alt/az stability

Just wondering; does it make any real difference if the OTA rotates 360 degrees around at the top of mount or if the mount as a whole rotates?
I'm thinking of making a mount with pillow bearings mounted on plates with the lazy susan bearings found in LightBridge dob mounts. I just don't know if the plates and bearings should be high at the top or at the bottom of the mount. The Max mount would sulely rotate at the top of a pier and most dob designs I've seen rotate at their bottom.

Thanks ro any input.

Scott

The Max mount would sulely rotate at the top of a pier and most dob designs I've seen rotate at their bottom.


That's it exactly.
It doesn't matter if it's top or bottom. Just that there is stability in the direction of rotation, when the scope is not rotating.
This is where I've had trouble with my homemade mounts. The mount, in the plain of rotation, must be stiff.
In a dob, the base must be stiff, the rocker box stiff in rotation, and the altitude bearings must be stiff enough not to flex in rotation either.
For a tripod, it too must not move in the plain of rotation. Only that part of the mount that is attached to the scope should move. The altitude part of the scope mount on top of the tripod must also not allow any flex.

Scott Anderson said:

Just wondering; does it make any real difference if the OTA rotates 360 degrees around at the top of mount or if the mount as a whole rotates?
I'm thinking of making a mount with pillow bearings mounted on plates with the lazy susan bearings found in LightBridge dob mounts. I just don't know if the plates and bearings should be high at the top or at the bottom of the mount. The Max mount would sulely rotate at the top of a pier and most dob designs I've seen rotate at their bottom.

Thanks ro any input.

Scott

Scott: The stability of any mechanical structure is not so simple to determine, there are a number of factors involved and the mount's stability and freedom from vibration depends how they all interact.

Dob mounts are stable because they sit on the ground, the distances are small between the supporting surface (the ground) and Telescope itself. When you start raising the mount off the ground, the lever arms become longer which makes it more difficult it is to make the mount rigid.

Consider your two choices:

1) A stiff tripod or pier with your lazy-susan bearings near the top.

2) The lazy-susan bearings at the bottom with a stiff tripod or pier on top of it.

If the bearings are at the bottom, the pier/tripod acts as a lever arm (moment arm) which magnifies any play/slop or compliance (opposite of rigidity) of the bearings. The forces on the bearings are greater, again because the lever arm is longer. So, clearly the bearings belong as close to the telescope as possible. A solid mount begins at the bottom...

Another factor is that if the bearings are at the bottom, they have to carry not only the telescope but the tripod/pier... they need to be larger.

Personally I don't know how effective the lazy-susan dob bearings are. Certainly the entry level scopes use them but the premium scopes all still use Ebony Star/Teflon bearings. Lazy Susan bearings are rather cheezy, using thin support plates and very small bearing surfaces, this in not the ideal combination for a stiff, stable mount, they probably exhibit some flex.

Placing a large Lazy Susan bearing at the top will probably create some awkwardness, it's gonna be a big thing that will get in the way of using the scope. Also, such bearings are not designed for "off-axis" loads, if you look at a DOB, all the the loads are vertical and centered right on the base bearing, the scope should not be off to one side.

That's my thinking.

Jon