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CEM60 vibration

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#1 adastranova

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Posted 07 December 2017 - 02:21 AM

Hello Members,

 

New to this forum but this seems to be a good place to get some decent info on astronomy equipment. Been doing amatuer veiwing for 20 years but within the last year have tried to move more into imaging. Added a Atik Infinity to my old 8 " LX200 GPS and liked some of the initial results. However the more I played with it I found the images were distorted to the edge of the field, and the 13-14 year old fork mount had a 2 or 3 second fast periodic error (making oval stars) I could not get rid of. So I decided to upgrade.

 

Recently I purchased an Ioptron CEM60 with a Celestron 925 EdgeHD and have only spent 3 partial nights with the equipment. Operation seems straightforward with easy rough polar alignment and finding objects is a breeze.

 

However, this setup seems to have some vibration issues I am trying to figure out. In the daylight I noticed that just touching to scope seems to make it wobble for quite a few seconds. My permanent pier is anchored into the ground with a 6 foot piece of railroad track surrounded by concrete. The pier is inside a 6x8 foot SkyShed with roll off roof. So the pier base is super solid and mostly shielded from minor wind gusts. When using the Luminos 23 mm eyepiece that was included the visual image is pretty good. However, the slightest tap makes a star wobble around in circles and doesn't completely disappear for over ten seconds.

 

When using a 15 mm eyepiece, there seems to be a lot of distortion. Star images seem like they are always oscillating in all directions to some degree. Every minute or two brights stars seem to increase in size for a few seconds and then shrink, like a larger vibration pattern which then dampens, perhaps due to wind or atmosphere? Carefully balancing payload and counterweight and using the mirror locks reduced this somewhat but I still am not satisfied. Even turning off the scope does not appear to resolve the problem. Have not even attempted imaging yet.

 

I contacted Ioptron and did not get any satisfactory answers. They asked if I was near some moving trains (no) and to try it for a few more sessions. I have read some forum posts with some having bad experiences with this mount while others are very satisfied. There are comments about what tripod to use and have heard some stuff about RA belt tension.

 

Any feedback would be appreciated. Not sure if this is mount, scope or me. The new scope out of the box appears very well collimated.

 

Thanks.

 

 



#2 Dimperev

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Posted 07 December 2017 - 05:24 AM

What you described is very similar to the resonance.
In any case, if you have oscillations of 10 seconds - this is a high-Q circuit.
This is not CEM60. This is your setup on the pier.
If the movement of CEM 60 without the sound of worm binding, then there is no movement with such frequencies.
Do you have no sewage or heating motors in the basement?


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#3 rmollise

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Posted 07 December 2017 - 10:01 AM

It may be the pier. It may not be. More than a few people have reported problems like this that have to do with the mount. You might want to do a search here...



#4 Chuckwagon

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Posted 07 December 2017 - 11:22 AM

So, untouched, with the mount power off, the vibrations are still noticeable?  It's possible they are coming through your pier.  If you have the tripod for the mount, try setting up somewhere quiet and away from people like a park or other more remote area, and see if the vibration remains.  If you get the vibrations on two different bases (pier and tripod) and in two different locations (home and away) then it's probably not the pier.  If the vibrations go away when on the tripod in a quiet remote location, then it's likely your pier that is transmitting the vibrations.


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#5 John Miele

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Posted 07 December 2017 - 03:59 PM

...However, the slightest tap makes a star wobble around in circles and doesn't completely disappear for over ten seconds..."

 

Is this a new CEM60 or did you buy it used? Some early users reported a "wobble" issue. I have an earlier model but I never had this "wobble" problem on mine. And I have not seen anyone mention this Wobble issue for quite a while in this forum.

 

Check that your azimuth adjustment knobs are tight...I'm not sure what else to try. Do you tighten your magnetic engagement screws all the way and then back off maybe 1/8 of a turn? That's how I set mine...John



#6 star drop

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Posted 07 December 2017 - 08:40 PM

Hi adastranova and welcome to Cloudy Nights and the Mounts forum in particular.



#7 adastranova

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Posted 07 December 2017 - 09:55 PM

What you described is very similar to the resonance.
In any case, if you have oscillations of 10 seconds - this is a high-Q circuit.
This is not CEM60. This is your setup on the pier.
If the movement of CEM 60 without the sound of worm binding, then there is no movement with such frequencies.
Do you have no sewage or heating motors in the basement?

