Re: Potential Issue: Mount Shifting on Pier After Setup




Something that immediately catches my attention is that you say that you polar align before putting the scope on the mount and balancing.   This is not the recommended procedure.    You always want to have everything balanced and THEN polar align.   This is especially true if you are setting up on grass or dirt, as both are very soft and spongy and will change the positioning.


The 5/16” screws that secure the mount can be tightened without fear of stripping the threads.   If you want, you can use longer screws to feel more comfortable that cranking down the screws with firm tightening will not possibly strip the threads, just be sure that the hex key is fully inserted and that it is not worn.  


If the holes in your pier have become oblong (you mention that you had to modify your pier), then it is possible to use a Flat Pier Plate (M9010-A) and Permanently affix it to your pier with massive torque and lock washers.   You can then attach a Flat Surface Adapter (119FSA) and you will have a new pier top that is fully CNC machined.  






George Whitney

Astro-Physics, Inc.

Phone:  815-282-1513

Email:  george@...


From: ap-gto@... [mailto:ap-gto@...]
Sent: Tuesday, November 06, 2018 6:57 PM
To: ap-gto@...
Subject: [ap-gto] Potential Issue: Mount Shifting on Pier After Setup



I want to make the group aware of a potential issue with the mount shifting on the base of the pier for those observers who set up and take down their mounts for every observing session. For additional background information, please refer to my post (#76952) on the AP User Group on August 7. Shortly after I started using my new 1100GTO mount I noticed a significant wobble when I placed the mount base on the pier, and I discovered that the top of my 1992 vintage pier was not flat. I was able to correct this by hand scraping the top of the pier until the mount base was making even contact all around. 


I thought that this would have cured any potential mount shifting issues until after a recent observing session I noticed that the base of the mount had shifted upwards on the south side. When I diagnosed how this could have happened the following factors came into play. Refer to the pictures in the link which I took of my mount and scope setup from my indoor observing site. 


The first picture shows my 1100GTO mount initial setup. This is probably what everyone who sets up their mount does before attaching the scope. The mount is assembled, and most likely polar aligned (I use the RAPAS), and the counterweights are attached in the positions that will balance the scope. In this case I am going to be attaching my 180 EDT. You will notice that with the counterweights attached, the center of gravity is well to the north side of the pier. This is creating a significant moment arm force that wants to try to pull the mount off the base on the south side of the pier. Only the mounting bolts are holding it in place. 


The next two pictures show the mount base as it looks when it is flush with the top of the pier, and when it has been lifted off the base. Due to the clearance between the mounting bolt holes in the pier and the 5/16" bolts, the base has shifted about .050". On my 8-inch pier, this is a shift of about 21.5 arc minutes. I am not sure how significant this is when it comes to guiding, but I would not want to have this shift happen after I have polar aligned my mount. 


What are the takeaway points from this ? 


Always make sure that the bolts that attach the mount base to the pier are sufficiently tight - but not too tight. You do not want to strip out the threads in the aluminum mount base. 

In my August 7 post, someone suggested that I should use toothed lock washers under the bolts. After this incident I exchanged the original flat washers that came with my mount with toothed lock washers, and these have made a big difference. 

Always check to make sure that the mount base has not shifted after everything has been attached. 


Finally, I would like to go back to the suggestion that I made in my August 7 post that the mounts should really have something to firmly fix the mount base to the pier in the same location every time it is set up. The bolts are simply not accurate enough. The best way I can think of to do this would be to use removable dowel pins in addition to the bolts. Replacing three of the mounting bolts with dowel pins would be ideal.





Here is the link to the pictures:

Join to automatically receive all group messages.