Polar Alignment Accuracy

[woodbourne]

I read the Sharpcap polar alignment documentation and I can polar align with sharpcap just fine. I noted that it was clear that trying to get the number down to zero was pointless. However, I'm wondering how accurate the PA can be in the first place. I'm not questioning that it works because it does. What I've found is that SharpCap (and every other method I've ever tried) will give me around a 1 arc minute difference between runs. Is that about the level of possible accuracy that can be achieved?

[admin]

Hi,

the procedure should be reproducable to a better accuracy than that, so you may have something reducing the efficiency in your setup - a common cause of this sort of thing is if something is flexing/moving/pulling as the telescope is rotated (even the weight of a loose USB cable pulling on the back of the camera being used to polar align can have a noticeable effect). If you rotate slowly and look carefully at the on-screen plot, you will see red diagonal crosses being drawn - these are the positions of the center of rotation that are calculated at various different levels of rotation. If everything is rock solid then those will all be in the same place and align with the red vertical cross which is the final estimate of the center of rotation. If those diagonal crosses are stretched out in a line or not near the final vertical red cross then something is very likely flexing as you rotate.

It's also worth noting that if your mount isn't perfectly balanced then the weight distribution on the tripod/pier changes as it moves around the sky - that can affect the alignment as you move away from the pole if on a soft surface like grass that could give under the weight of the mount.

cheers,

Robin

[woodbourne]

I have 3 mounts at my observatory - 2 AP Mach2's and a PlaneWave L350. They all are aligned to within one minute of the pole as far as I can tell. None have even a single dangling cable. The scopes are all 3D balanced. Yet there is a variance of 1 arc minute using any method.

So, again, what is the expected accuracy of the measurement? Please.

Rgrds-Ross