60 replies to this topic

[jbrandmeyer]

While the CEM bearing design improves the load distribution for the right ascension bearings, it is much harder on the declination axis bearings.  On a GEM, the declination axis bearings can be centered on either side of the center of gravity.  But on the CEM design the declination axis radial load is not balanced about the radial bearings at all.  Instead of Weight/2 on each declination axis bearing, they are experiencing 2*W and W or worse, depending on how close the saddle plate is.  The CEM's penalty here is offset somewhat by carrying about half the weight on the declination axis bearings, since the counterweight doesn't rotate about the DEC axis.

 

 

So, let W be the weight of the telescope.  Let the weight of the counterweights also be W, even though it may be somewhat larger depending on how close they are mounted to the RA axis.

 

I'll also accept the r and 4*r for the bearing distances on the RA axis.

 

CEM Right ascension axis bearings : W + W/4 + W/2 on one RA bearing, W/4 - W/2 on the other RA bearing, for 1.75W and 0.25W respectively.

 

GEM Right ascention axis bearings bear 2W + 2W / 4 on the heavy RA bearing, and 2W/4 on the light bearing, for 2.5W and 0.5W, respectively.

 

The CEM gets a 30% savings on the heavy bearing, and 50% savings on the lighter bearing.

 

 

Now, consider the declination axis.  The GEM bears both the counterweights and the OTA on a common shaft, but its radial bearings are located on either side of the center of mass.  Therefore, each bearing carries W.

 

The CEM is different. Only the OTA rotates in declination, so the declination axis bearings are cantilevered.  Unlike the GEM's right ascension bearings, where one bearing can be located much closer to the load, the CEM's declination axis bearings are located very far from the load.  Looking at pictures of a C11 mounted on a CEM60, it looks like the lever forces are 2R and R or worse, such that the bearings carry 2W and 3W, respectively.  As the load gets larger (say, a C14 on a CEM120?), this factor gets rapidly worse.

 

The CEM therefore pays a penalty of 200% and 300% additional load on its declination axis bearings.  This load factor gets worse as the telescope gets larger.  That's why the English equatorial mount also rotated the counterweight with the load on the declination axis.

[jbrandmeyer]

In summary you can balance a GEM so that the center of mass is located at the center of the tripod.  You can also balance a CEM so that the the center of mass is not at the center of the tripod.  So in practice either mount design can become center mass balanced or unbalanced.  CEM offers absolutely no advantages this way.  This is not about sacred cows, just practical physics.  DJ

 

If the load is balanced about both of the rotation axes, then you don't have a remaining degree of freedom to reposition the center of gravity as you choose.  The GEM CG will be at the intersection of the RA and DEC axes of rotation, biased a little bit by the weight of the RA axis drive.  The CEM CG will be on the RA axis, midway between the declination axis and the counterweight shaft axis.

 

Honestly, I think the slick part of the iOptron mount is the way the elevation adjustment rotates the entire assembly about the CG with that sliding mechanism.  Splitting the two halves of the load certainly makes this easier, but is by no means a requirement.  Maybe the GEM needs a few extra cm of pier clearance to be balanced over the tripod top, but that's it.

[gotak]

Sure would. For example, if you encounter an electronics failure on an iOptron, the entire head is going to be shipped back for repair. That leaves you mount-less in practice. With an AP Mach1GTO, you would simply replace the separate CP controller. Within a week of shipping, your Mach1GTO would be ready to use. Meanwhile the iOptron is still waiting on the repair involving the entire head assembly. This situation cost me 3 months of imaging once, and after this, I later upgraded to an AP mount. (for this reason and others)

The other thing: electronic port standards continue to change, will continue to change in the future, and here again the separate AP controller design offers advantages. With AP you can keep your same equatorial head and simply upgrade to a new, separate controller. (e.g. CP3 to CP4) Conversely, mounts using integrated electronics will not ever be upgradable this way. iOptron, and some other mount makers want you to buy an entire new mount later on, which ironically increases cost of ownership.

You get what you pay for at the old saying goes. DJ

In my experience ioptron just ships you the electronic parts and mechanical part should you want to do the work yourself. Not sure why you are under the impression it all has to go back.