Motors run smooth. Shed with pier is in yard with nothing close by but occasional cars on nearby street which never disturbed my old setup.



#8 adastranova

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Posted 07 December 2017 - 09:58 PM

...However, the slightest tap makes a star wobble around in circles and doesn't completely disappear for over ten seconds..."

 

Is this a new CEM60 or did you buy it used? Some early users reported a "wobble" issue. I have an earlier model but I never had this "wobble" problem on mine. And I have not seen anyone mention this Wobble issue for quite a while in this forum.

 

Check that your azimuth adjustment knobs are tight...I'm not sure what else to try. Do you tighten your magnetic engagement screws all the way and then back off maybe 1/8 of a turn? That's how I set mine...John

Knobs adjustments has no effect. CEM60 is brand new, supposedly new stock from manufacturer. I read about some problems like this in the posts from 2-3 years ago and it was my belief the bugs has been worked out by Ioptron.



#9 adastranova

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Posted 07 December 2017 - 10:04 PM

So, untouched, with the mount power off, the vibrations are still noticeable?  It's possible they are coming through your pier.  If you have the tripod for the mount, try setting up somewhere quiet and away from people like a park or other more remote area, and see if the vibration remains.  If you get the vibrations on two different bases (pier and tripod) and in two different locations (home and away) then it's probably not the pier.  If the vibrations go away when on the tripod in a quiet remote location, then it's likely your pier that is transmitting the vibrations.

Thanks to all who replied. Trying a tripod in a different location seems like an option. The pier however was used with my old setup, an 8"LX200 with Superwedge which weighed 62 pounds and did not have this issue. New setup total should be about the same range with counterweight. Wondering if the CEM60 RA drive belt could be acting like a rubber band, bouncing back and forth?



#10 bobito

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Posted 07 December 2017 - 10:04 PM

I'm guessing the new setup weighs a bit more than your old one.  Perhaps this is causing some flex in the pier? 

 

Edit, was typing this as you replied saying about the same weight.  LOL


Edited by bobito, 07 December 2017 - 10:06 PM.


#11 Chuckwagon

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Posted 07 December 2017 - 10:40 PM

 

So, untouched, with the mount power off, the vibrations are still noticeable?  It's possible they are coming through your pier.  If you have the tripod for the mount, try setting up somewhere quiet and away from people like a park or other more remote area, and see if the vibration remains.  If you get the vibrations on two different bases (pier and tripod) and in two different locations (home and away) then it's probably not the pier.  If the vibrations go away when on the tripod in a quiet remote location, then it's likely your pier that is transmitting the vibrations.

Thanks to all who replied. Trying a tripod in a different location seems like an option. The pier however was used with my old setup, an 8"LX200 with Superwedge which weighed 62 pounds and did not have this issue. New setup total should be about the same range with counterweight. Wondering if the CEM60 RA drive belt could be acting like a rubber band, bouncing back and forth?

 

The fork mount of the LX200 will transfer vibrations quite differently to the OTA than the CEM will.  The different structural setup of the mounts damps differently.  I don't think it's necessarily a weight issues, probably more a resonance issue, and structural differences.  But if you can do the tripod test that should help narrow it down.  Hopefully.  smile.gif


Edited by Chuckwagon, 07 December 2017 - 10:41 PM.

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#12 adastranova

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Posted 08 December 2017 - 12:18 AM

 

 

So, untouched, with the mount power off, the vibrations are still noticeable?  It's possible they are coming through your pier.  If you have the tripod for the mount, try setting up somewhere quiet and away from people like a park or other more remote area, and see if the vibration remains.  If you get the vibrations on two different bases (pier and tripod) and in two different locations (home and away) then it's probably not the pier.  If the vibrations go away when on the tripod in a quiet remote location, then it's likely your pier that is transmitting the vibrations.

Thanks to all who replied. Trying a tripod in a different location seems like an option. The pier however was used with my old setup, an 8"LX200 with Superwedge which weighed 62 pounds and did not have this issue. New setup total should be about the same range with counterweight. Wondering if the CEM60 RA drive belt could be acting like a rubber band, bouncing back and forth?

 

The fork mount of the LX200 will transfer vibrations quite differently to the OTA than the CEM will.  The different structural setup of the mounts damps differently.  I don't think it's necessarily a weight issues, probably more a resonance issue, and structural differences.  But if you can do the tripod test that should help narrow it down.  Hopefully.  smile.gif

 

Yes. But am hoping this may not be the case. Would hate to rebuild the pier. Plus I like the CEM60 features, designs and menus are a plus, so will keep trying to resolve this. Thanks.



