Polar alignment methods - experiments

Harmonic Geared Mount
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cwoodhou
Posts: 30
Joined: Fri Mar 26, 2021 9:27 pm

Polar alignment methods - experiments

Post by cwoodhou »

There are probably more polar alignment tools out there than anything else. I spent an entire night trying out different techniques. These are my conclusions so far and may be useful to others.

My mount has a sinusoidal PE of ±21", very similar to that of my Rainbow RST135, just a little smoother. I know from earlier experiments that this high PE causes problems with any method that uses modeling to work out PA. That include any drift methods that are not exclusively evaluated in DEC.

I found the best (and quickest) methods used the region around the celestial pole, (not always possible for some installations). Reason for this is that those methods that measure things rotating around the CP are unaffected by RA errors.
Mount was set up pointing North using a compass and angled at my latitude using a digital level. Accuracy is about 0.1 degree. I had a William Optics GT102 mounted on it, used the NYX tripod and pier extension, all on firm standing.

QHY Polemaster - worked well but needs a fairly dark sky to pick out the starfield.

Polemaster with lens swapped out for QHY mini guidescope and using SharpCap Pro. This is my preferred method. It plate solves the polar region and just requires a single rotation around RA to work out the center of rotation and hence the NCP position. It also works in lighter conditions than with the QHY polemaster.

TheSkyX TPoint add on. - on my Paramounts, this achieves remarkable PA and pointing accuracy, often just a few arc seconds. With the Pegasus, with a 100-point model, it failed completely. Even with the scope accurately polar aligned, the RA errors in the worm mess up the model calculations and even the super-model fails to tame things. I had RMS pointing errors of 1000", which is clearly wrong. It reported a large PA error, which was obviously wrong. I'm guessing the gear RA errors between close sample points extrapolates to a large assumed error. Need to investigate further but certainly, for those using some kind of model - it is much better to use widely-spaced samples for PA assessment.

Alt/Az adjustments: The Alt adjuster is on one side. If you put too much tension on the Alt lock knobs and adjust Altitude, it swivels the mount slightly (about 3 arc mins) in azimuth. For that reason, I follow this process:

Put only slight tension on all locking levers (but tighten M12 knob)
Adjust Altitude first
Lock altitude
Adjust Azimuth
Lock Azimuth

I also found the supplied M12 knob that holds the mount to the plate binds a little. I added a steel washer, which permits rotational movement under tension. I also added a nut and washer to one of the azimuth adjusters. In that way, once I have achieved a good PA, I can lock the adjuster on that side and use it for a reference. In that way, I can re-assemble the system and achieve fair PA without further adjustment.

hope this helps
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rimcrazy
Posts: 12
Joined: Mon Mar 01, 2021 12:03 am

Re: Polar alignment methods - experiments

Post by rimcrazy »

Thanks I will try your methods out. While I like the NYX, doing PA on it is more than a bit of a PITA. I spent 20 mins last night doing PA with NINA TPPA. First pass got me to about 1' and second pass to 19". I did a solid 5 hours of imaging on the Firecracker Galaxy and Phd2 reported a PA error of 13.8'. That's huge. My guiding was ok but not great. Total error was 0.83". I spent an obsessive amount of time going back and forth between Alt/Az as I'd lock one and the other moves. It's like playing "Wack-a-mole" with your PA. Quite frustrating. Across the 5 hours there was a fair amount of drift in my image stack too. PI can handle it post processing but that's not the point. I should be able to get reasonably locked images without the drift.

I have Sharpcap and I've used it quite a bit for PA among other things. While I have a pole master, I don't use it much anymore as even with SharpCap you can simply use your main camera to do PA. I'm not sure if that makes much of a difference but it's easy to check out. If the PoleMaster cam + SharpCap gives a more consistent PA I'm all for it.
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