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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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