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Unbalancing dec axis for better guiding

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6 replies to this topic

#1 Andre Moutinho

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Posted 14 October 2021 - 12:19 AM

I was adviced to unbalance the dec axis (camera in refractor) in order to have a better guiding in dec. I would to hear from more experinced people about this advice.
Thank you

#2 rockstarbill

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Posted 14 October 2021 - 12:45 AM

Beginner topic.

#3 StephenW

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Posted 14 October 2021 - 01:13 AM

>I was adviced to unbalance the dec axis (camera in refractor) in order to have a better guiding in dec.

 

If you are having issues guiding in DEC, then you are usually better to de-tune tour Polar Alignment and force DEC to only guide in ONE direction, instead of two.   The biggest issue DEC guiding usually faces is handling backlash when DEC guiding direction changes.   If you force guiding to only happen in one direction then the backlash issue goes away.

 

Not sure which guiding s/w you are using, but if it's PHD2 you should upload your guide log here and on the PHD2 forum to get some more detailed analysis of what DEC guiding issues you may be facing.


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#4 asanmax

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Posted 14 October 2021 - 01:04 PM

I second StephenW.

Most of the time, issues with the Dec axis come from backlash. You may need to inspect it and see if you can reduce it by adjusting the gear mesh.

PHD2 has a procedure to measure the Dec backlash, you may want to run it.

There is no need to make any of your axes one-side heavy if there is minimal to none backlash.



#5 avarakin

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Posted 15 October 2021 - 12:14 AM

Some mounts have pretty bad DEC backlash, so guiding calibration takes forever to work backlash. In such cases unbalancing in DEC helps a lot.

I have done with this with one of my mounts which has eccentric worm gear, forcing me to have large backlash.

The trouble with the method though is that force, caused by unbalancing changes while the mount tracks over multiple hours.

 

Alex



#6 WadeH237

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Posted 15 October 2021 - 08:42 AM

I was adviced to unbalance the dec axis (camera in refractor) in order to have a better guiding in dec. I would to hear from more experinced people about this advice.
Thank you

My advice is to never unbalance declination.

 

The issue is that RA balance is only stable when declination is properly balanced.  If declination is out of balance, then the RA balance will change with movement of the mount, which can interfere with RA tracking and guiding.  As an experiment to demonstrate this, try the following:

  • With the mount in home position (scope pointed north and counterweight shaft down), slide the scope back in the saddle so that it is noticeably rear heavy.
  • Move the mount so that the counterweight shaft is parallel to the ground and adjust the counterweights to be balanced in RA.
  • Return the mount to the home position and release the RA clutch to confirm balance.
  • Tighten the RA clutch and release the declination clutch.
  • Move the scope so that it's pointed east and re-tighten the declination clutch.
  • Check RA balance again and note that it's no longer in balance (it'll be west heavy, which you don't want).

By far, the best strategy to deal with declination backlash is to offset polar alignment and guide in only one direction, as noted in some of the replies above.


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#7 TelescopeGreg

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Posted 15 October 2021 - 12:20 PM

I'm inclined to agree with Wade.  Keep Dec balanced, but in addition, spend a little extra time getting a really good polar alignment first.

 

The decision comes down to the mount that you have, and in particular, how free the bearings are.  The theory is that the imbalance will keep the gears meshed, removing any backlash.  But that only works if the bearings are free enough for the OTA to actually rotate.  My AVX, for example, has rather stiff bearings, and the amount of imbalance necessary for it to spin on its own is far too much for the Dec motor to move smoothly. 

 

Better bet is to get a really good polar alignment, so that for the most part the Dec motor never needs to run.  That is, after all, the published reason why Celestron opted for the lower cost sleeve bearing on the Dec axis on this mount.  If the motor does have to run, it will be a nudge to one side or the other, and PHD2 seems to be pretty good about taking out the slack.

 

An imbalanced Dec axis might work for mounts with better (smoother, less stiff) bearings, but I would expect that those (likely higher priced) mounts would also be less likely to need the hack in the first place. 




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