Mesu 200mkII

[Luiscarss]

Hello everyone, I recently bought this frame, I have it supporting a 16" newton and about 30 kg of weight. The tube is made of carbon by Klaus, and the mechanics and optics are by TS.

I have it in an observatory with a drop-down roof, so it rises a third above.

The mount works well when the conditions are optimal, but at the slightest wind, just a little, the stars dance and I can't use it.

I'm struggling with the vibrations but I can't get them to go away

I did the following tests to see why the mount oscillated so much:
- Attach the tripod to the ground with cables to give it more stability
- Screw the equatorial wedge with more screws to the tripod
- I replaced the tube's losmandy tail with two new clamps and a reinforced losmandy tail
- I did tests with an iron bar of similar dimensions, to discriminate that the vibrations were not from the tube
- Record videos with a slight wind so that you can see the stars dance, and without wind so that you can see that the problem came from a slight breeze
- Change the center of gravity of the tube to center it in the mount by adding weight on the focuser
- I added against nuts to the screws
- They gave me a precision watch and I measured the oscillations in each part of the tube in reference to the ground, the tripod, the equatorial wedge and the tube.

https://www.youtube.com/watch?v=oj5BH5WElug
https://www.youtube.com/watch?v=8gMpb1jTTbc
https://www.youtube.com/shorts/JyqnIO7e1Kw
https://www.youtube.com/shorts/pYv-ZPqxhSw
https://www.youtube.com/shorts/uc6NYmI88JY
https://www.youtube.com/shorts/ZAXvKCNAoPs
https://www.youtube.com/shorts/nMZv75XJOgU
https://www.youtube.com/shorts/subK-L9pTGU
https://www.youtube.com/shorts/O5rpNEhX-jk

I made these videos to try to determine where the vibrations are produced

Can someone who has this mount help me?

[900SL]

Calling @ollypenrice

[ollypenrice]

I've had Mesus heavily loaded and never seen this.

How about the floor itself? Could the wind be moving the shed which then moves the floor? (Maybe try your metre on the floor itself?)

What about trying to check the vibration while the mount is not tracking? That might eliminate irregularity in the drive train.

Are you confident that the balance is correct? (I must say that my Mesus did not seem terribly sensitive to balance.)

How windy is windy at your location? 

Have you contacted Lucas?

Olly

[gorann]

I also have Mesu mounts but never seen this, but I have them on concrete pillars attached to half a tonne of concrete in the ground and isolated from the obsies.

There is a dedicated Mesu thread here on SGL, and you may consider posting your problem there.

 

[tomato]

That 16" Newt is presenting quite an area to the wind but I would echo the question how strong are the winds in your location? I have a Mesu Mk1 with 2 Esprit 150 refractors on there which never suffers from wind tremor, but the mount  is bolted via a substantial pier to a 1 cubic metre concrete block in the ground, and more to the point it is inside a dome.

I have crudely tried to replicate your touch test on my rig, I don't think there are any settling oscillations.

 

 I also have a 16" SW flextube dobsonian which I would say is on a stable base, but the eyepiece view trembles on that when the wind blows.

[SamAndrew]

You might find that the current distribution of weight has shifted the natural frequency of the system so that it is now resonating. I would add or remove weight from the counterweight shaft (and rebalance) and see what impact that has. Otherwise you may need to add something else to damped the vibration 

[whipdry]

Not sure the Mk1 can be compared to the Mk2 as the differences in design are significant, the counterweight support section & the bar are both hollow the first being a box & second a tube, both are solid on the Mk1... would anyone here know if this could be of any significance re: vibration when you suspend weights on hollow  structures compared to something more solid?

Peter

[SamAndrew]

The bar was solid on my Mk2, different structures will have different natural frequencies, it's the combination of the whole system that will decide if the system will oscillate.

[whipdry]

42 minutes ago, SamAndrew said:

The bar was solid on my Mk2, different structures will have different natural frequencies, it's the combination of the whole system that will decide if the system will oscillate.

