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CEM26 - first (cloudy) night

Astrophotography Celestron CMOS DSLR Imaging Mount Report Polar Alignment SCT Cassegrain
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#1 lpalbou

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Posted 04 March 2021 - 02:52 PM

Prelude

My mount died last week, at least the DEC motor.. I had some fun for a few nights trying to manually locate galaxies at 2000mm on a 35mm sensor.. Using my smaller setup AZ GTI + a laser to help me find the target. But realistically, when capturing small objects, one need a goto mount and plate solving, so I ordered the CEM26. I would have take the GEM28 if it was available, for the small gain in payload.

 

Setup

Everything was well packaged and everything needed to use and guide the mount was included (e.g. USB cable, and the mount also has built-in wifi).

CEM26 - unboxing 1/2
CEM26 - unboxing 2/2
 

I had bad experience with ioptron commander and the skyguider pro (never got it to work) but the CEM26 was recognized on first attempt. I got the 1.5" tripod as the literoc wasn't available. First impression on the tripod: small and light package ! Not much more than a camera tripod but fortunately more robust. The whole system tripod + mount can be lifted with one hand without much effort. The RA & DEC clutch are nice to lock the setup and also to test balancing (really smooth compared to skywatcher mounts). An allen wrench is included, both to block the RA axis for transport, to attach the mount to the tripod and to adjust alt/azi for polar alignment. Same experience with ipolar. Install the soft from ioptron website, connect, worked out of the box. Require to take 1 black frame (really, 1 ?), adjust your exposure and start the adjustments. ipolar will show you where you should be pointing at vs where you are pointing at. Once you get closer to a good alignment, the reticle is zoomed for fine tuning. I didn't see any mention of the polar alignment error though. A bit disturbing, but I trusted the soft when it said I was polar aligned. A note about the DC adapter: 12V 5A; however the adapter is closer to the mount end rather than the plug end.. I was hoping to put the adapter in my weatherproof connection box.. but too short. I have a DC 12V extender, so next night I will probably use that.

 

Even though the mount feel robust, it's still quite small and portable (4.5kg / 10 lbs) and I was a bit uncomfortable, especially with the CEM design, to put on it my C8 edgehd (14 lbs) + D850 (2.2lbs) + guide setup (2lbs). Fortunately, the saddle seemed robust and with the clutch on, RA/DEC are indeed quite robust. In terms of balancing, the single weight provided was not enough, but I had several from my other mounts. To be clear, I needed the CEM26 CW (10lbs), 2x11lbs and 1x3 lbs to balance the setup. So for a payload of 20 lbs, you feel that you are starting to ask a lot. Even though it's working, I will probably try to find bigger counterweights instead of 4.

 

Once everything is setup, the overall impression is rather good. Albeit the weight of the setup, no noticeable backlash in either DEC or RA. To clarify, there is a micro backlash, but probably not more than 1mm, it's very moderate. From the guiding graph (see after), it doesn't seem to impact, but I will report later it if it does.

 

The sky

Well, new mount = bad sky. Axiom verified.

New mount = bad sky

 

When the sky cleared up for 1h

First reaction: Yeah ! grin.gif cool.gif

 

I have a wireless security camera on my rooftop to check my scopes; useful for meridian flips, to check the sky or just to reassure you your setup didn't crash in an horrifying way (e.g. we do have some high wind from time to time in the Bay area). Anyhow, I did the quick polar alignment mentioned above, adjust the focus on Capella - also playing with the auto focus aid of APT which now detect the best focus and backlash of your focuser - and started imaging the sunflower galaxy... while the moon was rising from the horizon and without filters. In other words: ideal conditions lol.gif .

 

First impression: wow, this mount is silent and smooth ! I had some very noisy skywatcher mounts, this one is smooth and butter in comparison.

CEM26 - smooth goto

 

The guiding was surprisingly good for such a quick setup and with such bad sky conditions. To be clear, this is the best I got yesterday and there were peaks at 1.5", the average probably around 1.1":

CEM26: guiding on the first night

 

I didn't fine tune PHD2 as this was a quick first light between the clouds, but I believe that even with that payload, I could achieve consistent guiding of 0.8" or 0.9". PS: using zwo290mm mini + zwo 280mm guide scope. I know about OAG but it can complicate the back focus when using the 0.7 reducer + I never actually had any issue guiding at 1500mm before, even with other scopes. One has to be careful of flexures and badly attached equipment of course. Overall the mount responded nicely to correction pulses, both in RA and DEC (not surprising considering they both have Φ88mm, 144 teeth aluminum).

