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Ioptron CEM120: to EC or not to EC... that is the question

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#1 fmazzanti

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Posted 18 April 2021 - 10:50 AM

Hi folks,

 

I think the title of my questions says it all... I'm currently considering the purchase of a CEM120 to sit on the pier of

my permanent setup at the terrace of my home. My plan is to do AP using my GSO RC10" at the native focal length of 2000mm, so I want to take long exposures of 5-10min. I know this is demanding so I originally thought that the EC or EC2 version (with the encoders) was my preferable choice.

 

But then I started reading and watching youtube videos about this mount. That drives me a bit crazzy as I've seen people getting wonderful results with the encoders, but also read weird things about issues when trying to guide the EC/EC2 versions with PhD2. That makes me wonder what is the right version to purchase. In fact I plan to guide with TheSkyX Pro which I own, so if the bad interaction is only with PhD2, then that shall not be an issue. On the other hand, I know in posts you always read about bad experiences and hardly about good ones, so the impression one gets may be biassed.

 

So all in all, I'm in a mess. I may want to purchase a CEM120, but do not want to make the wrong choice. So what is the current status? What is the best choice now, to buy it with encoders or to stay away from them? Please notice I'm not really fond of unguided imaging, so I plan to guide, be it EC or not.

 

Best regards,

 

Ferran,



#2 Dynan

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Posted 18 April 2021 - 11:37 AM

A long, thorough discussion starts here:

 

https://www.cloudyni.../#entry10395108

 

I changed from EC to Non-EC and get better stars for my average seeing conditions. YMMV

 

Just my penny.gif penny.gif



#3 ravenhawk82

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Posted 18 April 2021 - 12:18 PM

I was debating this a while ago and ultimately decided against EC. There are SOME people who get excellent results with the EC versions, but given that there are only a couple hundred owners of this mount and I've seen at least twenty unique people complaining about poor encoder performance across various forums, that's a pretty high rate of failure and those are just the ones that speak up about it. If you're willing to take a chance on sending it back, check with some vendors and ask explicitly about their return policy for this mount and explain the issues you've seen mentioned. If they're reputable and have a good return policy it wouldn't hurt to take a chance, you just might be out some shipping money. 

For what it's worth, like you I settled on an RC10 scope with the goal of imaging galaxies with and without a focal reducer. I was originally planning on a CEM120, but after lots of reading I actually went with the CEM70 instead. That scope is well within its weight capacity and still allows room for me to mount a guide scope on top. I did however still choose the nonEC version as I didn't want to hassle with the encoders. I also don't have a permanent observatory, so the extra capacity of the 120 wasn't worth hauling a 60lb mount head in and out each night. Since you have a permanent pier this is a non issue... Were I in that boat I would have gone for the 120 and called it a day.

 

The CEM70 arrives today and I'll be ordering the 10RC once my bank account has had some time to recover. Later this week I'll post some long guided exposures from the new mount with my 6RC and see how it handles. 



#4 fmazzanti

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Posted 18 April 2021 - 01:17 PM

I see... such a bad thing :( Yes I can check with the vendor for his return policy, but I'm not sure I *want* to go through this game, I have to think carefully.

Well then I can ask the question the other way around: how well are the non-EC versions of this mount performing? Is there any guiding plot and/or image I can see, taken with a large focal length?

I ask because currently I own a Paramount MX (the original one) which has given me RMS guiding errors of around 0.3 arcsec/pixel in DEC but 0.6 arcsec/pixel in RA. With the native focal length of 2000mm that yields slightly oblong stars... not too much but I can't stand it (eccentricities measured with PixInsight of around 0.6, which is noticeable). I wonder if I may get something similar with the CEM120 and forget the purchase altogether, or can get better (with the non-EC version).

 

Thanks in advance,

 

Ferran.



#5 Dynan

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Posted 18 April 2021 - 02:08 PM

If you follow the (lengthy) thread I linked, it explains that the problem with encoders is not model or encoder specific. It is the way encoder signals are used. SDE (SubDivision Error) is intrinsic in encoder use. It's your dime, and your time, so get what you feel most confident with...and are comfortable possibly returning.

 

Keep us posted, please!



#6 benklerk

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Posted 18 April 2021 - 05:59 PM

Hi Ferran

 

I'm one of those people you got a bad 120ec mount. I have a 14" RC at 2850mm FL, it handles the weight pretty well. But I had one with the SDE high enough that I was getting inconstant guiding. 

