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Author Topic: PHD2 Dec error  (Read 2067 times)

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Offline Darth_Muppet

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PHD2 Dec error
« on: 21:11:43, 10 February, 2016 »
Scope is balanced but sometimes PHD2 Dec goes on a walk about then corrects itself..  Anyone else get this?

If the world was an orange....it'd be too small!

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Offline Roger B

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Re: PHD2 Dec error
« Reply #1 on: 22:00:13, 10 February, 2016 »
I don't,   but its been said before that the scope shouldn't be perfectly balanced,  should always have a slight offset so the gears mesh. 

Is yours like that ?
Rog

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Online chris.bailey

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Re: PHD2 Dec error
« Reply #2 on: 22:33:54, 10 February, 2016 »
Try increasing Dec Agg up to 100%.
Try dropping the Dec Min Move down a bit (0.15?)
Try increasing guide exposure (5 seconds) to get the guide star signal to noise ration up (the better it is the better the centroiding algorithm can get to work)
Would be nice to see the graph with corrections ticked so you can see the corrections that are actually applied.
As Roger says, try it VERY slightly nose heavy but being too well balanced normally gives a lot of reversals rather than a failure to keep up.
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Offline Einari

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Re: PHD2 Dec error
« Reply #3 on: 06:26:41, 11 February, 2016 »
Also try setting DEC guide to North or South (try which is right).
Tapio

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Offline tomhow

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Re: PHD2 Dec error
« Reply #4 on: 08:34:39, 11 February, 2016 »
Looks like backlash in the DEC gear.

Setting it to North or South so it never tries reversing the dec guiding as suggested above will help.
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Offline Darth_Muppet

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Re: PHD2 Dec error
« Reply #5 on: 12:10:13, 11 February, 2016 »
Cheers guys, will report back
If the world was an orange....it'd be too small!

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Offline mechanoid

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Re: PHD2 Dec error
« Reply #6 on: 19:23:49, 12 February, 2016 »
This looks to me like the scope taking up the backlash. If it only does it once, then that is probably it.  If it keeps doing it, it may be due to imbalance effects. Personally, I always aim to have the scope perfectly balanced.  I reason that if you load it one way, you have only a 50% chance it was the right way.  In general, if the worm is driving so as to oppose the torque, it will be fairly smooth. If however it is unloading the torque, then the drive will have a much greater tendency to move in jumps.  Friction will always keep the gears in mesh. Its just that sometimes you have to wait for the backlash to be eliminated after slewing to position.

I always find that things work best when the polar alignment is slightly off (as it usually is), so that a uni-directional drive can be used. Bi-directional drive works only if the scope doesn't wander too far before the backlash is eliminated.  In this case better polar alignment is required.

I dry lubricated all the bearings and gears on my dec. axis with PTFE, in order to eliminate slip-stick effects. No grease at all.  So far it seems to work much better than when it had the original grease.

I'd agree that for the RA axis a slight balance bias for the scope to move East may improve the drive for some mounts. Personally I find that perfect balance still works fine, and makes pier flipping easier.
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Offline Darth_Muppet

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Re: PHD2 Dec error
« Reply #7 on: 21:01:20, 12 February, 2016 »
Ok, had another go, doesnt seem to be affecting the subs taken but still getting the odd spike...
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Offline Nomis Elfactem

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Re: PHD2 Dec error
« Reply #8 on: 21:09:08, 12 February, 2016 »
What focal length are you guiding at... looks like it's chasing the seeing / a cloud towards the end of the run ? All you Dec corrections are very one sided.... worth checking the balance again and/or your polar alignment !

S.
Simon

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Online chris.bailey

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Re: PHD2 Dec error
« Reply #9 on: 22:16:25, 12 February, 2016 »
That last graph doesn't look too bad at all to me. Gentle nudges in the same direction is no bad thing, its the rapid reversals or nudges that don't keep up that are a killer. If that was earlier tonight or last night the seeing was pretty awful at times so an RMS under 1" would be good going in my book.
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Offline Einari

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Re: PHD2 Dec error
« Reply #10 on: 07:01:40, 13 February, 2016 »
All dec corrections should be one sided if correction is just north or south.
And i think that looks pretty good curve.
Tapio

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Offline Darth_Muppet

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Re: PHD2 Dec error
« Reply #11 on: 10:43:37, 15 February, 2016 »
OK cheers guys, I'll check the Polar Alignment again and also re balance the scope slightly nose heavy.
If the world was an orange....it'd be too small!

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Offline Oscar The Grouch

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Re: PHD2 Dec error
« Reply #12 on: 16:57:06, 15 February, 2016 »
I'm having enough trouble getting PHD1 to work without moving to 2!! Perhaps I should consider a synguider instead!!

Offline tomhow

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Re: PHD2 Dec error
« Reply #13 on: 17:03:02, 15 February, 2016 »
OK cheers guys, I'll check the Polar Alignment again and also re balance the scope slightly nose heavy.

As said above, avoid perfect polar alignment. Introduce some small error. This gives the dec guiding something to get its teeth into.
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Offline Darth_Muppet

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Re: PHD2 Dec error
« Reply #14 on: 11:36:52, 16 February, 2016 »
I tried the above. Checked and redid the Polar Alin, Rebalanced scope.  - now the Dec graph trace plummets downwards..  Will have to wait for another clear night to do another guide trace.
If the world was an orange....it'd be too small!

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