Re: [ATM] 40" mirror

["Jerry B. Hillman" <JBHillman@ev1.net>]

Hi Donald,
I said
>Adding weight to the base doesn't necessarily mean
> I have to add weight to the upper cage to balance it.
> Shifting the altitude bearings a little can move the COG an
> amazing amount.

You replied
>Actually, the COG is predetermined by the components of the telescope OTA
>(optical tube assembly).


This is true and I should have qualified my statement.  If the weight of an
existing OTA is changed by adding a heavier component, such as a heavier
mirror, then the altitude bearings can be shifted in the direction of the
end that the weight was added to, to compensate.  In the process of building
a new tube, your statement is perfect.  I might add that regardless of the
weight, 100 pounds or 100 tons, the position of the altitude bearings which
are centered on the COG will always be forward (between the mirror cell and
the upper cage) The separation of the mirror cell and the COG may be quite
small however.

I am not terribly worried about building and balancing the OTA, or making
the scope useable.  I have many obstacles ahead of me just to cast, grind,
polish, and figure a mirror this size, which incidentally is going to be
F3.5.  Focal length of 140" which puts the eyepiece close to10 feet from the
ground when pointed at zenith.  Thickness is still to be determined.


Bearing surfaces for the altitude and azimuth bearings have yet to be
designed, but I believe that they are subject to weight versus surface area
calculations.  So, more weight means more surface area.  Properly balanced,
it should be no harder to move than my 15" which moves at a gentle touch and
stays put when you quit touching.
Should be an interesting exercise in engineering.
Clear skies,
Jerry

BTW - I find it very interesting that the design of my 15", with it's eight
tubes and octagon frames. resembles the 1908 Mount Wilson 60 telescope
immensely

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