12 replies to this topic

[Tulloch]

Hi all, I recently picked up a 2nd hand LX-90 with the 12" OTA on board. Thanks to Andrew's (OzAndrewJ) advice, I've upgraded the firmware to A1F7 and set about reseting, re-balancing and re-training the scope.

 

However, when I use it for Planetary AP, the scope won't stay where it's put. It immediately reverses direction every time I release the directional button (on every speed setting including #1) and makes the whole process extremely frustrating. On Andrew's advice I've set the Az/RA and Alt/Dec Ratios  Percentages to 0% (which should stop this) and also tried a range from 10% to 50% with no effect. A link to a video showing the effect is below (turn the sound up for commentary).

https://drive.google...iew?usp=sharing

 

The OTA itself is able to produce some good images of the planets (for instance see hereand here) but I just cannot get the scope to stay where it's put. I admit that the focal length is quite long (around 6500 mm) and the camera sensor is quite small (1300x976 pixels) but the mount is unusable in its current state.

 

Apparently there's a way to tighten up the gears to reduce the amount of backlash, can anyone please point me to an online resource that describes how this is done? Failing this, I'll have to defork the OTA and use it on my Celestron CGEM mount.

 

Thanks,

 

Andrew

Edited by Tulloch, 11 November 2024 - 06:35 PM.

[OzAndrewJ]

Gday Andrew

I've set the Az/RA and Alt/Dec Ratios to 0%

Just a correction for others, its the percentages  that have been tweaked, not the ratios:-)

This was a normal test for stopping the "creep after beep" seen at times

but Andrew is imaging at 6.5m or so, so even small drift due to lash is affecting him.

I have never heard of people using an LX90 at this FL so if anyone has tried this

feedback would be welcome

 

Apparently there's a way to tighten up the gears

Not with an LX90

The worm to wormwheel is spring loaded

The gearbox itself is not user servicable ( unless you can print slightly oversize gears as reqd ),

The motor to Gearbox joint is flexible and could be tightened a bit

but it would require McGyvering.

Andrew Johansen Melbourne Australia

[Tulloch]

Gday Andrew

Just a correction for others, its the percentages  that have been tweaked, not the ratios:-)

Thanks, updated, still getting used to the Meade terminology . Why can't they just call it backlash compensation? 

 

Not with an LX90

The worm to wormwheel is spring loaded

The gearbox itself is not user servicable ( unless you can print slightly oversize gears as reqd ),

The motor to Gearbox joint is flexible and could be tightened a bit

but it would require McGyvering.

 

Aah, that's disappointing to hear. Thanks again.

[OzAndrewJ]

Gday Andrew

Just for info, have attached a piccy of the Gbx

As you can see, the gear axles are fixed into the plastic casing.

The motor to gear box fixing is "structurally fluid" ( and thats being nice )

Hotmelt glue is a favourite for making it more rigid.

 

Andrew Johansen Melbourne Australia

Edited by OzAndrewJ, 11 November 2024 - 07:02 PM.

[Tulloch]

FYI, here's what I got back on the M&MPI forum page...

https://www.cloudyni...024/?p=13791897

[OzAndrewJ]

Gday Andrew

OK, that is endplay in the worm, but that is something i have rarely heard about

and your drive train numbers were very small to begin with

so not sure if a "mechanical" tweakis going to fix that.

90% of people reported slop in the motor to gearbox joint, and that is hard to fix.

One made a video where they effectively had their units gearbox exposed

( like my first piccy )

They then slewed the motor at speed 1 each way and watched how much slop there

was before the worm gear started to move.

As i mentioned, the LX90 is a good introductory platform

but tracking in AltAz at 6.5m is probably pushing it a bit :-)

Polar would be a lot better, but you have the cost of the wedge

so its a game of diminishing returns

 

Andrew Johansen Melbourne Australia

[Tulloch]

Hmmm, not a lot of additional feedback from other users...

 

Here's some other thoughts I've been having...

