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George wrote

> I don't think that would work. With an open bottom, all the form-stability
> of a flat box shape would be lost. It would be a bit similar (but the other
> way up) to the loss of stability that occurs on roll-on vessels when they
> get water sloshing around on the wide vehicle deck.

Speaking of sloshing water, was this device protected from the wind in some
manner?  It is a significant problem if the water itself is not perfectly
still.

I had very good results with a roughly 10" x 4" plastic storage container
(with a positive 4-latch and O-ring-seal lid) filled with 50-weight motor
oil with black woodworker's oil-soluble dye added to the oil.  3/4 full, it
provided a long surface good for low altitude observations.  The ink-black
oil had no problems reflecting the navigational stars even with some light
pollution.

The drawbacks:

Wind would effect the surface of the oil, so it needed a glass cover.

No matter how careful I was about cleaning the rim of the vessel, oil got on
the outer regions of the glass and cleaning it up to my standard was a pain
in the rear.

If I put small shims between the glass and the vessel rim to stop the oil
transfer, wind could still cause minor ripples on the oil surface.

Testing was necessary to be certain the faces of the glass were parallel.

The oil was so highly reflective, it was too easy to mistake the image
reflected from the cover-glass surface for the image reflected from the oil
surface.

The glass surface was not necessarily parallel to the oil surface.  On the
plus side, this could assist in distinguishing between the glass and oil
reflected images. Not a big deal to use an inexpensive hardware-store bubble
level to get the container and glass parallel within cel-nav tolerances.

Summary:

Upsides: It is cheap, it works well, and no problems with water vapor
condensing on the glass during sun shots.

Downsides: It is messy, and you don't want to spill any of the oil/dye
mixture on your garments, shoes, floor, driveway or lawn.

Bill B.






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