These 120ec mounts are hit and miss, you could get a good one with no issues or you could get one with issues and spending quite a bit of time trying to diagnose the problem. My dealer didn't believe the mount have SDE so I had to produce the evidence. I sent almost months trying to find what the issue was. So I lost a whole year on imaging but instead doing troubleshooting instead. 

 

At this moment I have my new mount on order, just waiting for it to come by boat, and I can get back into imaging with my RC 14 which I haven't done yet, only test images.

 

If your going to use a 10" RC I would look at either the cem120 non ec, or a different brand.

 

regards

ben



#7 suvowner

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Posted 18 April 2021 - 10:44 PM

Hi folks,

 

I think the title of my questions says it all... I'm currently considering the purchase of a CEM120 to sit on the pier of

my permanent setup at the terrace of my home. My plan is to do AP using my GSO RC10" at the native focal length of 2000mm, so I want to take long exposures of 5-10min. I know this is demanding so I originally thought that the EC or EC2 version (with the encoders) was my preferable choice.

 

But then I started reading and watching youtube videos about this mount. That drives me a bit crazzy as I've seen people getting wonderful results with the encoders, but also read weird things about issues when trying to guide the EC/EC2 versions with PhD2. That makes me wonder what is the right version to purchase. In fact I plan to guide with TheSkyX Pro which I own, so if the bad interaction is only with PhD2, then that shall not be an issue. On the other hand, I know in posts you always read about bad experiences and hardly about good ones, so the impression one gets may be biassed.

 

So all in all, I'm in a mess. I may want to purchase a CEM120, but do not want to make the wrong choice. So what is the current status? What is the best choice now, to buy it with encoders or to stay away from them? Please notice I'm not really fond of unguided imaging, so I plan to guide, be it EC or not.

 

Best regards,

 

Ferran,

just save your money and sanity, and save up for a 10micron gm1000 if you want to have encoders. or get on waitlist from an astro-physics in 2 years. long subs at long focal lengths work so much better in the premium mount class, if you don't do it now, you probably will eventually. 



#8 fmazzanti

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Posted 19 April 2021 - 12:56 AM

Hi,

"long subs at long focal lengths work so much better in the premium mount class, if you don't do it now, you probably will eventually."

this is not always the case. I come precisely from there, as I have a Bsque Paramount MX, which I want to replace because it has never performed as expected (and my fighting with this mount is somewhat large... around 8-9 years). There are also quite a lot of reports of people complaining about faulty MyT's. Maybe 10micron is a better bet, I don't know. But I've already spent a terrible amount of money in something that is simply not up to the job. Trying that again is... I don't know.

Best,

Ferran.



#9 Orion64

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Posted 19 April 2021 - 05:06 AM

I'm also strong in the market for a cem120 and haven't done the purchase yet, only because I am asking the same question. Really not sure what the answer is re the encoders. Too many conflicting results from users. Especially for astrophotography where guiding is a must in any case

#10 fmazzanti

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Posted 19 April 2021 - 12:31 PM

yes, that's exactly how I feel. The more I read, the more confused I am.
I am ready for a non-EC if needed be, but ai have yet to see what RMS in both axes this one produces when guiding...

#11 amoncayo

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Posted 20 April 2021 - 02:42 AM

yes, that's exactly how I feel. The more I read, the more confused I am.
I am ready for a non-EC if needed be, but ai have yet to see what RMS in both axes this one produces when guiding...

 

I'm also strong in the market for a cem120 and haven't done the purchase yet, only because I am asking the same question. Really not sure what the answer is re the encoders. Too many conflicting results from users. Especially for astrophotography where guiding is a must in any case

Hello,

 

I've been using a non-EC CEM120 mounted on a permanent pier and occasionally on a tri-pier 360.

 

At times I was tempted to upgrade to an EC model but ultimately chose not too based on my experience with the smaller sibling, the CEM40-EC.

 

When seeing is very good, the standard CEM120 is stable and works very well, producing round stars with <0.5 score in PixInsight's eccentricity calculator.

 

I use a 12" truss RC exclusively with this mount. With an APCCDT67, I have a final FL=1850mm.

 

Attached is a brief guide log from last year, which is the only one I was able to find upon a quick look through my log files. The night was of better than average seeing. Note that I'm close to the pacific coast and subject to dramatic swings in seeing due to marine layer effects. I'm also in a class 5 Bortle scale area.

