RE: [ATM] 40" mirror

["Donald Good" <donald.good@comcast.net>]

Actually, the COG is predetermined by the components of the telescope OTA
(optical tube assembly).  The OTA consists of the mirror, cell, spider,
secondary, focuser, eyepiece, finder(s), any optional components (e.g.
holders, controls) and "tube".  The "tube" is either an actual tube (e.g.
sonotube) or in the case of a truss type, the truss, upper cage and mirror
box.  The moments of all these are added together and divided by the total
weight to get the COG.  Since the altitude bearings are circles which are
mounted such that the centers coincide with the OTA COG (required for
balance) they do not change the COG of the OTA.  NOTE: if the altitude
bearings are not full circles, they will affect the COG, but only a little
in the direction of the optical axis, and only a little more in the
direction perpendicular to the optical axis.  The centers must still
coincide with the total OTA COG, including the partial circle altitude
bearings, within a limit determined by the stiction of the bearings.  So you
don't got the COG.  The COG got you.

It just occurred to me that there must be a practical upper limit to the
weight of a telescope OTA using classical Teflon/Ebony Star altitude
bearings.  I know that the pressure on the Teflon can be held constant by
increasing the contact area with weight, but the total friction will
increase with increasing contact area.  Do all the big Dob's abandon the
classical for a roller bearing hybrid?

Don

> -----Original Message-----
> From: atm-bounces@atmlist.net 
> [mailto:atm-bounces@atmlist.net] On Behalf Of Jerry B. Hillman
> Sent: Thursday, December 23, 2004 6:00 PM
> To: matt; Jeff Anderson-Lee
> Cc: The Amateur Telescope Makers List; Jay Kirkland
> Subject: Re: [ATM] 40" mirror
> 
> Hello everyone,
> 
>... 
> balance.  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.
> Controlling the pressure on the altitude bearings can help 
> compensate also.
> 
> As far as the mirror itself, I'll keep the list posted on 
> developments concerning the thickness and ultimate design.
> 
> Clear skies, Jerry
> 
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