clearing backlash - what is it actually doing?

[Sandy MacPherson]

Perhaps someone can explain what is happening during calibration once RA is finished and the 'clearing backlash' phase begin.
Is it clearing both RA and DEC backlash?  My AVX mount will often drift a little after some east commands until backlash catches up.  How does PHD clear backlash?  How does it know it's cleared?   Some times I give a few north pulses beforehand to help but it doesn't seem to do much.  Also, why would my ra & dec ever not be orthogonal?  Should I always check that box?  sorry for the barrage of questions, I did read the help files but maybe I missed something - I can be slow sometimes!
Also, something to think about- I noticed that my AVX does not have consistent DEC backlash through it's entire revolution which is something I was sort of assuming.  What I mean is that the ring gear/ worm gear are not always the same distance apart - it may be greater at 90 deg than 270...maybe a flaw of my mount..it would be nice if PHD knew the backlash at different orientations for less than perfect mounts.

[bw_msgboard]

Hi Sandy.  During this part of calibration, PHD2 is making an attempt to clear only declination backlash.  RA backlash doesn’t matter for guiding so that is ignored.  We’d need to look at your logs to see what’s causing the orthogonality issue, but that’s often caused by Dec backlash.  And Celestron mounts are well-known for having a lot of that.  You may be able to see for yourself if you look at the graph in the Review Calibration tool.  You can make your life simpler if you just move the mount north for 20-30 seconds at guide speed immediately before starting the calibration.  That is likely to clear the backlash and get you a better result.  If you’re using pulse-guiding, you only need to get one good calibration and just keep re-using it.  PHD2 has to make an educated guess about how far to push the mount north to clear the backlash.  Nothing can be relied upon at that point in terms of guide rates or mount behavior.  As a result, it’s better than it used to be but still far from perfect.  The Guiding Assistant is much more aggressive during the backlash measurement process but even that is only good for maybe 10 seconds of backlash.  At such large values, it probably doesn’t matter – it will be difficult to guide in both Dec directions and get good results.  Uni-directional Dec guiding is probably a better option if you can’t improve the mechanics of the mount.  PHD2 doesn’t really care about the magnitude of the backlash in various sky positions.  You can measure it for yourself in different pointing locations if you want, but then what?  If it’s that large, I don’t think there’s much to be done about it in the software.

 

Good luck,

Bruce

 

---

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

[Andy Galasso]

On Tue, Nov 8, 2016 at 4:56 PM, Sandy MacPherson <funkys...@gmail.com> wrote:

Perhaps someone can explain what is happening during calibration once RA is finished and the 'clearing backlash' phase begin.
Is it clearing both RA and DEC backlash?  My AVX mount will often drift a little after some east commands until backlash catches up.  How does PHD clear backlash?  How does it know it's cleared?

RA backlash is not an issue for guiding since the RA gear remains engaged as long as the scope is tracking (and RA guide speed is <= 1x sidereal).  If you see some RA backlash after an eastward slew, you need only wait a moment until the backlash is naturally taken out.  It's important not to start a calibration until the mount RA tracking stabilizes after the eastward slew.

PHD2's backlash clearing only involves Dec. When PHD2 displays "clearing backlash" it is issuing North guide pulses and waiting to see the guide star move consistently in response. When it sees the guide star moving consistently it assumes the Dec backlash has been cleared and then starts the Dec calibration measurement.

 

  Some times I give a few north pulses beforehand to help but it doesn't seem to do much.

That is a good idea to do that. Nudge north until you see the guide star move, then start the calibration.  That way there will be no dec backlash when phd2 starts to calibrate Dec.

  Also, why would my ra & dec ever not be orthogonal?  Should I always check that box?

The main purpose of the "Assume Dec orthogonal to RA" option is to provide a workaround for mounts with very large RA periodic error.  If the PE causes excessive RA drift during the Dec calibration it would result an a bad Dec angle, so the option is provided as a workaround. The default setting, disabled, is appropriate for most of us.

Also, something to think about- I noticed that my AVX does not have consistent DEC backlash through it's entire revolution which is something I was sort of assuming.  What I mean is that the ring gear/ worm gear are not always the same distance apart - it may be greater at 90 deg than 270...maybe a flaw of my mount..it would be nice if PHD knew the backlash at different orientations for less than perfect mounts.

Although PHD2's backlash comp does not model the backlash, varying backlash should be handled relatively well since the backlash compensation is adjusted dynamically based on whether the backlash pulses are over- or under-correcting.   If you wanted to post a guide log, we could confirm whether this is working as expected for your mount.

Andy