CGEM DX dec balance issue
Posted 03 September 2012 - 09:44 PM
When I balance in Dec, I put the CW bar parallel to the ground and slide the guide scope back and forth (and the AMD counter weight) until the scope is balanced horizontally. While it is easy to achieve balance in this position, when I move the scope vertical it is very heavy on the saddle knob side of the OTA. I can't seem to find any combination of movements that will get it to balance when the scope is vertical and it seems very heavy to that side (left side if standing behind the scope).
When I was taking the scope down tonight I had a thought. Since the only part of the saddle that moves is the knob side - could it be that the saddle is optimized for a narrower dovetail bar? If both of my bars are wider that what it was designed for, it would appear that that would make the entire scope site a little left of center which could explain what I am seeing.
A quick search brings up some threads of the newer Celestron scopes having slightly narrower dovetails (the extruded ones) which seems to support this idea. I will need to take some measurements with the saddle fully closed vs. open to the width of my devetail to confirm, but I wanted to check and see if anyone else has noticed this before or if I am headed in the wrong direction.
I was wondering if changing to an ADM saddle might fix this (keep the OTA more centered) and help me get a better balance.
Posted 04 September 2012 - 07:22 AM
This will let you in on the secret. Works whether you're side-by-side, vertical, whatever. You must be balanced in all three planes. When you are, then the rig will remain wherever you place it with the clutches released. For side-by-side, this is the best set of instructions.
Posted 04 September 2012 - 09:33 AM
True balance is not indicated by the OTA staying one place but by the ability to turn the axis freely and not have it stop in any particular position. Frequently, the axes on these mount are not free enough to do this. Thus, from your desciption, I suspect that you are actually not balanced and if you turn the OTA vertical while the RA axis is horizontal you will probably see this.
With all this in mind, perfect balance is rarely a good idea unless you have very high precision gears with virtually no backlash. If you do balance the DEC perfectly, then you are likely to see it bouncing back and forth between the worm teeth. Just as in the RA, keeping the worm teeth loaded is important, it's just more difficult in the DEC because you are not dealing with one side of the meridian or the other. I would recommend that you do a good job of balancing the DEC but not worry about getting it perfect. The little bit of bias provided by the saddle (or finder or other things) does more good than harm.
Posted 04 September 2012 - 05:03 PM
This is a very good video. The only issue I have with it is that you would have problems observing with the tube rotated the way it is in the video. This may not be a problem for imaging.
This will let you in on the secret.
Another way to achieve balance of the tube is to use tube counterweights. This can be used with or without a rotating tube. On my cave 8" Newtonian I have a rotating tube setup but I use it all the time to position the eyepiece in a more comfortable position. My tube is balanced with counterweights so I don't have to depend upon its rotated position for balance.
If it's only a slight imbalance this might not be necessary. Also, as stated a tad of imbalance in the right direction can be a good thing.
Posted 04 September 2012 - 07:44 PM
Dave - That is a great video! It seems that the rotating of the tube would put the center of mass back over the centerline of the mount, which makes sense. That appears to be what my problem is (off center mass).
Ed - Excellent advice as always. I have hypertuned my mount via your kit and It really helped - it moves as sooth a butter now (many thanks for making this available)! I completely agree about the need to keep some weight on the gears, so I am not trying to get it perfect - just less than it is now. It is really heavy to one side. Like you mentioned, I kept the CW bar axis horizontal and after balancing the dec axis horizontally I moved the scope vertical - and it pulls pretty hard back into the horizontal position. I think I have the front/back balance pretty good (as it stays horizontal and doesn't tip toward the camera end or the business end). It is like a weight is hanging off the side of the tube. I was expecting this to improve when I moved from the SCT to the RC for exactly the reasons you mentioned. I was at a loss when it did not.
Jim - also very good points. I had thought about using some type of weight on the other side of the tube to counter the weight, but I want to explore some other options first. I actually tried something like that when I had the same problem with the SCT, but I couldn't get it right (was just trying to get a ballpark with some 1/4 lb weights taped to the side)
I thought about this quite a bit today and did some measurements and experiments today. What I found seemed to confirm my initial thoughts.
