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Payload capacity of mount

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#1 Clayk59

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Posted 01 March 2021 - 02:25 PM

Pretty basic question but I am new.  I have a mount, iOptron CEM25p and am now looking for an OTA.  The specs on the mount say payload capacity is "27 pounds excluding counterweight".  I am not exactly sure what that means. 

 

Is it that (theoretically) the mount can successfully track with a 27 pound payload if there is no counterweight?  And the addition of a counterweight and properly balancing the OTA on the mount might successfully track with a heavier OTA?

 

Or is it something else?

 

I am also curious what the maximum payload would be for that mount.  I hope to get an 8" reflector and do some astrophotography, hopefully up to 5 minute exposures if properly polar aligned.

 

If I am crazy hoping for this, [please tell me.

 

Many thanks to all

 

Clay



#2 SeattleScott

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Posted 01 March 2021 - 02:34 PM

No no no, you NEED counterweight or you risk damage or injury.

The counterweight is not included in the weight rating. So in theory the mount could handle up to 27lbs of scope and accessories, and however much counterweight is required to balance the load. That being said, a lot of the Chinese mounts have “optimistic” weight ratings because they are more concerned with selling mounts than how well the mount will actually work for the user. So you typically want to leave some cushion. Generally you don’t want to load it all the way to the maximum rating with Chinese mounts. Also the scope type matters. You might be able to get away with 27lbs of SCT but no way you can get away with a 27lb long tube refractor or reflector.

In general you can exceed the weight rating considerably without the mount imploding. The weight rating is supposed to be the most you can use and have the scope be usable. You could put a five foot long refractor on there and it will track and the mount probably won’t collapse, but it wouldn’t be stable enough for satisfactory viewing. Unless you enjoy boating and are used to watching objects bounce up and down.

Stability for visual is different than stability for imaging. For visual you just need the scope to stop vibrating for a few seconds. That is different than five minutes. Typically for imaging you cut the rated payload in half. That’s the rule of thumb anyway. So more like 13-14lbs, including all the camera gear. Personally I don’t have that mount. I suspect it would probably be satisfactory for visual. Pushing it but it would probably be ok if you aren’t too picky about vibrations. But I doubt you would be able to do much imaging with it. Maybe 30 seconds to two minutes might be realistic. But if you are serious about imaging you should probably look at a different mount. If you mainly want to do visual but want something with basic imaging capabilities to dip your toe and gauge interest, this might be good enough. Or it might just frustrate the heck out of you and discourage you from getting into imaging.

Scott

Edited by SeattleScott, 01 March 2021 - 02:41 PM.

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#3 rk2k2

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Posted 01 March 2021 - 02:36 PM

No, it means you don't count the counterweight.



#4 xiando

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Posted 01 March 2021 - 02:44 PM

What it means is "ignore the counterweight when determining payload weight". 

 

IE, 27lbs for scope and any accessories the mount will be carrying. (for viewing)

 

for astrophotography the rule of thumb usually offered by 'the community' is to never exceed 50% of the payload rating (although some question whether that is extreme and more of a upselling tactic than reality) 

 

For instance, my payload is about 17.5 lbs, and I have yet to see anyproblems. 


Edited by xiando, 01 March 2021 - 03:50 PM.


#5 kathyastro

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Posted 01 March 2021 - 02:53 PM

27 lbs of payload, plus the counterweights you need to balance it.



#6 Clayk59

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Posted 01 March 2021 - 02:55 PM

Very helpful.  Thank you all



#7 barbarosa

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Posted 01 March 2021 - 03:13 PM

The iOptron CEM line has a good reputation for stating a useful payload limit, useful for imaging and visual.

 

Speaking only to the case of the CEM60, the 60 pound payload maximum is in fact a useable maximum for imaging if you also guide. I know a CEM60 owner who normally exceeds the 60 pound maximum payload with excellent results. 

 

I also have a CEM 60 and can testify to exceeding forty pounds. At some point I begin to worry more about dropping something than I do about the payload spec.

 

Unguided imaging seems a sort of silly concern if the mount guides acceptably with the your payload. However CEM60 owners with whom I have spoken say, as I do, that unguided tracking is within or very close to the factory spec, at least for payloads under 60 pounds.

 

Hopefully someone who uses a smaller CEM will comment.



#8 DSOGabe

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Posted 01 March 2021 - 03:26 PM

Keep in mind that even though the mount is rated at 27lbs, any load approaching that limit will cause more unwanted vibration and movement while viewing. I had a C925 on my AVX (rated at 30lbs). Even though it weighed about 20lbs plus another 5 or so with diagonal, eyepiece and so on, it pushed that mount. Just adjusting focus or any contact with it would cause the whole thing to vibrate a lot. 



#9 johrich

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Posted 01 March 2021 - 03:36 PM

I have and use an Ioptron CEM25P for imaging.  It has a 4 inch refractor (Stellarvue SV102 Access) and all the associated guide scope, cameras, etc., which comes to about 12.5 lbs.  The CEM25P handles this load, which is slightly less than half its weight rating, easily.

 

John



#10 spereira

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Posted 01 March 2021 - 03:50 PM

Moving to Beginning Deep Sky Imaging.

 

smp



#11 imtl

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Posted 01 March 2021 - 04:04 PM

Like with anything in astrophotography, it's more complicated than just payload. The physical length of the scope+gear and weight will dictate arm moment and that is impotant just as the weight alone.

Also, it will depend on your image scale as well. If you are imaging with image scales much larger than your seeing + tracking errors then you will be able to tolerate tracking errors in your image and still get decent images.


