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Center-balanced Mounts . . . the big deal?

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#1 Oz Alfert

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Posted 16 September 2018 - 06:27 PM

Please explain to me the advantages, if any, of a center balanced mount, like iOptron's CEM25 and CEM60.

 

Thanks,

 

Oz


Edited by Oz Alfert, 17 September 2018 - 05:40 PM.


#2 astronz59

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Posted 17 September 2018 - 01:11 AM

Basically, high load carrying capacity versus size/weight of mount. Here's a link to a video made by Paul Chasse with an CEM25P carrying a 14.5 Kg payload with ease......

 

https://www.youtube....h?v=_cIOTROXEpQ



#3 epdreher

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Posted 17 September 2018 - 04:29 AM

Whatever happened to Paul, anyway?  Despite my wife owning a 45 Pro, I'm personally not a fan but his videos were informative.



#4 Real14

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Posted 17 September 2018 - 08:05 AM

Hello everyone. Not really my first post here, just a new account with my bff.

 

Please explain to me the advantages, if any, of a center balanced mount, like iOptron's CEM25 and CEM60.

 

Thanks,

 

Oz

There is also a CEM 120 waytogo.gif



#5 Real14

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Posted 17 September 2018 - 10:26 AM

Hi,

 

¿ honestly ? No Big Deal ...

 

GEM is German Equatorial Mount   flowerred.gif

 

CEM is Chinese Equatorial Mount   flowerred.gif

 

extract of the website listed below.

 

cemvsgem.JPG

 

http://www.skyatnigh...quatorial-mount

 

That is the only difference as I have been explained at NEAF 2018 by Dr. Jiang from iOptron. 

 

¿ are you in  the process of buying a Mount and if yes in which model are you interested ? Perhaps then we can help you answering your specific questions better.

 

 



#6 CharlesW

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Posted 17 September 2018 - 10:45 AM

In their latest issue S&T gave a pretty strong thumbs up to ioptron’s CEM120. 


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#7 Real14

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Posted 17 September 2018 - 10:48 AM

In their latest issue S&T gave a pretty strong thumbs up to ioptron’s CEM120. 

Hi Charles,

 

Is there an Online Version ?

 

Rainer

 

Correction:

 

After hitting enter and doing a search I found it and while reading this is a bit misleading

 

This one ? https://www.skyandte...rbalance-Mount/

 

Or is there a deeper one ?

 

Quote " Its spring-loaded saddle plate ... " for what would I want a spring loaded a saddle plate lol.gif


Edited by Real14, 17 September 2018 - 11:03 AM.


#8 sg6

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Posted 17 September 2018 - 12:53 PM

Really not sure, I would have said that the purpose of the weights and getting the set up balanced meant the weight was centered. If they "just" balanced then why the counter weights. and if not centered then scopes etc would rotate off one way or the other.

 

Maybe they just have a different mechanical arrangement - they do at least look different. And in looking different I would be cynical and suggest the rest is marketting.



#9 Real14

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Posted 17 September 2018 - 12:58 PM

Really not sure, I would have said that the purpose of the weights and getting the set up balanced meant the weight was centered. If they "just" balanced then why the counter weights. and if not centered then scopes etc would rotate off one way or the other.

 

Maybe they just have a different mechanical arrangement - they do at least look different. And in looking different I would be cynical and suggest the rest is marketting.

Hi,

 

The balance they refer too is a static one. Look at the GEM and look at a CEM and you will see that the center of gravity on a CEM is more to the center of the  Mount base whereas the center of gravity of a GEM is nearer to the front part of the Mount.

 

Make a simple sketch and draw vertical lines of where the weight goes through ...

 

You do not Need to use the weight Distribution of a telescope. Just simulate it with weight cubes where you can find in an easier way the center of gravity.

 

shocked.gif



#10 spokeshave

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Posted 17 September 2018 - 06:21 PM

In a traditional GEM, the center of mass of the rotating assembly (OTA, RA shaft and counterweights) is located well north of the center of support (see pic below):

gto.jpg

The red circle marks the center of mass and as you can see, it is well to the left of the center of support. In fact, this is the reason that the conventional wisdom is to orient the mount on the tripod so that one leg points north. That's to keep the mount from having a tendency to tip to the north. You can also see that the center of mass is cantilevered on the RA axis. This loads the RA shaft bearings unevenly. The cantilevered rotating assembly also makes altitude adjustments difficult, also requiring more clamping force to hold the ALT adjustment since the whole mount wants to pivot to the north.

