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On 12/13/05 7:05 PM, "MIKE BURKES"  wrote:

> 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
>
Mike,

Yes, and we have one here at Celestaire.  A surplus dealer located us and
sold it to us about 30 years ago.  Price was $100 plus $50 shipping.  Must
weigh about 150 lbs, and came factory sealed (never been opened)   Produced
by the Kollmorgen Co. of Brooklyn.  Serial # 22.  It is a three point
collimator which gives readings at 0, 45, and 90 degrees.  This allows the
technician to adjust the sector gear to minimize the IC error, or to at
least record it for the user to apply.  It came with cradles to support
about six of the most common aircraft sextants of the time (1943).

We also have another type of collimator which is simply a tube which emits a
collimated reticle.  I haven't taken the time to figure out exactly how that
is supposed to work.

Ken

   

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