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Goto Dobsonian: Balance & Alignment Issue?

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#1 Stargazer_Scotty

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Posted 16 March 2020 - 09:58 AM

Hello All,

 

I have an Orion xx16g goto dob I bought a couple of years ago for the last Martian opposition and also to do some serious DSO hunting.  Love the 'scope and its views, but have a chronic problem with the goto.  I perform a three star alignment properly but then can't quite get it to find my goto subject, say, for example,  M42.  It's off by over a degree or two.  I also notice when the friction clutches are disengaged the tube is front heavy and drifts fairly quickly to horizontal.

 

Could the balance issue be causing the alignment problem?  I inserted a meter into the circuit and observed when increasing the altitude the motor pulls more current (about 200mA) than when decreasing altitude.  If nothing else, this would seem to be harder on the motor circuitry.

 

I have tried the following so far, based on previous threads I have read regarding alignment issues:

 

  • Snug friction clutch knobs firmly.
  • Level telescope using a bubble level and pointing OTA to geographic north.
  • Make last fine adjustments on alignment stars up and right, to compensate for gear lash.

 

I was using one SLA battery previously, but last night I added an additional one in parallel to increase current capacity and extend potential battery life.  It clouded up on me so I didn't get a chance to try that one out yet.  I did perform the metering at that time, which is when I noticed the current differential.

 

Any suggestions?  Am I totally off base with this, or if not, any suggestions on a good lightweight counterweight system?  It looks like around two pounds added to the back seems to balance front to back.  (I know it needs to be balanced in the vertical as well.)

 

Thanks,
Scotty

 



#2 macdonjh

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Posted 16 March 2020 - 11:22 AM

I don't have any experience with go-to Dobsonians, so get a few grains of salt out for these ideas:

 

  • Make sure all your mount drive surfaces are clean to reduce slipping or binding.
  • I think you may be on to something with balance.  Perhaps a plastic baggie with the right amount of dirt in it taped to your scope could help you test your theory?
  • Try doing only a 2-star alignment and see if it's better.  I had trouble with a SynScan controller once, performing alignments with few stars helped, strangely enough.
  • Even though your scope is only a couple of years old, try updating the firm ware.  I eventually did that with my problem-child and it helped.  I couldn't tell you why.
  • Go back and reread the instructions for aligning your scope just to be sure you're not missing something.  Sometimes we forget steps in a procedure we've done a hundred times.
  • Use the PAE function in your hand controller: When you find an object, use PAE to add that location to your alignment model.  You should have better pointing while you're observing in that part of the sky.  When you slew to a different part of the sky, do PAE again to improve pointing while you observe in that part of the sky.

Good luck.


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#3 Jon Isaacs

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Posted 16 March 2020 - 11:31 AM

Balancing your Dob can't hurt and will likely improve things.  

 

When balancing a Dob, it needs to be balanced top to bottom and front to back.  I use Harbor Freight lifting magnets.  The 150 lb model weighs 2 pounds and costs about $10, the 250 lb model weighs 5 pounds and costs about $20. 

 

4618259-Balancing a DOB.jpg
 
Jon

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#4 howardcano

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Posted 16 March 2020 - 11:53 AM

I have the Skywatcher 8" GoTo.   It also has poor location and tracking if unbalanced.  Use Jon's suggestions and see if they help on your scope.


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#5 havasman

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Posted 16 March 2020 - 12:46 PM

Yes, balance the scope by adding a bit of weight as Jon indicates. Doesn't need to be perfect lab scales balanced but close is much preferred.


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#6 CounterWeight

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Posted 16 March 2020 - 02:19 PM

I've had the XX14g for several years now and no such issues, though mine came with small weights that attach on the rear of cell by the fans.


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#7 Stargazer_Scotty

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Posted 16 March 2020 - 06:50 PM

I've had the XX14g for several years now and no such issues, though mine came with small weights that attach on the rear of cell by the fans.

Mine has weights that screw on the back of the mirror cell as well.  I recently added a Telrad finder as I didn't like the EZ Finder that came with it - too easy to bump and knock out of wack.  I love the Telrad, but I know it added to the weight problem.  Prior to installing it, the OTA was back heavy.  No fans with mine.