I updated the RA motor board, replace two bearings they thought would be a good idea although wasn't necessary, replace one worm and have stripped the thing almost all the way down. All in my living room.

As for the other arguments.. so it's a device with a service life. At the cost it costs it's no big deal to replace it with the newer model in the future.

[555aaa]

The bearing loads on telescopes are tiny relative to the bearing capacity. What is generally the limiting parameter however is torsional stiffness, especially in RA. Torsional stiffness in solid shafts goes up with the fourth power of shaft diameter and with the cube for hollow shafts. It goes down linearly with length. Here's a numerical example from a spreadsheet I put together for this type of calculation [disclaimer - not a PE licensed mechanical engineer; use at your discretion]: you have a 50mm diameter solid steel shaft, 300mm long. Apply an upsetting moment (twist) of 2 Newton-meters to it, say from a gust of wind at the OTA. That produces a 2.6 arc second torsional deflection (twist) in the polar axis. Now change it to 3" (75mm), and that upset goes down to 0.5 arc seconds. If you get that 300mm down to 60mm, then you get a 5x reduction in torsional deflection.  In a traditional mount, the RA shaft has no static torsional load, but that's not true in the iOptron design. Torsional movement (in all mounts) in RA looks like guiding error. So that's why you often see a very large diameter North (upper) RA axis bearing and the worm wheel right there, or even a disk design for the RA to DEC connection in fork and GEMs. There is a traditional type of mount called a Cross-Axis or English cross-axis mount, that has a north "pylon" and a small bearing there, but that design requires a large cross-section for the long polar assembly. because the mechanical 'ground' for the worm is very far away from the OTA and has to travel all the way up the length of that big structure. Note also that they tend to put the counterweight right on the dec axis so it doesn't add torsional load to the RA axis. In the iOptron design, the counterweight load is (AFAIK) reacted directly by the polar shaft, whereas in a traditional GEM or fork mount, the polar axis simply carries dead weight and only reacts the dynamic load of the mount.

 

https://en.wikipedia...crocio_assi.jpg

 

Bending tends to be less significant in my experience. For example, taking the same 2" diameter (50mm appx) 300mm long steel shaft, hold it 45 degrees from vertical, and put 200lbs of load on it. You only get a deflection of around 0.07mm (70 microns), and in terms of upset stiffness, it comes out to only a deflection of something like 0.1 arc second for the same 2N upset load (a linear load in this case, not a moment).

 

But like I said before, the iOptron design is pretty compact and so it's probably just fine for the modest-sized OTAs used and I think the design has an advantage when you get close to the equator in terms of how the mount balances over the tripod. Other than that, I think it's actually a slightly less inherently stiff design than a traditional GEM.

[Real14]

Hi,

 

WOW, very interesting seeing all those different calculations.

 

The OP for sure has now got enough info to make up his mind and decide what he wants to buy

 

[Real14]

The bearing loads on telescopes are tiny relative to the bearing capacity. What is generally the limiting parameter however is torsional stiffness, especially in RA. Torsional stiffness in solid shafts goes up with the fourth power of shaft diameter and with the cube for hollow shafts. It goes down linearly with length. Here's a numerical example from a spreadsheet I put together for this type of calculation [disclaimer - not a PE licensed mechanical engineer; use at your discretion]: you have a 50mm diameter solid steel shaft, 300mm long. Apply an upsetting moment (twist) of 2 Newton-meters to it, say from a gust of wind at the OTA. That produces a 2.6 arc second torsional deflection (twist) in the polar axis. Now change it to 3" (75mm), and that upset goes down to 0.5 arc seconds. If you get that 300mm down to 60mm, then you get a 5x reduction in torsional deflection.  In a traditional mount, the RA shaft has no static torsional load, but that's not true in the iOptron design. Torsional movement (in all mounts) in RA looks like guiding error. So that's why you often see a very large diameter North (upper) RA axis bearing and the worm wheel right there, or even a disk design for the RA to DEC connection in fork and GEMs. There is a traditional type of mount called a Cross-Axis or English cross-axis mount, that has a north "pylon" and a small bearing there, but that design requires a large cross-section for the long polar assembly. because the mechanical 'ground' for the worm is very far away from the OTA and has to travel all the way up the length of that big structure. Note also that they tend to put the counterweight right on the dec axis so it doesn't add torsional load to the RA axis. In the iOptron design, the counterweight load is (AFAIK) reacted directly by the polar shaft, whereas in a traditional GEM or fork mount, the polar axis simply carries dead weight and only reacts the dynamic load of the mount.

