CEM120EC real-world review with actual guiding metrics

Read noise Astrophotography · Dec 7, 2025, 09:32 PM

When iOptron says zero backlash, they mean it.

I had to laugh just a little bit at your expense. When I was at NEAF last year, I put my hands on every single mount they had on display, and every one of them had very noticable RA gear slop. It wasn’t a good look tbf.


Jokes aside, I’ve been looking to move up to a bigger mount at some point. I haven’t ruled out trying a 120EC, simply because of the price point. Ultimate, I’m probably going for the GM2000. I know a few people that generally seem to like the 120 however. I also know alot of people that wouldn’t touch iOptron mounts with a 10 foot pole. I think what would really help us get a feel for the actual performance is sharing a guide log. I can show you screenshots of guide graphs that make my EQ6 look just as good as your 120EC, but conditions change, and in those poor conditions is where it will shine.

Brian Puhl · Dec 7, 2025 at 10:19 PM

Read noise Astrophotography · Dec 7, 2025, 09:32 PM

When iOptron says zero backlash, they mean it.

I had to laugh just a little bit at your expense. When I was at NEAF last year, I put my hands on every single mount they had on display, and every one of them had very noticable RA gear slop. It wasn’t a good look tbf.


Jokes aside, I’ve been looking to move up to a bigger mount at some point. I haven’t ruled out trying a 120EC, simply because of the price point. Ultimate, I’m probably going for the GM2000. I know a few people that generally seem to like the 120 however. I also know alot of people that wouldn’t touch iOptron mounts with a 10 foot pole. I think what would really help us get a feel for the actual performance is sharing a guide log. I can show you screenshots of guide graphs that make my EQ6 look just as good as your 120EC, but conditions change, and in those poor conditions is where it will shine.

Star eccentricity is usually around 0.30

It’s certainly obvious when you know what you’re looking at, I agree.

An RA OSC at 0.14 means PHD’s pretty much doing nothing. Your encoders are definitely doing the work. It does seem like the mounts doing extremely well. If it’s consistently like that, I’m guessing you just have some really good skies. Even on my 1 arc second nights, I still have a nice fine sawtooth action going, but I think I’m sampling the skies better with my 678M. Highly recommend the camera for guiding if you’re looking for an upgrade.

I got a bargain bin second hand CEM70ec-Nuc from Agena for under $3k. There is definitely some backlash in the DEC on mine, which is very manageable. For the price it is a ridiculously good mount.

Here is a full PHD2 log from my backyard for those interested.

https://drive.google.com/file/d/1iBGx0ZeL9eeCtweJYefz2ib4BpcjfeDD/view

Ashraf AbuSara · Dec 8, 2025, 01:27 AM

some backlash in the DEC on mine

I guess you know that you can adjust the worm/worm gear mesh…

📷 image.png

Brian Puhl · Dec 7, 2025 at 11:39 PM

It’s certainly obvious when you know what you’re looking at, I agree.

An RA OSC at 0.14 means PHD’s pretty much doing nothing. Your encoders are definitely doing the work. It does seem like the mounts doing extremely well. If it’s consistently like that, I’m guessing you just have some really good skies. Even on my 1 arc second nights, I still have a nice fine sawtooth action going, but I think I’m sampling the skies better with my 678M. Highly recommend the camera for guiding if you’re looking for an upgrade.

Hi sorry fell alseep

I’m currently using the ToupTek GMP462M. Honestly it’s been rock-solid: extremely sensitive, never fails to find stars. I also run a UV/IR-cut filter on it. Blocking IR keeps the guide star tighter and improves centroid positioning in PHD2.

You mentioned saw-toothing what gear are you using, and what aggression levels?

Sometimes that pattern shows up when the guide aggression is a bit high or when min-move is too low especially if it’s too low. Min move i adjust depending on seeing.

I’ve guided with both PHD2 and MaximDL, with and without AO, It’s alway the same fundamentals. PHD2 differs because you can select different algorithms. Maxim DL just works. I’ve lost Maxim DL due to PC death and I refuse to buy it again.

My workflow:

• PA first if that’s tight, everything else becomes easier.

• No PA offset that idea died with film.

• If I’m seeing oscillations, I increase min-move first; that normally calms the graph right away.

• Then I fine-tune each axis:
  

  • reduce aggression until it’s under-correcting (slightly slow to hit target),

  • then bump it up just enough so it lands corrections cleanly.

