Tripod spreader shift affects PA, PHD2

Using SharpCap's Polar Alignment feature
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Tripod spreader shift affects PA, PHD2

Post by devonshire » Thu Dec 27, 2018 3:41 pm


The Polar Alignment troubleshooting notes list some sources of mechanical movement that can affect Sharpcap's PA. I have another for the list, and it turned up post-PA and while guiding: My tripod spreader was shifting against the legs while the scope was in use.

- Perform Sharpcap PA while parked -> Good/Excellent
- Slew to target and begin guiding
- PHD2 complains about very bad PA
- Stop guiding and park scope
- Check PA w/Sharpcap. PA has gone from Good/Excellent to Poor.
- The mount's drive IS accurate, and I trust that it's gone back to the same Home position as the first alignment was done at.

I have a mount (EXOSII / PMC8) that uses the same sort of tripod spreader as LXD75/HEQ5/etc. The tripod is two inch stainless and very sturdy, with grippy rubber feet, but the spreader is basically a heavy eyepiece tray that pushes outward against the legs and cannot really immobilize them when tightened.

Even when apparently tight, I found that the legs must have been shifting a bit against the spreader as the load on them changed from the scope being slewed. This was on a concrete surface.

What I did to stop (or at least reduce) that shift was pretty simple. I put some short lengths of cut vinyl tubing over the ends of the eyepiece tray arms to improve friction between the arms of the tray and the leg. Then I tightened the spreader's bolt until the legs were unable to shift when tugged from side to side.

PHD2 still sees a PA difference, but it's much smaller and my guiding has improved.

I think I'd like a better spreader solution, but in the meantime what I've done was helpful. Photo attached.

- Bob

EXOS2_Spreader_Friction_Tubing_047.JPG (168.67 KiB) Viewed 1144 times

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Re: Tripod spreader shift affects PA, PHD2

Post by admin » Fri Dec 28, 2018 3:46 pm

Hi Bob,

thanks for sharing that with us – it will no doubt be useful to others if they encounter the same problem. I certainly haven't considered that as a possible cause of the mount shifting, but if you think about it the amount of friction between the steel of the spreader and the polished steel of the legs probably isn't really that high.

Cheers, Robin

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Re: Tripod spreader shift affects PA, PHD2

Post by devonshire » Fri Dec 28, 2018 6:05 pm

Oh, no - not high at all. The arcs cut into the arms of the eyepiece tray are closer to a V than a circle, so the actual points of contact are small as well as very hard, and if one leg shifts, the pressure on the other two changes... The tubing is a crude fix, but the added friction and increased contact area does do a better job of locking the legs in place.

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