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Old 18-01-2015, 05:31 PM
glend (Glen)
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Pier Tuners - Advice Needed

I have just assembled all of my pier adaptor components and mounted the NEQ6 on the pier for the first time, the idea being that I needed it all mounted so I can work out the wall height for the observatory.

The pier alone exhibits no vibration no matter how hard I slap it at the top of the pier. However, the higher the components get the more vibration enters the assembly. If I tap the dovetail mount point at the top of the NEQ6 I get a high frequency vibration but the amplitude seems very small. In comparison the same mount on the tripod when tapped with the same force, exhibits less vibration frequency but greater amplitude but damps quickly compared to the pier. The pier seems to resonate.

I had the tripod and pier side by side (tripod on the ground of course), and moved the mount head back and forth to try and work out why the pier seemed worse than the stock mount. Or perhaps its just different to what I am used to with the tripod, but I expected very little if any, certainly less.

The pier has three hold down bolts cast in the column, and the pier plates are 10mm aluminium plate sitting on 4 x 12mm bolts. The top pier plate is backed by a 100x100 x 10mm plate washer where the mount bolts to the adaptor.

As a mount pier adaptor I used the mount adaptor from the top of my Skywatcher Pier Extension as shown in this previous thread:

http://practicalastronomy.com/fittin...elescope-pier/

I suspect that this Skywatcher tripod head adaptor might be part of the problem, simply because it has much less load bearing surface area than a flat machined puck would. I did sand flat the bottom of the Skywatcher adaptor and it sits flush on the top plate (and is backed by that big plate section as a washer underneath the top plate) but it concerns me. Is anyone else using this method?

So any advice from the pier experts? Once I get up onto the mount section I start to get vibration (when slapped) and the higher up the worse it seems. No apparent vibration below the pier plates when the same force is used.
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Last edited by glend; 18-01-2015 at 05:49 PM.
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  #2  
Old 18-01-2015, 06:25 PM
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Bassnut (Fred)
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It all looks very solid, you may just have struck resonance by accident. Lower the top plate on the 4 bolts till they nearly touch the pier nuts.
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Old 19-01-2015, 10:11 AM
glend (Glen)
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Thanks for the replies, and especially the detailed PM from Rally. After much more reading, including the excellent debate on Pier Rat Cages started by Piota and Brendan in 2012, I have decided to redesign the rat cage and make it very short or eliminating it altogether.
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Old 19-01-2015, 11:23 AM
julianh72 (Julian)
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Quote:
Originally Posted by Bassnut View Post
It all looks very solid, you may just have struck resonance by accident. Lower the top plate on the 4 bolts till they nearly touch the pier nuts.
Definitely do this!

The four bolts will allow a torsional mode (twisting about the vertical axis), and a side-sway mode in each of the two orthogonal directions. The two side-sway modes will have effectively the same stiffness and frequency as each other, while the torsional mode frequency will be probably be a bit lower. If you can halve the free length of the bolts, this will dramatically increase the stiffness and reduce the vibration amplitudes (while increasing the vibration frequencies) of this part of the mount.
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Old 20-01-2015, 03:50 PM
glend (Glen)
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After trying a couple of other approaches I decided to get rid of the rat cage altogether. As I have three M12 bolts rising out of the pier I have gone with a single plate leveled and secured to the three pier bolts (120 degree apart). The mount centre bolt passes through a centre hole in the base plateand through the top section of the Skywatcher Extension into the mount. It is very strong now, and any vibration caused by a tap on the top of the mount has pretty much disappeared. It is a bit more awkward to setup as there is not a lot of room under the base plate, but attaching the base plate to the mount and then installing that entire assembly seems to be the way to go. Photos attached.
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Old 20-01-2015, 06:02 PM
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troypiggo (Troy)
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I didn't see the PM from Rally (obviously), but if he pointed to that rat cage, I'd agree with him. They create a "soft storey" of sorts. Glad you got rid of it.

I'm interested in what's going on under that bottom plate now. I'd grout that up with some non-shrink cementitious grout for even more rigidity.
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