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CofG or centre of mass calc

OK , so how DO you  calculate the centre of gravity of your telescope  ?
Actually we want the Centre of Mass CoM to get kinetic equilibrium set up OK,  but  "Mass " is for physicists and everyone understands the idea of a CofG.

You determine the height of the centre of mass (CoM) of the telescope set up  (OTA and Mount) as follows  :


You can determine the height of the CM of the entire telescope from the height of the CoMs of the tube and the mount simply like this :  

(height of the CoM of the tube x mass of the tube + height of the CoM of the mount x mass of the mount) / total mass of the whole system).


1) The CM of the telescope tube will be through the Altitude bearing position


2)You will need to determine the CoM of the mount by finding the point above the base of the mount about which the mount (WITHOUT the telescope in it! ! ! ) balances on 2 fingers placed horizontally to support it so that the mount acts like a well balanced see saw.


3) do the calculation above



an Orion 12  truss dob


1) CoM of telescope tube = altitude bearing height= 25" for tube assembly


2) CoM for the mount is 8" above the bottom of the mount. (  seems low, but this is  because the base  of the mount is very heavy compared to the sides etc )


Weights are as follows

OTA 50lbs

Mount  34lbs


So (25 x 50) + (8 x 34) / 84 =   1250 + 272  / 84 = 18.1 inches.


CoM of the whole setup of the Orion 12 inch dob on its mount is therefore 18 inches.


Very low profile mounts like those used in the US or by David Lukehurst, place the mirror box essentially in the altitude bearing and so have relatively low CoM figures.