 

Attached File  PHD2_GuideLog_2020-05-07_210653.txt   40.68KB   3 downloads

 

Also attached is an image taken recently (not completed due to changes in gear configuration) with my new ASI2600 camera. The guiding was in the range of 0.5-0.7" rms, as compared to the values shown in the guide log from last year. The image of M97 demonstrates what I typically achieve with this mount in decent (not great) seeing conditions. Things become a bit sharper with better seeing, but I've never been truly satisfied, not having obtained ultra-sharp stars so far with 3 years using this gear. Most likely I'd need to move the gear to a darker site with much better seeing conditions; or perhaps upgrade to a scope with better optics; or both.

 

210218_M97_ASI2600MCpro_33%.jpg

 

Al


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#12 niteman1946

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Posted 20 April 2021 - 06:58 AM

Nice image, Al.

 

Mark


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#13 fmazzanti

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Posted 20 April 2021 - 07:55 AM

Hi Al,

thanks for the guiding log and shared experience :) Your gear seems to be similar to mine: GSO RC10, ASI290mm mini for guiding and ASI294mm for imaging.

The image looks good and the results you get in terms of total RMS guiding errors are actually close ta what you'd get with a premium mount. One last question: how does the RA RMS guiding error compares to the Dec RMS guiding error? I ask because with my current mount the latter is half the former (0.6 in RA, 0.3 in Dec), and that leads to slightly oblong stars (at a resolution of 0.94 arcsec/pixel). That problem seems to disappear when both values are similar.

Best,

Ferran.



#14 amoncayo

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Posted 20 April 2021 - 08:45 AM

Hi Al,

thanks for the guiding log and shared experience smile.gif Your gear seems to be similar to mine: GSO RC10, ASI290mm mini for guiding and ASI294mm for imaging.

The image looks good and the results you get in terms of total RMS guiding errors are actually close ta what you'd get with a premium mount. One last question: how does the RA RMS guiding error compares to the Dec RMS guiding error? I ask because with my current mount the latter is half the former (0.6 in RA, 0.3 in Dec), and that leads to slightly oblong stars (at a resolution of 0.94 arcsec/pixel). That problem seems to disappear when both values are similar.

Best,

Ferran.

Ferran,

 

On my CEM120, I can't recall having seen RA be twice Dec on a typical imaging night. I agree that it would lead to oblong stars. That's precisely what I found with my CEM40EC. Even though the "rms" value of RA was low as expressed by PHD, the SDE mentioned earlier in this thread caused the RA to actually be much higher than Dec in mission mode and with the longer guide exposures that work better with the EC mounts. To combat that, when I was using >660 mm FL on that mount, I worked to purposely worsen Dec guiding. I found that if the discrepancy between RA and Dec was less than 40% or so, I'd get decent stars. Ultimately, I chose to only use a Redcat 51 on the 40EC. There, I don't ever see oblong stars due to the large image scale.

 

Back to the CEM120, I've generally seen the values to be similar or perhaps off by about 20% (RA usually larger) under good seeing. With poor seeing, they both worsen, approach, and even exceed 1" rms. Rarely have I seen it higher than about 1.2" unless there was ground vibration or high wind. At that level, I call it quits for the night.

 

My goal when I bought the mount was to achieve better than 0.5" rms guiding overall. While I've achieved it from time to time with great seeing, I typically achieve 0.5-0.7" rms on average nights. On worse than average days it will creep up to 0.8" or slightly higher. Images can still be impressive, with round stars. However, the images tend to be "softer".

 

I continue to wonder whether the softness is also in part due to my iOptron 12" RC..?

 

Al


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#15 amoncayo

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Posted 20 April 2021 - 08:51 AM

Nice image, Al.

 

Mark

Thanks Mark!

 

I wish I had been able to complete that image. That's one of my "difficult" targets for two reasons. My FOV is small and with OAG it is difficult to find a good guide star. Secondly, I've found it extremely difficult to draw out that reddish halo around the nebula. In this particular image, the ASI2600 did a very good job.

 

I'll definitely try this target again next time it is possible! (Seeing in my area has become dismal over the past year+. I've only been able to use my gear a total of 4 nights in the last 12 months!! frown.gif )

 

Al



#16 fmazzanti

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Posted 20 April 2021 - 08:53 AM

Hi Al,

thabks again for the input, that is quite clarifying things out.
Regarding softness of images, what is your resolution? Mine is 0.94 arcsec/pixel because I bin 2x2. If not binning, resolution is half that and I don't think the seeing (unless dam good) will allow for tight stars. Also are you sure about your RC collimation? Typically that's a reason for softness if not spot on, and RC's are difficult to collimate really well.
Best,
Ferran.

