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


Shailesh Trivedi
 

Hi,

I ran a 290 point APPM model last Friday and am having trouble centering to a target (I am off by several degrees). Ray suggested "Plate Solve and Recal" in APPM, but I noticed something odd in looking at the Model numbers. The polar elevation is -600 something arc seconds and the polar azimuth is -300 something, see attached.

I know that APPM is expected to correct for PA errors, but for a 530mm FL and image scale of 2.1 arc sec per pixel, is this error too much?

I prefer to dial it down to less than 10 arc sec, but the night I did my PA with PEMPRO, I was chasing the seeing in suburban Sacramento.



Please shed light. thanks.

Shailesh


Ray Gralak
 

Hi Shailesh,

I ran a 290 point APPM model last Friday and am having trouble centering to a target (I am off by several
degrees). Ray suggested "Plate Solve and Recal" in APPM, but I noticed something odd in looking at the
Model numbers. The polar elevation is -600 something arc seconds and the polar azimuth is -300 something,
see attached.
Provided polar alignment has not shifted, the model will compensate for polar alignment errors in both pointing and tracking.

I said "provided" because some people have noticed that settling in the ground can cause measurable changes in polar alignment.

-Ray

-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Shailesh Trivedi
Sent: Monday, September 27, 2021 8:27 AM
To: main@ap-gto.groups.io
Subject: [ap-gto] PA Error what is acceptable for 530mm FL & 2.1arc sec/pix

Hi,

I ran a 290 point APPM model last Friday and am having trouble centering to a target (I am off by several
degrees). Ray suggested "Plate Solve and Recal" in APPM, but I noticed something odd in looking at the
Model numbers. The polar elevation is -600 something arc seconds and the polar azimuth is -300 something,
see attached.

I know that APPM is expected to correct for PA errors, but for a 530mm FL and image scale of 2.1 arc sec
per pixel, is this error too much?

I prefer to dial it down to less than 10 arc sec, but the night I did my PA with PEMPRO, I was chasing the
seeing in suburban Sacramento.



Please shed light. thanks.

Shailesh


Shailesh Trivedi
 

Thank you Ray. I have a permanent pier in concrete that has been set up since 2013, so I do not believe it shifts, especially since I have constructed it with steel Rebars in concrete.

Shailesh


Ray Gralak
 

Thank you Ray. I have a permanent pier in concrete that has been set up since 2013, so I do not believe it
shifts, especially since I have constructed it with steel Rebars in concrete.
There are two types of concrete: concrete that is cracked, and concrete that hasn't yet cracked. :-)

Yes, the ground can shift in a permanent setup like yours. Just not as quickly.

-Ray

-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Shailesh Trivedi
Sent: Tuesday, September 28, 2021 6:37 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] PA Error what is acceptable for 530mm FL & 2.1arc sec/pix

Thank you Ray. I have a permanent pier in concrete that has been set up since 2013, so I do not believe it
shifts, especially since I have constructed it with steel Rebars in concrete.

Shailesh


Shailesh Trivedi
 

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.

Thanks,

Shailesh





Ray Gralak
 

Shailesh,

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.

-Ray

-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Shailesh Trivedi
Sent: Tuesday, September 28, 2021 8:25 AM
To: main@ap-gto.groups.io
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.

Thanks,

Shailesh






Shailesh Trivedi
 

Ray,

It has been cloudy and rainy in Sacramento last few days (yeah we need the rain in California). I will first try the Plate Solve and Recal before proceeding further. 

Thanks,

Shailesh