Importance of East-Heavy in Mount Balancing

[BigDan]

My RA guiding looks a little too spiky to me.... at times it can look pretty good, and at times it looks pretty ragged.  I use Polemaster.... and I know my PA error is pretty low, as confirmed by running the GA periodically.... error regularly less than 0.5.  In the past, I used PEMPro to adjust a curve and upload to the mount for PEC.  I need to do that again.... it's been several years.

I have never tried going the east heavy route when balancing my mount.  I've got 4 x 21 lbs. of weight on the shaft.  My understanding is for targets to the east, you move a weight slightly down the shaft.  For western targets, move weight up the shaft.

Two questions:

Is east-heavy balancing really worth doing, and if so..... it can be done without screwing up the calibration (don't have to re-calibrate)?

It seems like east-heavy would keep constant tension in the same direction for all mount positions of the RA axis.

Dan

[Andy Galasso]

On Mon, Jan 14, 2019 at 8:22 PM BigDan wrote:

Is east-heavy balancing really worth doing,

It really depends on the mount. Some types of mount benefit, others do not.  In your case--I recall you have Losmandy mount--I believe it should help.  I'd check in one of the Losmandy forums and get some opinions from the experts there though.  You can also measure it yourself -- run the guiding assistant on both sides of the meridian with and without shifting the weight and look at the RA curve which will be showing the un-guided tracking performance. (Use a log viewer such as PHD2 Log Viewer to view the curve.)

and if so..... it can be done without screwing up the calibration (don't have to re-calibrate)?

No need to re-calibrate.   You need to re-calibrate only if you rotate the guide camera or change the guiding equipment -- different camera, different camera binning, change the mount guide rate, etc. Calibration also varies with declination, but PHD2 will compensate for that automatically as long as it knows the current Dec via an ASCOM or INDI mount connection.

Andy

[BigDan]

WOW.... this is sounding like I could use the GA to check unguided tracking error, both sides of Meridian, and play around with moving some weight to get the error down as low as possible.

I never knew about this option.  Thanks, Andy.

[Brian Valente]

Hi Dan

Generally for Losmandy Mounts, they benefit from having an east bias weighting, unless you have spring-loaded worm gears.

You can definitely check the GA on both sides of the mount to see how it may benefit you. 

The "less effort" way i've seen a lot of losmandy users go about this is to add a smaller removable weight, and then slightly east bias the mount. when you flip sides, you just remove the small weight and you are western bias. 

there are also some folks who do this 'hang a weight on the RA" which eliminates the need for changing weights around. there's some discussion and links to pics on that setup here: https://www.cloudynights.com/topic/465843-mount-imbalance-for-tracking/ 

However, if I recall your guide logs correctly, you had choppy guiding in general, including DEC, which to me suggests variable and sometimes poor seeing conditions and less related to the mount performance. 

Brian

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

--

Brian 

Brian Valente

portfolio brianvalentephotography.com 

[BigDan]

I'm kind of wondering if I can use the GA to tweak in my RA balancing for each object.  I think I've got one counterweight that is farther out on the shaft than the others.  I could put a mark on each side of the weight on the shaft as starting point.  I'm going to slew to a target, start the GA, and see what the graph shows for RA.  I'm guessing, maybe I'm wrong, that if RA has pretty good balance, the RA plotting should be pretty horizontal.  If not, I stop the GA/stop the camera, then I slightly move the outer counterweight... maybe going east-heavy, and start up the GA again.  Do this until I get RA to go horizontal.

If I do this on enough targets that I can see a pattern where east-heavy generally makes things better, then I would say I've narrowed in on a procedure that has helped.

Someone can correct me as to whether the RA plotting on the graph should be horizontal when running GA, and changing the CW balancing can affect that.

Dan  

On Monday, January 14, 2019 at 8:22:04 PM UTC-5, BigDan wrote:

[Andy Galasso]

Dan,

Changing the balance will not affect the RA drift.  The slow drift is a function of polar alignment error and where the scope is pointing in the sky.

The balance may affect the RA periodic error profile (how much the RA oscillates back and forth.)  You may have smoother RA and lower PE with a slight east weight bias. With an east weight bias each element of the gear train (including the worm and worm wheel) is engaged on one side, lifting the equipment against gravity. With a West bias the gears are engaged on the other side holding the gear back from falling with gravity.  The difference in how the gears are engaged can cause different periodic error characteristics for some mounts.

Here's an example of somebody else's GA run.  Notice the overall downward trend of the RA (blue) -- that's the drift.   Declination (red) is also slowly drifting upward due to ordinary polar alignment error.  The high frequency fluctuations in the Dec graph are the result of atmospheric seeing.

Superimposed on top of the RA drift is an oscillation -- that's the periodic error.  Here's the same data with the drift corrected (removed), showing just the periodic error (and seeing):

HTH,

Andy

[BigDan]

Thanks, Andy.... that sets me straight on RA drift with the GA..... would not have worked.  I'm just going to try going east-heavy by moving the outer 21 lb. CW by 1/2 to 1 in. up or down the shaft depending on the pointing direction.  I probably will loosen the RA clutch first and test by hand at horizontal how much I have to move the CW each direction to go a little bit east-heavy.

 

Dan

[john bartucci]

Just throwing in my 2 cents:  I have a much lighter load on my G-11 but I keep the heavier weight as high up on the shaft as I can and use a 7# weight as a vernier of sorts near the end of the shaft.  I move the 7 pounder a few inches up when the OTA is on the East side of the mount.  But I really can't say I've noticed a huge difference when I've forgotten to move the weight.  I modded my brass RA worm with the belleville washers and it helped a lot, too.

John

[Alex P]

The issue is more complex.. as Andy commented, an E bias keeps all the gears constantly engaged in lifting the weight. ( a good thing )

HOWEVER,  when the system is W load biased, there are two possibilities :

1) The worm pushes the RA to the West as normal

2) Or, the main RA bearings and grease stiffness is overcome by the offset load and the RA 'falls through' the backlash free- space so that the worm is now Holding the ring gear from continuing to fall ..  If the bearings/grease sticks again, the mount will not move until the worm moves through the backlash free-space and begins pushing the mount Westward again..

3) Unpredictable variations of 1 and 2  ..

Summary : a Westward Bias  can really stress the PhD control loop corrections !

-Alex P

" The balance may affect the RA periodic error profile (how much the RA oscillates back and forth.)  You may have smoother RA and lower PE with a slight east weight bias. With an east weight bias each element of the gear train (including the worm and worm wheel) is engaged on one side, lifting the equipment against gravity. With a West bias the gears are engaged on the other side holding the gear back from falling with gravity.  The difference in how the gears are engaged can cause different periodic error characteristics for some mounts. "