#17 amoncayo

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Posted 21 April 2021 - 02:17 AM

Hi Al,

thabks again for the input, that is quite clarifying things out.
Regarding softness of images, what is your resolution? Mine is 0.94 arcsec/pixel because I bin 2x2. If not binning, resolution is half that and I don't think the seeing (unless dam good) will allow for tight stars. Also are you sure about your RC collimation? Typically that's a reason for softness if not spot on, and RC's are difficult to collimate really well.
Best,
Ferran.

Hi Ferran,

 

My image scale for the M97 image was 0.39"/px. I will try binning at some point, given that I have 26 mega pixels available. That would put it at 0.78"/px and closer to where you are. The ASI2600MCpro allows binning in color. It is the first OSC camera that I've used that can do this.

 

The guiding was at 0.6"/px.

 

The collimation has held very well for my RC. It could perhaps use a very fine tweak, but others have noted that the softness is just an expected artifact of the very large central obstruction.. I suppose I wouldn't know unless I upgraded to a Planewave or other "premium" OTA, as those too would have large obstruction.

 

Al



#18 Beaverpond Astro

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Posted 21 April 2021 - 09:46 AM

My seeing varies quite a bit over the course of the year, and literally being next to a large beaverpond doesn't help, but on a night of decent seeing my CEM120 will guide consistently in the .4" to .5" total rms range with occasional periods of .35".  Elongations measured in PI at .5 or slightly better..

 

A poor night of seeing will yield guiding anywhere from .6" to 1.2"...

 

I run a WO 6" with guide scope, sometimes a 2nd scope (81mm).  About 45-50 lbs..

 

So I am quite happy with my non-EC 120; I think I got a good one...the factory PE curve was almost flat (.6" P-P), for whatever that is worth..


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#19 amoncayo

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Posted 21 April 2021 - 02:54 PM

My seeing varies quite a bit over the course of the year, and literally being next to a large beaverpond doesn't help, but on a night of decent seeing my CEM120 will guide consistently in the .4" to .5" total rms range with occasional periods of .35".  Elongations measured in PI at .5 or slightly better..

 

A poor night of seeing will yield guiding anywhere from .6" to 1.2"...

 

I run a WO 6" with guide scope, sometimes a 2nd scope (81mm).  About 45-50 lbs..

 

So I am quite happy with my non-EC 120; I think I got a good one...the factory PE curve was almost flat (.6" P-P), for whatever that is worth..

It is interesting that your results seem to align well with mine. waytogo.gif

 

On the topic of PE, there's what iOptron provides (as measured by encoder -- whatever that means), then there's what we actually see in the field. My mount is typically in the range of 1.5-2x what is specified for the mount. It is relatively well behaved, so PHD's Predictive PEC works very well to tame it. My payload exceeds 70 lbs and I have 88 lbs of counterweights on the mount.

 

Al


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#20 fmazzanti

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Posted 21 April 2021 - 03:53 PM

Should I understand then that the actual PE specification from iOptron is about half the real one? They say a maximum peak to peak of 3.5 arcsec... so that can really be 6 or 7?

#21 amoncayo

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Posted 21 April 2021 - 04:17 PM

Should I understand then that the actual PE specification from iOptron is about half the real one? They say a maximum peak to peak of 3.5 arcsec... so that can really be 6 or 7?

That has been what I've observed with my particular mount. In fact, if you use the PHD log viewer and reconstruct the PE (basically removing the corrections applied by PHD), then you will see the intrinsic PE of my mount. In that particular session it was about 9-10" peak-to-peak.

 

While it looks bad, it has been something that can be tamed. I came to the CEM120 from an Orion Atlas EQ-G. That mount had about 30" peak-to-peak PE..

 

Al



#22 astrohamp

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Posted 21 April 2021 - 10:34 PM

During an exceptional period of good seeing (for New England northern Vermont) my CEM120 (non-EC) mount guided at 0.35"/0.26" (RA/Dec) RMS if I am not mistaken.   I see double these amounts often, again I believe due to poor seeing.  

My instruments are set up using ZWO OAG PHD2 guiding with an ASI174 camera at 1.22" per pixel un-binned through the main OTA at 990mm Fl.  

Early in 2020 during a discussion with the iOptron marketing manager I learned that at that time something over 300 CEM120 mounts had been sold/delivered to the USA market.

If I were making the decision again the CEM70 (non-EC) would be high on the list, (didn't exist when I acquired my 120).  With payload issues the 120 in a non-EC form would be my choice again.  

I believe it was something I read from an Astro Physics discussion (made by 'he' himself) regarding using encoders that helped steer me away from the added expense.

 

 

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  • CEM120guidingB.jpg

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