Here is what I did:
1. measured and marked the dovetail center with a pencil
2. found the center of the saddle as lined up with a yardstick along the CW bar, through the middle of the polar scope hole and past the worm adjustment hole (the best I could get - I think it was very close)
3. Mounted the scope and marked the dovetail where the saddle center passed.
What I observed (in the attached pic) is that the scope is indeed setting about 0.25" off of center. I think with the weight of the RC, guide scope, etc. all setting off center, it is causing the issue I am seeing.
1. After reconfirming my front/back balance horizontally, I rotated the RA back to center and when releasing the dec clutch, the scope falls to the left (viewed from behind - the side to which it is offset).
2. I then loosened the dovetail, moved the saddle 180 degrees (now the knobs are on the right side) and re-attached the scope.
3. As I thought it would, it now falls to the right (because the scope is now off-set to the other side)
If it was the scope that was heavy to one side I would have expected it to continue to fall to the left, but the direction changes with the saddle - so that 0.25" off-set is what I think is causing the problem. If I could get it centered, I could then use the position of the guidescope (or the CW on the underside of the scope) to unbalance in either direction as much as needed to keep the gears meshed, but as it stands it always pulls to one side (and a lot harder than what seems reasonable - but I am not and expert in this - just what it feels like to me)
I think I will investigate a different saddle and see if Anthony (ADM) can confirm the centering of the dovetail in the saddle. If not I may have to try tube weights or just live with it.
Does my little experiment and conclusion seem reasonable? I rather enjoy trying to work some of these things out - just kind of the way I am.
Posted 07 September 2012 - 03:02 AM
So the moral of this little story is that a 1/4" offset between a scope's longitudinal axis and the projection of the Declination axis centerline is not negligible and can generate a significant moment arm force, especially with larger scope weights. This can cause havoc with the autoguider.
The solution to this was to offset the Losmandy dovetail plate by 1/4" so the Dec axis intersects the scope's longitudinal centerline. The scope could then be balanced normally. The 10"SN tube rings were mounted on a 1/2" oak wood board and then the Losmandy dovetail plate was attached to the other side. Addition photos of the arrangement are posted under YahooGroups/CelestronCGEM/Photos/DKsolar32N94W.
Unfortunately, this did not completely solve the ccd sub-frame star drift problem, so I'm still working on that elusive one.
Posted 07 September 2012 - 03:51 PM
I have an ADM saddle on the way and i will post up any new findings after I have it installed on the mount. I did notice that the Losmandy dovetail was not mating very well with the stock saddle and It may be contributing to the offset I am seeing. If I sight down the dovetail and look at the interface with the saddle, it is not fitting well and keeping the dovetail from fully seating - thus keeping the dovetail displaced a bit. I can't tell if it accounts for the full 1/4 inch, but it may be close.
Another reason I decided to go ahead and replace the saddle is after seeing how poorly the dovetail fit the stock saddle, I began to have concerns that it may tip out of the saddle if enough weight is pulling on it. It seemed to grip it pretty well, so it may not be a big deal - just another reason to make the change.
Thanks for the feedback and I am glad you are seeing some success with your fix. If I still need to compensate for any offset after the saddle change, I will try something similar.
Posted 08 September 2012 - 02:15 AM
I took another picture to refresh my memory of how the pieces fit. As I perceive it, the problem is totally due to the saddle. Pretending the mount is setup in the Home position with scope looking at Polaris, the photo view is from the North top looking down to the South. The ADM plate centerline is offset to the West by the ~1/4". If the scope is mounted directly to & centered with the dovetail plate, then the scope will also be offset. For larger scopes, the resulting torque force is substantial. To correct this, the scope needs to be moved laterally towards the East by 1/4" or away from the jaw screws. A solid piece of oak wood is one method for making the offset.
Posted 10 September 2012 - 04:15 PM
I installed the saddle, mounted the scope and double checked the centers of the dovetail, saddle, mount centerline and they all matched.
After doing a quick balance of the RA and the Dec (horizontally), I held my breath and loosened the clutch and rotated the OTA vertical - when I let go...no movement - wow. I could push the scope around in dec with the slightest amount of finger pressure (in either direction). There is only the slightest preference for the knob side, but is tiny and that seems normal. This is a huge change from the way it previously behaved. I now feel like I can actually get it balanced (and have some influence on how out of balance I want either axis).
Thanks to everyone for giving me feedback and many good things to ponder while getting this solved.
Posted 13 September 2012 - 01:56 PM