 

https://en.wikipedia...crocio_assi.jpg

 

Bending tends to be less significant in my experience. For example, taking the same 2" diameter (50mm appx) 300mm long steel shaft, hold it 45 degrees from vertical, and put 200lbs of load on it. You only get a deflection of around 0.07mm (70 microns), and in terms of upset stiffness, it comes out to only a deflection of something like 0.1 arc second for the same 2N upset load (a linear load in this case, not a moment).

 

But like I said before, the iOptron design is pretty compact and so it's probably just fine for the modest-sized OTAs used and I think the design has an advantage when you get close to the equator in terms of how the mount balances over the tripod. Other than that, I think it's actually a slightly less inherently stiff design than a traditional GEM.

Hi 555aaa,

 

Where is the spread sheet ?

 

Thanks

[Real14]

 

Sure would.  For example, if you encounter an electronics failure on an iOptron, the entire head is going to be shipped back for repair.

Hi djhanson,

 

That is absolute nonsense what you wrote. Exchanging the boards does not take more then 15 minutes and as another poster said iOptron ships them to you without a problem.

 

I am in contact with them and have got for testing purposes some boards. The boards arrived in Mexico in less then 4 days ...

 

 

Conversely, mounts using integrated electronics will not ever be upgradable this way.  iOptron, and some other mount makers want you to buy an entire new mount later on, which ironically increases cost of ownership.

Also not true ... 

 

regards Rainer

Edited by Real14, 19 September 2018 - 08:01 AM.

[Real14]

Hi,

 

I would say all in all the Consensus could be

 

Buy whatever you want and be happy ...

 

In my case having had to repalce two mounts my decision to buy two CEM 120EC2 so far has been correct for me. Having spent US $ 13,996.00 for two Mounts with 115 lbs Instrument capacity, Encoders on 2 axes, 2 counterweights, good saddle plate, through the Mount cabling sufficient for many devices and a good after sales Service, I am happy even if as detected or calculated or guessed, I have microns of Flex and huge amount of load on DEC bearings, but looking at my results, what more can I have ?

 

No problems at all and just colaborating with iOptron as a Beta tester for new Firmware developments.

 

Same Investment with " Premium Mounts " (still not very cear for me what a Premium Mount is, but does not matter, I like my Premium Mounts) would have cost me more then the double and also no guarantee that I will have no problems. Every mount producer also only cooks with water.

 

If I would have had the " Premium Mount " investment, I even could have invested in three CEM 120EC2 mounts and have one as full spare and still saved ~ US $ 7,000.00 (A nice Takahashi TOA 130 NFB US $ 6,950.00or a Takahashi FSQ-106ED4X US $ 5,350.00)

 

Again, would be nice to hear an opinion of the original OP and what he thinks now about this interesting opinion, knowledge and experience exchange ...

 

 

regards Rainer

Edited by Real14, 19 September 2018 - 09:05 AM.

[jbrandmeyer]

To be clear, I'm not attacking iOptron-the-company.  Customer service reports vary widely from thrilled to grumpy, but that's true for some of iOptron's competitors as well.  In an online forum, we don't have access to statistical measurements of RMA rates or anything like that, all we can get is the most vocally thrilled/grouchy customers.  I'm not a customer, and therefore have nothing to say about their customer service.

 

I'm trying to answer the question, "why didn't the GEM manufacturers adopt this layout decades ago?" based solely on basic principles of mechanical engineering design.

 

Stating that a particular bearing must bear a lower or higher load translates into "all things being equal, this bearing is less/more expensive."  Some folks have lauded the split arrangement specifically for the improvement in bearing loads.  But on balance, it is clearly worse for the bearings in the CEM layout.  As the design is scaled up, the disadvantage is more pronounced.