 

I did 3mn exposure at ISO 400 (sweet spot for D850) for slightly less than an hour at f10. I saw some drifts so my polar alignment was not perfect, still each 3mn exposure at 2000mm was good with round stars. My focus isn't fantastic either, but the seeing between the clouds was not that good.

 

First (quick) light

Sunflower galaxy from the light polluted sky of San Francisco, no filter, with the moon rising and between the cloud - 1h exposure at f10 so obviously not enough for a good image, but sufficient to judge the tracking with that setup:

Sunflower galaxy - quick first with CEM26

 

Conclusions & Perspectives

Overall, good/great impression. I am still impressed by how quiet and smooth the mount is. The whole setup was done without issue and except from the additional counterweights, the package contained everything needed. So far, I believe this is a better mount than skywatcher EQM35 (same weight) and HEQ5 pro (10kg) but not as robust as the tank EQ6r pro. And that makes sense, remember we are speaking of a 4.5kg mount instead of 17kg...

 

I am waiting for the next clear nights to:

- refine polar alignment and compare ipolar to phd2 drift align / sharpcap : my polar alignment was "good enough" but not perfect

- see if i can achieve a consistent 0.8" guiding

- if I observe backlash (there may be a few, hence peaks at 1.5" ?)

- didn't try PPEC yet

- also I would like to get an unguided graph to check the peak to peak PE


Edited by lpalbou, 04 March 2021 - 03:06 PM.

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#2 randcpoll

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Posted 04 March 2021 - 03:04 PM

Thanks for this nice review. My GEM28 is supposed to arrive today!



#3 readkonrad

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Posted 04 March 2021 - 03:22 PM

Nice to see these mounts out in the wild. Please keep us posted!

#4 Al_N

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Posted 04 March 2021 - 04:15 PM

Thanks for the review. Hoping my GEM28 ships soon.



#5 RossW

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Posted 04 March 2021 - 08:05 PM

Thanks for the review. I hope someone does a review for the CEM26-EC too.


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#6 CarolusRix

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Posted 05 March 2021 - 12:24 AM

Thanks for this nice review. My GEM28 is supposed to arrive today!

Hope to hear about it!



#7 Feccy

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Posted 05 March 2021 - 03:28 AM

My CEM 26 arrived last week - had 2 clear nights initially but been 100% cloud cover since. There area few people (including me) that have had issues with a wobbly RA axis - easily cured by tightening 2 grub screws but not expected on a brand new mount. Interestingly mine arrived in a metal flight case - didn't order that but hey. Another owner on FB has managed 5 min subs unguided so it is a very good mount. Hopefully clear skies tonight so I can get the camera hooked up and see what I can do unguided. Thanks for the review


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#8 lpalbou

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Posted 05 March 2021 - 06:03 PM

Night 2 - still too many things fluctuating to be sure

 

I was lucky enough to have clear skies yesterday, so I gave it another try. First, I redo the polar alignment with ipolar to get the best possible result. As a note, tightening the two screws on the side of the mount after adjusting alt/azi does affect slightly your alignment, be sure to screw gently one side, then the other. I am not also super comfortable that the same screws are used to hold the mount on the tripod and do the adjustment.. the screws are long enough not to go out (hopefully), but still. It's also probably why when tightening them the adjustments is not perfect and require what I would call "iterative tightening on each side".

 

I did some guiding and the results were to my surprise not better than yesterday, varying from 0.8" to 1.2". I decided to do some drift align and.. discrepancies between ipolar and drift align. It's a bit difficult to know which to trust at this point. In theory, the ipolar being perfectly aligned with the polar axis should be very accurate, yet the drift was clearly observed (had to scale the graph to y+/-16 after < 200s). So drift align it is - it's usually what I trust the most as it's what we use to guide. Got a nice polar alignment and decided to run the guiding assistant to see what was going on there:

CEM26 - Guiding Assistant after Drift Align for PA

 

I let it run 600s to really have stable numbers. PA: 0.4 (rather happy). DEC backlash.. 1420ms that's not great. RA peak-peak 32.77, that seems really really high. Ok, I try to guide with that calibration and.. guiding about the same, maybe even slightly worst than with ipolar. Albeit the excellent number I get from both drift align and guiding assistant, I try to check on sharpcap.. from zero to 90° and 90° to zero. Consistent bad polar alignment numbers:

CEM26 - albeit great PA in drift align and guiding assistant, poor alignment in sharpcap

 

Ok, this reminds me of all the messages on this forum about inconsistencies between drift align, sharp cap and ipolar (rarer as fewer users). I know what people says: flexure. Right. But still, got PA of 0.4 confirmed by drift align and guiding assistant. It's not my scope, it's well attached. The cables ? I redo my cable management for next night (see perspectives). Bad balancing affecting more at some orientations ? (see perspectives too). Still puzzling to me, so waiting for another clear night to see what's going on there. I also fear the screws to attach the mount to the tripod are a bit weak for such an heavy load, considering they serve both the role to attach the mount and adjust the alt/azi (really surprised by that design).

 

Sharpcap it is then. New polar alignment, sad for my PA of 0.4. Same thing, ensuring I achieve excellent PA from zero position to 90° and double checking from 90° to zero position:

CEM26 - new PA with sharpcap, double checked to ensure no flexure

 

I do like that sharpcap is fast and allows to check for flexure by just reiterating the PA check. Out of curiosity, I check what ipolar is saying:

CEM26 - excellent alignment in sharpcap but no go in ipolar
 
 
Not good.. time is passing, I check again sharpcap who keeps on telling me I have an excellent PA.. so let's trust it. I decided to re-run the guiding assistant to check what changed:
CEM26 - Guiding Assistant after sharpcap PA
 
Interestingly, the PA error is 0.8 instead of 0.4. Not that far, even though the adjustment I did IMO would warrant a bigger delta.. But ok. Total RMS not so different either. DEC backlash completely different: 184ms instead of 1450ms. It's difficult to trust that number and it probably means the DEC backlash is larger than what I initially thought. Yet it's not constant. I have seen perfect guiding with really good DEC and guiding with near periodic DEC backlash of up to 2-3" (rapidly guided out, but too late of course). RA peak to peak is now 13.82, much more manageable, still greater than the advertised 10". I also really don't like that I get such different values. I of course think of flexure, but I did tighten everything. Once more, I think at the way the mount is attached to the tripod. Is it the culprit ?
 
Summary of guiding
So far, the average guiding is really around 1". I wouldn't say it's bad but it does limit the resolution you could get with excellent seeing. With very short exposures on Sirius, I did notice the elongation, which is probably just round up with wobbles around that center with longer exposures. At high resolution, this does mean some blurs. On shorter focal length, you would probably not notice it. It still performs better than an EQM35 and probably better than an HEQ5 pro at half the weight. A bit disappointing in comparison of an EQ6r pro or probably even the CEM40.
 
Imaging tests
Quick 12x2mn exposures of Orion (always useful as a reference as we all imaged it multiple times):
Orion 12x2mn - cem26 - 2000mm
 

Longer 76x3mn exposures of the Sunflower galaxy (really faint for the LP of San Francisco):

Sunflower 76x3mn - cem26 - 2000mm
 
Perspectives
* I adjusted the cables (dew heater on both C8 & guide scope, focuser, guiding camera, etc) to run along the scope, be secured and don't put any weight on either side of the scope
* I adjusted the weight distribution on the scope as my guiding scope is on the side rather than on a dovetail bar on the top (I really have to order that bar). I adjusted the DEC, then the RA. Both axis are smooth and stable, I can push them with a finger without forcing
* I will do again the polar alignment. I really would like to see some consistency between ipolar, drift align and sharpcap
* I also would like to get some consistency with the Guiding Assistant, where I got both terrible peak-peak error and more or less acceptable (albeit beyond specs) values
* I have to check a bit more the DEC backlash; when I checked it the first time, it seems minimal and I would have agree with a value of 184ms. But after seeing some 2-3" DEC backlash while guiding, I would tend to agree with the worst value (1423ms)
 
Notes on design
* once again, I think using the same screws to both attach the mount to the tripod and to adjust the alt/azi is a design flaw. The center pin seems only to be here for aligning the tripod and the mount head
* I forgot to mention something important also regarding the saddle knob to attach the scope. The design looks strong but... they use a knob too big and weren't cautious enough about the spacing between the mount and the saddle. I noticed it the first time when balancing the DEC and felt some resistance due to that knob scratching the mount (!):
CEM26 - saddle know design flaw
 
After some adjustments:
CEM26 - saddle knob adjusted to avoid collisions with mount head
 

As you can see, even with that orientation, the spacing is minimal.. And it's really not ideal to have to adjust such a knob with your payload on the mount. So it's definitely a design flaw and that knob should be changed.