#13 Waldemar

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Posted 08 December 2017 - 01:25 AM

Is your mount attached to a so called 'ratcage'? A mountingplate elevated by threaded ends, so there is a space between the top of your pier and the mounting plate?

If so, the size and length of the threaded ends maybe the cause of boosting any vibrations by an effect called resonance frequency, a kind of swinging of the construction. Changing the length of the threaded pillars will change this resonance frequency. The shorter and thicker they are, the higher the resonance frequency will be. Since the effect you are describing sounds like you have quite long 'pillars', making the distance between the top of your pier and the mounting plate smaller should immediately result in a change of effects. This resonance frequency is different for anything you put on the mounting plate, that explains why it did not occur with your older mount. Weight, construction and size are decisive factors in these effects, as are the lengths and size of the threaded 'pillars'. My advice would be to make them as short as possible, to get away from low frequency effects.


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#14 adastranova

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Posted 08 December 2017 - 02:27 AM

Is your mount attached to a so called 'ratcage'? A mountingplate elevated by threaded ends, so there is a space between the top of your pier and the mounting plate?

If so, the size and length of the threaded ends maybe the cause of boosting any vibrations by an effect called resonance frequency, a kind of swinging of the construction. Changing the length of the threaded pillars will change this resonance frequency. The shorter and thicker they are, the higher the resonance frequency will be. Since the effect you are describing sounds like you have quite long 'pillars', making the distance between the top of your pier and the mounting plate smaller should immediately result in a change of effects. This resonance frequency is different for anything you put on the mounting plate, that explains why it did not occur with your older mount. Weight, construction and size are decisive factors in these effects, as are the lengths and size of the threaded 'pillars'. My advice would be to make them as short as possible, to get away from low frequency effects.

Yes. I have this common setup. The plates have about 3 inches between them with 3 sections of 7/16" stainless threaded rod as the leveling mechanism. Will definitely give dropping the height a try. I suppose any piece of metal will resonate given the proper configuration. Many thanks. I should try to post a pic for all to see.



#15 bobito

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Posted 08 December 2017 - 10:50 AM

... Would hate to rebuild the pier...

 

If it turns out to be the pier, you could probably just put some braces around it to firm it up.



#16 RedLionNJ

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Posted 08 December 2017 - 11:14 AM

If it does turn out to be pier-related, and if your pier is hollow (vs solid), filling the cavity with sand may have a positive effect on your situation.

 

For what little it's worth, I upgraded over the years from an 8-inch fork-mounted SCT to currently a 12-inch fork-mounted SCT (on a permanent pier). It's on a wedge and the darned scope rings like a tuning fork if you as much as tap it - it can take well in excess of 10 seconds for the vibration to die down.  But once it's settled, it's fine as long as you don't touch it.



#17 adastranova

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Posted 08 December 2017 - 10:54 PM

If it does turn out to be pier-related, and if your pier is hollow (vs solid), filling the cavity with sand may have a positive effect on your situation.

 

For what little it's worth, I upgraded over the years from an 8-inch fork-mounted SCT to currently a 12-inch fork-mounted SCT (on a permanent pier). It's on a wedge and the darned scope rings like a tuning fork if you as much as tap it - it can take well in excess of 10 seconds for the vibration to die down.  But once it's settled, it's fine as long as you don't touch it.

Yes. Thanks. The main post is 4 inch steel pipe which I filled sand from the start. I could live with taking some time to let the vibes settle down. It is the other oscillations, making stars change in size at irregular intervals, which concerns me more for imaging. Will need to consider bracing and perhaps adding more mass to the pier. With poor winter weather approaching, this all may take some time to work out.



#18 psandelle

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Posted 08 December 2017 - 11:13 PM

I don't know anything 'bout anything, but isn't 4" a little narrow for stability? I mean, the iOptron piers are 5.7", and I wouldn't do anything less than 8" if I were building an obsy. But, and I'm sure others who know about this will chime in. I know the 4" pipe is stuck on a railroad tie, but how long is the 4" part?

 

Maybe I'm not reading all this right.

 

Paul



#19 adastranova

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Posted 09 December 2017 - 12:54 AM

I don't know anything 'bout anything, but isn't 4" a little narrow for stability? I mean, the iOptron piers are 5.7", and I wouldn't do anything less than 8" if I were building an obsy. But, and I'm sure others who know about this will chime in. I know the 4" pipe is stuck on a railroad tie, but how long is the 4" part?

