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Balancing on equatorial mount with optics on the side

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#1 TheChaosMachine

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Posted 03 March 2021 - 09:12 PM

Hello!

 

I posted a bit ago about using my Celestron SLT130 while I wait for my backordered Orion 6". It turns out that the SLT130 already has a dovetail so I don't need to worry about making one (yay!).

 

I have attached the scope to my EQ6-R Pro and am trying to balance the Declination (R.A. is balanced just fine using a single counterweight).

 

With the optics being on the side of the tube it seems to be impossible to balance. I assume having optics on the back of the scope would be ideal, but how does one achieve balance when the optics are on the side?

 

No matter how I move the dovetail in the mount, there seems to be no way to balance on declination. The imbalance seems significant enough to make the mount work too hard and the Orion I ordered also has the optics on the side of the tube. 

 

Does anyone have experience with this? I tried searching the forums and have done a lot of YouTube searching, but haven't found anything that addresses this.

 

Thanks in advance!



#2 photoracer18

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Posted 03 March 2021 - 09:41 PM

What exactly do you mean by "optics on the side of the tube"? Its a Newt right? So the optics are at the bottom of the tube.



#3 adosaj

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Posted 03 March 2021 - 09:44 PM

The idea is sort of the same, but in the opposite direction. Ideally you would attach small weights on the rear of the OTA or a counter weight bar that attaches to the length of the OTA. Maybe you could use Velcro to attach the weights.  For my refractor I just bought a very long dovetail bar and moved the scope forward on the saddle. You would need to do the opposite, moving the OTA further back on the saddle. 


Edited by adosaj, 03 March 2021 - 09:59 PM.


#4 TheChaosMachine

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Posted 03 March 2021 - 09:48 PM

Sorry, I wasn't very clear.

 

The optics aren't directly across from the dovetail, but are rather close to the dovetail. This means that both the main eyepiece and the finder scope are close to the dovetail that attaches to the mount. 

 

Most of the videos have the optics directly across from the dovetail, so a balance seems straightforward. 

 

I'm going to try to attach a picture, not sure if I'm allowed with a young account age.

 

oLbIG59.jpg



#5 photoracer18

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Posted 03 March 2021 - 10:01 PM

OK I looked the SLT130 up. So the dovetail is very short so you can't just slide the tube down to a better position by loosening up the clamp. Short of getting a longer dovetail and maybe drilling different holes you need some weighted magnets to stick on the side of the tube to balance it. Its like balancing a Dobsonian. Ed Ting's Youtube series on telescopes talks about this when discussing  Dobs as they have the same kind of issue.



#6 photonroundup

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Posted 03 March 2021 - 10:29 PM

I would agree with that assessment. Looks like you can’t reach the balance point with the short dovetail, but you could try to add weight opposite the camera and guide scope (small weight at longer distance) to shift the balance point over the dovetail.

#7 TheChaosMachine

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Posted 03 March 2021 - 10:32 PM

I don't understand how a longer dovetail can account for the weight being all on one side of the OTA. Am I misunderstanding something? I hoped the picture would help, but I may be failing to grasp something simple. 

 

Won't a longer dovetail just allow me to move it forward and backward in the mount rather than rotating it so that the weight of the optics is directly opposite the dovetail?

 

Another angle, directly from the end of the OTA:

A1364BQ.png


Edited by TheChaosMachine, 03 March 2021 - 10:37 PM.


#8 adosaj

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Posted 04 March 2021 - 02:26 PM

It helps by moving the center of gravity of your setup to be more centered on the mount. You basically are trying to get an equal amount of weight forward and backwards of the center of your Dec axis. By getting a longer dovetail bar you can attach the bar so more length is available to move your OTA further back. 



#9 Stelios

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Posted 04 March 2021 - 02:57 PM

I don't understand how a longer dovetail can account for the weight being all on one side of the OTA. Am I misunderstanding something? I hoped the picture would help, but I may be failing to grasp something simple. 

 

Won't a longer dovetail just allow me to move it forward and backward in the mount rather than rotating it so that the weight of the optics is directly opposite the dovetail?

 

Another angle, directly from the end of the OTA:

A1364BQ.png

You are thinking (correctly) about the *3rd* axis of balance. Even if you balance in RA and DEC, you have the left-to-right imbalance, and you will most definitely have issues as a result.

 

You can solve the problem by getting rings and a dovetail attached to them. Then you can rotate the tube so that the camera and guidescope are on opposite sides of the 3rd axis. 

Even better would be to get a top dovetail for the rings (two dovetails) and mount the guidescope on top. Then you would rotate so that the camera is on the declination plane (the camera, counterweights, and scope are on the same plane as the guidescope). 


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#10 Alex McConahay

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Posted 04 March 2021 - 03:32 PM

You have several problems running all at the same time. But, they can be solved.....

 

First off, since you are using a newt, you will want to consider moving to an OAG. This lessens the amount of stuff you have hanging off there. And, in the long run, you will want to do this anyway, since that arrangement you have will eventually drive you crazy. (Not for a while.....it will work. Just not what you want in the long run.) 

 

Secondly, consider tube rings. These fasten to the dovetail, and allow the scope to move forward and backward. Shifting the weight, rotating, etc.  

