RE: [ATM] Putting digital setting circles on a massive existing mount

["Robert Haler" <bhaler@lymax.com>]

Guy,

I once put digital circles on a very large mount using an external friction
drive to run the encoders. It was impractical to remove the giant shafts for
drilling - even if the proper machinery had been available. Fortunately, the
clutches on each shaft included well-machined collars 5 to 6" in diameter
(I've forgotten the exact size). The encoders were mounted on short
spring-loaded arms. Each arm carried a pressed-in bearing and a short shaft
ending in an aluminum disk. One end of the shaft was coupled to the encoder
and the aluminum disk was wrapped with fine grit waterproof sandpaper to
ensure against slippage. As the mount rotated on either axis, the aluminum
drive disks would ride against the clutch collars and accurately rotate the
encoders. Obviously, this system will only work if you have access to some
part of the shafts or some concentric part mounted to the shafts. A few
points to keep in mind if you build a friction drive:

1. The encoders can't tolerate side load. A separate shaft must be used to
withstand the substantial pressure needed for accurate readings. Both the
load shaft and the arm pivot passed through press-fit bearings in the swing
arm.

2. The surface of the clutch collars were anodized and rather slick. I used
plastic backed waterproof sandpaper glued to the drive disks. They sell this
stuff at hobby shops for model making in very fine grits - 400 and finer. I
used 400. The sandpaper was glued to the disks with an extremely thin layer
of gel superglue - the kind that requires primer. I put the glue on the
wheels and rolled them against the back side of the sandpaper. A helical cut
was then made to trim away the excess - leaving a perfectly smooth surface.
The temptation is to skip all of this and use small diamond coated grit
wheels - but the surface must be smooth and regular or you will get encoder
errors. 

3. The various available setting circle boxes have a maximum number of
"ticks per rev" they can handle. If I recall correctly, it's 32,000 on the
Tangent boxes. You must make sure that the ratio you get between the drive
surface and the drive wheel does not exceed this number.

4. The entire assembly must be reasonably square. Any tilt angle between the
mount shafts and encoder drive shafts will introduce errors.  

    
If you don't have access to a round concentric part, you can bridge the face
of the mechanical setting circles (like DEC circles on an SCT) or drive
gears with an aluminum bar mounted on a couple of standoffs with a hole for
the encoder drilled in the middle. As long as the encoder shaft is held over
the center of the mount shaft and is square with it, it will work. I guess
worst case, you could use some of that PC-7 epoxy putty to bond a coupler to
the center of a shaft or a setting circle. The only requirements are rigid
coupling, square mounting, and no side-load on the encoders. I've mounted a
lot of encoders on some fairly strange mounts and haven't encountered one I
couldn't skin (although, I will admit that some solutions were not
aesthetically pleasing).  

Hope this gives you a few ideas.

Bob
 

   



-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Guy Brandenburg
Sent: Saturday, April 23, 2005 10:16 AM
To: atm@atmlist.net
Subject: [ATM] Putting digital setting circles on a massive existing mount

I know of a 14" Schmidt-Cassegrain telescope on a very
large, old Ealing mount. It has simple analog setting
circles. I have an interest in  installing digital
setting circles on it, so it is no longer necessary to
star hop. Because of the way the mount was built, it
is not possible to 'reach' the axes of the mount to
install typical encoders. I was wondering whether it
might be possible to install encoders on the OUTSIDE
of the telescope mount... Any thoughts? 

Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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