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Ken,

     I would like to suggest you try building a square raft out of PVC
and for right angle PVC corners. If glued properly this should be both
water tight and symmetrically buoyant. I have seen such floats used to
support sea kayaks above water to prevent bottom growth. Home Depot
should have everything at a reasonable price.

Greg

On Sep 15, 9:04�am, "Ken Muldrew"  wrote:
> Thanks to everyone for helpful suggestions. I do like the idea of getting
> a plastic "donut" from a toy store (one of those stacking, tower-of-hanoi
> like toys should provide several sizes to choose from) and mounting the
> mirror using some kind of adjustable centering mechanism.
>
> In the meantime, I rebuilt my box-raft using epoxy and more care in
> construction. I floated some small pieces of wood between the raft and the
> sides of the box to keep the raft more-or-less centered. Of 5 trials so
> far, using Jupiter and a couple of stars (emptying and refilling the box
> each time), the error has been between 0 and 3 minutes. I think the main
> source of inconsistency is, as George suggested, due to air bubbles
> underneath the raft. If I float the raft in a large sink and look
> underneath, I can see several bubbles, perhaps 2mm in diameter, each time
> I float it. My next step will be to try a wetting agent, as Bill
> suggested, to see if that helps. I would like to keep it as simple as
> possible, so my first thought is to clean the glass with a mild chromic
> acid solution, but even if something like a drop of detergent is needed,
> that is still a lot less messy than a tray of motor oil.
>
> To protect it from wind, one would need a cloche with plate glass windows,
> but the raft seems to be more immune to a light breeze than a plate of
> water, especially with spacers to keep it from touching the sides of the
> dish.
>
> With careful construction, especially if you have machining equipment
> available, I think it would be quite realistic to think that one could
> build a mirror raft that floats level within a tolerance of 15". Getting
> it to do so in a dish of tapwater without any other contrivances may be a
> bit more difficult.
>
> Ken Muldrew.
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