EQ Platform height of centre of gravity ??

[Astro-Geek]

I'm trying to fully understand the theory in the design of EQ platforms for Dobsonian telescopes.  🤔

I understand the need to restrict the arc to an hour or so, to avoid the Dob falling off the platform (sideways) and/or straining the original alt/az bearings.

I also understand the principle of the two rotating radii on the polar access to tilt it to the latitude (52 degrees for me), whilst keeping a horizontal base.

I've attached a diagram I've cobbled together from existing diagrams on the web to explain my thoughts so far.

I'm now trying to rationalise the importance of the centre of mass of the OTA and its alt/az base being at the exact same height as the polar axis, (sorry about my gobbledegook description, but hopefully my diagram shows more clearly what I'm trying to explain).

Finally, my question -  when one buys a ready made "off the peg" eq platform, (eg Geoptiks), how can they allow for the varying heights of centre of mass of the various Dobs that may be placed upon them ?

[vlaiv]

What you have depicted in your diagram is just ideal case, and ideal case is never going to happen in real life.

Just the fact that one will be adding eyepieces and messing with focuser (racking in / out) makes center of gravity move around. Such a small deviation won't make system unstable and prone to tipping over.

If people have any issues with center of gravity - they can always change it, and often do when using heavy EPs. Some eyepieces and coma correctors combined can have more than 1-1.5kg of weight and often people attach counter weight on other side of OTA to balance it and stop it from "dipping" down. In similar way you can always add some weight on your dobsonian base to move center of mass up and down if you think your rig is not stable enough.

[Astro-Geek]

Thanks Vlaiv, I can see your point about the CG moving anyway, cameras and big eyepieces all having an effect.

Surprisingly, I hadn't thought of using counterweights in the base for the vertical plane balance, even though I'm well used to them on the OTA for horizontal balance. 👍