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On 2/27/2014 2:41 PM, Sean C wrote:
> I'm close to being able to buy my first metal sextant. I will most
> likely get the Astra IIIb. I was just wondering if anyone has used the
> "practice bubble horizon" sold by Celestaire and others. If so, what did
> you think of it? Is it worth purchasing? Thanks in advance for any advice.

I found it worthwhile. Like almost anything Celestaire sells it performs
as well or better than advertised. If I recall the last time I reviewed
it Ken thought I was being unkind. That was not my intention, but rather
to point out its strengths and weaknesses from my vantage point.

For the price, it is hard to beat (with the exception of an artificial
horizon). The experience somewhat like being on the water on a choppy
day. To paraphrase one reviewer, "It's like trying to take a shot while
balancing atop a beach ball in a swimming pool." For me, it is closer to
making observations from a light to medium displacement sailing vessel
with a fin or wing keel in 5' to 7' waves. One has to keep himself or
herself stable, keep the sextant vertical, keep a body which is moving
up and down and left and right centered in the scope and in the sweet
spot of a traditional mirror while bringing the body tangent to the
horizon, quite possibly with a zero-magnification tube. This all has to
come together while at the crest of a wave. Doable, but an order of
magnitude more difficult than an AH or standing near the water's edge.

The largest problem I have is centering the relatively large horizontal
bubble on the center line while doing the same for a zero mag body.
Bisecting the sun with a relatively thick line is not too difficult. I
feel the addition of limiting lines to the reticle would be a huge plus.
Even if the bubble size changes slightly, bracketing it between two
lines with a slight overlap or gap would be more precise and accurate
for me. (Not unlike centering a bubble in a circle on an aircraft
sextant.) I have had some success placing masking tape on either end of
the light portal to create a bracket of sorts. The pitfall here I
suspect may be that the masking tape is not on the reticle or spirit
level, so it casts a shadow that may change position based on the
direction of the incoming light. More testing is called for.

I do keep a piece of masking tape on the body, as there are times when
the alignment of the sun to the light portal creates way to much
brightness on the bubble. In that event I place the tape over the light
portal to act like a sextant shade.

Admittedly I have not played with it much since I've been living within
a 30 minute drive to Indiana's north shore. I plan to take it to Lake
Michigan once the Midwest deep freeze subsides, and the pack snow and
shelf ice melt off.  Much easier to obtain an accurate SIC with a
natural horizon than being landlocked and playing around with LAN
observations.

Bottom line, I feel it is a good training aid. I do not feel it is
accurate enough to provide me feedback for fine tuning my skills with a
metal sextant at this point. That is best done with an AH or natural
horizon.

You've done very well indeed with your Davis. It is time to reward
yourself. Drink like a Greek god and purchase a metal sextant. IMHO you
will be more than pleased with an Astra IIIB should you decided to go
that way.

Hope that helps

Bill B

   

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