Mount balancing

[Philip Benson]

I have an EQ8 which is carrying two 6" refractors (TMB and Lunt) side by side using an ADM dual saddle system.  The weight of these scopes is similar and to account for the small difference I use a Losmandy balancing system.

Over the weekend I installed my QSI683 camera onto the TMB.  This is quite a heavy camera and I expected to have to rebalance the system.   I have now got it set so that when the mount is in its polar home position it is balanced and also when the mount is aiming at the horizon.  However within a small zone of about 20 degrees in between I am losing balance in declination.    The RA axis appears to be balanced throughout however.

Should I be concerned about the balance problem in declination within that small range?   Needless to say the scope is not tracking in declination while I am imaging and so surely any balance issues in declination are not as significant as RA.

[Astroboffin]

Hi,

If a mount is balanced in one position, it should be balanced in all, are you adding something when in the small out of balance zone, even something as small as a cable ? As it will make a difference.

I have found that my set up has to be perfectly balanced all the time to get perfect tracking,

if the DEC is very slightly out, it could in fact help keep,the gears in the mount meshed, some people say you should have the mount very slightly out of balance in both directions to keep the gears tight together in the mount, but alas I am not one of them.

Regards

AB

[StuartJPP]

Agree with above...balance should be the same in every position/orientation...

I had a problem a while ago where I had a similar problem, in one orientation it would move on its own...with the clutches released.

It turned out to be the vixen/losmandy clamp that wasn't quite true when clamped...there was a slight offset parallel to the OTA (very slight) but with many kg. of kit it is enough to tip the balance...literally.

It can be hard to visualise where the imbalance is sometimes...

[swag72]

Is balance really the same throughout the entire movement of the mount? I ask as mine certainly isn't balanced across the complete movement of the mount.

[Astroboffin]

Yes mine is, when I balance mine I get it absolutely perfect in both axis, then when imaging I add the wire loom which puts a slight bias on the dec axis but keeps the gears engaged, the Az axis is perfectly balanced all the time.

Works for me anyway....

AB

[laser_jock99]

Is balance really the same throughout the entire movement of the mount? I ask as mine certainly isn't balanced across the complete movement of the mount.

No I don't think it is either.

Bear in mind the mount needs to be balanced in two axes. This can get quite complicated if the mount is loaded with multiple scopes and cameras.

For instance to achieve a good balance with this rig, notice how I have had to offset the guide scope  to counterbalance the heavy DSLR.

It is definitely worth moving things around to achieve the best equilibrium.

[swag72]

I was advised to balance the mount for about 60 degrees as that would give the best over all balance for the whole sky coverage - That way it's always a little out on the horizontal and a few other places too.

[steppenwolf]

With two telescopes on an ADM bar, you are in a good position to get true balance in all positions (it is harder to achieve with a single telescope with a finderscope attached as these are (normally) offset). This is how I balance my dual rig on an ADM bar but bear in mind that I have a small counterbalance bar on my lightest 'scope to allow for the weight of cameras with relatively short focal length instruments. Excerpt from a little book wot I wrote :-

A side by side mounting arrangement for the imaging and guide ‘scopes can prove particularly challenging when carrying out the DEC balance as there are two planes of balance, side to side and front to back but tackling the task methodically will result in a good balance being achieved.
    Start by attaching the two telescopes in a ‘best guess’ position and install the cameras complete with filter wheels and other accessories. Adjust the focus and set it as close to the normal extension for both guide and imaging cameras as you can determine. Remove the dust caps and extend the dew shields if you have refractors. Attach the cables and loosely route them to their final positions at the centre of the DEC axis and lock the DEC axis.
    Rotate the RA axis until the counterbalance bar is horizontal and roughly adjust the counterbalance weights to balance your telescopes. This balance is not critical at this stage so don’t waste too much time getting it perfect. Tighten the RA axis clutch so it won't move.
    Loosen the DEC clutch and let the telescopes rotate to find their natural position. Now adjust the position of the side by side plate on the mount so that the two telescopes balance each other pointing either up or down (if it helps, tighten the DEC clutch before each adjustment).
    Rotate the telescopes horizontally and slide them backwards or forwards until they balance (if it helps, tighten the DEC clutch before each adjustment). It may also help to use the telescope that is located closest to the centre of the DEC axis for making this adjustment as this will be the heavier of the two and will have the most impact on horizontal balance. The centre of gravity is now aligned with the DEC axis and the telescopes will now be balanced at every point of rotation of the DEC axis.
    Finally, balance the RA axis as you would for a single instrument.
 

