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Dampening the Shakes

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#1 Asbytec

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Posted 15 January 2020 - 06:41 AM

So, my Dob is flocked, shielded, weighted, collimated, soaped, and ready to go. I think the next project is to dampen residual shaking when focusing at high magnification. I can focus, no worries, but it would be nice if the mount/scope could be modified for better vibration dampening. 

 

I guess my first question is, what are some of the sources of shaking so I can isolate and strengthen those sources? The feet, the solid tube length, torque on the riser boards, the ground itself, any or all of the above? Second question is, if you've modified your mount to reduce shaking, what worked for you?

 

I have the ES Firstlight 8" f/6 with the IKEA style fasteners. That might be a source, but I keep them pretty tight. Sometimes I set up on patches of grass, and sometimes on a small brick surface (custom built to keep the Scope out of the tall grass). 

 

Thanks in advance. 


Edited by Asbytec, 15 January 2020 - 06:45 AM.


#2 Jon Isaacs

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Posted 15 January 2020 - 07:27 AM

Norme:

 

Two questions:

 

Does the scope shake when tracking at high magnifications?

 

Does the scope have a two speed focuser?   A two speed does help focusing because it offers a finer feel and motion but in many ways the real advantage is a two speed is a gear reducer so it dramatically reduces the force required to focus.  This means a light touch and much less jiggling.

 

There are any number of possible reasons a scope can jiggle when focusing..  Loose spider vanes are a possible cause.  A heavy finder that's cantilevered can result in the jiggles.  

 

Jon



#3 cmooney91

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Posted 15 January 2020 - 07:37 AM

Reducing the force applied to the telescope's structure during focusing is another approach. 

 

You cold make an enlarged focus knob or lever that goes over the existing knob. I've seen people use the lid to a peanut butter jar. I think a lever is better because you don't have to grab it (shake)  to use it.  Increasing the radius by 3X  decreases  the force by 1/3X

 

Another more expensive option would be a 10:1 reduction knob

 

Edit: Jon beat me to it


Edited by cmooney91, 15 January 2020 - 07:38 AM.

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#4 Starman47

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Posted 15 January 2020 - 08:17 AM

My answer may not be what you want, but almost any part of a telescope can introduce vibration into the whole system. So, it is best to work systematically starting at one end of the telescope (or the other end). And keep in mind that there may be an issue with more than one part of the telescope. And remember most of the people on CN will help. Test their suggestions as necessary, and work systematically,

 

The feet on the groundboard may be small points. I replaced mine with hockey pucks.

The rocker box sides (left and right side) may be under engineered for the task.

 

Mirror cell may not be attached to the tube firmly

 

The truss system my be under engineered (which is not an issue for the ES Firstlight 8" f/6).

The focuser may be loosely attached to the scope.

The draw tube may be wobbly,

The spider may be wobbly.

The secondary holder may be wobbly.

 

I worked through a similar issue last year. I used a laser collimator. I was able to watch the spot on the primary move as I touched/moved different parts. I identified one major component that moved when it was not supposed to move. But in fact, several components needed work to resolve the problem.

 

Clear skies


Edited by Starman47, 15 January 2020 - 08:20 AM.

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#5 Asbytec

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Posted 15 January 2020 - 08:22 AM

Does the scope shake when tracking at high magnifications?

 

Does the scope have a two speed focuser? 

Yes, it shakes when tracking, but settles pretty quickly when I remove my hands. No two speed focuser, thought about getting one (in the meantime, I crafted a larger focuser knob to test). I agree, a light touch makes a difference. I usually give it a light touch and approach focus nice and easy.  

