you with my telescope's dimensions and the problems I found.

Splitring diameter            : 893mm  (35.16")
Roller-scate wheel diameter   : 54.6mm  (2.15")
Additional worm gearbox ratio : 60:1  (ie. between stepper motor & r/s-wheel)

I found that even with this overall ratio of 982:1 (893/54.6 x 60) one can
still see the telescope "jerking" at meduim magnification in certian parts of
the sky (most noticable near the equator).  This is quite OK for visual use,
but would be a problem if used for photography.  I then read an article
somewhere which said that the criteria here is the angle the telescope
moves/step. 

I half-step my 200steps/rev motor giving 400steps/rev.  This translates into
a step angle of 3.3 arc seconds/(half)step ((360 x 3600)/(982 x 400)). I
cannot remember the exact number quoted in this mentioned article but recall
that I still had to reduce mine by at least half (giving about 1 arc
sec/step) to be used photographically. 

The above mentioned values are for tracking mode.  To work out slew (fast
motion) speeds, you would need to know the maximum motor speeds possible from
your drive.  Here steppers are not ideal since they cannot be driven very
fast.  Having a good enough gear ratio for tracking, makes slewing so much
slower again.  The max. slew speed I could get out of my system was about 80
times track speed which translates into 20degrees/minute. If I have to cut
this in half to track better it will take even longer to move between
objects.  Since I have built a slip clutch into my drive wheel, I merely slew
by hand but with a computerised system where steps are counted to know the
telescope position, you cannot have any slippage - unless you have shaft
encoders. 

There are tricks like running the motors above their rated voltage, etc.  to
gain some speed, but I have not played with these yet.  I would recommend
Mel Bartels's home page as well as some of the other ATM links on my
ATM-links page for more info.  Mel would also be a good one to ask your
questions to. 

Hope this helps.

Best regards and let me know how you get on.

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 -
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