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Reducing mount vibrations.

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#1 jimsmith

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Posted 19 May 2013 - 09:30 AM

I have a WO EZTouch mount on a fixed steel pier. There are two telescopes mounted permanently on it...a 250mm mak and 120mm refractor. The views through the mak are quite stable even at higher magnifications, but the refractor wobble can be quite excessive at higher magnifications. It takes about 2-3 seconds for it to settle down after focussing.

The mak is mounted on the "native" vixen side of the mount. The refractor is mounted using an accessory vixen dovetail attachment screwed/bolted to the other side. I am using the SkyWatcher provided vixen bar and tube rings. The attached photo shows the relevant bit of the set-up.

Would I be likely to get better stability if I mounted the refractor using a Losmandy type plate or perhaps tube rings spaced further apart...or closer together...or...?

I wonder if anyone has any recommendations before I start investing in new kit that might not make any significant difference.

Thanks, Jim

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#2 Whichwayisnorth

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Posted 19 May 2013 - 01:39 PM

I wish I could help. Looks like you are already doing what I would have done. :(

#3 EFT

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Posted 20 May 2013 - 09:45 AM

This is really a mount issue and not a scope and dovetail issue. The majority of the problem that you are having comes from the fact that the refractor has a much longer moment arm than the MAK. On a light weight mount like this, that equals stability problems. A refractor of weight equal to that of the MAK, but much greater length will be less stable than the MAK in most cases. While you might see some flexure in the refractor mounting, it is not likely to be anywhere near the amount of flexure or axis instability that you are seeing from the mount. Unfortunately, for this load, you are looking a needing a more hefty and stable mount.

#4 gdd

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Posted 20 May 2013 - 12:22 PM

Hi Jim,

Maybe motorizing the focuser will allow this mount to work with the 120mm refractor.

Gale

#5 dhaval

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Posted 20 May 2013 - 12:25 PM

The best way to reduce mount vibrations is to try and not have them in the first place! I would say you are probably under mounted. You really need a beefier mount!

#6 jimsmith

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Posted 20 May 2013 - 12:35 PM

Yes, I'm afraid that EFT and dhaval are probably right. I've got too big a load on the mount. Are there any similar but beefier mounts out there?

#7 jimsmith

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Posted 20 May 2013 - 02:47 PM

Motorising the focuser is an interesting alternative, but I'm not really enthusiastic about anything that makes my set-up more complicated.

I carried out a few experiments...
Removing the Mak seemed to make no difference.
Moving the refractor to the slightly shorter mount arm made the vibrations worse. (I wasn't expecting that.)
Sliding the refractor up and down in its rings made no difference to the vibrations.

It looks like I may have to live with the problem, or spend my way out of it. The prices on the aokswiss.ch site for the bigger mounts look a bit scary.

#8 Mark9473

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Posted 20 May 2013 - 03:16 PM

Is the steel pier filled with sand?

#9 jimsmith

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Posted 20 May 2013 - 03:50 PM

No, the steel pier is not filled with sand, although I did have it made with a hole in the top in case filling it turned out to be necessary. The pier does ring if you hit it. but that's at a much higher frequency than the visible vibrations. (About 6 or 7 wobbles per second.) The Mak seems fine so I assumed the pier was OK. Am I perhaps mistaken?

#10 StarStuff1

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Posted 20 May 2013 - 04:02 PM

It certainly would not hurt to fill the pier with sand. Years ago I had a ROR observatory with a 5-in apo with an EQ mount on top of a 4-in square hollow steel pier. Sand definitely helped dampen vibrations in that case.

#11 Dwight J

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Posted 20 May 2013 - 04:26 PM

One problem with using sand is moisture retention and this can cause the pier to rust if it is steel. No doubt it would add mass. An alternative would be low expansion foam which will absorb some of the vibration and pose no rust risk. You could add turnbuckles that go from the top of the pier to the end of the pier legs/feet and once tightened, can reduce movement. Separating the tube rings further apart could also help.

#12 jimsmith

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Posted 21 May 2013 - 01:12 PM

Thanks for more good ideas.
The steel pier is concreted into a deep hole, so it has no legs or feet.
As a long shot, I have just ordered some 3mm Sorbothane sheet which I will cut into strips and use between the scope and the tube rings. It might make some difference I suppose...

