[troypiggo (Troy)]

Originally Posted by rally

Troy,

You might like to add extra in for load imbalances and cable drag.

Absolutely. I've been saying all along that there are many variables that affect the outcome of this, and it's just not worth the effort of going into detailed calcs. What I was doing above was directly responding to Bojan's model for the sake of it. It was a quick and simple model to post.

Originally Posted by rally

As the mount swings through meridian the cables and any other eccentric load (side mounted) guide cam, finder scope, etc) can change by double.
You cant balance these out completely except with a very complicated set of eccentric counterweights - which are not practically used.

When the camera rotates 180° after a meridian flip a similar thing can happen.
If you are using a large camera, say with 8 filter wheel and depending on your camera angle (the eccentric load may be pointing out or in toward the pier and its cables protruding yet further out or in) this eccentric imbalance can end up being quite a lot more than your wind induced loads - I would estimate by a factor of maybe 100 or more based on my own experiences.

I'm not denying that any of those actions occur. However, this thread is about the pier design. My thoughts on this are:

Wouldn't your mount normally be pretty well balanced about both axes when in a static position? If not, you'd be stressing your mount quite a bit.

If your balance is out by enough to affect the pier, I'd seriously be concerned about your mount. In your mount is one of those massive ones like a Paramount ME with huge capacity, we wouldn't be talking about such small piers and again the pier wouldn't be an issue. It's all relative.

I think the actions you are talking about are perhaps momentum of the moving mount? If all of your gear was reasonably close to perfect balance, there should be negligible lateral loads on the pier. The reaction at the connection of the mount/pier should be practically nothing in shear. There may be some torsion, but that is not a lateral load.

I'd expect there'd be far more movement/slop in the mount than there would be in the pier. By the time the mount settles down, the pier would well and truly be static.

And if you're talking about side by side setups versus piggy-back setups, you need to consider deflection in the plates and all connections too. It's all additive.

And you wouldn't be imaging in those milliseconds/seconds after a bit slew anyway, would you?

Originally Posted by rally

Depending on where the scope is pointing that imbalance differential can be a little (say at zenith) or a lot (say lower on the horizon) - so it is very dynamic.

I am not sure where you calculated the position of the wind load, but depending on the OTA, it could be well in excess of 1m above the top of the pier and I suspect that will increase the deflection in your calcs.
But I agree 25kmh winds mean the end of any imaging session !

As I said above, I was simply addressing Bojan's numbers directly. If the centroid of whatever is catching wind is higher, the deflections will indeed increase by the difference of the square of the heights.

Originally Posted by rally

750mm height for a pier may be OK for some piers with small scopes where the floor level is also the pier footing level, and low walls, but for most amateur observatories constructed of wood above the ground, and for larger scopes the pier needs to be taller by maybe 300mm to 1000mm to get above the obs floor and maybe another 500mm to provide clearance for the scope and wall clearance to get a reasonable view (say 30-35°) above the horizon.
(much depends on the particular Obs design)

Since the deflection is proportional to the square of the pier length (height) this will affect things considerably, Amateurs need to know that they cant just scale it linearly.
Double the length and the deflection increases 400%, triple it and its up by 900%.

So I would argue that the 8" Nominal Bore pipe (8.625") you used 219mm OD with 6.35 wall thickness may not be sufficient for many situations.

If you look at that CN link we keep referring back to, he correctly provides different pipe dimensions for different heights, for the reasons you mention here.

The 219 diameter pipe I mentioned was quoted specifically for the example used for Bojan's model. I think I've said this many times in this thread before. Change the height, or change the wind speed, or change the area, or change any of the other factors used in the calcs, and the pier size should be adjusted accordingly.

But again, it appears to me that the CN article adequately addresses most of those issues for amateur imagers.

Originally Posted by rally

I am truly not trying to be a trouble maker !

No worries. As others are saying, it's an interesting discussion. I'm learning too.

Originally Posted by rally

But the issue is not always so simple and I am not only trying to talk theoretically here - these are all problems I have had to deal with either for myself or calculations of real life piers that have been constructed at lengths greater than 1.75m - even as much as 2.7m.

Yes Alex - TPoint is wonderful - but it doesnt understand camera rotation yet ! (aka eccentric load rotation). Only rigidty can help that one - in the OTA, adapter Mount and pier !!!

Cheers

Rally