 

The spider vanes are tight. I do have a heavy focuser, the Hexafoc 2.5" is pretty stout, and a 50mm RACI...then some counter weight on the other end (USB Power bank at 600 grams and two speaker magnets). Probably doing that cantilever jiggle. Good point. But it dampens in short order. I dunno, about a second after I let go...it's done shaking. Maybe it's just me...all shaky in old age. :) 

 

 

You cold make an enlarged focus knob or lever that goes over the existing knob. I've seen people use the lid to a peanut butter jar. I think a lever is better because you don't have to grab it (shake)  to use it.  Increasing the radius by 3X  decreases  the force by 1/3X

 

How did you know I used a peanut butter lid, too? You know, a lever might be worth a try. Thanks. Gotta scrounge around the kitchen and laundry room to find something. :)



#6 Asbytec

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Posted 15 January 2020 - 08:29 AM

My answer may not be what you want, but almost any part of a telescope can introduce vibration into the whole system. So, it is best to work systematically starting at one end of the telescope (or the other end). And keep in mind that there may be an issue with more than one part of the telescope. And remember most of the people on CN will help. Test their suggestions as necessary, and work systematically,

 

<snip>

 

But in fact, several components needed work to resolve the problem.

 

Clear skies

Got ya, I agree...more than one cause working together. Right on. One of the first things I looked at was pushing slightly on the tube from side to side against the rocker box. It does seem to rock slightly. Even the ground board seems to rock a tiny bit suggesting maybe the feet aren't as solid as they look. Not sure yet...but yea, I suspect the feet as one issue. Easy fix. I cannot tell, really, if the risers are rocking separately from the ground board. The whole mount may rock a tiny bit maybe as one solid piece. A tiny bit at high magnification is a lot. 

 

Thank you. 



#7 SeattleScott

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Posted 15 January 2020 - 03:53 PM

One second dampening time is pretty good. Setting up on grass instead of pavement helps.

Scott

#8 Asbytec

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Posted 15 January 2020 - 06:13 PM

One second dampening time is pretty good. Setting up on grass instead of pavement helps.

Scott

I've wondered about that. Tried both, but undecided which is better. Out here in the very rural area, the grass is wild and it grows in lumps. Sometimes it can grow to waist high, the goats cannot keep up. :) So I have to find or make a suitable patch to set up on. One where the feet actually touch the ground (lol.gif) instead of the ground board resting on a lump of grass. I usually place the scope and test the stability beforehand. 

 

Yea, the dampening time is tolerable, just when I am focusing it can be a little shaky for my liking. The image is moving when we slew anyway, anyway, so I pay little attention to any shakes. Just focusing trying to use a light touch which sometimes works well enough. The feet seem to be made of some strong rubber, but not completely stiff. They might compress and give a little (maybe more so on a hard surface?). 

 

My previous 12" Dob, the rocker box was a bit tall and I thought there might be some torque on the riser boards, so I added a cross member to stiffen them against any torque applied by the tube. It did not help much. My Mak, I could observing with my hand on the scope or tripod with no visible shaking. I don't expect that kind of stability with my Dob, but some dampening would be nice to help with focusing. 


Edited by Asbytec, 15 January 2020 - 06:18 PM.


#9 Oberon

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Posted 17 January 2020 - 04:05 PM

One second dampening time is pretty good. Setting up on grass instead of pavement helps.

Scott

Yech! How is that possible unless the paving is loose?

 

For grass I use spikes. Flat feet on grass are useless. You want a solid 3 point connection to the ground.


Edited by Oberon, 17 January 2020 - 04:06 PM.


#10 Oberon

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Posted 17 January 2020 - 04:11 PM

I've wondered about that. Tried both, but undecided which is better. Out here in the very rural area, the grass is wild and it grows in lumps. Sometimes it can grow to waist high, the goats cannot keep up. smile.gif So I have to find or make a suitable patch to set up on. One where the feet actually touch the ground (lol.gif) instead of the ground board resting on a lump of grass. I usually place the scope and test the stability beforehand. 