#13 jimsmith

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Posted 25 May 2013 - 02:11 PM

The sorbothane sheet was too thick to use in the way I intended. So no progress there.

However, I did change the position of both scopes slightly (I moved one forward and the other back) and that seems to have reduced the vibrations somewhat.

I think I can live with the slight wobbliness now.

#14 Per Frejvall

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Posted 26 May 2013 - 03:44 AM

I notice a quite long Vixen bar... Apart from pier oscillations, that will be the source of problems. I haven't used a Vixen bar since I discovered the Losmandy bars - no comparison!

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#15 jimsmith

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Posted 26 May 2013 - 06:00 AM

Thanks Per. Your thoughts match my original thinking on the problem. I will find out how much the Losmany alternative would cost me.

#16 jimsmith

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Posted 11 April 2014 - 09:39 AM

Nearly a year on and I'm finally going to get around to fixing this problem. Mars would look even better without the wobble. It's pretty bad at x180. This week is going to be the week I fill the steel pier with sand. Best to try the cheap option first, I think.

#17 jimsmith

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Posted 16 April 2014 - 01:02 PM

So, I filled the pier with sand. Now when I tap the pier it no longer makes a metallic ringing sound. So high frequency vibrations are now being suppressed. Unfortunately, it has made no difference to the visual wobble. I wonder what to try next.

#18 Xeroid

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Posted 16 April 2014 - 03:45 PM

Ok, you asked what's next, here goes a crazy suggestion:

1. Replace the metal ring dovetail bar with a LONGER HARDWOOD (White Oak) bar shaped to mimic the dovetail angles (12 degrees?)

Why?
Increasing the space between the telescope rings should help to reduce flex/wobble.
Longer dovetail made of wood will dampen vibration while reducing weight.

#19 gdd

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Posted 16 April 2014 - 06:03 PM

No matter how long the dovetail the length of the attachment area on the mount remains the same. Try suspending a chain from the long end of the dovetail or from either end of the OTA to absorb vibrations. Let the chain drag on the ground. Be sure to protect the OTA from the chain.

Here is a CN thread containing the chain idea and a few others to dampen telescope vibrations.

Gale

#20 Dan Finnerty

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Posted 16 April 2014 - 07:12 PM

So, I filled the pier with sand. Now when I tap the pier it no longer makes a metallic ringing sound. So high frequency vibrations are now being suppressed. Unfortunately, it has made no difference to the visual wobble. I wonder what to try next.


I have never tried this myself but I have heard reports of this working well. You can make a diagonal brace that extends from the objective end of the telescope tube to the counterweight shaft. This can very effectively damp out vibrations due to a long telescope tube with a lot of weight (the objective lens) at the end of the tube.

There is a name for this strut, but I cannot recall it. I am sure others will be able to remember it and then you can do a google search and get more information than you ever wanted!

#21 gdd

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Posted 16 April 2014 - 09:23 PM

Hargraeves strut.

Gale

#22 Dan Finnerty

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Posted 17 April 2014 - 12:46 AM

Hargraeves strut.

Gale


That's it! Do a google search on that and you will be immeasurably enriched.

I should take my own advice and build one.

#23 jimsmith

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Posted 17 April 2014 - 03:47 AM

Thanks for all the suggestions. I may relegate this scope to low power, wide field duties when my new Dobsonian arrives. That won't fix the problem, but it will get round it, assuming the new scope's mount is stable!

#24 Rustynuts

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Posted 17 April 2014 - 08:34 PM

I second the foam idea, I just finished pouring my 12 inch dia pier with 8 lb per cubic foot closed cell foam from US composites. Decided to use the medium density foam for 4 reasons. 1. to re gain strength that was lost when i had to cut into the pier 2. to dampen vibration, 3. to prevent rust and not to cause it. and finally not to add too much mass to the pier it already weighs in at around 100lbs this 8 lb stuff is hard when cured. I had to use a angle grinder with a wire wheel to trim it. very happy with the result as a test I hoisted the pier and did the ring test all that I got was a thud that lasted less than a 1/4 when taped with a hammer second before it would ring for 5 or 6 seconds. Jon






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