• Once RA and DEC are stable, I zoom in on the graph and continue to fine tune until it’s correcting only what’s needed no chasing seeing.

If you want to discuss this further just DM me.

CS!

Rainer Ehlert · Dec 8, 2025 at 01:28 AM

Read noise Astrophotography · Dec 7, 2025, 09:32 PM

CEM120EC

Hi,

Which year did you get them?

I got them in April 2018 and are two CEM 120EC2 and those were the first two mounts with encoders leaving the iOptron premises.

What is your longest focal length you work with?

I do image at 3200mm focal length

Wow how do they handle 3200 mm ?

I’ve got 2048mm 1624mm 550mm 328mm

It’s currently running the 550mm and I’ll swap out to the longer focal length for galaxy season I don’t anticipate any problems at all. Worst case I have a AO unit to hand. But that’s planned to go on the CEM 70 with RC 8 and RC 10 Astrograph for the 120EC

Rainer Ehlert · Dec 8, 2025 at 01:44 AM

Ashraf AbuSara · Dec 8, 2025, 01:27 AM

some backlash in the DEC on mine

I guess you know that you can adjust the worm/worm gear mesh…

📷 image.png

There is a bit of an art to fine tuning backlash. I find the best average by listening to the motor as it drive the axis . Binding and stiction are the enemy here.

At 550mm, you are probably rock solid.

My only concern would be moving up to longer FLs and the effects of SDE at very fine image scales. SDE won’t just show up in your PHD2 graphs - you have to go looking for it: Make a recording with PHD2 guiding corrections turned off, 0.5” exposures, at around Dec=0° & the meridian, then do a frequency analysis of the log file. The SDE is there, and you’ll see it.

My CEM70EC is unimpeachable at 910mm, but it does have about 0.4” peak-to-peak SDE, which has never been evident in any subframes at my current focal length. I’ve never tried a longer FL, and maybe never will. The mount guides reliably between 0.3-0.4” RMS on steady nights, somewhat better at higher declinations.

I’ve had three iOptron mounts: the CEM70EC, which is outstanding and reliable, night after night. I briefly had a CEM40, which had some wild Dec swings, that couldn’t be corrected by adjusting the worm meshing or belt tensions. I ended up having to return it, sadly. And then finally a CEM120 (vanilla, no EC) which, guided, is just as rock solid as the CEM70EC, without any risk of SDE. Two outta three ain’t bad.

I lust for a 10Micron mount, but right now, between the CEM70EC and the CEM120, which have both been more or less plug & play, I’m having a very hard time justifying a $12k-18k outlay.

Jesse Priolo · Dec 8, 2025, 05:12 AM

At 550mm, you are probably rock solid.

My only concern would be moving up to longer FLs and the effects of SDE at very fine image scales. SDE won’t just show up in your PHD2 graphs - you have to go looking for it: Make a recording with PHD2 guiding corrections turned off, 0.5” exposures, at around Dec=0° & the meridian, then do a frequency analysis of the log file. The SDE is there, and you’ll see it.

My CEM70EC is unimpeachable at 910mm, but it does have about 0.4” peak-to-peak SDE, which has never been evident in any subframes at my current focal length. I’ve never tried a longer FL, and maybe never will. The mount guides reliably between 0.3-0.4” RMS on steady nights, somewhat better at higher declinations.

I’ve had three iOptron mounts: the CEM70EC, which is outstanding and reliable, night after night. I briefly had a CEM40, which had some wild Dec swings, that couldn’t be corrected by adjusting the worm meshing or belt tensions. I ended up having to return it, sadly. And then finally a CEM120 (vanilla, no EC) which, guided, is just as rock solid as the CEM70EC, without any risk of SDE. Two outta three ain’t bad.

I lust for a 10Micron mount, but right now, between the CEM70EC and the CEM120, which have both been more or less plug & play, I’m having a very hard time justifying a $12k-18k outlay.

Hi Jesse,

You touched a very delicate topic and yes, I am a big victim of SDE on both of my CEM 120EC2 mounts.

During two years having had the first EC2 mounts from iOptron I helped them to solve a lot of their software bugs but the SDE problem in my mounts is still rpesent and especially at 3200mm focal length it is visible in the RAW images but thanks to Russell Cromans BlurXTerminator my images do have round stars.