 

Here it is for day 2.. not a bad mount, but some design flaws and still some things I have to figure out (eg RA peak-peak).

 

 

 

 


Edited by lpalbou, 05 March 2021 - 06:13 PM.

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#9 larryjh

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Posted 06 March 2021 - 10:42 AM

Hi, and thanks for the great review.  I am considering this mount as well.  I currently own the ancient predecessor ZEQ25.  Noticing the mechanical differences, I have a couple questions for you.  On the ZEQ, the mount is attached to the tripod with a center screw, and there is also a tightening screw on either side of the mount which tightens up the altitude axis for polar alignment.  What I have found is that I can tighten the center screw fully and still make azimuth adjustments for polar alignment.  What I'm wondering is if, on the CEM26, the two screws that hold the mount to the base can be fully tightened, or maybe nearly so, and still allow for azimuth adjustments during polar alignment, so that there is no need to tighten them again after polar alignment. 

 

Also, on the ZEQ there is an adjustable "post" for altitude adjustment during polar alignment.  Some posters have noted that this added sturdiness to that design, but there is a different, possibly less sturdy method on the CEM26.  Does the altitude adjustment seem sturdy to you on the CEM26?   Similar to the azimuth, I find that on the ZEQ I can tighten the two side screws almost fully and still do the altitude adjustment for polar alignment with having to further tighten afterwards.  Is there anything on the CEM26 altitude axis that needs to be tightened after polar alignment?

 

My comment on polar alignment accuracy is that, with my ZEQ, I use the optical polar scope to get "close" and then tighten up the accuracy using Sharpcap.  Similarly, I think I would just use iPolar to get "close" and still use Sharpcap for final accuracy.  To me, the big advantage of iPolar is that you don't have to stick your head down next to the mount and squint up through the scope.  I'm sure it's more accurate than the optical scope too, but I still wouldn't trust it as much as Sharpcap.



#10 Feccy

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Posted 08 March 2021 - 08:30 AM

 

Perspectives
* I adjusted the cables (dew heater on both C8 & guide scope, focuser, guiding camera, etc) to run along the scope, be secured and don't put any weight on either side of the scope
 
 

 

 

 

I'd appreciate another photo of how you run your cables - I too have a CEM26 and as this is my 1st goto mount I haven't done any 'cable management 'as yet



#11 Feccy

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Posted 08 March 2021 - 08:34 AM

What I'm wondering is if, on the CEM26, the two screws that hold the mount to the base can be fully tightened, or maybe nearly so, and still allow for azimuth adjustments during polar alignment, so that there is no need to tighten them again after polar alignment. 

 

Yes they can. I have found if fully loosened, the polar alignment goes 'off' when re-tightening - so a mostly tightened mount can still be moved with the Azi screws. The Alt adjustment is slightly course so fine adjustments are tricky. There is a locking handle that gets tightened after PA is achieved. Similar issue as above with PA going off a wee bit.


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#12 dx_ron

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Posted 08 March 2021 - 09:23 AM

 

 

I'd appreciate another photo of how you run your cables - I too have a CEM26 and as this is my 1st goto mount I haven't done any 'cable management 'as yet

 

Hi Feccy,

 

Obviously not a C8, but in case it's useful here's what I settled on for my CEM25P after much fiddling around with options. I'm getting the impression that the internal mechanics of the 26 might be different, but I think the external geometry is pretty much identical.

 

(the mass of cables bundled up at the top are the usb dew heaters, there's a usb hub and a raspberry pi mounted just behind the guide camera)

 

I bought a bag of these cable clips. I don't use the adhesive, though I guess you could. I ran a zip-tie through a hole at the rear of the dovetail + one of the clips. There's only enough cable looped there allow for strain relief. A zip-tie on the cable bundle keeps the loop the right length.

 

To tear down, I just unplug the three cables in the bundle (12v for usb hub, mount interface cable and a dummy dslr battery) and open the clip on the back of the dovetail. Everything else stays on the tripod.

 

To avoid using adhesive on the base of the mount, I ran two-sided velcro around it, then used the adhesive to attach a square of velcro to the 2nd cable clip. The loop between the two cable clips is just long enough to allow the dec axis to rotate as far south as I figure I will ever point. There's very little movement from there as the RA axis rotates, so that just hangs down. Most every other configuration I tried had issues with cables brushing up against the tripod when pointing near the meridian and that was causing noticeable guiding issues.

