AVX Backlash

[fatboy1271]

I uploaded my last session's PHD2 log and apparently my DEC Backlash is out of control. I'm seeing YouTube videos on opening the gearbox to make some adjustments. Before I go too deep into such an endeavor is this something that AVX owners commonly do? I'm pretty handy if I have good instructions, so I'm willing to try if it is "recommended" by people I trust. Yes, recommended is in quotes because I know this would still be a do at your own risk adjustment.

fat

[JayTee]

I need to do the very same thing to my AVX mount.

My backlash is fierce. I just haven't gotten the gumption to tear into it for fear of what I may or may not do right or wrong. I even have a bookmarks folder with nothing but how to address / repair the backlash in your AVX mount. I keep saying, one of these days.

Cheers,
JT

[fatboy1271]

Glad to know my trepidation is well warranted... Seems easy enough; however, that's what scares me!

[fatboy1271]

I did it! I followed AstroForum's YouTube video and it was a piece of cake... Well, minus the one screw that just vanished? I took some shaky cell phone video that I'll try to edit and post tomorrow. Honestly, the hardest part was getting the plastic casings back together I won't be able to test it tonight, but the amount of slop I saw in the gears was what I would consider pretty bad. Obvious delay in both the DEC and RA when slewing the opposite way.

Here's the video I followed:

[bobharmony]

Nice job on digging into the gear mesh. Getting the motor gear and the worm gear is half the battle. There is another adjustment that can be made with the plastic covers off that our friend in the video didn't do.

The worm gear that he refers to meshes with a cylinder attached to the axis itself to actually rotate the mount. if you look closely at the video you will see three allen bolts in a row underneath the dec motor. Those can be used to adjust the fit of the worm gear to the teeth on the cylinder. If I recall correctly, the two outside bolts are there to hold things in place, and the center bolt controls the spacing. When the center bolt is loosened it will allow the worm to mesh more tightly to the teeth, and if you tighten the center bolt it will create more space between the worm and the teeth.

The idea is to loosen the two outer bolts, make a small adjustment to the center bolt, then snug the outside bolts again. Run the motor through as much of a turn as you can. The idea is to get the mesh tight without making it so tight that it binds at any point in the rotation. It may take several iterations to get the setting just where you want it.

Oh, and when you are done with the worm adjustment, go back and do the motor gear-worm gear adjustment again, as doing the worm adjustment will affect the relative position of the motor and worm gear

About the plastic covers, yeah they are a pain to get back on, and I have managed to strip the screw threads on at least one of the connections points. Go gently with those too!

Bob

[fatboy1271]

Thanks Bob... I think Did you see a noticeable difference after these adjustments?

[yobbo89]

I think I've realised why most mounts are poorly adjusted, it's most likely a 1 size fits all for temperature, temperature affects tuning a bit of clearance for different climate zones and seasons, you should find that the tune will differ over the course of the year ie summer/winter and its best to have a bit of play or you might run into binding when the temperature changes

[fatboy1271]

Well said yobbo! Let me ask this; in the summer heat things expand, correct? If that is true then a tighter fit in the summer "should" mean a looser fit in the winter...?

I'm also thinking that with how simple of a process this was that I could set a Calendar reminder to make adjustments every two months or so. Just to keep an eye on things...

[yobbo89]

Correct with the expansion, you should be fine to tune it in summer and leave it through winter, when summer comes here I'll have to retune, as I only just tuned it due to restoration. But I tend to give it a little play and leave it all year round, I use a pretty tight tune to get the most out of the mount, this removes the need to shift my counter weights on a meridian flip given that one sides tracking is still not as good as the other, this is the worm on the Ra, Dec gears I'm tuning

BTW I had a look for a belt kit for your mount and no luck, but there are off the shelf parts that should match it, you'll need to rewire the motors in reverse. I just in case you want to think about going deep into mount tuning

[bobharmony]

Thanks Bob... I think Did you see a noticeable difference after these adjustments?

