Re: PA Error what is acceptable for 530mm FL & 2.1arc sec/pix

Ray Gralak


I was trying to make the point that even permanent piers can shift, especially if placed in soil with any amount of clay. For example, it's common here in California, where clay soil is abundant, that as the clay gains or loses water the clay expands or contracts. This can happen very quickly. Because of this, cracks can form in sidewalks and exterior (and interior) walls in homes. Clay soil can even unlevel homes, so a pier can be easily affected. So despite your pier being solid, the ground around it might shift and move the pier. I hope that makes sense?

That said, it could be that the mount's polar alignment has shifted for another reason (e.g. alignment changed when the mount bolts were tightened).

BTW, did you perform the Plate Solve and Recal in APPM? If so, did it help? If not, something physical in your setup may have changed. If so, you could do another APPM run and compare pointing terms to see what has changed.


-----Original Message-----
From: [] On Behalf Of Shailesh Trivedi
Sent: Tuesday, September 28, 2021 8:25 AM
Subject: Re: [ap-gto] PA Error what is acceptable for 530mm FL & 2.1arc sec/pix

Thanks Ray.

If the ground shifts in the matter of a few days or weeks, my setup is no good. I have used it with longer FL
eg 1200mm and other scopes even up to 1650mm FL for many years with no issues in models, though not
APPM model but Tpoint on another Bisque mount that I no longer have, so I am down to AP1100 with AE (I
am glad have the AP1100 with user installed AE). In summary, the concrete is not cracked, and to the best of
my prior data points noted, the 10inch steel pier I have is also rigid on rebar reinforced concrete.

I will redo a better PA (seeing and clouds permitting) and then redo the model just to be sure.



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