 

Yea, the dampening time is tolerable, just when I am focusing it can be a little shaky for my liking. The image is moving when we slew anyway, anyway, so I pay little attention to any shakes. Just focusing trying to use a light touch which sometimes works well enough. The feet seem to be made of some strong rubber, but not completely stiff. They might compress and give a little (maybe more so on a hard surface?). 

 

My previous 12" Dob, the rocker box was a bit tall and I thought there might be some torque on the riser boards, so I added a cross member to stiffen them against any torque applied by the tube. It did not help much. My Mak, I could observing with my hand on the scope or tripod with no visible shaking. I don't expect that kind of stability with my Dob, but some dampening would be nice to help with focusing. 

There is no reason why a Dob can’t deliver excellent stability. Feet should not be rubber, so get rid of those, but thats probably just the first step on a systematic effort to track down the wobbles.


Edited by Oberon, 17 January 2020 - 04:12 PM.


#11 Asbytec

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Posted 17 January 2020 - 05:22 PM

There is no reason why a Dob can’t deliver excellent stability. Feet should not be rubber, so get rid of those, but thats probably just the first step on a systematic effort to track down the wobbles.

Thank you for chiming in, Jonathan. I replaced the feet yesterday, testing later today or tonight. Another CN member with the same scope said it helped, so I am hopeful.

 

But, yes, the beginning of a systematic approach. Until it's dampened to an acceptable level, anyway, at high magnifications. To the extent possible and feasible... 


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#12 Asbytec

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Posted 18 January 2020 - 08:04 AM

It looked to be clear tonight, so I sat up on my brick pad carved from the tall grass. Some of grass is 8 feet high. Seeing looked promising, too, but the scope had not yet cooled (as it cooled, the star images got nicer).

 

So, the trapezium was my test subject. I am not convinced the hard feet on bricks made much difference. I guess I should have been more clear, I was concerned with the amplitude of the shakes. The stars were flying around the field of view making it hard to follow and focus. Dampening after focusing turns out to be a little closer to 2 or 3 seconds.

 

Anyway, I was not convinced anything was any better, so I spied a clear grassy spot and set up there. And I realized my focuser friction lock was, well, not loose. I think this may have required me to use more than an easy touch on the peanut butter lid causing a lot of the shakes. So, I kind of broke with tradition and fixed two things at once so I have no idea which worked. :lol:

 

But, on the grass, I guess some folks are right. It does seem to dampen a little nicer and with the friction lock loosened, I could reduce the amplitude of the shakes significantly. Bottom line is, it's manageable and I can approach focus more cleanly. 

 

Broken clouds rolled in, so I swung over to 52 Ori to split and practice focusing on it. Not bad and got a clean split at 330x. Much easier to focus. I also played with my hand position touching the scope in different places. Turns out, the biggest shakes are when I first lay a hand on it, then it dampens and might shake a tiny bit. 

 

I bet it will be crystal clear by morning, as usual, so I may just wait a few hours and set up, again, after the movie tonight. Thanks to each of you...



#13 brentknight

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Posted 18 January 2020 - 09:42 AM

I replaced the plywood feet with hockey pucks on my Discovery 10", but noticed the wiggles were worse. I later realized it wasn't so much the pucks as the placement of the feet. It's important to make sure the feet are directly supporting the teflon pads above.
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#14 Asbytec

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Posted 18 January 2020 - 09:47 AM

I replaced the plywood feet with hockey pucks on my Discovery 10", but noticed the wiggles were worse. I later realized it wasn't so much the pucks as the placement of the feet. It's important to make sure the feet are directly supporting the teflon pads above.


OK, I'll check that. Didn't cross my mind. Thanks
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#15 jtsenghas

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Posted 18 January 2020 - 06:02 PM

Yes,  I am reading this thread for my first time and I was planning on suggesting that you check that the feet are directly below azimuth pads. 


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#16 doug mc

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Posted 18 January 2020 - 08:07 PM

I solved the shakes on my scopes with this simple attachment. Just pushing on the spokes instead of grabbing the focusor, cures the shakes. Works well on single speed as well.