 

But, I am sure you did not want to be told to go out and spend more money......(Consider, though, that a set of rings, or counterweights, or whatever, may well cost nearly as much as an OAG! They can be had for not all that much. )

 

So, start by analyzing what you really need. You will want the center of your front-back load directly over your Dec axis. Where is the center of that load? To check this, take the (fully loaded, camera, guider, and all) tube and dovetail off the mount, and take a nice round piece of something (broomstick, length of pipe, whatever. big enough to work with, but does not have to be real large). Put that round thing on the floor, and put your dovetail on it, moving back and forth until the tube/dovetail assembly balances front and back. Note that while you are doing this, you will have to manage the front load from tipping over sideways without really adding or relieving any weight up there. When you find that balance point, mark it on the tube/dovetail with a piece of blue tape.

 

If you cannot find such a point (that is, it is in front of, or behind the dovetail, then things get a little trickier. You can:

 

....add weights to the light end. Never a good idea unless you have lots and lots of equipment capacity. And you do not. 

....Move something further out. In your rig, the only thing that could move further out in front is the guide scope, etc  Of course, the further out something is, the more moment of inertia you have to work against.  

...Get a longer dovetail, long enough so that you can move the weight of the tube further forward or back relative to the dovetail.

...Remount the existing dovetail such that is it centered under the balance point of the front-back weights. (This is the best free solution.) 

 

You have to do something along these lines first, or you will never be balanced front and back. 

 

Now, lets move in that third axis. 

 

The best solution is to go OAG, which puts the weight pretty much in line with the imaging camera, is lighter, and does not require as much setup each time you go out (and gives better guiding!). And then line that imaging train up with the counterweight bar. That is, you want to mount everything in one line. You can mount the guidescope opposite the focuser/imaging camera. This helps to minimize the inequality of the weight.

 

Next, you could mount them next to each other. This helps minimize inequality also. While you are at it, consider mounting the focuser pointing "downward" (in line with the couterweight bar--pointing towards the counterweights, not on the far side). This helps minimize the number of weights you have out on the counterweight bar, and the moment of inertia for the motor that has to push the whole thing around. BEWARE, though that this could put your rig in a position to crash into itself more, since the load on the bottom is extended toward the tripod, etc.    

 

Okay, unless you have everything symmetrical, you need to balance in that third axis. That is difficult to do when your tube is actually screwed to the dovetail. You need to be able to rotate the tube. This requires rings to do it the right way, or weights, etc. Or redrilling and such. 

 

To do it with rings, simply mount the tube in the rings, lock the dec axis, make the counterweight bar level with the ground, and move the weights as needed to balance. Lock the RA axis, and unlock the dec. Move the tube forward and backward until it balances. NOW....and here is the part that balances in the third axis....Point the tube so that it is vertical, with the dec axis unlocked. rotate the tube as needed until the load sits still. You should be able to now unlock the RA axis (the dec is already unlocked), and nothing will move. YOu are balanced in all three axis. Alas, you need to be able to both slide and rotate the tube to do this. 

 

Alternatively, if you have a round tube, simply lie it on the ground. (or a table if you need to stick the camera and all off the end) and roll it around until it balances. Mark where it balances. This, along with the test you did to measure center of balance forward-backward, will give you an overall center. This is where you need to drill and mount your dovetail!!!.

 

If you can't do that, you may wish to consider weights......

 

Good Luck. 

 

Alex


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#11 kathyastro

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Posted 04 March 2021 - 03:39 PM

With the hardware you currently have, it can't be done.  For imaging, the configuration you want is to have the focuser / imaging camera down and the guider on top.  With a normal mounting system, you would just rotate the OTA in the rings and bolt the guider to the top of the ring.  The problem is that you don't have rings.

 

I would suggest getting a set of rings for the scope, another set for the guider, and two matching, longer dovetails.  The rings will allow you to rotate the OTA, and the longer dovetails will allow you to (a) move the scope forward so the camera clears the motor head of the mount, and (b) mount the guider well back to balance the forward weight of the OTA.


Edited by kathyastro, 04 March 2021 - 04:12 PM.


#12 Alex McConahay

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Posted 04 March 2021 - 06:25 PM

You know, that picture is really disturbing. Even with a camera on the front, I do not see how it could possibly balance with the mirror that far away from the center of the tube....Mirrors and their cells are really heavy. Nearly every newt I have seen has the center of balance with the mirror one third to one fourth of the tube length (maybe less) away. I'm not saying it is wrong. But it certainly does not look right. 

Alex



#13 photonroundup

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Posted 04 March 2021 - 09:44 PM

Just curious - are you able to achieve balance without the guide scope and camera installed, or is the mirror cell already that much heavier that is is an issue?

#14 Alex McConahay

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Posted 04 March 2021 - 09:51 PM

Just curious - are you able to achieve balance without the guide scope and camera installed, or is the mirror cell already that much heavier that is is an issue?

If this is a newt, we can assume that at the far end of where we see the focuser is a very heavy mirror. I already don't see how he could hope to balance it if that heavy mirror is so far away. But, if you take the stuff up front away, there will be an even greater imbalance forward to back----much less anything about being off centered left to right. 

 

This is a newt, right?

Alex



#15 TheChaosMachine

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Posted 06 March 2021 - 04:40 PM

Wow, thanks for all the amazing responses. I am overwhelmed that so many folks would help out, y'all are incredible.

 

I have decided that I'm just going to wait for the Orion 6" f/4 Newtonian to arrive, it's supposed to be in stock some time in April (about a month away now). I think forcing my existing scope to work is going to be either too costly or too much of a jury rig. I also don't want to do anything that might damage my scope since it's a lightweight go-to that works well for lunar and planetary observing (and some larger nebulae). 

 

Thanks again for everyone who took the time to read and respond (Alex - that was an amazingly detailed reply). I'm so happy that there are passionate people here willing to help out a total amateur like me.

 

Cheers,

Greg




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