Re: [ATM] dob balance

["Scott Ewart" <scott.e@quicksurfer.com>]

Lance,
    Using a larger diameter bearing should indeed help stability, but the 
spacing of the pads it rides on will determine how smoothly it moves.  A 
wider spacing (more arc, say 100 degrees versus 90) will increase stiffness, 
while closer spacing will ease stiffness.  You may just have to experiment 
to find the spacing that is just wide enough to hold the scope from moving 
while changing eyepieces, etc., and also give you the stability and feel 
you're looking for.
    Using a bearing that is not a full circle will change the center of mass 
slightly, so if it's mounted with its center at the same place as the old 
bearing, the scope will be slightly bottom-heavy (assuming it was exactly 
balanced before).  BUT, I must emphasize the word SLIGHTLY.  The missing 
mass is so close to the center of mass that you may never even notice.  Just 
mount them SLIGHTLY lower on the OTA.  If you want to get it exact, try 
mounting the bearings with tape, or something very temporary, and set the 
OTA down on the edge of a 2x4 or something, and move the bearings around 
till you get the balance you want.  This is your new center of mass.  That's 
where they go.

Scott Ewart


----- Original Message ----- 
From: "lance clarke" <alaskawolfjoe@hotmail.com>
To: <atm@atmlist.net>
Sent: Wednesday, July 06, 2005 9:49 AM
Subject: [ATM] dob balance


| This will likely be obvious to some of you, but I just wanted to make sure 
I
| get things right.
|
| I've just retrofitted my sonotube dob as a truss dob.  The mirror box is
| deep enough to store the secondary cage, so it's easy to break down and
| transport in the car when we go camping.  Right now I've got a 5 inch
| diameter alt bearing, and as you can guess it doesn't move as smoothly as 
I
| would like, and it is also quite succeptible to vibration.  I intend to 
cut
| larger diameter bearings so the alt will be seated much lower and with a
| broader base.  These bearings will be arcs, not circles or semi-circles.
| Will this help fix the mentioned problems?  As well, and this is what I'm
| much more uncertain about, will my center of gravity change?  That is, 
since
| the bearings are attached below the center of gravity of the OTA, will 
this
| change the center of gravity?  Or does it remain the same as before since
| the  bearings are supported?
|
| I hope I've made my question clear.
|
| Thanks
|
| Lance
| Toronto.
|
|
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