dec guiding seems not working properly with dither

[Jin Qian]

Hi -

I tried dither with byeos (aggressiveness 5) + phd2 (scale 1 - 2) but dec guiding seems not working. dec line just drift up or down regardless which dec guiding direction I choose. I have avx mount, dither RA only, dec guiding in one direction. 

Here're logs.

https://drive.google.com/open?id=0B4WmA86bp3WFQTV6UzRaMUFZeVk

Thanks!
jin

[Andy Galasso]

Hi Jin,

The problem you are having in Dec is unrelated to dithering.  When Dec Guide Mode is set to North or South, then PHD2 will automatically only dither RA (your selection of Dither RA only does not hurt, but it is not necessary either.)

When you select uni-directional Dec guiding (dec guide mode = North or South) you have to pay attention to the direction of the Dec drift and make sure your dec guide mode is selected to be the direction that will correct the drift.  The correct direction can be determined by looking at the guiding graph in PHD2. For example, in the image below you can see the Dec trace drifting downward, South, away from the direction labeled GuideNorth. In this example we would select dec guide mode = North.

In the logs you posted, the direction you chose was generally in the same direction as the drift, so PHD2 was unable to issue the Dec corrections that would have corrected the drift.

I think if you set your Dec guide mode to the opposite direction of what you were using, you will have some success.  The direction of drift can change over time depending on where the scope is pointing, so you need to monitor the dec drift and make sure your Dec guide mode is set accordingly.

Andy

[bw_msgboard]

Hi Jin.  Also, the GA run shows you have quite a poor polar alignment – something in excess of 26 arc-min.  That makes it easier to see the direction of the drift, but it may cause you problems with field rotation at some point.  I wonder why you didn’t measure the Dec backlash in the mount – do you already know it’s bad and is that the reason for doing uni-directional Dec guiding?

 

Bruce

 

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[Jin Qian]

Hi Andy,

I tried both directions but no luck. In guidelog, scrolling down in log sections, there's a 1h4m period followed by a 43m period. They're where I noticed the drift. If I didn't mess up anything, I should have changed guiding direction from south to north or the other way around. But dec drifted on different directions in those two runs. They're on same side of meridian.

Thanks,

jin

[Jin Qian]

Hi Bruce,

Yes, polar alignment was very bad initially and I ran a drift alignment before the 1h4m section. I didn't let it drift for long but I thought alignment should be much better. Do you see 26 arc-min for the long section?

It's AVX mount : ) I measured backlash a while ago and it was beyond compensation so I only do uni-direction since then. I heard that slight inbalance on east side would reduce backlash but haven't tried it yet. Maybe I can get better dither result doing both RA and Dec?

Thanks,

jin

[bw_msgboard]

Hi Jin.  I agree with Andy, I think your problem is Dec guiding, not dithering.  I’d fix one problem before moving on to the next.  I was just looking at the GA result, but it looks like the polar alignment was much better by the time you did the 1 hour run.  I’m going to leave this alone and let Andy help you out – we don’t need too many cooks in the kitchen. J

 

Bruce

 

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[Andy Galasso]

Jin,

Unfortunately you chose the wrong direction both times. In the 1h4m period dec was drifting South and you selected South dec guide mode. In the 43m8s period dec was drifting North and you selected North guide mode.

Even on the same side of the meridian the direction of drift can reverse.  You'll need to observe the direction of the drift by looking at the guiding graph and set the dec guide mode to the direction that will correct the drift.  With a little practice it should be pretty straightforward to learn which setting you need to choose: watch the direction of the drift and look at the "GuideNorth" label on the graph. You can think of it this way: North corrections will "pull" the dec line toward the GuideNorth label on the graph.

Andy

[Jin Qian]

Thanks Andy! It's great to learn direction of dec drift can reverse. Does the direction change base on some factors? Or I need to watch guiding graph in entire session and change direction accordingly from time to time?

jin

[Andy Galasso]

On Mon, Jan 2, 2017 at 6:53 PM, Jin Qian <jin.q...@gmail.com> wrote:

Does the direction change base on some factors? Or I need to watch guiding graph in entire session and change direction accordingly from time to time?

The direction of drift changes based on your polar axis alignment error (error in altitude and azimuth) and where the scope is pointing. In principal if you knew the direction of your polar axis relative to the pole, you could calculate the direction of drift.  In practice we do not usually know that and it is necessary to watch the graph.

It may be worthwhile to look into tuning the dec gear mesh so you can get the backlash under control enough to use PHD2's backlash compensation.  If you can do that, it would allow you to set you dec guide mode to Auto (bi-directional). I don't know enough about AVX mounts to say how feasible that would be.

Andy