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  #1  
Old 15-03-2018, 11:39 PM
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Bart
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Why does PHD do this?

PHD just drifts off in Dec every so often and then comes back. I have adjusted the mesh, balanced the scope and mount and played with PHD settings, however; occasionally it does this and ruins a subframe.
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  #2  
Old 16-03-2018, 12:16 AM
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billdan (Bill)
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You running a guide scope or an OAG Bart? Could be some sort of flex going on or something moving slightly. Like Mirror shift due to gravity, or the focuser drawtube shifting because of camera weight and gravity, or even cable drag.
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  #3  
Old 16-03-2018, 12:56 AM
luka
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Backlash.

See how the correction pulses (red lines on the bottom) try to do the correction but noting happens for a while and then suddenly there is a correction. This is a clear indication of backlash.

1. Which mount?
2. Run the guiding assistant in PHD. It will measure the DEC backlash for you to confirm my guess.

By the way, the popular HEQ5/EQ6 almost always have some backlash in DEC that cannot be removed. The usual cure is to guide in one direction only and also to keep the mount off balance to keep the gears engaged.
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  #4  
Old 16-03-2018, 09:59 AM
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I'm using an FLT 132 with an overhauled and adjusted standard focuser with no slop or image shift on direction reverse, running an OAG. The OAG prism has recently been re-glued.

As far as backlash is concerned, WOW! how much tighter do I need to get the worm and wheel? I had it feeling pretty good and it was just stalling the motors so I have backed it off a fraction, I can barely feel any slop in DEC or RA. It is an AZ-EQ6.

As for the guiding assistant, tried that, not impressed. I can get better guiding results fiddling myself. I also tried to guide in one direction as per guiding assistant, and that did not work so well, either.

I have to dump my alignment and start again anyway so I will re-balance everything and try again.

Thanks for your thoughts guys, I'll play with everything mentioned anyway and see what happens.
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  #5  
Old 16-03-2018, 10:19 AM
Imme (Jon)
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I went from a guide scope to a pricey OAG recently....battled for a month or so to try and get it working after getting very similar results to your graph. I was thinking backlash, PHD settings, mount issues, alignment....tried everything....adjusted everything....pulled things to pieces and rebuilt......nothing helped.

Tuesday night I cracked it and swapped the ZWO guide scope back in, did a calibration and immediately did a 10 minute sub with almost perfect stars (it was very windy which wobbled the scope)

I don't know what the OAG issue was but I tell ya.....I wont be going back down that road again!

They say OAG is the way to go.......but with my setup a guidescope is much better....honestly think it depends on what you have and how things interact with one another
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  #6  
Old 16-03-2018, 11:26 AM
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I generally get good guiding and RMS figures aren't too bad, ergo I have no issue with the OAG as such. I do agree though, your mileage may vary re OAG and guide scope. I think sometimes in this game it is what works for you.

I guess my real issue with PHD is that it does this when I have never seen MaxIm do it, ever. So I'm kinda thinking this is something in PHD.

The reason I'm using PHD is that I want to run automated focusing so I'm using SGP which does this very well with out moving from your target area. I'd rather waste a couple of minutes between frames focusing rather than slewing, acquiring, focusing, slewing back etc. What the real time difference is I'm not sure, although the slewing and acquiring method sounds longer straight up and my scope is very temperature sensitive so focusing every 2 frames is quite often required.
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  #7  
Old 17-03-2018, 11:02 AM
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Shiraz (Ray)
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Quote:
Originally Posted by luka View Post
Backlash.

See how the correction pulses (red lines on the bottom) try to do the correction but noting happens for a while and then suddenly there is a correction. This is a clear indication of backlash.

1. Which mount?
2. Run the guiding assistant in PHD. It will measure the DEC backlash for you to confirm my guess.

By the way, the popular HEQ5/EQ6 almost always have some backlash in DEC that cannot be removed. The usual cure is to guide in one direction only and also to keep the mount off balance to keep the gears engaged.
yep, that's backlash, not software. if a mount doesn't have spring loaded worms, it actually needs a bit of backlash to stop the gears from binding. above procedure fixes the problems it causes.

Last edited by Shiraz; 17-03-2018 at 12:24 PM.
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  #8  
Old 19-03-2018, 08:22 PM
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Thanks for the confirmation, Ray. I'll give it a go.
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