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On 2/28/2014 6:26 PM, Alan S wrote:
> Re the Practice BH, while I suppose they work, in daylight anyhow,it
> takes a steadier hand than I have. Also,as I mentioned, I never could
> understand the BC Factor, perhaps I'm dense.

First a correction. Brackets on the reticle would not help much with the
bubble as the reticle lines do not show up on the bubble. The limit
lines would have to be on the spirit level, or perhaps the mirror that
reflects the level.

Regarding the bubble error/correction or SIC (Sextant Index Correction
as Celestaire calls it):

As an analogy, imagine you purchased a 4 ft plastic carpenter's level at
you local big-box store. You bring it home, use a magnifying glass to
center the spirit level and some ingenuity to secure the level int that
position. Then you check level with a section of water-filled clear
tubing.  You find it is a 1/16" off of perpendicular (no island gravity
mound). You can compensate for that in use, or figure, "What the heck,
it's a carpenter's level so 4.5 minutes of an arc doesn't really mater
when you have seen 8' walls 1" out of plumb. Clearly that would be too
much error for a sextant.

To obtain bubble error I first doubled checked IE using tangent images
of the sun's limbs. Then I did a LAN observation with an AH to use as a
sanity check/baseline. I also establish a true south a range. For
several days following that I tripod mounted the sextant using
photographic grip hardware and pointed the scope true south. Then I did
my best to center the bubble, lock it down, and do LAN observations
(time and Ho precalculated). (I found it easier and more repeatable for
me to bisect the sun with the reticle line rather than bringing a limb
tangent to the line, but that's a personal preference.)

Once you reduce the bubble LAN observations (no dip correction) and
compare the Ho to Hc, you obtain an error that includes any index error.
Back out the IE and you have your bubble error. That's my story and I'm
sticking to it ;-)

Note: I did several observations and averaged them as precisely
centering the bubble from day to day was not a given for this operator,
even with a tripod. In some situations lighting may cause the bubble to
have a highlight on one long edge and a shadow on the other producing
fluting. This optical illusion further complicates matters.

I will follow up with questions I have regarding determining this units
bubble error with a natural horizon.

Hope that helps

   

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