 

Maybe I'm not reading all this right.

 

Paul

The 4" pipe is thick wall steel, not just a tube as on a tripod. It's about 36" long, bolted to a length of railroad track weighing about 450#, with the last 2 feet above ground and the in ground 4 feet surrounded by about 6 inches concrete on all sides. The pipe is mounted with a leveling head. On top of that is an Ioptron Mini-Pier, about 6 in. tall. I couldn't knock the pipe over with a sledgehammer if I tried. So how I get vibrations in all this mass is perplexing, perhaps some type of harmonic, like twanging a ruler off the end of a desk. Perhaps 6 in. pipe may be a consideration. Thanks.



#20 bobito

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Posted 09 December 2017 - 01:03 AM

I re-read your original post.  You said "makes a star wobble around in circles".

 

I can't think of how a gem mount could wobble in a circle, but it seems the effect of a mount gyrating  (is that the right word?) would do exactly that.



#21 adastranova

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Posted 09 December 2017 - 02:12 AM

I re-read your original post.  You said "makes a star wobble around in circles".

 

I can't think of how a gem mount could wobble in a circle, but it seems the effect of a mount gyrating  (is that the right word?) would do exactly that.

Yes. There are two separate things. An obvious gyration if the scope is tapped or touched, with the star doing loops. The other is when the star increases in size for a second or two every few minutes, like a rapid buzzing in all directions, very symmetrical and centered . Have never seen anything like the latter before. Have seen some pics of atmospheric distortions, but that type of pattern is more irregular than what I am seeing.



#22 Waldemar

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Posted 09 December 2017 - 03:42 AM

What you are seeing every few minutes is probably the resonance frequency effect of a periodic error.

Try to shorter the threaded rods of your rat cage and see what happens as I suggested before. I bet you will be amazed....

It would be a good thing to fill the remaining thread with nuts, so they will be more solid, which will Change the res. freq. as well.

The whole rat cage construction is a mostly unnecessary thing anyway. If it is absolutely needed for positioning the mount, keep the rods as short as possible. For levelling it is not absolutely necessary, because EQ mounts do not need a perfect levelling. Direct mounting the head on the pier if possible is a far better option. 

 

Levelling is nice for a mobile set up to have a starting point for further adjustment, but that is about it.

 

Edit:

4" diameter steel pipe is not a solid construction for a pier anyway, even if it is thick walled.

An extra solution to dramatically change the res. freg. may be the use of a thick plywood plate between the pier and the upper mounting plate.


Edited by Waldemar, 09 December 2017 - 04:16 AM.

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#23 Waldemar

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Posted 09 December 2017 - 04:21 AM

Like this:

 

IMG_8607-1.JPG


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#24 Dimperev

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Posted 09 December 2017 - 10:43 AM

Just for an example.
I have a pier with a diameter of 219 mm and a height of 1.5 meters.
If I apply a horizontal force of 1 kg to the top of the pier, the top of the pier will deviate by 0.002 mm. This corresponds to a deviation of 0.3 "arcsec.
If your pier has a diameter of 4 inches thick steel pipe (diameter 101 mm and wall thickness 5 mm), then the force of 1 kg will tip its top by 0.023 mm.
This corresponds to a deviation of 3.4 arcsec and this is already a lot.

 

Ranger 4 - thanks for the idea of the plywood plate. I will do it myself.



#25 Waldemar

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Posted 09 December 2017 - 11:09 AM

Just for an example.
I have a pier with a diameter of 219 mm and a height of 1.5 meters.
If I apply a horizontal force of 1 kg to the top of the pier, the top of the pier will deviate by 0.002 mm. This corresponds to a deviation of 0.3 "arcsec.
If your pier has a diameter of 4 inches thick steel pipe (diameter 101 mm and wall thickness 5 mm), then the force of 1 kg will tip its top by 0.023 mm.
This corresponds to a deviation of 3.4 arcsec and this is already a lot.

 

Ranger 4 - thanks for the idea of the plywood plate. I will do it myself.

I have no idea what kind of plywood is available to you, but my experience is that 40 mm thick 5 or 7 layers bamboo works the best. Next best one is Finnish or Russian birch, at least 24 mm (1 mm layers) thick, thicker is better. The dampening quality works so well because of the cross layering of the wood nerves. I did build loudspeaker boxes for a living and the same kind of natural laws apply.




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