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Best method for balancing SBS setup?

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#1 ShawnPreston

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Posted 04 May 2011 - 12:12 AM

So, my quandry... I am side-by-side mounting two scopes of very different weights and lengths. One is a C14 with dew shield, and the other is a much lighter and shorter WO 98 FLT. I'm having a heck of a time finding ant sort of consistent sweet spot with DEC balance.

I'm wondering, should I mount each scope alone on the mount first to determine and mark it's balance point on the individual dovetails, then put the 2 together (how the were balanced individually) and try to move them around as a unit?

I'd appreciate any tips from my SBS brethren.

Shawn

#2 jmiele

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Posted 04 May 2011 - 12:25 AM

Is the SBS balanced? Meaning, is the weight of the two different weight scopes centered over the DEC axis? I've never done a SBS but have often wondered about this issue. To me, it would seem the balance would be affected by where in RA the scopes are. Do I have that correct? Joe

#3 Dmitri F

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Posted 04 May 2011 - 12:49 AM

I was thinking of switching from a piggyback to SBS mount and did some tests with homemade setup. So my experience is limited but I think I understand the theory. You need to put the center of mass (CM) of the whole rig on the DEC axis. By sliding the scopes back and forth (either together or just one) you move the CM along the same direction. I don't think it matters if you balance them individually or not. The problem is that you need a way to move the CM in the perpendicular direction. I don't know how people usually do this. ADM has a SBS saddle that allows you to do just that.

When you position your DEC axis horizontally and release the DEC clutch, the scopes will rotate so that the CM is below the DEC axis and this will tell you which of the two directions is not balanced.

Hope this helps a little.

#4 ShawnPreston

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Posted 04 May 2011 - 10:52 AM

The difficult part I'm finding is that since the scopes are SO different in weight, the heavier dominates everything. For example, I have my C14 on the west, and 98FLT on the east. Even if the 2 scopes are rotated clockwise and pointing to the east, when I release them, everything will rotate counter-clockwise to settle with the heaviest part down.

I think what I'll try to do is get the C14 balanced in DEC, then add the 98FLT to the mix and try to maintain balance.

I'll keep everyone posted.

Shawn

#5 gillmj24

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Posted 04 May 2011 - 12:49 PM

I have three scopes on a Cassidy triad bar and I also move them forward and backward with the rings loosened to help balance.

Every scope combination has a balance point, you just have to find it. Even taking 10 or so iterative steps I get very close to balance (the AP mounts handle a bit of unbalance just fine) in about 15 minutes.

When starting out I bear hug the biggest scope just in case things go flying when I release the clutches.

#6 gillmj24

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Posted 04 May 2011 - 12:52 PM

If you're looking for a good method, try this:
http://www.wilmslowa...i.htm#balancing

#7 Bowmoreman

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Posted 04 May 2011 - 03:26 PM

If you're looking for a good method, try this:
http://www.wilmslowa...i.htm#balancing


That pictorially describes exactly how I do my SBS setup of a C11 and FSQ106 (and sometime soon may swap one out for the AT6RC...)...

as long as you're methodical, it works fine.

#8 ShawnPreston

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Posted 04 May 2011 - 04:40 PM

Excellent methodology gillmj24.. I'll do exactly what's described.

Thanks!

#9 Dmitri F

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Posted 04 May 2011 - 05:45 PM

That's a nice guide. Much clearer with the pictures.

Shawn, from what you describe it looks like you start with the C14 too far from the center. Since it is so much heavier, it should be almost at the center, and the WO to the side. C14 should be as many times closer to the center as it is heavier.

C14 is about 45lbs and WO 98FLT is 7lbs. So C14 should be 6.5 times closer to the center. If the OTAs are 15 inches apart then C14 should only be 2 inches away from the center, and the refractor 13 inches away.

#10 averen

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Posted 04 May 2011 - 09:40 PM

Here's the method I used to use when I had a sbs setup

Start with everything off of the mount. But have the telescopes as close to how you plan on using them (eyepieces and diagonals in or cameras)

Find the balance point of one of the scopes using a round object under the rail. A ball point pen or similar works well. Mark the balance point with a wax pen or tape. Do the same for your other scope.

Place both scopes onto your sbs setup with their balance points centered over the dovetail bar of the sbs. Now everything should be balanced fore and aft.

Take your sbs bar with both scopes attached and find the left-right balance point using the same method as above. Mark the sbs dovetail bar and place it on your mount with the balance point directly centered over the dec.

Move the ra counterweight as needed and you're good to go!

I had great luck doing it that way. It sounds complex but I used to be completely mobile and could get it balanced in well under 5 minutes. Let me know if that works out for you

Jared

#11 Dmitri F

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Posted 05 May 2011 - 01:09 AM

That's an interesting idea Jared, and not just for SBS. Useful when some friction in the mount axis prevents good balance.

Also, I always felt uneasy about putting all the expensive and fragile stuff like a binoviewer on the scope, and then releasing the clutch and sending it all for a spin.

I'l give it a try next time I set up.






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