 

 

The static load on the RA shaft shouldn't matter much in practice.  In both systems, the RA drive is mounted close to where the load joins the RA drive shaft, and the offset is about the same on commercially available units.  On the CEM's split design, the RA drive's location is also balanced between the two half-shafts.  So what if a static load adds a few arcsec or even arcmin of deflection?  This changes much slower than other sources of nonperiodic error, and should be guided out in practice.

 

In the GEM layout, the saddle plate can be located closer to the RA shaft than the CEM design, which reduces many of the loads, including periodic and nonperiodic loads on the RA drive.  This is directly due to the declination bearing separation needed to keep the cantilevered load reasonable.  In principle, anyway.  In practice, 45-60 lb mounts have a tall saddle plate to clear the RA drive, so it mostly washes out at this size.  But on the CEM120, the tall saddle plate leads directly to much heavier counterweight.

[Real14]

 In the GEM layout, the saddle plate can be located closer to the RA shaft than the CEM design, which reduces many of the loads, including periodic and nonperiodic loads on the RA drive.  This is directly due to the declination bearing separation needed to keep the cantilevered load reasonable.  In principle, anyway.  In practice, 45-60 lb mounts have a tall saddle plate to clear the RA drive, so it mostly washes out at this size.  But on the CEM120, the tall saddle plate leads directly to much heavier counterweight.

Hi,

 

Well that depends on the size of the worm gear and the Meridian angle I want to achieve. The nearer the saddle plate to the RA shaft the less Meridian angle clearance I will have. The bigger the RA worn gear the farther away my saddle plate from the RA shaft. Compromises, compromises and more compromises.

 

That is why I used in my first Mounts, GEM design, a DEC riser as well as on the CEM 120 EC2 I have also. On my first Mounts I raised it by 2" and on the mounts now I raised by 63 mm.

 

Anyhow  a possible disadvantage of the CEM is that the DEC assembly in its movement is limited due to the mounts lower Body, and in this case on the CEM 120 to 14° degrees past the meridian.

 

OK the GEM in a certain Position can achieve more but when it comes to a position where the camera hits the pier there is no difference to a CEM.

 

So an Advantage of the GEM over the CEM also does not exist here. IMHO

 

Below some pictures of a GEM limited by the Pier.

 

 

Edited by Real14, 19 September 2018 - 10:12 AM.

[gotak]

Hi djhanson,

 

That is absolute nonsense what you wrote. Exchanging the boards does not take more then 15 minutes and as another poster said iOptron ships them to you without a problem.

 

I am in contact with them and have got for testing purposes some boards. The boards arrived in Mexico in less then 4 days ...

 

Also not true ... 

 

regards Rainer

It's also often said that only the premium (and I hate that term, it's a term that associates with the BMW logo in a **** meme) will stock parts etc for the long term but:

 

https://www.cloudyni...rs#entry8476193

 

If you can get a new motor of an out of production mount after 9 year of ownership (granted the MT 2 is very similar and likely uses the same motors) I don't know if that argument stands always. 

 

Personally I do know for a fact they still stock the original IEQ45 (non-pro) parts like worms because they have gotten confused and offered me a non-pro worm once. And historically they have offered upgrade path with part swap when it's possible (zeq24 to cem25 for example, and ieq45 high precision worm, ieq45 to pro). 

 

In the end I think maybe someone just had a bad experience for whatever reason and has their own point of view. From what I have seen bad experience doesn't seem common. Yes, they don't fix everything (PEC, the older IEQ45 had meridian flip issues with ASCOM that were never addressed) but on the balance of things considering the price discount vs performance it's not exactly a big problem.

Edited by gotak, 19 September 2018 - 12:58 PM.

[jbrandmeyer]

Anyhow  a possible disadvantage of the CEM is that the DEC assembly in its movement is limited due to the mounts lower Body, and in this case on the CEM 120 to 14° degrees past the meridian.

Thank you for sharing your use-case.  That much clearance lets you take meridian-crossing subframes over 40 minutes long with margin to spare.  Is this really very common for you?

[WadeH237]

I'm amazed that they don't consider that voiding the warranty.  But some companies have different quality standards.  From a quality control perspective, most companies frown upon consumers working on fixes to a new product, as the consumers aren't properly trained in many disciplines. (e.g. ESD control, etc) 

I know from first hand experience that Astro-Physics will on occasion ship parts that involve disassembling components of the mount, including opening the controller and replacing chips.  I've also replaced a bolt inside the gearbox under recall and transplanted absolute encoders from one mount to another.  They don't consider any of this voiding the warranty, and will happily walk you through it.