Attached Thumbnails

  • IMG_20210303_175108368_HDR.jpg

Edited by dx_ron, 08 March 2021 - 09:26 AM.

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#13 lpalbou

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Posted 08 March 2021 - 02:14 PM

Hi everyone, thanks for the interest in that review. Here is the follow up

 

3rd partial night

 

It was more windy (up to 15-20kph) than the other days and when I started guiding, I observed large peaks, especially in RA. They stabilized a little bit after some time and thought at first it could be just the time for PHD2 to stabilize. Unfortunately, I had that later on too. I went up on the rooftop to check the mount one more time. The wind was moderate - to illustrate a big blue ikea bag wouldn't fly away -, but you could feel a small breeze. No cable issue (to answer @Feccy, I use cord protector to bind all the cables together: https://www.amazon.c...e?ie=UTF8&psc=1 ; and I attach them to the scope also using some elastic velcro), no level issue, checked the polar alignment with sharpcap and still ok. I was thinking about that odd guiding assistant results with very different peak-peak errors in RA. So I tested again: the DEC backlash is really not that big, nothing I would be concerned about.. but the RA backlash.. was enormous. I don't understand how I didn't realize this before, so apologies for not reporting it before too. To clarify, I have tested about 10 mounts in my life and I never saw that level of backlash in any mount, especially in RA were precision is crucial (you could guide without DEC).

 

And I want to illustrate what I mean by enormous and unacceptable:

CEM26 - enormous RA backlash

 

I believe this explains why I had those two very different RA peak-peak errors, as well as why the mount is so affected by even the slightest breeze. Remember the mount is advertised as zero backlash. I don't believe in zero backlash for that kind of design, but that's once again the worst RA backlash I have ever seen. I have stumbled across two videos on youtube:

- CEM26 RA backlash: https://www.youtube....h?v=P-78L2rao7I

- CEM26 DEC backlash: https://www.youtube....h?v=LXrCpLOLnvU (I believe mine is fine)

 

I hope this is not a general issue with the CEM26 (Feccy and dx_ron, do you have that issue ?). At this point, I am contacting telescopes.net where I bought the mount to ask either for a repair or replacement as the mount is unusable unless I voluntarily unbalance the RA axis to minimize that backlash.

 

Conclusion for this post

I still believe it's a good mount: when it guides successfully, I can achieve 0.8", which for the form factor is great. The mount has some surprising flaw design (saddle knob too big). But the backlash I observed is unacceptable (also check the youtube video). I don't know if it's just my mount, a few mounts or all of them. This backlash makes any guiding/imaging very inconsistent (see my two guiding assistant results). I will update this post when I get some news from telescopes.net and/or ioptron.


Edited by lpalbou, 08 March 2021 - 02:19 PM.

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#14 larryjh

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Posted 08 March 2021 - 03:08 PM

I've ordered a CEM26 but not yet received it.  I've been reading through the manual though.  In regards to the RA backlash, I was just wondering if you have tried the RA mesh adjustment described in the manual.  Unfortunately I can't get your video to play, but the youtube video sure looks like the mesh is too loose.  You would hope that a brand new factory mount wouldn't have this problem, but maybe the adjustment would fix it.



#15 dx_ron

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Posted 08 March 2021 - 03:19 PM

lpalbou: that video looks exactly like what I have seen posted in the iOptron owners fb group. If you have the same issue, the fix is apparently very simple. It apparently involves just tightening a couple of grub screws, possibly the ones you can see on the face of the RA axis in that youtube video.

 

I have a CEM25P, so have not encountered this particular thing, but it seems a lot of CEM26s shipped with those screws too loose


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#16 lpalbou

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Posted 08 March 2021 - 04:42 PM

@larryjh, thanks for the suggestion. Indeed, people at telescopes.net asked me to try the RA adjustment, It's neat to have those screws outside and easily accessible (for reference for others having similar issues, see https://www.ioptron....EM26_Manual.pdf page 36). Also thanks dx_ron for reporting others with similar issues on FB. It' surprising that a number of those mounts out of the factory have not been more calibrated..

 

However in my case, this did not solve my issue. At best, it made the axis stiffer (harder to rotate), but it can still wobble about 1-2°. If it wasn't for that issue, I would be happy. Now discussing about how to repair / pick it up / exchange. I wonder if the GEM28 will have the same issue; I don't think I have seen anyone reporting that issue for the CEM40 or GEM45 ?