• I trained the motors by pointing at a stop sign down the road about 200m away using an 11mm eyepiece which pretty much filled the FOV. This gave me a good handle on when I was back to my original position. However, when the scope is horizontal it is slightly front heavy, while when it's looking up (at the planet) it's slightly rear heavy. Will this affect the training values?
• Should the scope be front (or back) weighted all the time so the gears are always engaged?
• My main issue with the tracking was that I needed to press the "down" button most often to keep the planet on the screen, to which the mount would then rebound back "up" again. Does this point to anything re balancing?
• Will changing the training values manually from the handbox help?
• I'll spend some more time playing with the Alt/Az Percentages to see if I can find anything that works in my case. I haven't given up on the LX-90 (yet), I really want to make it work if possible.

Anything else I should try?

 

Thanks again,

 

Andrew

[OzAndrewJ]

Gday Andrew

 

Will this affect the training values?

No

The drive train measures gear lash in total.

When a mount is running, even if slightly unbalanced, it will reach a position of equilibrium

on which side of the worm is loaded.

Using slew keys after that merely loads the gears to go faster or slows down/reverses the gears to slow down.

The lash "percentages" will only ever cut in if the motor has to reverse, and that is not affected by the balance.

 

My main issue with the tracking was that I needed to press the "down" button most often

But you were also on one target ( based on our earlier discussions )

If you try on a star on the opposite side of meridien, is the effect the same????

Trying to analyse AltAz can be a minefield due to the fact that the "pure theory" motor rates

change based on position.

 

Will changing the training values manually from the handbox help?

What are you going to change em to??????

Remember, any time you reverse, the mount uses this value to determine when to start tracking.

I need to re look at the motor code, but it gets very complex on how to calc a motor rate.

 

Anything else I should try?

Based on your FL, a wedge :-)

 

Andrew Johansen Melbourne Australia

[Tulloch]

What are you going to change em to??????

Something that works?  

 

I dunno, I'm hoping I don't have to de-fork it, willing to try just about anything...

 

Thanks again, you are the Meade guru 

[Skywatchr]

Something that works?  

 

I dunno, I'm hoping I don't have to de-fork it, willing to try just about anything...

 

Thanks again, you are the Meade guru 

Many years ago I had a 10" LX90 with virtually the same "drift after slew" issue.  I watched the gears and discovered the worm housing to casting was "flopping" causing the worm housing to "climb" on the wheel gear, then settle back after the slew.  I removed the worm block and discovered "slag" on the top of the mounting posts.  I carefully filed them flat, and had to choose the right shim washers to keep the block "floating" on the pivot, yet had no lateral play.  After that, it worked much better with very little drift after slew.  I had to do it with both the DEC (ALT) and RA (AZ) worm mounts.

I also used it on a wedge instead of ALT-AZ. 

[Tulloch]

Many years ago I had a 10" LX90 with virtually the same "drift after slew" issue.  I watched the gears and discovered the worm housing to casting was "flopping" causing the worm housing to "climb" on the wheel gear, then settle back after the slew.  I removed the worm block and discovered "slag" on the top of the mounting posts.  I carefully filed them flat, and had to choose the right shim washers to keep the block "floating" on the pivot, yet had no lateral play.  After that, it worked much better with very little drift after slew.  I had to do it with both the DEC (ALT) and RA (AZ) worm mounts.

I also used it on a wedge instead of ALT-AZ. 

Thanks for your reply, much appreciated.

 

Looks like quite a process to disassemble the gears and housings to get to where I need get to to fix my issue. I know I said I was willing to do just about anything to fix it, I didn't realise that I'd have to do everything   . I'll need to psych myself up to this disassembly/reassembly procedure, I was really hoping the fix would be more about software adjustments than mechanical ones. 

[Skywatchr]

Thanks for your reply, much appreciated.

 

Looks like quite a process to disassemble the gears and housings to get to where I need get to to fix my issue. I know I said I was willing to do just about anything to fix it, I didn't realise that I'd have to do everything   . I'll need to psych myself up to this disassembly/reassembly procedure, I was really hoping the fix would be more about software adjustments than mechanical ones. 

That's not to say it is the problem, but if software adjustments don't compensate you may have to dig in.  I'm a mechanic so for me it was a breeze, except for those blasted plastic fork covers.

[Tulloch]

That's not to say it is the problem, but if software adjustments don't compensate you may have to dig in.  I'm a mechanic so for me it was a breeze, except for those blasted plastic fork covers.

All good, thanks again.

 

I'm all thumbs generally, just concerned that if I pull it all apart I'll do more damage than good .