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On Balancing a GEM

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#1 Dave Lee

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Posted 31 March 2014 - 05:45 PM

I have a G11 (non-Gemini - late 90's model) that was serviced and put back into operation last summer. This was/is my first experience with a GEM (fork mount background). This mount typically carries ....

1) Older C11 OTA with Losmandy D-style dovetail

2) Lumicon 80mm finder with an inexpensive 1.25" diagonal and Celestron X-Cel 25mm EP

3) Telrad or Howie Glatter laser 1x finder

4) Astrozap flexible dew shield

IOW, a pretty conventional setup.

I'm having a couple of minor (possibly related) problems and this question is about DEC balance.

Balancing as typically practiced is pretty simple - horizontal RA axis and adjust weights until balanced in RA (with scope parallel to the ground). Then adjust the scope dovetail's position in the saddle until (with horizontal tube) DEC is balanced.

What I have noticed is that when you do this the scope is severely unbalanced (in DEC) in other positions (at least for my setup). For example put the RA axis parallel to the ground and point the DEC axis at say 45 degrees from vertical (to the north). The scope is now severely unbalanced (EP end is too light). It is enough that I have to tighten the clutches (so the OTA will not 'slip') so much that I can't achieve smooth push-to motion. Additionally it might be part of the random 'DEC stalls' that I get with OTA movement via the hand controller and it certainly makes EP changes iffy in some cases.

I am considering adding some kind of sliding weight to the forward portion of the scope dovetail that I can move as a function of where the scope is pointed. Or maybe I should change out the RA/DEC clutch pads so that 'pretty darn tight' on the clutches allow smoother push-to motion. Or maybe I should balance DEC at some other angle (God knows I pretty much never view with the OTA parallel to the ground).

Am I unique in having this problem? I don't recall hearing of others having to do anything special to 'fix this'.

Thanks.

dave

#2 T1R2

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Posted 31 March 2014 - 06:02 PM

that is common, if your just visual that will probably be ok to be imbalanced(unless you hear cracking sounds), but if AP, you want to balance for the part of the sky your scope is pointed. I would rebalance the dec, specially since you have the long dew shield

I wouldn't "push To* with the clutches engaged, balance at the angel you generally observe and see how well balanced you are in the south sky also, that is one reason your having slips is the clutches are not fully engaged with the weight of your scope when using the motors.

#3 james7ca

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Posted 31 March 2014 - 06:54 PM

I think this is happening because you have equipment that is hanging off of the centerline of the polar axis. When you have your equipment attached to the mount and you stand behind the mount (looking toward the north pole) does anything hang to the left or right of the main scope? If so, that will change the balance point depending upon where the scope is pointed. You can try to rotate the optical tube to better balance the load, or you can just make sure that everything is on the same centerline (which is what I do, my finders are mounted directly over the main scope).

Here is a YouTube video that illustrates the problem:

https://www.youtube....h?v=hGduG2jB9ec

Check out the tip at timeline 5:20

#4 Dave Lee

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Posted 31 March 2014 - 10:09 PM

James - I'm pretty sure that is the issue. All that I have offcenter (of any weight) is my finder. Unless I'm willing to drill holes in my OTA I'm kind of stuck with that. But maybe there are some clever counter-balancing things (or possibly weight assemblies made to fix this issue for AP as stated earlier).

dave

ps. I guess that I should add those heavy EP's (when using a diagonal) to the off-center list.

#5 EFT

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Posted 31 March 2014 - 11:10 PM

Hi David. In regards to the side to side balance on the DEC, this is very common, especially when the DEC axis of the mount rotates fairly freely as the G11 does with the clutch loose. There are many things that add weight off of the centerline of balance, like the focuser, finder, etc. Some of these are substantial while others are not. In general, because of the dynamic movement of the DEC axis, it is difficult, if even possible, to perfectly balance the DEC axis. The good news is that perfect balance on the DEC (or either axis) is not necessarily a good thing since it allows the worm wheel to bounce around between the worm teeth. Being off balance in the DEC is fine as long as it does not strain the drive system. With a Gemini, straining the drive system would result in a drive stall fault. With a non-Gemini system I'm not sure if there is anything to give you a warning other than the axis not moving when it should be.

Basically, just get the DEC axis balanced as well as you can and as long as the mount can move accurately and not stall or sound like it is otherwise straining or doing something it shouldn't, then it is not much to worry about. If the off-balance is sufficient to cause problems, then rearranging things like your finder and Telrad to counter each other will usually do the trick.

Front to back balance on the other hand can be more of a problem with the SCT since it is so tail heavy to start with and finders, diagonals and eyepieces only compound that problem. It is not uncommon to run out of room on the dovetail to move further forward in the saddle. What I typically do is attach a small 1 or 2 pound weight to the underside of the dovetail up front. There is typically a 1/4-20 screw hole there for a safety stop on both stock and after-market dovetails. I have some small weights from Scope Stuff that I got back when I had a fork mounted scope and they have a 1/4-20 threaded shaft and work very well for this purpose and also act as a safety stop. There are also some counterweight kits available that attach to the bottom of the dovetail and can be slid back and forth. Using these techniques will take care of any significant front to back imbalance problems.

#6 james7ca

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Posted 01 April 2014 - 02:51 AM

James - I'm pretty sure that is the issue. All that I have offcenter (of any weight) is my finder. Unless I'm willing to drill holes in my OTA I'm kind of stuck with that. But maybe there are some clever counter-balancing things (or possibly weight assemblies made to fix this issue for AP as stated earlier).

dave

ps. I guess that I should add those heavy EP's (when using a diagonal) to the off-center list.

Yes, you can get counter weights that will attach to the tube. However, have you considered adding another dovetail to the top of your C11 (on which you could mount your Lumicon finder)? You might also consider removing your Telrad or Howie Glatter laser 1x finder after you have finished your alignment (that is, if they are on quick disconnect mounts).

As was previously mentioned, having a certain amount of imbalance can actually be a good thing. In any case, if that imbalance changes in direction or degree every time you move to a different point in the sky it can be a real problem (with astrophotograph).

#7 Dave Lee

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Posted 01 April 2014 - 05:59 AM

Ed, thanks for the comments. I do believe that my Lumicon finder with relatively heavy diagonal and EP is the culprit here. And there are several simple solutions.

dave

#8 schluterdude

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Posted 01 April 2014 - 01:23 PM

Yup, completely normal. It is a result of different torque moments of the accessories on your tube shifting its CofG. You'll never get it perfect....

Visual, get it as good as you can, and relax.

AP, balance it at the point where you plan on using it, and relax!

Being relaxed is key ;-)

#9 Dave Lee

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Posted 01 April 2014 - 06:13 PM

FWIW, I had an old 'saddle' and small weight/mount screw in my box of "where did these parts come from?". It is a pretty heavy weight (5 pounds maybe) designed to thread up/down a 6 inch mounting shaft that attaches to the saddle that attaches to the scope dovetail (see pic below).

I need a lighter weight as right now it comes way too close to the DEC stepper housing for comfort. But this addtional weight makes a HUGE difference in the consistency of balance in various positions.

dave

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#10 MHamburg

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Posted 02 April 2014 - 07:56 AM

Dave,
That ADM accessory really helps my setup for AP.
Michael

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