 

Regardless of the pros and cons of GEM vs CEM mounts, it seems like we should at least apply the same standards to all companies.  If this is some kind of criticism of iOptron, it should also be a criticism of Astro-Physics.  Personally, I consider it a good thing that both of these companies will support an owner making reasonable repairs when it makes sense. 

[fmozza]

I purchased a CEM60 in May 2018 and have spent a fair bit of time learning about the mount. Disclaimer: I bought the mount, no special deal, and no-one is paying me to say anything - just my experience. 

 

Bottom line: the CEM60 is a very good mount for the $$.  My other mount is a 7 year old CGEM which I hypertuned, and works well when the electronics isn't giving me fits. But the CEM60 is a significant step above - the best I've been able to get out of the guiding the CGEM is an RMS of 0.8 arcseconds, whereas I routinely get around 0.5 arcseconds on the CEM60. You have to take care to balance, route cables and polar align, etc, but it is consistent. See the attached image as an example of this (object was Andromeda with an alt of 53 deg, and dec of 41 deg.)  This gets me into the range for my imaging at about 1arcsecond per pixel which is very good for this class of mount.

 

Regarding the dynamics of a CEM vs a GEM, I think its a moot point.  For me, a key metric is what does it cost to get to a low(er) guiding RMS.  I didn't expect the CEM60 to be able to do 0.5 on a regular basis.  

 

The CEM60 does have its quirks, but I can live with them - I'd like to have the PEC fixed so I can use it with guiding, and there are a few other niggly things, but its a good mount.

 

And by the way, one should ignore all marketing claims for all products - marketing is where rubber hits the sky, testing is where rubber hits the road. Proof is in the pudding.  And ignore the trolls....

 

 

jmh

 

[mvas]

The CEM60 does have its quirks, but I can live with them - I'd like to have the PEC fixed so I can use it with guiding, and there are a few other niggly things, but its a good mount.

I am looking for a new GEM / CEM mount.

Please help me understand the highlighted phrase, "... I'd like to have the PEC fixed ..."

What do you mean by "fixed" ?

a) Are you saying the PEC corrections are not permanent ?

b) There is too much PEC in the worm ?

c) Other ?

 

EDIT:

I found this ...

https://www.cloudyni...is-figured-out/

Edited by mvas, 20 September 2018 - 09:25 AM.

[fmozza]

From what I understand is the CEM60 firmware ignores guiding corrections when PEC is engaged. It's not a big issue for me - PHD2 with the PPEC algorithm for RA and Z-filter for DEC gives me what I need and I don't know for sure if PEC + guiding would yield a better RMS.  The PHD folks recommend it as a best practice at this point which is why I'd like to try it.

 

Using the Guiding Assistant with PHD2, I've seen a range of RA Peak-to-Peak on my CEM60 from 3.75 arcseconds to 8.3 arcseconds, and the average is around 5 arcseconds which is what iOptron claims for the mount. I'm happy with the numbers I'm getting.  Your mileage may vary.

 

I am looking for a new GEM / CEM mount.

Please help me understand the highlighted phrase, "... I'd like to have the PEC fixed ..."

What do you mean by "fixed" ?

a) Are you saying the PEC corrections are not permanent ?

b) There is too much PEC in the worm ?

c) Other ?

 

EDIT:

I found this ...

https://www.cloudyni...is-figured-out/

[rgsalinger]

I may be unique on this forum since I own both an SB mount (MX+) and a CEM120EC2. (I also owned a Mach 1 for quite a while. I'd still own the Mach 1 if I hadn't scored a share in a local observatory. Here's what I think about the issues in this thread. The Mach 1 does not have the same capacity as the MX+ or the CEM120EC2. So, to me, it's better to compare it to the MYT and the CEM60EC which I can't personally do from experience although I did use an MYT once.)

