How can I make my telescope pier more rigid?

[Mike S]

SUMMARY:

I had a pier fabricated for my telescope but the final product is not rigid.  A small amount of force or a bump will cause excessive movement of the telescope - think "upside-down pendulum".  This is BAD FOR ASTROPHOTOGRAPHY ☹️.  I need to find a way to make the pier more rigid.  I realize that I can start over with a redesign, and it may come to that,  but I want to see if I can save the existing pier as I already invested in its fabrication.

DETAILS:

The pier consists of a 1/4" steel plate, 10" diameter, bolted to a concrete pad. A 3" by 34" square steel tube is welded to the base plate and the mount head is welded to the tube. The mount head consists of two 1/4" steel plates, 8" diameter, separated by three 2" square tubes, 6" long. I have since added an equatorial wedge to the top of the mount. This wedge converts the telescope mount to an equatorial mount. But it added another 12" to the height of the telescope, which has exacerbated the unwanted telescope movements. The telescope assembly that sits on top of the pier weighs about 70 lbs.

I have since found (yes, I wish I found this before I had the pier fabricated) some guidance that says I should have used a "thick" base plate and a 1/4" steel tube 8" in diameter.  https://www.cloudynights.com/articles/cat/articles/how-to/pier-design-fundamentals-r1236

 

MY QUESTION:

As I stated I am looking for ways to increase the rigidity of the original 3" pier post. 

Would bolting four 3" square tubes to an existing 3" square tube (one per side) increase rigidity? I would need to bolt vs weld because the extra tubes would cover over the bolts that mount the whole pier to the concrete base. If I weld them I can no longer remove the pier, if needed.

Does anyone have another idea that would stiffen the pier?  I would appreciate any advice you may have.

Below is an image of the original design and a photograph of the current setup: 

[G B Reid MIMechE, SIMarEST]

The problem you are experiencing is a combination of a number of relatively small things being magnified - not so ironically - by the telescope itself. 

From the bottom up:

1. You have the stretch in the bolts - this can be reduced by pretensioning these as tight as the bolt diameter will allow. Effectively this is the effect of an adult standing behind a door and a child trying to push the door open...until the child matches or exceeds the force with which the door is being held closed by the adult, there will be no movement....in this analogy, you are increasing the weight of the adult behind the door.

2. the flex of the upright. Contrary to popular believe, everything bends....the effect of the upright of the unit is effectively the same as a diving board....a stiff one alright, but a diving board none-the-less. the simplest way of increasing the stiffness of this section is to add a few additional supports going from the base to the topmost mounting plate...the more the better as they will increase the "second moment of inertia" of the beam an result in its stiffening. 

3. The columns on the top are - alas - not overly rigid, but the #2 should resolve this, alternatively place a couple of flat plates between them to eradicate any lateral movement.

4. Optical parallax...alas, there's not much you can do about this one other than, perhaps, look at the streetlamps instead of the cosmos as the distances are closer and the result of any imposed vibration significantly lower.

For reference, were I to design such a stand, my baseplate would be many times larger to spread the load and the structure supporting the unit would be a lattice work to increase the rigidity...at very least consisting of three widely spaced columns with connections between...with this application it is not about the mounting weight, but the rigidity of the mount that is the critical factor.

Hope this helps

Bruce

[Mike S]

Thank you for the suggestions!   

One thought I had prior to my post was to use something equivalent to telescoping jacks between the concrete base, which is below the base plate,  and the bottom of lower plate on the top assembly.   I was originally hoping to be able to bolt in additional support and avoid having to take everything apart to have components welded in. Not sure if that would be enough.

I can try what you suggest, going from the base plate all the way to the top plate but my concern is I need to leave sufficient room to get in between the 6" posts - as the primary knob to lock down the telescope mount to the pier is under there.  I can do three 2" square tubes that overlap the existing 2" tubes.  I suppose I can fit six 1" square tubes around the assembly if I replace the knob underneath with just a bolt head, and use a wrench instead of my hand to tighten the telescope mounting bolt.

I can also try adding a few flat bars between the 2" tubes in the upper assembly.  Again space is a concern but the bolt and wrench idea (mentioned above) should remove the need to get my hand in there.

 

[DrD]

GB Reid has given some very good advice. The question that remains is simply, is it worth doing? Will the modified support be stiff enough, or should you simply scrap that which you have and start again. I'd advise looking seriously at that question before investing much more time/money in this support.

DrD