Ok thanks, I've never understood the thinking behind supplying a tube rather than a solid counterweight bar, seems counterintuitive... I did ask but never got a answer to that particular question!

[Tomatobro]

I watched the videos and I think the issue is with the tripod.  In particular the structure above where the legs attach to where the Mesu sits.

Tomato used a portable mount with his Mesu for quite a while and had no issues with wind even though the setup was in the garden. That mount had a 125mm diameter steel tube with a wall thickness of 5mm.

 

 

[Luiscarss]

Hello everyone, thank you very much for your interest, I try to answer all the questions and see what I can try

 

On 5/10/2023 at 9:32 PM, ollypenrice said:

How about the floor itself? Could the wind be moving the shed which then moves the floor? (Maybe try your metre on the floor itself?)

What about trying to check the vibration while the mount is not tracking? That might eliminate irregularity in the drive train.

Are you confident that the balance is correct? (I must say that my Mesus did not seem terribly sensitive to balance.)

How windy is windy at your location? 

I have tried to measure the oscillations from the ground to the tripod, and there is none, from the tripod to the equatorial wedge there are no vibrations either, the vibrations begin behind the three screws, and are more intense on the DEC axis

The winds that particular night were 7 or 8 km per hour

I have been writing with Lucas for five months, plus the year that I was waiting for the frame, and he is very ambiguous in his emails, I already did the tests that he recommended but they did not solve anything. The last thing I know about Lucas is that he didn't have time for any more of my experiments, and when I asked him for a refund, he didn't answer me again.

21 hours ago, tomato said:

I have crudely tried to replicate your touch test on my rig, I don't think there are any settling oscillations.

I made an iron bar to rule out flexing in the tube, and the frame kept oscillating at the slightest touch

 

21 hours ago, SamAndrew said:

You might find that the current distribution of weight has shifted the natural frequency of the system so that it is now resonating. I would add or remove weight from the counterweight shaft (and rebalance) and see what impact that has.

I'm going to try what you tell me to see if I can improve it

 

2 hours ago, Tomatobro said:

I watched the videos and I think the issue is with the tripod.  In particular the structure above where the legs attach to where the Mesu sits.

If it were the tripod, shouldn't it also vibrate with respect to the ground?

On 5/11/2023 at 1:02 PM, gorann said:

I also have Mesu mounts but never seen this, but I have them on concrete pillars attached to half a tonne of concrete in the ground and isolated from the obsies.

 

I did not feel those oscillations, although I did not do it for much either time, I was afraid that the EQ8 would break, that's why I bought the Mesu

 

Thank you all very much, the truth is that these days there has been no wind, and I have been able to use it, I have also tried it with a smaller tube, and it works well, the problem is when the conditions are not optimal

[whipdry]

I'm pleased to hear the mount can work well for you... guess the size of the scope/sail is the main factor where wind is concerned, with a much more compact sail/scopes etc. my Mk2 works suppressingly well in quite strong gusty winds!

Peter

[dan_adi]

On 10/05/2023 at 21:31, Luiscarss said:

Hello everyone, I recently bought this frame, I have it supporting a 16" newton and about 30 kg of weight. The tube is made of carbon by Klaus, and the mechanics and optics are by TS.

I have it in an observatory with a drop-down roof, so it rises a third above.

The mount works well when the conditions are optimal, but at the slightest wind, just a little, the stars dance and I can't use it.