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#17 Feccy

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Posted 08 March 2021 - 05:07 PM

lpalbou : had exact same issues. 2 grub screws under the cover on the RA axis shaft were loose. Quick tighten and all ok. Have had cloud cover since so haven’t had it looking at the sky but the wobble has gone. Thanks for the cable pointers. 


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#18 lpalbou

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Posted 08 March 2021 - 09:16 PM

@Feccy: I think it worked but I will have to wait for some clear sky to test it. I will document this in this post / review so that it can serve others too.

 

FIX RA "BACKLASH" OF CEM26

 

Step 1: hopefully the only step you need

The RA axis is governed by several mechanisms. The simplest one is explained in page 26 of the manual https://www.ioptron....EM26_Manual.pdf and relates to the normal / conventional backlash due to mesh / gear contacts. It's a simple mechanism where you unscrew half a turn to unblock the mesh and you adjust the mesh up/down with another screw:

 

CEM 26 - RA axis, mesh/gear adjustment

 

I would say kudo to ioptron for a simple mechanism who doesn't require to open the mount. Be sure to retighten the mesh screw (1) to block it in that position when you are done adjusting.

 

Step 2: if it doesn't work, open the mount

However, in my case, this didn't change anything and as mentioned by Feccy, one has to remove 1) the counterweight shaft,  2) remove the cover of the RA axis (3 screws) and 3) adjust 2 grub screws

 

1) remove the counterweight shaft: 1 hex screw + 2 grub screws

CW shaft
 
2) remove the RA axis cover: 3 screws
Remove RA cover

 

3) the RA axis was turning freely of 1-2° within the front tube of the RA axis. It's a weird design and I wouldn't be surprised those 2 grub screws unscrew from time to time and would produce the same issue. It was probably simpler for ioptron to assemble the mount without having the same tube rotating on the back and the front.. and considering the importance of that junction, I would have do more than 2 grub screws that just connect the two axis by tightening two grub screws.. I don't know what's the opposite of kudo, but NOT kudo on that one.

RA axis grub1
RA axis grub2

 

By doing so, the RA axis back and front rotate together. I also realigned the mesh after doing that and my RA axis looks solid and without backlash. This could explain also some discrepancies I observed in polar alignment with the 3 methods. I will report how the mount is working after that fix. Hopefully I won't have to return it for repair/replacement. Surprising design.

 

Note on tripod / mount

For those interested, that's how the mount is connected to the tripod. The center knob is not a screw, so the 2 screws you see on the tripod are the one that hold the mount and are used also for alz/azi adjustments. I would have preferred something a bit stronger but since the weight is mostly going down, hopefully it should be ok. To answer someone question, yes we can untighten them just a little and still adjust alz/azi.. however I am unsure that's a good idea as 1) if your mount can move even a little it will (flexure) and 2) on the long term, adjusting alt/azi that way could damage the screws

CEM26 - mount connection
CEM26 - mount tripod
 
Important note: please be sure to tighten everything when you have done your adjustments, in particular the front counterweight shaft.. and if you are unsure of what you are doing, you are probably better off returning the mount for a quick repair

Edited by lpalbou, 08 March 2021 - 09:18 PM.

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#19 limeyx

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Posted 08 March 2021 - 11:07 PM

 

@Feccy: I think it worked but I will have to wait for some clear sky to test it. I will document this in this post / review so that it can serve others too.

 

FIX RA "BACKLASH" OF CEM26

 

Step 1: hopefully the only step you need

The RA axis is governed by several mechanisms. The simplest one is explained in page 26 of the manual https://www.ioptron....EM26_Manual.pdf and relates to the normal / conventional backlash due to mesh / gear contacts. It's a simple mechanism where you unscrew half a turn to unblock the mesh and you adjust the mesh up/down with another screw:

 

 

 

I would say kudo to ioptron for a simple mechanism who doesn't require to open the mount. Be sure to retighten the mesh screw (1) to block it in that position when you are done adjusting.

 

Step 2: if it doesn't work, open the mount

However, in my case, this didn't change anything and as mentioned by Feccy, one has to remove 1) the counterweight shaft,  2) remove the cover of the RA axis (3 screws) and 3) adjust 2 grub screws

 

1) remove the counterweight shaft: 1 hex screw + 2 grub screws

 
 
2) remove the RA axis cover: 3 screws
 

 

3) the RA axis was turning freely of 1-2° within the front tube of the RA axis. It's a weird design and I wouldn't be surprised those 2 grub screws unscrew from time to time and would produce the same issue. It was probably simpler for ioptron to assemble the mount without having the same tube rotating on the back and the front.. and considering the importance of that junction, I would have do more than 2 grub screws that just connect the two axis by tightening two grub screws.. I don't know what's the opposite of kudo, but NOT kudo on that one.

