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How to balance on a CEM25P

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#1 devong67


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Posted 17 August 2019 - 09:37 PM

Hello. I've been a long time astronomy fan. I currently own a Meade LX200 10" and and ETX 90. Lots of accessories. As an avid hobby photography enthusiast, I have at time tried to do astrophotography from time to time. The LX200 is probably close to 20 years old and when I started doing astrophotography, I used film and actually did some hyperring of film (showing my age!) and manual guiding. You have no idea what astrophotography really is if you haven't stood at your telescope with your hand on the controller watching a faint star in a double-crosshair eyepiece and doing all you can to ignore the back pain, leg cramps, whatever just to keep the star in the little box at the center of the eyepiece for 30 or 60 minutes! Thankfully, those days are over! Of course, I switched to DSLR from film a while ago and continued to dabble when time allowed. Usually not as often and I would like. I've been out of the hobby for a few years now and want to get back in. Unfortunately, the skies in northeastern Ohio are very light polluted and worse yet, the trees around my house (mostly neighbor's trees) are bigger and taller, so I'm thinking a need to get a more portable setup. Plus the LX200 10" is a beast to move around. I love the scope and the views I get from it, but it is probably time for me to sell it to someone who will use it more than I currently do. Anyway I digress, but just wanted to give you some background on my experience.


So for a more portable setup, I have been reading a lot on here lately about mount recommendations and after weighing all of the pros and cons I decided to purchase and iOptron CEM25P mount. It arrived the other day along with a Sky-Watcher ProED 80mm APO Doublet refractor and field flattener. I realize the pros and cons to both of these, but for my budget, they fit nicely and I think I'll be able to get more use from them. The mount seems really good and is definitely more portable than the LX200. Also, it is a lot quieter coming from a Meade fork mount background. As it is easier to see during the day, I set everything up today in daylight just to get a feel for it, and to make sure it was going to work. Had some concerns about getting it to work with my DSLR (Canon EOS 77D) and wanted to see how to balance the scope on the mount as don't have my experience with GEM (or CEM in this case) mounts. Got everything setup and was able to focus with the DSLR on a TV station tower in the distance so I guess that is a plus.


When it came to balance the scope however, I had some difficulty. In RA, I was able to get 'close' but I think since the finderscope is offset from the center line of the scope it messes with the balance. Let me try to explain. Thinking of the dial of a clock, If I put the telescope at 3 and the CW at 9, I could get balance and it held. If I moved the scope to 1, then the mount wanted to move it to 12 and slightly past. Likewise, I could put the scope at 0 with the CW at 3 and it would hold but if I moved the scope to 11 it wanted to move and fall back to 9. Not sure I am making this clear or not. If I remove the finderscope then I can get it to balance. Is this a big deal or do I need to figure out how to offset the weight of the finderscope somehow?


I found a youtube video to illustrate this:



and his solution:



For the DEC axis, I was able to achieve balance for visual setup (diagonal and eyepiece), but as soon as I added my camera and field flattener, I didn't have enough room on the vixen rail to slide the scope forward enough to get it to balance. I guess I need to add weight to the front of the scope. I'm looking to get a smaller guidescope to mount for guiding so not sure if that will help of not with this depending on how/where it gets mounted.


Once again, how critical is getting the balance on these mounts?


Anyway, even with my balance issues, the scope slewed fine while I was testing just moving it around with the controller with and without the camera attached. There was no noises from the mount, in fact since I'm used Meade mounts, I had to check to make sure that it was actually moving it was so quite. Now to get some clear skies.


Thanks in advance for your comments.



#2 OldManSky



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Posted 17 August 2019 - 10:02 PM

Balance is very important on this mount. The goto is accurate and the tracking great, but the motors have limited torque, and if you’re not balanced you will have poor tracking results.

For the fiderscope being offset from the axis, rotate the scope in its rings to tut it right on top. The focus knobs will be tilted, but it will balance.

And if you can’t loose the tube rings and move the scope further forward in them to help with dec balance, you might have to add some weight up front...or get a longer dovetail!