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#12 Alex McConahay

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Posted 01 March 2021 - 04:22 PM

Hey, while we are at it......are you sure the reflector you are thinking of putting on it can handle a camera. Many Newts do not come to focus without some modification of the focuser or Primary mirror mounting system.

 

I don't mean to say it cannot happen. But you have to be prepared for the adjustments, or it could be frustrating. 

 

Alex


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#13 RogerM

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Posted 01 March 2021 - 05:28 PM

Like with anything in astrophotography, it's more complicated than just payload. The physical length of the scope+gear and weight will dictate arm moment and that is impotant just as the weight alone.

Also, it will depend on your image scale as well. If you are imaging with image scales much larger than your seeing + tracking errors then you will be able to tolerate tracking errors in your image and still get decent images.

Just to add to the physics side of things.  It is very important to take into account another facet to a mount's weight ratings and this consideration can have a significant impact on a mount's actual tracking capabilities that go beyond simply balancing.  A mount's drive system is designed to move a certain amount of mass but to move that mass requires a mechanical force known as torque.  To exceed a particular motor's torque rating and you will either stall or quite possibly burn the motor out (armatures in the case of old school brushed DC motors.)  Since many types of mounts have no internal positional feedback (except for those higher end mounts with encoders), what's known as 'open-loop systems' in the systems and controls world, any slippage in the drive mechanism, motor stall in the case of a conventional AC or DC motor or even missed steps in the case of a stepper motor will result in poor to no tracking (increased hysteresis response), even offsets in pointing (goto) as well as returning to home/park/zero position.

 

In short, overloading a mount's design can/will open a Pandoras Box.


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#14 terry59

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Posted 02 March 2021 - 06:14 AM

I may have missed it but has anyone talked about moment arm?



#15 Alex McConahay

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Posted 02 March 2021 - 09:11 AM

And sail area.....

 

I think at the Beginner level, the important thing is the 50% rule. 

 

There is no set criterion for "mount capacity." Like "apochromatic," "ease of use," "distortion free," and a few other terms, they vary from manufacturer to manufacturer. So, be conservative. 

 

And if you already have the stuff, and are wondering if it is worth a try, go for it. Your pictures will tell you if it will work for you. (assuming your skills are not the limiting factors).

 

Alex



#16 xiando

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Posted 02 March 2021 - 09:32 AM

I may have missed it but has anyone talked about moment arm?

I think someone mentioned long tube scopes above. so...yes-ish?

 

 

And sail area.....

True, if one isn't using any sort of wind protection, which in my opinion is uh... not intelligent. If you have a wind shield, sail area is somewhat moot, although you bring up a good point for unprotected setups.



#17 TelescopeGreg

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Posted 02 March 2021 - 07:05 PM

I may have missed it but has anyone talked about moment arm?

Moment arm (aka moment of inertia) is a really big deal if you're pushing things.  Even if you're not, paying attention to it is a hygiene item.

 

The base physics is that inertia is the mass times the square of the distance to the pivot point, while balance is simply mass times distance.  When you square a number, it gets big fast.  So mass being equal, a shorter and squatter configuration will outperform one that's longer or taller.  Think about how an ice skater spins more easily when their arms and legs are pulled in.  Your mount is trying to control the movement of all the stuff on top of it, and that's a job of fighting inertia.

 

And remember there are two components, one for each axis (RA, Dec).

 

Dec axis is affected by the length of the telescope, and how the weight is distributed.  We put a heavy mirror or set of lenses at one end, and a heavy camera at the other, with a long tube in between.  That's a problem.  Unfortunately, focal length demands a certain length, so there is a minimum.  But be sure to not make the problem worse by positioning a Finder or guide scope at either end, for example.  Put them in the middle, over the Dec pivot point.  If you have an option to use extra light weight extension tubes vs racking out the (heavy) focuser, do it.  Just be careful about not overdoing it and causing vignetting of the camera sensor.  An off-axis guider adds weight clear out at the camera end, which makes guiding in Dec harder. 

 

RA axis is affected by the height of the telescope and stuff on top of it.  Again, the diameter of the scope is fixed; just don't make the problem worse by mounting a guide scope up on stilts, or putting a USB hub or power distribution box on top.  Add them below the OTA or even better, don't put them on the moving parts at all (i.e. down on the tripod is best).  Also consider ADDING counterweights, and moving them up the shaft.  2x the weight at 1/2 the distance cuts the inertia in half (2w/4d).  On a Newtonian, hanging the camera down instead of sideways or straight up will help lower the center of mass.

 

I went from an 8" Newtonian with all the bristles (camera, finder, guide scope) totaling 27lbs to a 5" refractor with nearly the same focal length.  The refractor was actually heavier than the Newt, but with a smaller (and lighter) guide scope, the net came in at 26 lbs.  Not much change.  But when you look at the weight distribution, the refractor was smaller in diameter, and had lower mounting rings.  I mounted the guider low and in the center, and I moved the finder (when used) to the center instead of at the end.  The weights needed to move up about half way for balance.  My overall guiding improved dramatically.  I could probably do even better with additional counterweight up higher.

 

This was on an AVX mount, which tops out at 30lbs "rated" capacity.  Note that I'm way over the 50% guideline.  By focusing on the weight distribution, I was have been able to continue to use the AVX, saving me well over $2.5k and a lot of hassle that would have been planned for a new mount, investing that in the really nice refractor (*).

 

Physics works.

 

 

(*) I wish I could say I planned this, but the opportunity to grab the Stellarvue OTA came up right before Christmas, and I couldn't resist.  The improvements in guiding were a very pleasant consequence, totally obvious only in hindsight.




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