#8 Stargazer_Scotty

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Posted 16 March 2020 - 06:52 PM

 

Balancing your Dob can't hurt and will likely improve things.  

 

When balancing a Dob, it needs to be balanced top to bottom and front to back.  I use Harbor Freight lifting magnets.  The 150 lb model weighs 2 pounds and costs about $10, the 250 lb model weighs 5 pounds and costs about $20. 

 

 
 
Jon

 

Thanks, Jon, I will look into that.



#9 CounterWeight

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Posted 16 March 2020 - 07:47 PM

There is an issue that some folks had with the goto, and I cannot remember what it was, something in the mechanical transfer part of drive mechanism.  I think if you can search in the reflector forum it hopefully will bubble up?  It was not all that long after the line was introduced... maybe a year or two?  Sorry I can't be more help.


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#10 Stargazer_Scotty

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Posted 17 March 2020 - 06:33 AM

There is an issue that some folks had with the goto, and I cannot remember what it was, something in the mechanical transfer part of drive mechanism. I think if you can search in the reflector forum it hopefully will bubble up? It was not all that long after the line was introduced... maybe a year or two? Sorry I can't be more help.



#11 Stargazer_Scotty

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Posted 17 March 2020 - 06:43 AM

Thanks, Counterweight. I will look into that. I remember someone writing something about grime in the gears and cleaning and re-lubricating fixing the issue. I can’t remember if I saw that here or somewhere else. I’ll exhaust all the simple fixes first, then delve deeper. Even though I’m a pretty good shade tree, I hesitate to start digging into the gearbox too soon. I don’t want to inadvertently damage anything. I will keep you guys updated as to my progress.

#12 junomike

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Posted 17 March 2020 - 03:08 PM

My Orion xx16g is top heavy also, although it's never hindered the GOTO function of the telescope.

Some people recommend turning off the dual encoders as this can be the source of bad GOTO's, however that hasn't been

my experience.


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#13 Eddgie

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Posted 18 March 2020 - 06:01 AM

I have the 12" and I have routinely run the scope with hugely imbalanced loads without issue when everything was right with the scope, with the emphasis on the "everything is right" part. I have used everything from image intensified eyepiece to Binotron B27 with Nasser zooms and when everything was right, never had any problems caused by imbalance.

 

If everything is not right, you can have problems.

 

Here are the things to check.

 

Check that all base hand knobs are tight. If you have not done this in a while, I will tell you that they can loosen.

 

If the altitude clutch is tight but the scope still moves up and down some (you feel some play) remove the truss assembly from the base and check to see that the screws that hold the dovetail saddle plate (the casting that the dovetail on the scope slides into) are all tight.  If they are not, you may want to pull them out and put some non hardening Loctite on the screws before putting them back in, and when you put them in,  get them tight. 

 

Next, while you have the scope out, look at the bearings on the opposite side from the dovetail saddle casting; the bearings that support the non drive side of the scope.  These bearings may have rubber tires on them.  These tires were originally glued to the bearings. If the glue has failed, these tires will deform under the pressure of the scopes weight and walk or squirm on the bearings.  If you detect that they are loose, my advice is to just rip them off and let the bearing races run directly on the plastic drum.

 

Now your scope may be different than mine in design but in the pictures the design looks similar.

 

Next is backlash. Unless something has changed, the manual gave about zip with respect on how to adjust backlash.

 

Before I type this instruction for about the fifth time on CN, let me apologize in advance for typo errors.  I am traveling in NZ and the letters on my keyboard is upside down and th horrible virtual keyboard is painful to type on.  Also, since I am not at my scope, I have to go from memory. This means I might get something reversed,  but some trial and error on your part should sort it out.

 

First, go to a star low and south (as close to south as possible but it does not have to be exactly south).  This star will be used for the as backlash adjustment. I forget but I think it is Utilities/backlash/azimuth. You might have to look around for it.  As I recall, when you find it, it should give a value.

 

What you are going to do is to use your reticle eyepiece and center a star in the field.  Set your slew speed to 3. Now, using the Right button on the controller to move the star away from the center of the field maybe half way to the field stop. Next, watch carefully... Using the Left button, bring the star back to the exact center of the field and exactly as the star hits center, let go of the button. If you overshoot, start over.  Once you let go, wAtch the star.  If the backlash is set properly, the star should stay exactly centered.  If the star appears to drift a little before it starts moving, you need to increase the backlash value.  If you see the star appear to bounce back in the direction it came from, you need to decrease the value (I believe the value is arc seconds?) increase or decrease as necessary so that when you release the button, the star stays centered and does not drift or bounce back in the direction it came from.  When you have this right, exit the AZ backlash routine.

