EQ6-R Dec backlash command cause RA deflection

[JonathanA]

Really frustrating problem with my EQ6-R. What's happening is that every time a backlash compensation pulse is sent in pHD2 I am getting a large deflection in RA. The graph below shows it quite well. The frustrating thing is that guiding is pretty good in between times (RMS about .6-.8). Is this some sort of vibration that's being set up in the mount by the backlash correction? Would reducing guide speed help? Can anyone suggest an adjustment I could make that might improve this? I know that I can set it up to guide dec in one direction only but I'd rather avoid this, if I can. Any thoughts would be much appreciated.

 

Edited September 2, 2019 by JonathanA
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[discardedastro]

It is certainly mechanically possible, more or less depending on exactly what you've got on the mount and where.

Reducing guiding speed could help, yes. Those corrections also look quite large - it may be worth tweaking the Dec algorithm to prefer more, smaller movements over larger movements (smaller min-move would be a good start). I'd start there and then look at reducing guide pulses to .3x or .2x if the problem persists.

[michael8554]

First off your black on black text is quite hard to read.........

You could setup to avoid dec backlash comp being sent at all.

Balance to be slightly heavy one way in Dec, then Dec shouldn't cross the axis and require big corrections.

Michael

[JonathanA]

On 01/09/2019 at 00:42, discardedastro said:

It is certainly mechanically possible, more or less depending on exactly what you've got on the mount and where.

Reducing guiding speed could help, yes. Those corrections also look quite large - it may be worth tweaking the Dec algorithm to prefer more, smaller movements over larger movements (smaller min-move would be a good start). I'd start there and then look at reducing guide pulses to .3x or .2x if the problem persists.

Yes, my minmove was quite large (but it was the recommended value at the time). The behaviour has been quite consistent for a little while though even when using quite small min move values.  I'm mostly thinking for now that there is some mechanical vibration that is large enough to disturb RA everytime I get a dec backlash correction.  The Dec axis has always been quite stiff and holding my ear up to the mount I could hear a faint scraping at some points in the cycle when manually turning the axis. Anyway, I have loosened up the thrust bearing slightly and I've now got freer movement and no noise. I'll have to wait an see whether this has improved matters (or created a whole bunch of other problems!)

[JonathanA]

17 hours ago, michael8554 said:

First off your black on black text is quite hard to read.........

You could setup to avoid dec backlash comp being sent at all.

Balance to be slightly heavy one way in Dec, then Dec shouldn't cross the axis and require big corrections.

Michael

Yes, sorry about that - it wasn't intentional!

In know that with a slightly poorer PA in dec I could engineer things to guide in one direction only and that may well get round the problem. At the moment that is my backstop - which I'm guessing will work quite well. I'll try to get to the bottom of the issue first though if I can.  Can you explain what you mean about being heavy in dec - I'm can't quite see how that would help (I understand how it helps with RA)?

[michael8554]

You have Dec backlash. 

So to force the mount to take up the backlash by itself, balance it to "flop" in the same Dec direction as the slight PA error you mention. 

So PHD2 will send constant small Dec corrections to straighten out the Dec trend, and with judicious choice of MinMo and Aggr the Dec axis should remain on one side of the axis instead of yoyo-ing over and under the axis. 

Michael 

[JonathanA]

24 minutes ago, michael8554 said:

You have Dec backlash. 

So to force the mount to take up the backlash by itself, balance it to "flop" in the same Dec direction as the slight PA error you mention. 

So PHD2 will send constant small Dec corrections to straighten out the Dec trend, and with judicious choice of MinMo and Aggr the Dec axis should remain on one side of the axis instead of yoyo-ing over and under the axis. 

Michael 

Got it, thank you, Michael. That makes perfect sense. First time out it will be a pain working out which end to bias towards but after that it seems quite straightforward.

[michael8554]

From memory:

Run the mount for a while with Dec guiding off. 

If the trend line is down, I think this means you need to guide North. 

So the scope needs to be slightly nose heavy so that it sags to the south. 

Then north pulses will raise the trend line. 

 

Michael