 
 

 

By doing so, the RA axis back and front rotate together. I also realigned the mesh after doing that and my RA axis looks solid and without backlash. This could explain also some discrepancies I observed in polar alignment with the 3 methods. I will report how the mount is working after that fix. Hopefully I won't have to return it for repair/replacement. Surprising design.

 

Note on tripod / mount

For those interested, that's how the mount is connected to the tripod. The center knob is not a screw, so the 2 screws you see on the tripod are the one that hold the mount and are used also for alz/azi adjustments. I would have preferred something a bit stronger but since the weight is mostly going down, hopefully it should be ok. To answer someone question, yes we can untighten them just a little and still adjust alz/azi.. however I am unsure that's a good idea as 1) if your mount can move even a little it will (flexure) and 2) on the long term, adjusting alt/azi that way could damage the screws

 
 
 
Important note: please be sure to tighten everything when you have done your adjustments, in particular the front counterweight shaft.. and if you are unsure of what you are doing, you are probably better off returning the mount for a quick repair

 

Great news, I hope it works for you ! 



#20 Starlancer

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Posted 08 March 2021 - 11:27 PM

Ioptron really needs to find a better way to mount the head to the tripod, a toolless method will always be far superior then having to mess with a Allen key.  On a very portable mount like this, it’s even more important.


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#21 lpalbou

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Posted 09 March 2021 - 01:16 AM

Ok, starting a rain period of 2-3 days in SF, so no more update until the next clear sky.

 

Unfortunately, there was far too many clouds to do a proper test after this fix, but it seems at least the mount could now handle breeze, not like yesterday. Since the backlash seems now minimal, I am also much more confident of the mount tracking and I believe the guiding was less erratic. A guiding assistant of 400s still evaluate the RA peak-peak error to about 15", so still out of specs, but it could be just the sky conditions.

 

Cheers


Edited by lpalbou, 09 March 2021 - 01:17 AM.

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#22 GoldSpider

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Posted 09 March 2021 - 08:48 AM

Great review and follow-up info.  I've got one on order and it's great to have this kind of reference for it's shake-out.

 

Regarding the saddle bolt, is a "fix" for this possibly as simple as using a longer bolt?



#23 lpalbou

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Posted 12 March 2021 - 07:45 PM

4th night is a charm (wasn't it: 3rd time's a charm ?)

 

Several nights beneath the clouds, wondering if that fix worked and if the mount would reveal at last to its true potential.

 

Then finally, the clear night I was waiting for, yesterday.

 

Polar alignment: same story, ipolar & sharcap didn't agree. I aligned first with iPolar, then SharpCap, double check the alignment was "excellent" over two tests, then pointed towards Sirius. Plate solving to align a bit more.

 

CEM26 - PA - Sharpcap vs iPolar

 

Collimation: using my tri-bahtinov mask.. noticed again some collimation issue. I do have been carrying the scope around a lot these days between the tests with the mount and the rain. 5 or 6 bob knob rotations later, scope collimated.

 

Recalibrating PHD2, quick guide.. some stable results, then, guiding assistant. I did 3 as I wanted to see some consistency. I have done better for the polar alignment, but this will be sufficient.

 

Guiding Assistants

 

First Guiding Assistant (pointing at orion nebula again):

CEM26 - after fix - Guiding Assistant 2
Ok, 20.3 arc-sec peak-peak, so around the +/- advertised 10 PE.
DEC backlash 766ms, not perfect but acceptable. Overall it's not excellent, but it's good.
 
Second Guiding Assistant (pointing to the cigar galaxy, to a slightly darker area of the sky and later during the night, also closer to Polaris):
CEM26 - after fix - Guiding Assistant 2
DEC is wobbling, strange. DEC backlash lower, 280ms. From the different tests I have done, it seems an average would be around 350ms.
RA peak-peak: 10.49 arc-sec, that's actually really good. The relative proximity to Polaris may also explain the better accuracy ?
 
In a nutshell, it's difficult to have consistent results, but they now look good enough. After the fix I indicated in step 2, the mount was much more consistent and resilient to wind/breeze. I have talked with iOptron and they confirmed the fix, those screw should always be tighten and for future mounts, they will probably add some LocLite/glue to avoid further issues. It was a bit difficult at first to discuss as everyone was pretty sure it was a question of just adjusting the mesh gear (step 1). I am pleased I solved it without sending back the mount.
 