On a CEM mount, the counterweights and OTA are separated, with the CWs to the north and the OTA to the south:

cem60.jpg

Here the center of mass is centered over the center of support and the mount is far more stable as a result. The CoM is also centered between the RA shaft bearings, loading them evenly. The CoM is also centered over the altitude adjustment making that adjustment easier and since the mount is balanced and the weight is not cantilevered to the north it will have no tendency to move in altitude thus requiring less clamping force.

As you can probably see, a CEM mount does not need to be engineered to compensate for a substantial cantilevered mass. It is a good design and frankly, is how GEMs should have been designed.

As a side note, I see the "Chinese Equatorial Mount" comment in the article as a compliment. The GEM is named after the country of origin. It only makes sense for the innovative improvement on the GEM to also be named for the country of origin.

Tim
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#11 555aaa

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Posted 17 September 2018 - 08:47 PM

The load path in the iOptron between the counterweight and OTA passes back thru the polar axis which is detrimental to stiffness in RA. In a conventional GEM, the load path for counterweighting the OTA is independent from the polar axis and the N polar bearing can be made as large as desired or can be a flat disk. I don't think it is an issue with the modest loads mostly used. It looks like the c.g might sit farther back as noted.

#12 Guest_djhanson_*

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Posted 18 September 2018 - 12:40 AM

That's to keep the mount from having a tendency to tip to the north.

If this were really true and significant I couldn't place my AP 1100GTO on its tripod adapter, without bolts, and still have it stand upright. (which it does fine)  CEM is a marketing gimick at best.  Not significant.  By the time you load your OTA (short moment arm, long moment arm whatever the case may be) and load the shaft with counterweights you will already change the center of mass of the entire system.  (which is why counterweights are meant to slide on the counterweight shaft)  CEM makes no difference here and if it were significant every GEM type of mount would have adapted something like it decades ago.  It offers absolutely no advantages.  DJ


Edited by djhanson, 18 September 2018 - 12:56 AM.


#13 spokeshave

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Posted 18 September 2018 - 07:15 AM

If this were really true and significant I couldn't place my AP 1100GTO on its tripod adapter, without bolts, and still have it stand upright. (which it does fine)


No argument here. However, you took my quote completely out of context. If you consider this context - "In a traditional GEM, the center of mass of the rotating assembly (OTA, RA shaft and counterweights) is located well north of the center of support" - which includes the actual load the mount is designed to carry, your statement falls apart. Note that I am not recommending the following because it would most definitely have disastrous results. Having disclaimed that, if you were to load your AP1100GTO (or any other GEM) with OTA and counterweights without bolting it to the tripod, everything would come crashing to the ground. On a CEM mount (not recommending this either) everything would stay put. So, my point stands.
 

By the time you load your OTA (short moment arm, long moment arm whatever the case may be) and load the shaft with counterweights you will already change the center of mass of the entire system.

On a GEM, yes. On a CEM, no. By virtue of the design of the CEM, the center of mass will always be centered over the center of support, assuming that the system is balanced of course. On a GEM, on the other hand, the more payload that is added, the more the center of mass moves away from the center of support. 
 
It may not be an advantage that is meaningful to you since you have a mount that is well-engineered to overcome the shortcomings that are inherent in the GEM design, but there are indeed valid engineering advantages to the CEM design far beyond what you characterize as "marketing hype". 

 

Anyway, I tried to answer the OP's question with an honest and collegial response. Since it looks like some people are misinterpreting my response as goring their sacred cows, I'll bow out of the conversation.

 

Tim


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#14 Arie

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Posted 18 September 2018 - 07:51 AM

Whatever happened to Paul, anyway?  Despite my wife owning a 45 Pro, I'm personally not a fan but his videos were informative.

It's been a while since my last contact with him.

They had just escaped hurricane Irma last year.

 

He and his wife are "enjoying the golden years" as he said.

Paul is taking it easy now.

 

But that was exactly one year ago . . . . . 



#15 Pauls72

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Posted 18 September 2018 - 08:02 AM

In their latest issue S&T gave a pretty strong thumbs up to ioptron’s CEM120.

Does any magazine ever give an advertiser a poor review?

Just keep in mind magazine reviews are extremely biased, so I wouldn't give them a lot of value. Actual user reviews would be a better source of information.


Edited by Pauls72, 18 September 2018 - 08:05 AM.

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#16 gotak

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Posted 18 September 2018 - 08:41 AM

Does any magazine ever give an advertiser a poor review?

Just keep in mind magazine reviews are extremely biased, so I wouldn't give them a lot of value. Actual user reviews would be a better source of information.

I think they just tend to only use the thing they know will get a good result instead of just about any crap out there. As they say if you have nothing good to say then say nothing.

