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Vibration Problem with CGEM?

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#1 jacobmarchio

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Posted 22 March 2013 - 08:56 AM

Hi all,
A few nights ago, I wanted to do some lunar imaging with my 6", as seeing wasn't all that good and I wanted a wider field. So I put the 6" on the CGEM (there's an ADM saddle that goes from the CGEM down to that small, since my C9.25 dovetail is also that size [CG-5, I think?]) Anyways, put that on there and balanced it. It looked funny, y'all. A (comparatively) dinky little 6" F/5 riding a huge CGEM, with the counterweight almost all the way up the bar. So I did the GoTo alignment, and slewed to Jupiter. The moons were all lines, like this " - ". I thought, "Surely, the seeing isn't THAT bad". I also noted that the last time I saw something like that was when I ran my computer fan on the back of the scope and caused vibrations all over the place. It really looked like vibrations. Oh, I must mention I'm on grass, not concrete. So I put my ear to the mount, and I could hear it tracking. Nothing seemed abnormal. So shut down the mount and started it again. No improvement. At this point I'm slightly bewildered; I'm walking around and around the mount and scope, looking for problems. So finally I look down the scope. I then touch the secondary spider vanes, and the were vibrating VERY noticeably. In fact, I could hear the vibrations in the tube, it sounded like a very low note on a musical instrument. Then, I put my hand on the outside of the tube, and feel the vibrations. So, now I'm really confused, as this never happened with my heaver C9.25. So I take the 6" Newt off, and put the C9.25 on, and balanced it. I aligned the mount (again) and slewed the mount over to Jupiter. It finds Jupiter, and I look through the eyepiece. The moons are pinpoints, even at a higher magnification. I'm thinking, "How can this be? Astrophotographers always recommend overmounting the load." It didn't (and doesn't) make sense. Does anyone have any suggestions?

If you're wondering, all the knobs were tightened down. No lose knobs.

I also balanced the mount with both the C9.25 and AstoView ota.

#2 dickbill

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Posted 22 March 2013 - 09:41 AM

Be sure the spreader plate (between the tripod) and the bar knob over it are tight.

#3 Cliff Hipsher

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Posted 22 March 2013 - 11:23 AM

As a former US Navy Submarine Sound Analysis Tech it is my pleasure to welcome you to the world of harmonics, resonance and standing waves.

Your 9.25 SCT does not exhibit this behavior because of its mass and construction which define its "natural frequency".

With the Newt, the open tube acts like an amplifier in much the same way the sound board in a piano works.

Apparently the motor vibration frequency is at some multiple of the optical tube natural resonant frequency.

To prove this, mount the Newt, turn the mount on and watch the spider vanes. In fact, if you can, look at the vanes and then turn the mount on.

If this is an issue with resonance you will see the vanes begin to vibrate and at some point the amplitude will peak.

When you see the vanes vibrating you are witnessing resonance induced standing waves.

The best way to deal with this is to dampen either the vibration at its source (ie the offending motor(s)), or dampen the vanes/secondary mirror assembly.

#4 jacobmarchio

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Posted 22 March 2013 - 03:52 PM

@dickbill- I actually checked that, and tightened it down anyway.

@Cliff Hipsher- thanks for taking the time to write your informative post. I don't have a way to dampen the secondary or vanes, so... I guess I'll just leave it on the AstroView (EQ-3) mount.

#5 EFT

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Posted 23 March 2013 - 11:22 AM

I would agree that you probably have a harmonics issue occurring here. One thing you might want to try is finding and using a lighter counterweight that can sit further down the bar. Even though the balance would be the same, it is possible that a different weight distribution might make the difference. Also try changing the balance of the mount, i.e., less well balanced, and see if that changes things with the motors working a little harder.

#6 Starhawk

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Posted 23 March 2013 - 12:46 PM

Try adding something to the scope to make it heavier and rebalance. A guide scope or super finder sized short focal length refractor would do nicely, but a scuba weight would do the same thing. Then rebalance in this mode.

The idea would be to lower the natural frequency of the scope.

Natural frequency = sqrt (stiffness/ mass)

Lowering the natural frequency will make it less responsive to high frequency vibration. Lowering stiffness would do the same, but is harder to arrange and will cause other problems.

Just for the sake of discussion, increasing stiffness and decreasing mass will make it more able to transmit high frequency vibration.

-Rich

#7 Cliff Hipsher

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Posted 24 March 2013 - 06:22 PM

I don't have a way to dampen the secondary or vanes, so... I guess I'll just leave it on the AstroView (EQ-3) mount.


Try placing a layer of black electrical tape on one side of each vane...

#8 jacobmarchio

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Posted 26 March 2013 - 07:18 PM

Thanks everyone for your incredible help :bow:

Funnily enough, I just tested it and found no vibrations. This is in our shed, on concrete, and I did a fake alignment. The only other thing I did different was I didn't use the battery, I used a power adapter. Can the power source have something to do with it?

If I go out in the field and run into the vibrations, I will definitely try your suggestions.






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