I'm struggling with the vibrations but I can't get them to go away

I did the following tests to see why the mount oscillated so much:
- Attach the tripod to the ground with cables to give it more stability
- Screw the equatorial wedge with more screws to the tripod
- I replaced the tube's losmandy tail with two new clamps and a reinforced losmandy tail
- I did tests with an iron bar of similar dimensions, to discriminate that the vibrations were not from the tube
- Record videos with a slight wind so that you can see the stars dance, and without wind so that you can see that the problem came from a slight breeze
- Change the center of gravity of the tube to center it in the mount by adding weight on the focuser
- I added against nuts to the screws
- They gave me a precision watch and I measured the oscillations in each part of the tube in reference to the ground, the tripod, the equatorial wedge and the tube.

https://www.youtube.com/watch?v=oj5BH5WElug
https://www.youtube.com/watch?v=8gMpb1jTTbc
https://www.youtube.com/shorts/JyqnIO7e1Kw
https://www.youtube.com/shorts/pYv-ZPqxhSw
https://www.youtube.com/shorts/uc6NYmI88JY
https://www.youtube.com/shorts/ZAXvKCNAoPs
https://www.youtube.com/shorts/nMZv75XJOgU
https://www.youtube.com/shorts/subK-L9pTGU
https://www.youtube.com/shorts/O5rpNEhX-jk

I made these videos to try to determine where the vibrations are produced

Can someone who has this mount help me?

I use a 8 inch refractor with the older Mesu. It does ok during light winds, but it has a smaller surface area than the big newton. One thing you could try is placing a longer saddle on the mount, so the contact surface between scope and mount increases. I use a 60 cm long saddle. It could also be that the scope is simply too big for the mount. It's not always about the weight.

 

PS: I saw you use a tripod. Don't. Make a steel pier or concrete

Edited May 13, 2023 by dan_adi

[windjammer]

Hi 

I don't have a mesu but a couple of points:  your setup to my eye looks wrong - the scope is outsize for the mount.  You seem to have 4x 10kg weights, two of them a twice as far from the centre of gravity of the OTA.  The load of weights and OTA at 80kg+ are hanging off tiny bearing surfaces.  The pics of scopes others have posted here are tiddlers in comparison !  When the wind catches the end of the very long OTA the torques on the drive mechanics must be very large - as I understand it the mesu is a direct pressure drive with a small diameter drive shaft pressing against the edge of a larger disk on which the scope sits.  So it is only the tiny pinion that can resist wind-driven turning moments of the disk.  

Everything flexes, so the important thing is how fast do the vibrations die away.  If the mount settles in 2 or 3 seconds after an impulse that would generally be considered a reasonable performance.  If the wind kicks at a higher rate then even a good mount would not function adequately.  Looking at your clever tests with the test dial indicator, the settling time does seem a bit long.  Can you improve the damping so the mount settles faster - sometimes loosening up nuts and bolts can help detune the system.  Some foam packing of the OTA to the mount might absorb some vibration.  Vibration pads at the tripod feet might also help to drain away the energy.  Lots of small improvements to the damping are probably more likely that one big fix, unfortunately.

Your OTA is a huge windsail - if performance is good as you say with no wind, then perhaps raise the observatory walls to shelter the OTA (even a removable canvas wind break might do).  The usual remedy for wind is a dome with observing slit, perhaps that is the way to go.

Simon

[Luiscarss]

I'm going to try your options, to see if I can improve the system. I think that the mount cannot handle so much sail, the traction system that it has seems not to be the most suitable for my tube. It makes me a little angry because before starting the project I asked if there would be any problem. I have in mind to build a dome.

 

Thank you all very much for your interest, because I'm a little frustrated with the manufacturer's troubleshooting and the fear that if something breaks I don't know where to go.

[Zakalwe]

Get shot of the tripod and tube. Carbon fibre is very rigid *along the strands of the fibres* but can be flexible in other axis. You also need mass to damp down the vibrations. That tube, mount and counterweights represent a heck of a lot of mass and surface area, all mounted on a tipod.

I would wager that a decent steel or concrete pier will get rid of a lot of your problems. You could try hanging a substantial weight directly underneath the tripod, suspended off the ground to see if more mass will help with the damping.

[Luiscarss]

I'm going to try to write a column, to see if I improve a bit, although I don't have much hope. I'll tell you.