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High elevation shakiness on StarNavigator

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#1 Gmb1830

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Posted 15 April 2024 - 10:49 AM

I’ve been using an SN102 for double star observing. The other night when observing near zenith the normal shakiness never dampened out. I was left with a high-frequency wobble of maybe 10-20”. At lower elevations, no issues: Castor is cleanly split, for example, as it was getting lower in the west, with enough space for a third star in-between. Pointing and tracking nominal.

I would have noted this in previous sessions (I’ve had the scope since December), so I’m guessing this is a setup issue (balance? Tightness of clamshell?). Looking for ideas so I don’t get a repeat.

When looking into the eyepiece, the vibration was mostly left-right, so maybe it’s not in the elevation axis. There was some light wind (5 mph?), but rotating in az by 90 deg didn’t help. Thinking the tube is swinging like a pendulum along that plane.

The one thing I wanted to try, but forgot at the end of the night, was to turn off the drive and see what happens. I was also thinking about skipping the OTA a bit in the clamshell to see if a balance change fixed things. Instead, I just stuck with targets closer to the horizon.

Note I’ve modded the tripod a bit. I’ve built a triangular tray to replace the stock tray. It bolts onto all six holes of the spreader, but on short nights like that one I’ll just let the bolts slide into the holes without tightening. I also weigh down the three corners of the mount with a brick on a paracord loop so a corner of the brick is on the grass.

Sorry for the length, but wanted to provide enough details for those who may have seen this before. Next clear night (looks like at least a week away), I’ll experiment more, mainly with balance and looking for looseness.
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#2 Skywatchr

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Posted 15 April 2024 - 03:42 PM

Without you trying turning off the drive, there isn't much we can suggest other than the breeze was shaking it when the scope was more or less vertical.



#3 MikeBY

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Posted 16 April 2024 - 11:26 AM

It sounds like you are experiencing "float" . This is what happens when the OTA is at a true balance point, where there is no load on the drivetrain in one or both axis of travel. 

In most drive systems there is an inherent amount of space or 'free play' in the gearing to prevent binding and excessive friction.

You may notice this as a delay when changing the direction of motion as the motors take up this gap, which is called backlash. It's normal to have some backlash. Meade has some advanced settings to compensate for this, at least in some mounts.

 

So you'll notice if the OTA is at an exact balance point,  it takes very little to cause OTA movement and there is almost no dampening as the OTA is floating in the backlash and the drivetrain is actually not connected to the OTA, so no pressure is applied on that axis. 

 

One more important note about balancing. When you balance the OTA, you are balancing the moments of inertia. So, when the OTA is horizontal,  the calculation is simply mass x distance from the rotational axis.  

 

A refractor like you have had most of its weight at the two ends.  The large objective lens at one end and the diagonal and eyepiece and finder at the other. The diagonal and eyepiece move. When focusing. So you should focus on a very distant object before adjusting the scope position for or after in the clamshell to balance it. Because you're horizontal,  the force of gravity is at right angles to the OTA and the fact that the finder and eyepiece lens is sitting above the center line of the OTA is of no consequence.  That you are at a balance point while horizontal is of little consequence because we rarely observe objects near the horizon.  It's rare that the mass of the objective lens matches the other components, so we adjust the distance away from center so that the distance x mass at each end is equal.

 

What happens though as the telescope angle changes and it points higher?  

Gavity is now pulling at an angle to the OTA. You can look at that as part of the force still being at right angles to the OTA and another part is along the axis. The combination of those two forces being the same as gravity pulling at an angle. The part that wants to make the OTA rotate (the part at right angles to the OTA) s reduced.  As you approach vertical, the objective lens end approaches zero force at right angles and all of the force pulling along the axis. On the other end, the same thing is happening,  however, the finder and eyepiece ( in the diagonal) are still contributing some force due to the offset from the center axis. 

 

So at the vertical point you've become a little bit "back heavy". But, looking exactly straight up is also just one specific position. The real problem that you're facing is that you are crossing another balance point on the way towards vertical. 

There are several ways to solve the issue.

1st) Start ' back heavy' to avoid the transition from front to back heavy where the balance point occurs. The problem you may have is that by moving the OTA iback n the clamshell you may not have clearance from the mount bar to get close enough to the zenith for your observation. 

 

2nd) Add a counterweight to the system so you can control the balance independent of the OTA clamping position. 

This can as simple as a small weight attached along the bottom of the OTA with strong velcro. I suggest along the bottom to offset the off center finder and eyepiece assuming you're using a diagonal. Put a safety strap on the weight so that it can't fall off if the velcro gives way.  Even a small weight falling few feet can break a toe. 

I don't know the weight of the OTA or any dimensions, not the angle or limits (some scopes have a blind spot near 90°)

 

3rd) go to a 3D counterweight system.  The idea here is similar to #2 except you set up the counterweight on a threaded staff so that you can position it off axis to offset heavier finders,  camera gear, and eyepieces that are off axis. The DIY way is detailed here on CN, but it can be fine cheaply with a U channel, T-,nut, a few hex nuts washers  and some threaded rod.. The weight needs only a hole to pass rod and can be fixed in position with nuts and washers. No need to tap threads on the weight.

This one is probably more than you need,  but I gather you might be the type to tinker a bit. 

 

Two other thoughts, 

When observing near the meridian watch out for the 'Meridian flip' At some point the scope will need to flip both axis 180° to continue to track a star. If may catch you by surprise and if the clearance isn't there you don't have limits set right collisions can occur.  

 

Good idea to do extra brace on the tripod. Tighten the screws,  and add mass to it, center as well  legs.  Keep everything as rigid as possible, but beware the limits of the materials you're working with. Bends, warps or overtightening can make things worse.

Vibration is a problem for everyone.  

 

Sorry if you didn't need all this detail. 

I'd rather error on this side and make it useful for those at any level who may read it. 

This issue comes up for people with all types and levels of equipment.

It's part of the ongoing learning curve. 

 

Clear skies, 

 

Michael

 

 


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#4 Gmb1830

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Posted 24 April 2024 - 08:56 AM

Thanks. I appreciate the detailed reply.

Monday night was predicted to be an abnormally good seeing night, and the first clear night in a while, so I set up with a bit more attention to where the OTA sits within the clamshell. That is, I want to find a reproducible location that yields reproducible results.

I started simple: I use a bubble level as part of the alignment technique. It’s about 6” long., and I aligned the tube such that the level just spanned the distance between the clamshell and the front lens holder. An arbitrary pick (with the idea of adjusting from there), but in the end it worked well.

Before powering up I pointed it towards the zenith to check. At 133X it didn’t exhibit the previous behavior, even with a few light breeze ‘gusts’, so onward to alignment, etc. Overall a good setup, but there’s still this arcsec-level vibration that takes a long time to settle. The bigger shakes from focusing and pointing drop reasonably fast. The next “alpha night” (eg, the only thing visible is alpha Leo, alpha Vir, etc) I’ll schedule some engineering time and see if further adjustments help. I’m now curious if there is a dec component to this—if I point at Polaris, the motors will barely be moving (and engaging the gears less) as compared to when pointing near the equator.

To answer your question, the OTA with nominal accessories (like the 6 mm good line) is about 5 lbs. A lot of fiberglass and plastic in that 800 mm FL. Wish there was enough metal to add magnet weights when needed.

The tripod, especially the legs, are really a weak spot. I’ve considered making my own legs (lots of good examples on this site), but given the need for adjustability in my uneven yard, it’ll be a project only if all else fails.


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