Enjoy the new mount. I love mine.

#3 RandallK



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Posted 17 August 2019 - 10:03 PM

I am having the same issue with my CEM40. My Z103 will perfectly balance horizontally along with the image train (camera, flattener) guidescope atop. But at other positions it wants to favour one direction or the other but very little. I really don't think it matters as long as it is in balance on enough on the horizontal. My guidescope is centered and I use a laser for a finder and IS of center a bit but very very light weight.

Edited by RandallK, 17 August 2019 - 10:09 PM.

#4 mewmartigan



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Posted 18 August 2019 - 08:58 AM

When I had my CEM25p I did the offset washers in the video above to get perfect balance.


I think the perfect balance may have accentuated the backlash whereas being slightly imbalanced hid that. So I ended up make things a bit east heavy during setup with the washers on.


My autoguiding was actually fine without the washers and being a bit imbalanced. I probably would have left it that way (i.e. if I still had it, I would remove the outrigged washers). I had done the washer trick just so that the scope wouldn't immediately start to fall when I unlocked the gear switches at zero position.



To RandallK's point above, I don't plan on doing any washer setup or perfect balance in all direction with my CEM40. I have had it out once so far and the autoguiding didn't suffer at all. One less thing hanging off the mount, haha.

#5 gotak



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Posted 18 August 2019 - 09:26 AM

You should try your best to balance it in all orientations. Once you do it'll work that much better.

In theory you want to do that with any mount provided you don't have back lash issues.

#6 kathyastro


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Posted 18 August 2019 - 09:44 AM

Balance declination first.  Declination needs to be balanced in two directions: longitudinally and laterally.  You have already established that you need either a longer dovetail or an auxiliary counterweight to balance dec longitudinally. 


Once you have that balance set, you need to balance laterally.  If the scope is in rings, rotate the OTA, with all its accessories mounted, in the rings until it balances from side to side.  Typically, your heaviest accessory (guide scope, probably) will be near the top.  If your dovetail is mounted directly to the OTA without rings, then you will need an accessory weight to balance laterally.  If you opt to use an auxiliary weight for balancing longitudinally, the same weight can also balance laterally.


If the OTA is steel, you can use a magnet as the auxiliary weight.  Otherwise, you might have to get creative with a DIY mounting system.


Once you have declination balanced in both directions, then balance RA.  You will find that, having already balanced dec laterally, you should have no trouble in the 12 o'clock position from off-centre loads.


To minimize off-centre loads that need to be balanced out, consider replacing your finder with a mini guider.  You don't need both.  Many mini guide scopes will fit directly into a standard finder moutning shoe.

#7 Tristarcapt


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Posted 19 August 2019 - 10:29 AM

Hi Devon, I have a very similar setup to yours, including the 77d. As you have discovered with any off center weight on your mount (including guide/finder scope setup, focuser, or any other stuff you’ve mounted on your scope) it tends to be very difficult to achieve good balance in RA on both sides of the mount. You get it balanced with the scope on the east side and it’s heavy or light on the west side or visa versa. I’ve got 18.5 lbs on the mount and I initially was getting .7-.9 total RMS with PHD2. I got the balance as good as I could get it. Really good on one side and pretty good on the other side. I had a powered usb hub, guide scope/camera, moonlite autofocuser, and focuser control box attached to the scope (ES ED102CF). I was getting decent results but wanted to get better balance but didn’t want to add any more counter weight to scope or fiddle with any off axis counterweight on the counterweight bar. I found that the largest contributor to RA balance issues was the off center guide scope/camera. So I decided to go OAG. Ditched the guide scope saved some weight, with the 290mm mini I never have a problem finding guide stars. After that I get very good balance on both sides. One side still slightly different but my Cem25p doesn’t seem to notice Guiding now is consistently .4-.8” total RMS. Which results in around half a pixel on the guide camera. At my imaging scale of 1.075”/pixel I never have any star issues. My DEC and RA RMS are both pretty close.

