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Bill wrote
> If your location is known, make a series of observations.  After adjusting
> for refraction, the difference between your observation and calculated Hc
> will be the IE.> Hi Bill,

Mike wrote:
> wading in here without full knowledge...however: Won't the intercept be a
> combination of all of the errors? The IE, Observational Error, Timing
> Error....etc?
>
> As an example using a standard marine sextant with known IE and known position
> of the user, once corrected for IE my intercepts represent my capacity to
> execute an accurate sight. Is it different with a bubble sextant?


Mike is correct of course. Hopefully a series will average these variables
out.  Even doing an IE check with a natural horizon, I doubt I could do
better than plus minus 0.1' to 0.2'.

I was in error for another reason.  That being the error seen with a bubble
horizon by observation would be the combination of bubble error and IE.

So to answer Michael's question about IE, use the sun.  Turning the drum in
the same direction, do a 5 to 10 tangencies of the disc off and 5 to 10 on
the arc.  Subtract the off-the-arc readings from 60'. Average each series.
Now subtract the on-the-arc and adjusted off-the-arc averages and divide by
2. That is your IE.

In calibrating my cardboard sextant with bubble attachment, I first
determined index error (building roof line about 1.5 miles away, horizon
later), and then from a known position preset calculated Hs for a time in
the future.  Calculated Hs was calculated using calculated Hc as Ho with
"backwards" corrected for refraction, IE, dip etc (it helps to use a high
body as you are working refraction backwards so want to minimize errors).
The sextant was set for the pre-calculated Hs. If your preset Hs requires
adjustment to align at the given time, the difference is bubble error.

Of course it helps to use a series to average out the errors Mike mentioned.
And what better time than local area noon when the sun's elevation remains,
for all practical purposes, static for a minute or so.

Now if you go to the shore and determine index error, and look at the bubble
and it is not dead center when the horizon is aligned, one might think they
have bubble error.  No necessarily so.  The scope is pointing down and not
level unless your eye is at water level.

I do not know whether a real bubble sextant has adjustments other than
bubble size  that would "level out" the bubble, or if it is treated as a
correction (if present) to Hs like IC. If one leveled out the bubble with
height of eye of 6 ft, then one would have to adjust subsequent bubble
observations for the additional 2.4' dip that was "built in."

My best suggestion is to establish IE using the sun.  Then preset as above
and using the bubble do a quick succession of observations from 30 second
prior to LAN, to 30 seconds past LAN (using a tape recorder or assistant to
record the observations). Average the observations, and any discrepancy
between Ho and Hc should be very close to any bubble error.

Hope I got that right.

Bill

   

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