Now some actual guiding while imaging
 
I have played a lot with PHD2 parameters. I decided this would not be a night of imaging and really more of testing, to know the mount better. I have tried 0.5s, 1s, 1.5s, 2s and 3s guiding. MinMo for both RA and DEC was suggested between 0.22 and 0.5 depending of the GA run. While it would work at let's say 0.25 or even 0.3, you do observe some deviations which result in slightly more blurred images at high magnification. So I lowered that to 0.18, both in RA and DEC. I did that as the backlash on DEC doesn't seem so bad and if you remember my first post, the worms have the same specs so it's not unexpected they would behave the same. Guiding was roughly around 1". It was difficult to get better, some peaks at 0.9" but not really stable. Some other peaks at 1.1" up to 1.3" that were more stable than those at 0.9". Increasing Hysteria to 25 (I was at 5, due to the previous wobbling in RA & wind from the other night) made the graph more stable. 
 
CEM26 - Guiding on Orion

 

Then I pointed towards the cigar galaxy, closer to Polaris, and also in a darker area of the sky from where I am. Along the night, with darker skies, less wind, the mount achieve what I believe is its peak efficiency... 0.58" over 600s (!).
CEM26 - actual guiding
 
Even though the guiding assistants were good but not excellent (not consistent enough IMO), when guiding without wind, the mount performed admirably with a payload of around 20 lbs (C8, D850, focuser, guide camera & scope). I believe this could be the best stable guiding I ever had in a long time (0.58" on average over 600s), especially when considering the size and weight of the mount. Once again, I want to stress out.. without wind. Even though the fix made the mount much more stable, it's still very prone to wind and you will probably average to 0.9" or 1".
 
Conclusions

* I am happy of the mount and can recommend it. If properly balanced, polar aligned and if no wind, the guiding can be impressive, even with a 20 lbs payload. I can confidently say that the mount is better than the EQM35 and HEQ5 pro and the form factor / weight is really nice. The trade off of course is that the HEQ5 pro is probably more robust to wind as it's twice the weight

 

* It would have been a better experience if iOptron had design those two screws differently. They are reporting those tests to the factory and new mounts should hopefully not have that issue. If you do, either follow step 2 or see with your seller if you have an issue (your mount could be fine too, so don't stress out !). I remember reading somewhere that some people thought their mounts were not performing as good after some time.. I can't help but wonder if that's not the issue why. I could easily imagine those 2 screws untighten after some wind and over time leading to the same crazy tracking I saw. The loclite solution is hopefully good enough but doesn't seem ideal either. I would have prefer a more robust design on that part.

 

* I am still a bit surprised by the wobble in DEC observed in the guiding assistant, even when there were no wind. I don't know where it comes from. If it wasn't there, I could certainly increase the MinMo to 0.4 as suggested by the assistant

 
* The saddle knob is IMO a design flaw and I hope they will fix it at some point. I am not sure using a longer bolt would help, but putting a different and smaller knob would do the trick
 
* I saw that noted before, but you may want to wobble the saddle a little when you attach your scope, as I've found you can often tighten it 3 or 4 times after you did it the first time.. not ideal but not a deal breaker either
 
That's it.. I think this conclude my review. You can guide at high magnification with that mount (2000mm focal length on a 35mm sensor with pixel size of 4.35 => resolution of 0.44" / pixel). It's pushing a little, but it works.
 
Have fun and happy imaging !

Edited by lpalbou, 12 March 2021 - 08:04 PM.

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#24 ppg677

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Posted 16 March 2021 - 03:54 PM

I read elsewhere that imaging with a C8 on a CEM26 is a nonstarter. Sounds like this is *not* the case, but why didn't you go for the CEM40? Save $700 or so?  

 

I'm currently trying to decide between a GEM28 and CEM40 and am leaning towards the latter "just in case" I upgrade to an OTA that needs the extra capacity.  The 5-lb weight difference doesn't seem to matter...what matters is the extra $700-$800.


Edited by ppg677, 16 March 2021 - 04:32 PM.


#25 chanrobi

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Posted 16 March 2021 - 10:33 PM

What I would love to see is just a video of the mount go-to'ing with a ipad or what not...

 

Literally still TWO videos on youtube about this mount. Crazy ridiculous


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