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#17 Asbytec

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Posted 19 January 2020 - 12:30 AM

I solved the shakes on my scopes with this simple attachment. Just pushing on the spokes instead of grabbing the focusor, cures the shakes. Works well on single speed as well.

I walked through the hardware store looking for inspiration, for something like that...large knob or some kind of lever. 

 

From above, it turns out the azimuth bearings are not directly over the feet. Seems to me they should be so to better transmit any vibration directly to the ground. The bearings are midway between the feet, I suppose to avoid the mounting screws holding both from hitting each other from opposite sides of the board. So if there is any pressure on the bearing (even the slightest pressure), stability relies on the unsupported strength of the base board to transmit the vibration, shaking, or any torque in a less direct path to the ground. I think that is right. It might help. 

 

Was thinking about those rubber feet, too. They should compress slightly under the load of the rocker box and the tube assembly. So, if there is any torque (even the slightest), the feet in the direction of the motion probably won't compress significantly. But, the the feet opposite the torque, shake, or vibration probably decompress slightly as the load is translated to the other feet. Then they re compress as the vibration cycles to and fro. Being compressible allows the rubber feet to give a little and probably allowing the load to shake a tiny bit. I think that's right. 

 

Both of the above are probably very small influences, but magnified at high magnification. So, seems every little bit might add up to something less shaky. 

 

Edit: both modes done. Stronger feet and placed over the azimuth bearings. An initial test pushing on the tube does seem a lot more sturdy, though, and I am encouraged already. The bearings were cleaned and soap applied. We'll see tonight. Thanks, again. 


Edited by Asbytec, 19 January 2020 - 01:33 AM.

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#18 Asbytec

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Posted 19 January 2020 - 05:32 AM

Holy cow, vibration is down to almost nothing. Something worked very well, I believe aligning the az bearings with the feet made a big difference. I'm a very happy camper. smile.gif

My heartfelt thanks to each of you. I'm done with this project. smile.gif

 

Edit: Even tracking is greatly improved, and my scope just became more of a pleasure to use. I was focusing on Uranus and tracking it at 600x last night with the greatest of ease. 


Edited by Asbytec, 19 January 2020 - 06:47 PM.

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#19 jtsenghas

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Posted 19 January 2020 - 07:35 PM

This is SO OFTEN an issue with dobs, both homemade and commercial.   The entire baseboard becomes a bit of a springboard when feet aren't close to support pads. Small flexing gets amplified in angle by the magnification of the scope. I'm glad most of your issues were simply that. 

 

Another common issue with vibration on dobs that have tall rocker boxes is insufficient stiffness of  the rocker box.  This is far more common in homemade closed tube scopes than truss dobs that tend to have lower and stiffer rocker boxes.  When it does occur the symptom is more oscillation in the azimuth.direction.  When this is the case,  triangular braces to the sides of the rocker box can do wonders. 


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#20 Asbytec

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Posted 19 January 2020 - 07:42 PM

The entire baseboard becomes a bit of a springboard when feet aren't close to support pads.

 

Yep, the effect is amazing. 

 

Another common issue with vibration on dobs that have tall rocker boxes is insufficient stiffness of  the rocker box.

 

Yep, again, I found this to be the case in my previous 12" solid tube Dob. 

 

Man, I cannot reiterate enough how pleased I am with the fix and each of you for assistance. 

 

1. Stiffer feet (wood drawer handles)

2. Feet relocated below the azimuth bearings. 

3. Large focuser knob for slow motion and an easy touch. 

4. Set up on the grass (but may verify that by setting up on my small brick pad). 

5. I also replaced one of the IKEA style locking bolts in the rocker box that was coming loose over time. 

6. I turned my azimuth bolt upside down to provide an additional 1/4" of clearance for the tube to ride a bit lower. (I was able to view Venus without any additional weight, better than previously). 

7. Cleaned and soaped the bearings...

 

Done! 


