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Side-by-side Losmandy dovetail size question

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I am looking for advice on losmandy-style mounting bars and the sensible loading vs length to achieve a side-by-side mounting that doesn't show deflection under load. 

I have an existing side-by-side losmandy arrangement using Altair TMS parts to carry an 8" Cass at one end and a 5" refractor/80mm guider pair at the other end.  This is on an EQ8 and uses a 300mm bar to separate the scopes, along with a FLO adjustment mount for co-alignment.

The weight of each of the Cass is about 10Kg with filters and cameras and mounting gear and the other end maybe about 13Kg, that seems to work fine.  

I am moving to a 12" Cass that has a 16" tube and weighs about 13Kg all up. That means I need a longer central rail.  I think I need one at least 15= 9" + 6" long or one slightly longer where I can move the guider down to between the other two. 

Is this too much for a central rail ? Is there a heavy-duty rail I can get for this sort of arrangement ?  I am worried about deflection between the two ends of the side-by-side bar due to the weight. The EQ8 central dovetail is about 8" long  so the middle is well supported and the cantilever distance will be about 5" at each end if balanced in mass..









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Not quite the simple answer you were hoping for. But here goes.

I use Losmandy fittings on my Alter D6 mount and 'play' with different configurations. Up to the mount capacity (40Kg) I have not had any issues with Losmandy rigidity for visual use.

Early on I discovered clamp and rail parts from different manufacturers (both scope and accessory) often did not fit together properly.

Looking for a 'standards' documents, I asked around various  manufacturers importers and retailers.

At the end of the day ADM and FLO between them confirmed the absence of an agreed standard and ADM sent me a mechanical drawing to show their profile.
Other manufacturers and importers did not respond.

I therefore modified my existing parts to suit ADM and have always bought ADM since then.
If other people are not prepared to publish their dimensions, how do I know they won't change with time?

I have previously (years back) posted the ADM dimension on SGL and will dig it out if there is interest.

Hope this helps, David.

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Thanks David

Not sure I am looking for a simple answer but it all helps. 

The key issue I think I have, notwithstanding the fit of dovetails to each other, is the degree of mechanical droop for each weight at the end of a lever arm. What size of dovetail bar doe I need ? I think this is measured in thickness of the bar. 




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Hi Mike the overall assembly could be made a lot more rigid if you add a brace between the two sets of scopes.  Just something solid (like another dovetail?) that would link across the top of your scopes - the bottom of your scopes would already be connected by the main dovetail.  Adding a brace wont make it absolutely solid but it will massively reduce droop and inter-scope movement.  I'm always drilling & tapping 6mm holes in aluminium struts for allen screws and finger bolts for use with my equipment.

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