I was excited to use PA for the first time a few nights ago and it worked really well (version 4). Tonight for some reason, it did not (version 3.2). Although I was able to get a solution that SC reported as GOOD, there was no Celestial Pole (CP) or RA axis noted in the image. I could see the concentric rings, but the center one was off of the image. That did not seem correct to me. I did not actually test the alignment since after fiddling for a while, the fog came in.
Is it possible to have a good PA without seeing the mark for the CP?
No CP during Polar Alignment
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Re: No CP during Polar Alignment
Hi,
yes, that can work fine with those two points out of view (it's just not as satisfying visually). SharpCap knows where they are and can work out when they have been lined up.
Normally the cause of this is some misalignment between the guide scope in the home position and the RA axis. It doesn't affect the calculations, but it means that the scope is pointing off to the side a bit. If you detach / reattach the guide scope then that could be it, otherwise its possible that the guide scope mount may flex. Also check that your home position is really at +90 dec (or -90 if you are SH).
cheers,
Robin
yes, that can work fine with those two points out of view (it's just not as satisfying visually). SharpCap knows where they are and can work out when they have been lined up.
Normally the cause of this is some misalignment between the guide scope in the home position and the RA axis. It doesn't affect the calculations, but it means that the scope is pointing off to the side a bit. If you detach / reattach the guide scope then that could be it, otherwise its possible that the guide scope mount may flex. Also check that your home position is really at +90 dec (or -90 if you are SH).
cheers,
Robin
Re: No CP during Polar Alignment
In my case, the view was through the main scope since I don't use a guide scope (yet). The FOV is small. Only 47 x 24 arcmin. But it seemed to work fine with version 4 (and tracking afterward was really good). Still seems counter-intuitive to me that the scope could be properly aligned when it is not pointing at the CP.
Re: No CP during Polar Alignment
The point you miss is that it’s not the scope that you are aligning to the celestial pole, but the mount’s RA axis… the scope may or may not point at the pole, depending on whether it is fully aligned with the mount’s axes
Re: No CP during Polar Alignment
Good point. However, if that is the case, then I've got another issue since the mount was in the home position. Perhaps I just got 'lucky' the first times I tried it. Upon reflection, I think it would not take much at that FOV to knock the scope's aim off-axis, so it is another thing I will adjust during the process. Thanks!
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admin
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Re: No CP during Polar Alignment
Hi,
yes, two things can stop the scope being aligned with the RA axis in the home position
1) The home position is slightly off in Dec - it won't take much with a narrow FOV to put polaris out of view
2) Cone error - that is the front of the scope pointing out away from the RA axis slightly (or in towards the axis slightly). If you have cone error then you can correct it - typically with a small shim/washer under either the front or back mounting bolt that connects your tube rings to the dovetail plate. I created a program called 'ConeSharp' to help measure cone error - https://www.sharpcap.co.uk/conesharp - it's a bit primitive, but it does the job.
cheers,
Robin
yes, two things can stop the scope being aligned with the RA axis in the home position
1) The home position is slightly off in Dec - it won't take much with a narrow FOV to put polaris out of view
2) Cone error - that is the front of the scope pointing out away from the RA axis slightly (or in towards the axis slightly). If you have cone error then you can correct it - typically with a small shim/washer under either the front or back mounting bolt that connects your tube rings to the dovetail plate. I created a program called 'ConeSharp' to help measure cone error - https://www.sharpcap.co.uk/conesharp - it's a bit primitive, but it does the job.
cheers,
Robin