Re: [ATM] 40" mirror

["matt" <mariusrf@bellsouth.net>]

Jerry,

Have you considered a folded Newtonian with a replaceable secondary to be
used also as  Cassegrain at Nasmyth focus ?
That would make the scope more versatile, it would considerably reduce the
eyepiece height and make it comfortable to view , and would be easier to
balance since all eyepiece or equipment changes that normally happen at the
focuser would coincide with the OTA COG .

best regards,
matt tudor

-----Original Message-----
From: Jerry B. Hillman <JBHillman@ev1.net>
To: Donald Good <donald.good@comcast.net>
Cc: 'The Amateur Telescope Makers List' <atm@atmlist.net>
Date: Saturday, December 25, 2004 3:46 PM
Subject: Re: [ATM] 40" mirror


>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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