Tripod spreader shift affects PA, PHD2
Posted: Thu Dec 27, 2018 3:41 pm
All,
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.
Scenario:
- 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
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.
Scenario:
- 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