From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


Hi
 We, hacked over this idea before but this is from
a slightly different angle.
 What I'm proposing is a cell for thin mirrors using
only the three normal support points plus a simple air bag system. This is
primarily intended for a Dobson arrangement. First, The system I'm
proposing compensates for both the angle and changes in air pressure caused
by temperature ( it may require a simple pump/bleeder to keep it in range
for large temperature changes and altitude changes when moved ).
 The idea is for the air bag to support most of the
weight of the mirror and the three point system to just keep it located
right. The problem is that you need to keep a constant pressure for any
particular Alt angle and still change the pressure with changes in Alt of
the scope. The way I propose doing this is with a diaphragm and a circular
weight. The weight is suspended such that when the telescope is pointed
straight up, it pushes its full weight on the diaphragm. When the tube is
horizontal, the weight is supported by swing support lines and just rest
against the diaphragm with no pressure.
 One would make the entire air volume as small as possible.
This keeps the effect of temperature to a minimum. The weight/diaphram
could exist in many of the mirror boxes I've seen used. Of course, the
larger the surface area of the weight on the diaphragm, the larger the
weight and space required. Smaller surface areas require longer travel to
compensate for angle. Some simple gas law equations should determine this
ratio for any specific mirror. It could be connected by a small tube and
mounted anywhere along the OTA to help balance things.
 Some might think that instead of air that you can use
a fluid like water or something. This has the problem that water has quite
a bit of weight across width of the mirror when looking at the horizon. Air
compressed, to the amount needed, has almost no change across the width of
the mirror. Also, the natural pressure of the system at the horizon is
zero.
 The support bags can be made of thin mylar sheet and
still provide good support. The three point mount can poke through the
mylar. If made with good tolerances, the main cell bag need only be about
1/32 or so thick.
 So, what say ye! Can one use PLOP for different effective
gravities?
Dwight

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