After I did the initial adjustment to the motor gear/worm gear combination (same as you). I saw some reduction in DEC backlash. After adjusting the worm/cylinder fit, I was unable to get a decent mesh on the motor/worm combination and lost all the benefit.

I have started playing with the DEC backlash settings in the Hand Controller and had some decent results in April. Unfortunately I haven't had another session since, so am unable to report if it was real or just a fluke of the one night.

Bob

[fatboy1271]

Thanks for the extra research! If I had better overall seeing I would probably think quite a bit about it... My issue is that I live in a Bortle 9 and have never taken this setup to a dark sky. Now with a soon to be 6 month old it is going to be even more difficult to do that.

I'm pretty much limited to 300sec maximum exposure lengths; if that. I've achieved as high as 600sec but at the cost of bumping my ISO all the way down to 400. While I was happy with the tracking accomplishment, in the end I feel like that was a waste of time for the data I got out of it.

Maybe you can convince me otherwise

[fatboy1271]

Looks like I need to investigate the Backlash settings in the hand controller! Thanks Bob.

[JayTee]

Just remember, PHD2 specifically warns against using backlash control in your hand controller and trying to use PHD2 at the same time. It causes an epic battle like Clash of Clans.

Cheers,
JT

[fatboy1271]

Just remember, PHD2 specifically warns against using backlash control in your hand controller and trying to use PHD2 at the same time. It causes an epic battle like Clash of Clans.

Cheers,
JT

Thanks for the tip JT!

[bobharmony]

Just remember, PHD2 specifically warns against using backlash control in your hand controller and trying to use PHD2 at the same time. It causes an epic battle like Clash of Clans.

Cheers,
JT

I have read that too, JT, but I don't know if I believe it based on one nights experience. I know that my guiding that night was such that I didn't have to throw away any subs, my dither settle times were the best I have ever seen, and the calibration data showed the smallest orthogonality error I have ever experienced at 0.1 and a difference of .02 arcseconds difference in the RA and DEC guiding rates. I am normally luck to get the axes within 3 degrees of each other . My thought is to take in as much data as you can, and feel free to experiment. I have also read advice that says using the HC to tune out backlash has helped with PHD2 guiding has been very helpful to others.

In short, it pays to read the advice that is out there, but it can also be useful to experiment. I was moved to experiment this time because I had been reduced to dithering in RA only in my previous session, and the pattern noise that produced was very difficult to work with. I had to try something to improve the situation, and mechanical tuning had not worked well for me.

Bob

[JayTee]

Hi Bob,

I hear what you're saying and applaud your efforts at experimentation. I will caveat what I said by saying that the advice I proferred comes from Bruce and Andy over at PHD2. It is not from my own empirical experience.

Cheers,
JT

[fatboy1271]

So Brian on the PHD2 forum has been reviewing my logs and had this to say after my Backlash experiment:

"here's the good news - you crushed it on improving DEC. if i hack out the parts of your poor guide stars (presumably branches, etc.) DEC is significantly better

here's the bad news - you crushed it on improving DEC, so much that now your RA is more of the limiting factor!

you just can't win right?

But i'd say you did a great job taming that, well done

for RA, investigate how to reduce these couple of major fundamentals:

these are at 297 sec and 602 sec. your mount should have PE correction built in, might check that out

might give you some clues on your mount."

I'm pretty darn happy about that! Now I have to learn PEC Training...

[JayTee]

Hi Patrick,

Congrats, and yes, it's time to investigate PEC training and also predictive PEC in PHD2. The two work together really well.

Cheers,
JT

[fatboy1271]

Hey JT,

Do you use CPWI? I find it to be pretty awesome in conjunction with my AVX! It has Train PEC built-in, rather than the separate PECTrain app. I'm hopin' to get out tonight or tomorrow to run that! I've got the PHD2 Predictive PEC info up on another tab so I'll read about that too.

Once again, I can't thank you enough for all the input and help

fat

[yobbo89]

What is your tracking pixel arc sec before and after on average ?