After a time they left me standing alone in the rain with no solution. That is the only reason why nowadays I would stay away from iOpton due to them not compying with their responsability.

At the beginning hte SDE was terrible and after updating until the last firmware version it looks like the SDE got smaller but it is still there.

OK, I am imaging at 0.245” arcseconds per pixel. Takahashi Mewlon 250S (Uncorrected Dall-Kirkham) with a ZWO ASI 1600MM Pro.

You can see my images under my profile.

Of course if I would have had the money I would perhaps bought 2 AstroPhysics but never a 10 Micron as there is no representative in Mexico and the US representative, well I will not say why I would not have bought 10Micron from him.

There are a few AstropHysics in Mexico and the service is quite acceptable. This are the issues one has to deal with living in a country far away from the technical possibilities.

All in all I would be careful to recommend the encoder mounts from iOptron but the mechanical mounts with NO encoders are very well built and so I would say reliable.

If one wants to buy encoder mounts from iOptron I would want to get a written confirmation that there is no SDE problem with them and if yes I have the right to return them at their expenses, or get them for a 3 month trial period.

I recently purchased a CEM 40EC based on my excellent experience with Ioptron customer service on my previously acquired CEM25P. I have only gotten it out once due to endless overcast here in Florida and some obligatory travel.

On that occasion I struggled to balance the mount and was shocked to find that the counterweight bar was not 180 degrees from the dovetail saddle. (It wasn’t even close! Probably between 170 and 175 degrees!) Apparently on the CEM70s the precision which which this is set at the factory is so low that there is actually an adjustment screw for this, but mine was so far out of kilter that the range of adjustment provided by the screw was insufficient.

I sent the mount back to iOptron and they replaced the mount head without charge, but overall I was astounded to see a problem of this magnitude and obviousness on inspection leave iOptron’s factory. A Google search revealed that others have run into this problem as well. I was also surprised that iOptron required a copy of my receipt to apply the warranty to repairing what could ONLY be a manufacturing defect and a severe one at that. On a $5000 mount (which is less than the other mounts discussed on this thread, but still…)

Looking forward to getting out and trying out the mount, but I’m a little nervous given the fact that a defect of this magnitude got through inspection.

John Tucker · Dec 8, 2025 at 10:14 PM

I recently purchased a CEM 40EC based on my excellent experience with Ioptron customer service on my previously acquired CEM25P. I have only gotten it out once due to endless overcast here in Florida and some obligatory travel.

On that occasion I struggled to balance the mount and was shocked to find that the counterweight bar was not 180 degrees from the dovetail saddle. (It wasn’t even close! Probably between 170 and 175 degrees!) Apparently on the CEM70s the precision which which this is set at the factory is so low that there is actually an adjustment screw for this, but mine was so far out of kilter that the range of adjustment provided by the screw was insufficient.

I sent the mount back to iOptron and they replaced the mount head without charge, but overall I was astounded to see a problem of this magnitude and obviousness on inspection leave iOptron’s factory. A Google search revealed that others have run into this problem as well. I was also surprised that iOptron required a copy of my receipt to apply the warranty to repairing what could ONLY be a manufacturing defect and a severe one at that. On a $5000 mount (which is less than the other mounts discussed on this thread, but still…)

Looking forward to getting out and trying out the mount, but I’m a little nervous given the fact that a defect of this magnitude got through inspection.

Oh, that must have been extremely frustrating to discover totally understand the shock. The good part is that iOptron sorted it out quickly and replaced the head. At least that gives confidence that they stand behind the product when something slips through.

I actually thought the adjustable counterweight bar was a clever feature for fine-tuning OTA balance. On my CEM70 it was perfectly fine from the factory, but my focuser shifted the balance a little, so I added a small saddle weight and then used the CW bar to compensate. You really don’t need to move it much. I also made a slight DEC tweak, but that was just because the mount was built in a much warmer climate than where I’m using it now.

My CEM70 has been a very solid performer. I posted a full one-year write-up with tips, tricks, and guide graphs to show the methods work. I’ve owned many mounts since 2010 back then 14″ peak-to-peak PE was considered good. After trying a lot of brands, this is the first time I’ve genuinely felt like I got something of real value for the money. The CEM70 did so well for me that a year later I stepped up to the CEM120EC.

Hoping your next clear night goes smoothly once dialled in, these mounts can surprise you in a good way.

CS!