So my recommendation would be to go OAG then balance as good as you can and go for it. My experience has been the mount can handle it. While certainly achieving “perfect” balance would help, once your balance is good enough to guide you significantly below your imaging scale why waste time fiddling with it.

On a side note: I think many underestimate the robustness of this mount (CAUTION, some may have a coronary if they continue to read past this point!). I never break my setup down. I carry it fully assembled about 25’ from my basement through my slider to the patio and back every night I image. I spend very little time leveling, just get it pretty good. I frankly, after initially balancing with my current configuration, haven’t checked my balance in months. I do use an Ipolar to get an accurate polar alignment. I do a one star alignment and my gotos, tracking, and guiding are all spot on. My plate solves are always successful and it’s never taken more than 2 attempts to center on my target. Everything just seems to work (I hope I didn’t just jinx myself!). My goal was to spent as little time as possible on the setup and as much time collecting photons as possible.

Again, if you’d like to improve your balance and ditch some weight, I’d go OAG. Teleskop express sells a great canon t ring OAG combo with the correct backfocus for the 77d which works great with the zwo 290mm mini.


I can also provide PHD2 guide logs from the last couple of nights if you’re interested to verify my results.

Good luck and most of all, have fun,

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#8 MrRoberts


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Posted 19 August 2019 - 12:14 PM

 I agree with Tristarcapt 

All I had to do is get it close. I had to adjust to OTA within the scope rings some.

I've had my C-8/E and ES 102 FCD100 on it. Never a problem.

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#9 F.Meiresonne


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Posted 19 August 2019 - 04:11 PM

What is the weight with your C8, seems rather heavingly loaded....

#10 devong67


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Posted 19 August 2019 - 07:08 PM

Thanks everyone for the information and hints. I will try some of them next time. Also, I hope to have my autoguider in the next few days so that will be another item in the mix and why I didn't spend a lot of time getting perfect balance for now. I was able to get out Saturday night for a few hours before the clouds started to roll in about 1:30 AM. I must say I was impressed with the mount and setup. Coming for Autostar/Audiostar (I got the Audiostar electronics upgrade for my Meade LX200 when the caps fried) the hand controller seemed pretty intuitive to me. The one thing I totally forgot to do was to level the mount but it must have been pretty close. I polar aligned - pretty easy with the illuminated polar finder - and did a one star alignment on Vega. I setup on my deck and from that location the eastern sky is pretty obstructed but I decided to plunge ahead. I did a goto to M57 and it was in the field of view. Also goto'd to the Moon and Jupiter. I then hooked up my DSLR and decided to try some unguided exposures just to see what would happen. I was only able to go about 30 seconds but was able to get some encouraging shots - nothing worth really fussing with but I was able to see what I was aiming for. Like I said, didn't level, balance was a little off and I was learning the new mount, scope, field flattener, etc. but it wasn't totally a waste of effort. Once M27 came in to view I got a few shots of that as well until it got to be cloudy and I had to call it a night. I definitely need longer than 30 second exposures but I'll have to wait for the autoguider for that.


Overall I am really impressed with the mount. It's goto capabilities seem pretty decent to me - every object was within the frame on my crop sensor DSLR. It is pretty light and easy to setup and transport. Something that will definitely be a plus when I take the setup on vacation or to remote sites. Now for some clear skies!

#11 F.Meiresonne


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Posted 20 August 2019 - 01:25 AM

I guess those Ioptron mounts are great or can be great , once you get them running. There may be some issues wich IMO unevitable with electronics, but once you overcome them one should be ok.

Yesterday I got my alignment right and every target I slewed to with goto was in the eyepiece, some centered even , some more to the edge, I guess I went over 20 objects in one hour , it was a test, and succeeded great. I only did a polar alignment and a one star alignment afterwards (all though it suggested a two start alignment which I actually ignored)

I find also that those mounts are real sensitive meaning you have to balance them good, they move real smooth and their axes are light to turn...I remember my former HEQ5 being stiffer many years ago.

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