Help! Mesu 200 problem

[old_eyes]

Hi all Mesu users. Got a problem and looking for advice.

I have had a Mesu 200 MkII for a couple of years. I bought it from Bernard at Modern Astronomy in December 2020.

So far it has worked extremely well, but I have just developed a major problem. Last night I was using it in my automated observatory as usual, and parked it at the end of the session. Today, I set up another run and found that the RA axis was making some strange noises and seemed to be slipping. The scope would not goto the area of the sky selected. I released the servo motors and reset the scope to the park position. However, when I engaged the servo motors again the DEC axis moved OK, but the RA axis did not. Neither under computer control of with the handset. The LED on one axis of the Sitech servo controller was blinking and the other steady.

The RA axis was floppy, as if the servo was not engaged. I switched all power off and turned the mount back on again. Both LEDs on the Sitech controller were on.

The DEC axis responded normally to the hand control pad, but the RA axis would not move. I tried releasing and re-engaging the servo motor several times but with no effect. Listening carefully, I could hear the servo motor rotating when it was engaged or not engaged. I also noted that the RA axis servo motor seemed loose in its housing. The encoder end of the servo moves by about 3-4mm side to side with a light touch. There is no movement in the DEC axis servo.

Has anyone seen anything like this before? Are there any adjustments I can make to make the RA axis servo engage with the RA axis disk?

I have emailed Lucas Mesu to see what he thinks, but wondered if there was anything I could do myself.

old_eyes

[Jonk]

Hmmmm....sorry to read this. What sort of 'noise', is it the motor or mechanical noise from the RA part of the mount?

I would certainly not attempt a mechanical repair, Lucas should also tell you this. He will probably ask for the mount to be sent to him. I have done this myself when I thought I had an issue that turned out not to be. The delivery costs about £20 each way from memory.

Having said that, what I would do is swap the connections on the Sitech controller, both the motor connections and encoder connections. Then see if the noise is the same and the issue is infact with the RA mechanically.

Another thing to check is your power supply. Can you try another? If the output of the PSU is dipping for example, the current would increase and this might be causing the controller issues, hence the blinky mode.

I can't think of much else to try immediatly, but just wait for Lucas' reply.

Send him a video if you can with the noise.

[tomato]

I have a Mk 1 Mesu and the drive motors are solidly mounted, no play in them whatsoever. However, this may not be  helpful as I know you can disengage the drives on the Mk 2. It does sound mechanical to me and would endorse @Jonk’s suggestion to send a video clip of the mount to Lucas.

[old_eyes]

Thanks all. Lucas is on the case and has asked for video evidence. That's the next step.

But any thoughts from a Mesu owner welcome.

[Jonk]

Would you be happy to share a video of the noise here too? Does it make the same noise in both directions?

[old_eyes]

23 hours ago, Jonk said:

Would you be happy to share a video of the noise here too? Does it make the same noise in both directions?

Hi Jonk,

We have moved on a bit doing some basic diagnostics. I will update when I have a solution. Situation MAY be different, and there might be an explanation, but I prefer to wait until Lucas gives me some confirmation.

[ollypenrice]

I agree with Tomato. It sounds mechanical and shouldn't be touched without a Lucas say-so. All the Mesus here are ones with the fixed motors so I can't help. I hope you get it sorted quickly.

Olly

[old_eyes]

UPDATE.

So I think (deep breath) we are OK.

Immediately after the fault started I contacted Lucas Mesu, and he was his usual helpful self.

Checking over the mount, I realised that the RA servo (the RA axis was the problem) was sticking out much further than the DEC servo and seemed loose. Lucas asked for some pictures and video, and when I touched the RA servo to show the problem it fell out of the mount. Fortunately not completely, as the cables and the equatorial wedge stopped it coming right out. Worrying that it was not a good idea to have an open hole into the RA mechanism, I pushed it back into place.

Lucas sent over some pictures and measurements to show exactly the depth the servo should be inserted, and the orientation to prevent strain on the encoder connections. The servo is held in place by a grub screw and this was loose. I put the servo back as instructed, tightened the grub screw to what felt like a similar torque to the DEC axis servo, and nervously tested the mount. It seemed OK in daylight, slewing to roughly where I expected a target to be in the sky and returning to the parked position (at least to the accuracy of a spirit-level).

My remaining worry was that the servo being out of position might have damaged the drive shaft or the encoder connections. So a live test was needed.

Last night I was able to test under relatively clear skies and I got about 6 hours run time in total. At first, the guiding curve was poor, but I realised I had been knocking the whole system about a bit, so I recalibrated in PHD2, which settled the guiding down enormously. Polar alignment also seems to be a bit off, so there may be more to gain from sorting that out. Slewing, centring, meridian-flips etc all worked fine.

I imaged the Horsehead Nebula (as this is a project I am working on at the moment). This is  a PHD2 log chart for part of the session:

Not great guiding, but the Horsehead is low down in my skies and the seeing was not great. For comparison, here is the guide chart for the same target on 2nd January before the problems started.

Very similar results. I get better guiding with higher targets under better sky conditions, so I can be hopeful that the mount will be as before.

It is tribute to the solid construction of the Mesu 200 that it was possible to put the RA drive back together and have it work normally. That and my luck. I have not explored the whole sky yet, so it is possible that there is a problem lurking there, but that would mean I had damaged the RA axis disk. Hopefully that is the stronger part of the mechanism and would be OK.

If you have a Mesu mount, it might be a good idea to check that the grub screws holding the servos into their collars are not coming loose. I think I will check mine every year, just to be on the safe side.

Anyway, panic over. Now I just have to deal with the North Wales skies!

old_eyes

[ollypenrice]

I think, 'Phew,' might be the word we're looking for.  An RMS of 0.6 is very unlikely to be your limiting factor for resolution.

Olly

[old_eyes]

19 minutes ago, ollypenrice said:

I think, 'Phew,' might be the word we're looking for.  An RMS of 0.6 is very unlikely to be your limiting factor for resolution.

Olly

Yes very lucky indeed. I was already planning a road trip to Sint Oedenrode for repairs and wondering what sort of paperwork/carnet I would need for a temporary import for surgery!

The rig does go below 0.5" RMS for the right target on the right night, but I am happy with 0.6". As you say - not a limitation.

[Jonk]

Thanks for sharing the reason - this serves as a good reminder to check over equipment we use - a small loose grub screw and suddenly the large expensive kit cannot work as intended!

Also, the RMS you see is probably the conditions - I had my AZEQ6 at <0.4" a few nights ago for a couple of hours.

[tomato]

Good to hear of a happy outcome. My Mk 1 delivers varying guiding performance, on a good night, and the right target it can hit 0.3” total RMS. However I have a big load on there so it is sensitive to balance, especially when I keep putting things on it! PHD does it’s best but I can see the error double from the best performance.