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Hi folks. Wasn't a " collimator" also used for index error determination! I
realize the device is quite rare. Has anyone come across one? Would like
info.
Mike Burkes


>From: Bill 
>Reply-To: Navigation Mailing List 
>To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
>Subject: Re: bubble sextant index error
>Date: Mon, 12 Dec 2005 01:44:09 -0500
>
>Ken, Mike and Michael.
>
>To my chagrin, it occurred to me after several (lengthy) posts that I have
>no idea if a dedicated bubble sextant has the ability to observe the body,
>bubble, and horizon simultaneously (as Celestaire's German
>build-it-yourself
>cardboard unit does).
>
>Ken's suggestion makes sense on many levels.  As the bubble is illuminated
>(mine is not) Polaris will remain relatively static for all practical
>purposes for a much longer period than the sun at LAN so is a better
>target.
>Additionally, of what practical importance is it if you have an IC and
>bubble error correction if you cannot view all three?  (My cardboard unit
>was used for both on-the-water and bubble observations, so both were of
>importance to me). What is relative is the combined affect on readings if
>used in bubble mode only.
>
>As to Mike's calibration by observation concerns, average. I have no idea
>how to calibrate a sextant other than by observation.  Mike's point on
>reducing variables remains valid, as does Einstein's suggestion to keep it
>as simple as possible, but no simpler.  In this case, observation seems to
>be about as simple as it can be.
>
>I would still caution, subject to Ken's review, that if using a natural
>horizon and the eye is not at water level, dip would have to be adjusted
>for
>in calibration.
>
>Bill
>
>
>
>
> > On 12/10/05 4:55 PM, "Bill"  wrote:
> >
> >>> So, how do I determine the index error of my bubble sextant without a
>water
> >>> horizon?
> >>
> >> Michael
> >>
> >> 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.
> >>
> >> Bill
> >>
> >
> > Bill and Michael
> >
> > A method I have used is to prop the bubble sextant up on a table
>outside.
> > Center the bubble in the field of view more or less (it is not
>critical).
> > Then bring Polaris down until it just winks off and on, and take the
> > reading.  Then proceed to the bottom of the bubble.  Average the two
> > readings, apply refraction and compare to the Hc for your position.
>This
> > gives you  the error of the bubble at that particular altitude.  Time
> > passage while doing this is not critical for Polaris.
> >
> > Unlike marine sextants, bubble sextants can have different errors of up
>to 3
> > or 4 minutes at different altitudes, due to misadjustment of the
>internal
> > sector gear.  This makes taking sights at varying altitudes advisable to
>see
> > if there are any changes from the accurate IC using Polaris.  But for
> > goodness sake, plot an Hc curve instead of working out lots of sights.
>The
> > raw data you plot will not only reveal IC changes, but backlash, and
>even
> > the personal error you may have in estimating the center of the bubble.
> >
> > Ken Gebhart

   

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