 

For Alt, you will use a star low in the west. Does not have to be exactly west.  The process will be very similar, but this time you will center the star, use the Down button to move the star away from center, then the Up button to bring it back, and once again, work until you have a value that keeps the star centered.

 

What you are doing in both cases is driving the scope into position against the direction of movement and seeing if the motor driver takes up the proper amount of slack to get the star moving again as soon as you let go of the button. If the star bounces back in the direction it came from, the motor driver is turning too far in the driving direction.

 

The reason I use the south and west stars is just to isolate the different axes as much as possible.

 

Now again, I can't be sure that the mechanical are the same and it is possible that the controller routines have changed since I last did mine.

 

Record the values (I just wrote mine on a little scrap of paper and clear taped it to the controller).  If you reset the controller for any reason, just reenter the values.

 

There is more.  PAE Values. If you are using the PAE values, be aware that these are saved at power off. Now if you always left the scope in exactly the same position then once set, the PAE values would never have to be cleared.  If though you are using PAE, and you never ever resetting them, you can accumulated incorrect data.  I recommend clearing PAE at the start of each session and rebuilding the PAE data for each session as you go along.

 

Last.......... If you are manually skewing, be sure to sync your encoders frequently. Procedure in the manual for this.

 

Ok, I am not saying any of these things are your problem or that any of these things will fix your problem. What I am saying is that if all of these things are right on my scope are right, my pointing is good regardless of my imbalance with any load I use, and there is a huge range of Payload on my scope. My scope just does not care about imbalance in the slightest and when all of these things are right, my go2 performance is pretty good (though there is always gimbal error near zenith, but this seems to be a general characteristic of alt as go2 mounts.)

 

Darn that was hard to type on a virtual! Keyboard.  I hope something here was valuable to to you and I hope I have the button directions right.

 

Good luck with this.  I probably will not come back to this thread but I would appreciate a PM if this helped, if you have corrections, new data, or found some other problem.

 

Stay safe..,


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#14 Dave Bush

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Posted 21 November 2020 - 04:51 PM

...I perform a three star alignment...

How are you getting to do a three star alignment?  Are you using the hand controller?  If so, what is the firmware version?  I've got the latest V5 hand controller with the latest firmware and there is no three start alignment available.   Now there is via the Synscan app but I've not yet tried that.



#15 Stargazer_Scotty

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Posted 22 November 2020 - 12:50 AM

How are you getting to do a three star alignment? Are you using the hand controller? If so, what is the firmware version? I've got the latest V5 hand controller with the latest firmware and there is no three start alignment available. Now there is via the Synscan app but I've not yet tried that.



#16 Stargazer_Scotty

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Posted 22 November 2020 - 12:51 AM

How are you getting to do a three star alignment? Are you using the hand controller? If so, what is the firmware version? I've got the latest V5 hand controller with the latest firmware and there is no three start alignment available. Now there is via the Synscan app but I've not yet tried that.



#17 Stargazer_Scotty

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Posted 22 November 2020 - 12:53 AM

I am using the Synscan Pro mobile phone app. I found it easier to manipulate than the hand controller. It has a three star alignment.

#18 Stargazer_Scotty

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Posted 22 November 2020 - 01:04 AM

I am attempting to apply some of these potential fixes to the goto issue. I certainly appreciate all the responses! I’m pretty new to goto being a “push to” guy most of my visual observing career. Unfortunately the weather here in Florida has been terrible. On top of that I work a lot. I need clear nights when I’m free to run these tests and there sadly have not been very many of them. I confess there have been a few good nights, but I want to spend my time observing and not fiddling with equipment. Ironically when I got the post about the three star alignment I was actually out there trying to do just that - fix the goto that is. I am plugging through it and will get a solution I hope soon.
Thank you all for your help! I will let you know what the outcome is as soon as I can.

Edited by Stargazer_Scotty, 22 November 2020 - 01:10 AM.



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