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Greg R wrote:

> I'm already having enough of a challenge trying to get sights with
> Celestaire's bubble horizon for the Astra - it's a lot like trying to
> stand on a beach ball in the deep end of the pool.  :-)

Agreed.  I have the $8 bubble attachment for the cardboard sextant that
Celestaire sells, and it is superior IMHO to the Johnson practice model they
carry.

The cardboard unit's bubble has limit lines, so you can calibrate bubble
error by placing the bubble tangent to a line.  Even though the cardboard
sextant's scale only reads out to 5', I have consistently gotten within 2'
or less of Hc with it, hand held or tripod mounted.

With the Johnson practice unit (on my Astra), there are no limit lines, or
line across the center of the bubble, so not only am I trying to center the
body (sun) in a zero-mag tube, I am also trying to center that huge bubble
against two lines on either side (which don't necessarily align to
themselves in practice).  If I tripod mount the sextant, I can get my
standard deviations down to within a a few tenths of a minute, so very good
precision (repeatability).  Accuracy kills me though.  Even if I do my best
to center the bubble before clamping it down, in many cases my results are
clustered around 2' or greater off of Hc (3' to 4' is not uncommon).

Would it cut too far into profits to put two limit lines in the bottom of
the bubble vial?  I don't care if it makes the bubble correction 10' or more
off, as long as I can measure it and correct for it.

Bill


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