 

I'd sum up my experiences with these points-----

 

1. There is simply no practical advantage between a GEM and a CEM design, when you are talking about these two mounts. Period. Theory is very nice, but in practice good manufacturing technique wins out over theory. The thousands of SB and AP observatory class mounts out there are running just fine. The CEM is no easier to balance than any other equatorial mount I've ever used. There are certain to be edge use cases in favor or one or the other but I haven't found one yet. I've put four systems on both and just haven't seen any difference that I can attribute to CEM/GEM design differnces. In fact, the CEM is 7 pounds heavier than the MX+ to achieve the same weight capacity. Alt/AZ adjustments are smooth and accurate on both mounts.

 

2. Support is good from iOptron and they will send you parts. I just changed out the USB3 board and sent them back the old one. Just one example. However, both SB and AP have much better support models. I sent an email to AP about buying a field flattener and received a response within 24 hours from Roland himself. You will never hear from iOptron in that time frame. I'm still waiting for answers to three questions I posed weeks ago about bugs I've found in the software they provide with the mount. The SB forum can provide you with answers in the middle of the night - it has done that for me. SB and AP have full repair facilities in the USA (where I live). I'm not sure that iOptron can repair a CEM120EC2 here.

 

3. Despite how annoying my current bug list is, in a practical sense, the CEM120EC2 works at least as well as the MX+. I've put up to 80 pounds on both mounts and they both track (same location) at around .3 arc seconds of error. Can even be a bit better on good nights. The mounts are seeing limited. The mechanicals are superb on both. The CEM has a little squeak when slewing my MX+ burbles when tracking. 

 

4. The CEM120EC2 is a bargain. It's almost 3000 dollars cheaper than the MX+ once you add in the necessary bits and pieces to give you a full system. This assumes that you buy them and don't build things yourself. As a practical matter you can buy an ASI1600MM-C, the companion filter wheel, a nice OAG and a guide camera for the difference. Or fly to China see the Great Wall and fly back. 

 

5. You get a full through the mount power and USB cabling implementation with the CEM. Right now I have my wide field system (WO71 Star) on the mount. It's got a ZWO 1600MM-C, companion 7 position filter wheel, moonlite electric focuser, a ZWO290 mini guide camera plus an Optec Flatman. All five components are attached to the saddle plate and power is supplied to the main camera and focuser from the plate as well. I have had ZERO USB failures over 4 complete nights of imaging since I set it up and zero USB failures over several weeks using the same stuff on a different OTA. The same setup on the MX+ required an additional, a USB hub, bypassing the Versaplate and some power splitters to boot to get the same result. 

 

6. The big advantage of the MX+ is (to me) the seamless integration of the system with the SKYX. I love the SB forum, I get tips there all the time about how to use the system better. Trying to use it with the CEM120EC2 has been problemmatic. This is probably a separate post but, for example, the CEM will not accept custom tracking rates in DEC and RA. This makes imaging things like comets, satellites and asteroids more difficult. Even though the SKYX supports this generally (I've used it on non Bisque mounts) an error is thrown with the CEM120. SB owns problems with the mount and the software. It's really this kind of integration that led AP to build out APCC (I think). 

 

7. I personally find the CEM120EC2 driver software GUI for the mount a reall dog's dinner. It has three different panels to deal with. It is slow to load. Some features don't work. It interferes with my (T-Point) pointing model and it doesn't support custom tracking rates. They should have looked at what others provide and not tried to build something entirely different.  

 

As always, YMMV, IMHO and please correct anything you think I've got wrong (PCAYTIGW?).

 

Rgrds-Ross

[star drop]

The thread title and topic is Center-balanced Mounts . . . the big deal?. Please refrain from politics and off topic replies. 

[rgsalinger]

Errata in my previous post:

 

1. The specified capacity of the CEM120 is 15 pounds more than the MX+. For those the believe specs, that might be important.

 

2. The CEM120EC2 is equipped with high resolution relative encoders. This might also be important to those who are partial to encoder mounts. Equally spec'd the CEM120 without encoders is 5k less than the competition equally equipped. 

 

3. The CEM120EC2 has a hand controller. This can be a huge convenience if you want to use the mount visually.

 

I'm hoping that I'm not the guy who was "off topic". I thought it was interesting to compare two similar mounts of different designs but I may have left some things out that I should have included. 

 

Rgrds-Ross