[StuartJPP]

Is balance really the same throughout the entire movement of the mount? I ask as mine certainly isn't balanced across the complete movement of the mount.

I didn't think so after spending what felt like a couple of hours trying to figure out where the imbalance was. However once I found it, it was possible to balance it. What makes things difficult to balance is when there are "outriggers", like a finder scope etc. in reality these have to be compensated for.

Everything has to be symmetrically balanced in all planes and it can get tricky. Personally I don't think a tiny bit of imbalance will make a difference, though better to be safe than sorry.

I know for a fact that the Vixen compatible clamp on my mount does not clamp symmetrically even when using the manufacturers Vixen compatible dovetail with the clamp. There is definitely a slight imbalance.

[Astroboffin]

The problem with balancing an Equatorial mount perfectly is that you can only really balance in 2D and you require 3D, by that I mean you can put the correct amount of weight on the shaft, move the weight up and down the shaft to get balance, but ideally you need to be able to move the shaft at different angles to counterbalance items that are offset on top of the scope such as finders and guide scopes, if these were exactly on the top of the scope then 2D balance would be enough.

No with an SCT on a wedge and a 3D balance kit, I can get absolute perfect balance in all axis and directions, by adding the correct amount of weight, sliding up and down and angling the weight bar either side of the OTA, and then perfect balance in all directions.

Regards

AB

[ollypenrice]

You can and should get a dual rig balanced at any and every position. (Finding 'dynamic balance.') I think that Steve's explanation above would be quite impossible to beat. It could be neither shorter nor clearer!  Chapeau, Sir.

Olly

[Bubbles82]

I've always found balancing my dual bar rig quite easy, but then again having to set up every time has forced me to do this over and over again. But I have always wondered that once balanced at start up once you start moving the camera and filter wheel out on the focus tube how much out of balance does that put the whole rig ?

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[Astroboffin]

I've always found balancing my dual bar rig quite easy, but then again having to set up every time has forced me to do this over and over again. But I have always wondered that once balanced at start up once you start moving the camera and filter wheel out on the focus tube how much out of balance does that put the whole rig ?

Sent from my iPad using Tapatalk

It certainly will alter the balance point, I tend to do a load of test set ups and write down the weight needed and mark the bar with a marker, so when you go out imaging you know what weight to put on what mark for a certain set up, obviously you need to have it set up roughly in the focused position for the testing purpose.

Works for me

AB

[Philip Benson]

Thanks all. 

I will try Steves balancing method as listed but moving the side-by-side plate in the mount saddle is not easy when you have two 16kg telescopes attached!  I will need to get a 2nd pair of hands to help with that I think. 

[ollypenrice]

Thanks all. 

I will try Steves balancing method as listed but moving the side-by-side plate in the mount saddle is not easy when you have two 16kg telescopes attached!  I will need to get a 2nd pair of hands to help with that I think. 

No Indeed. It's a two person job. When doing anything which involves loosening the dovetails on anything big I run a safety strap to the observatory roof, loose enough to allow adjustment but suffiently short to prevent any hittings of floors or tinklings of glass...

Olly

[Astrobug]

I speak for myself on a newton scope.

The weight of my finder/focuser need to be balanced with a weight on the other side of the tube.

It is the only way I found to be balanced in all position with the mount.