 

As for actual users, the reviews have been generally positive. A copy of the CEM60 has been used with some success swinging a 12" meade SCT for imaging. And the various CEM120 results has been generally good.

 

The design itself is not proving to be head and shoulder above traditional GEM designs. The real story is that they are reasonably priced mounts that seems to work very well for imaging.


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#17 mandarpotdar

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Posted 18 September 2018 - 09:50 AM

>> That's to keep the mount from having a tendency to tip to the north.
>> If this were really true and significant I couldn't place my AP 1100GTO on its tripod adapter, without bolts, and still have it stand upright. (which it does fine)

I am at 17° latitude, have an Orion Sirius EQ-G (= HEQ5) and it does tip to the North when placed without attaching the bolt on tripod. The balance of a regular GEM worsens at lower latitude.
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#18 Stelios

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Posted 18 September 2018 - 10:26 AM

I've never owned a CEM, and probably won't as I'm happy with the mounts I have. However, there's no question that many are happy with them, and that they are lightweight for the payload, an important consideration.

 

However, looking at the second illustration in Tim's post (#10) it seems to me that the CW shaft will experience significant torque when the CW bar is loaded to capacity. Is that a concern? 


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#19 Real14

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Posted 18 September 2018 - 11:06 AM

it seems to me that the CW shaft will experience significant torque when the CW bar is loaded to capacity. Is that a concern?

No, no concern at all. Look here. The only concern could be that if that part is not correctly designed it could break but as you see it supports 54.1 kg ~ 119 lbs of counterweights and is still one piece.

 

https://www.cloudyni...ngth/?p=8756529

 

Where is the difference of the way a CW shaft is attached to a GEM and a CEM ? Just because there is nothing else up there ?

 

The loaded CW shaft of a GEM exerts as much torque on the RA as the CW shaft on a CEM and the torque on the RA axis on a GEM is much higher as the whole DEC assembly also exerts torque on the RA axis whereas on the CEM being the load of the DEC assembly between 2 bearings the load is distributed onto the 2 bearings evenly.

 

Somewhere here on CN many Moons ago, I explained with Images how the load is distributed on a GEM as well as on a CEM.

 

Rainer



#20 555aaa

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Posted 18 September 2018 - 11:40 AM

I think the iOptron design does have an advantage if you are trying to use a tripod and are near the equator. You know, they could have put a second counterweight on the S end of the polar axis, and then you'd have less moment on the part of the shaft between the worm and the dec axis, and it would move the C.G. even closer to the center. Right now you have this long Z shaped load path when you are on the meridian which is where most people observe.



#21 Real14

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Posted 18 September 2018 - 11:55 AM

I think the iOptron design does have an advantage if you are trying to use a tripod and are near the equator. You know, they could have put a second counterweight on the S end of the polar axis, and then you'd have less moment on the part of the shaft between the worm and the dec axis, and it would move the C.G. even closer to the center. Right now you have this long Z shaped load path when you are on the meridian which is where most people observe.

confused1.gif



#22 Real14

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Posted 18 September 2018 - 11:58 AM

Hi,

 

I found it

 

https://www.cloudyni...hl=losmandy cem

 

Any questions ?



#23 ChrisWhite

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Posted 18 September 2018 - 04:26 PM

I'm amazed at how easily these cem mounts balance, and how nicely the gears mesh.

I have the cem25 and the cem60. I like them both. They are lightweight and very capable.

Would I rather use a mach 1? Yes... but they are tough to come by and 3x the cost.

I think it's a great design and someday if I build an obsy I'd be tempted by the 120, even with mounts around that are considered higher quality at similar prices!
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#24 Real14

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Posted 18 September 2018 - 05:16 PM

 

Would I rather use a mach 1? Yes...

Hi Chris,

 

I do not think so. A plain Mach1GTO costs US $ 5,490.00 and does not include a saddle plate nor a counterweight and for US $ 10.00 more you can get a CEM 120EC ...

 

A lot of already installed through the mount connections, a saddle plate and 2 pieces  10kg counterweights

 

Still want a Mach1 GTO ?

 

undecided.gif

 

Look how nice the cabling is ... At the moment you can not get it better ... waytogo.gif

 

post-51924-0-51952400-1533741939.jpg


Edited by Real14, 18 September 2018 - 05:19 PM.

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#25 Real14

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Posted 18 September 2018 - 06:17 PM

Hi,

 

I leave this two drawings here and you can make your own deductions about the differences ...

 

 

 

GEMdesign.JPG

 

 

CEMdesign.JPG

 




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