Edited by Asbytec, 19 January 2020 - 07:49 PM.

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#21 Asbytec

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Posted 19 January 2020 - 07:53 PM

Yech! How is that possible unless the paving is loose?

 

For grass I use spikes. Flat feet on grass are useless. You want a solid 3 point connection to the ground.

Sorry, I missed your comment earlier. 

 

I hobbled together a small brick platform outside in the wild grass thinking a firm surface would help. Maybe it does. I need to confirm it as I tested solely on the grass last night. I actually toyed the idea of using feet that stick into the ground, too. Not good for my tile floor, though. :lol: The wooden feet do seem to be doing the job. 



#22 jtsenghas

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Posted 19 January 2020 - 08:49 PM

You do indeed not want to be on soft springy turf, but most ground is surprisingly firm. If you are setting up on grass,  you want to be sure at least that the turf is not contacting the baseboard except to the feet. 

 

Grassy areas tend to be favorable to paved areas because there is far less heat typically radiating from it at night to add to the seeing. 

 

It may be useful to establish a small area of compacted soil or to make a small observing pad if your grassy area is springy. Jonathan's suggestion to drive solidly into the ground is also a good one,  in my opinion. 


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#23 SeattleScott

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Posted 19 January 2020 - 10:57 PM

Yech! How is that possible unless the paving is loose?

For grass I use spikes. Flat feet on grass are useless. You want a solid 3 point connection to the ground.

Ok I guess i am used to tripods.

Scott

#24 Asbytec

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Posted 20 January 2020 - 04:13 AM

You do indeed not want to be on soft springy turf, but most ground is surprisingly firm. If you are setting up on grass,  you want to be sure at least that the turf is not contacting the baseboard except to the feet. 

 

Grassy areas tend to be favorable to paved areas because there is far less heat typically radiating from it at night to add to the seeing. 

 

It may be useful to establish a small area of compacted soil or to make a small observing pad if your grassy area is springy. Jonathan's suggestion to drive solidly into the ground is also a good one,  in my opinion. 

The ground here is dry and pretty hard, it's compacted somewhat by the last rains we had and the drying effect of the sun. Still the idea of driving the feet into the ground (like a tripod, not sharp but pointy enough) is not lost on me. The wooden drawer handles I found in a hardware store (gotta work with what you can find) are rounded at the end, so not exactly flat. Best $2.50 I spent in a long while. I'll keep an eye out for something more pointed. The small brick pad was meant to create some clear level ground, and yes, thermals were a concern as well as a trade off looking for stability. 

 

Whatever the case, the scope is significantly more stable and beyond what I hoped for. I really believe the most significant contribution came from aligning the feet with the azimuth bearings. I noticed it right away after the Mod was complete and even before testing. Now, I cannot resist walking by the scope and giving it a little push to feel it move (or resist movement) as one solid piece instead of being spongy. Such a dramatic improvement. And it looks clear tonight, so gotta do an observing plan and get back out there and enjoy it. smile.gif

 

Thanks again, I cannot tell you enough how happy I am with the assistance in this thread. This one really helped a lot. 


Edited by Asbytec, 20 January 2020 - 04:14 AM.

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#25 gwlee

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Posted 22 January 2020 - 07:19 PM

Unless there’s some part on your scope that’s loose and needs to be tightened down to eliminate the shakes during focusing, it’s likely that a 2-speed focuser will provide more immediate and obvious improvement than anything else you can do.
 

I have a 2nd base for my XT8 that’s constructed from Baltic Birch. Otherwise, it’s dimensionally identical to the original factory base, uses exactly the same hardware, and performs identically. For two years or so, I have been experimenting by making various reversible changes to the BB base that I think might help to reduce the shakes and then doing side-by-side testing with the original factory base. So far, I haven’t seen much improvement with this approach, but have a few more ideas that I want to try when I get around to it. 

 

 

 

 




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