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Old 02-04-2001
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Calibrating a Sextant

I have a 1945 aviation bubble sextant that I plan to use on my sailboat, but it needs to be calibrated. Do you have any leads on who could do this sort of work?

Jim Sexton responds:

The USAF aircraft bubble sextant can only be calibrated in a military-maintenance facility. The standard procedure is to mount it in a calibration device, time the averaging mechanism, and then check out the HS reading by setting the elevation wheel to various angles of 0, 30, 45, and 60 degrees. You'll then need to check them against preset marks on the opposite wall. If the sextant passes all these checks, it is certified for use.

Up until the late 1950s a sextant was assigned to each Air Force navigator who was responsible for determining any personal error due to his eyes and ability to keep the celestial body centered in the bubble and the bubble centered on inscribed crosshairs. This personal error was noted and used in all sight reductions. After the 1960s, the sextant was assigned to the aircraft and usually remained with it until it was due for maintenance or recalibration, when it was replaced with another one. Each aircraft that carried a navigator had two sextants assigned to it and these were part of the aircraft's inventory. Assigning sextants to the aircraft instead of each navigator was a cost-saving measure, since there were many more navigators in the USAF than aircraft.

Whether the sextant was assigned to a navigator or an aircraft, it was always inspected and checked by the navigator prior to each flight. These checks consisted of verifying the accuracy of the timer by starting the averaging mechanism and checking it against a stopwatch to see if it ran for two minutes. It was then checked for azimuth alignment by sighting on the aircraft's vertical stabilizer to ensure the azimuth read 180 degrees. The final check, if the sky was clear, was to take an observation on any available celestial body (sun, moon, planet, or star), reduce the sight, and plot the LOP to see how close you came to the aircraft's parking spot. Most Air Force bases also had pre-surveyed bearings and elevation readings from all parking spots to the beacon located on top of the control tower. This was an option you could use when the sky was overcast to crosscheck the sextant readings.

An averaging mechanism was needed since the bubble was affected by any change in aircraft speed, heading altitude, or attitude. All aircraft have a periodic gyration about all three axes, making it next to impossible to take an instantaneous sight with any kind of accuracy. So the navigator had to maintain the body centered in the bubble and the crosshair for two minutes to average out these errors. The time of the shot was the midpoint, or one minute, after the shot was started. All celestial in an airplane was very carefully preplanned and it took about nine to 12 minutes to take a three-body sighting—two minutes for each shot and two minutes between each shot for stars. After the shutter dropped at the end of the averaging, you needed to align the averaging mechanism and record the Hs. It then took a minute or so to preset the Hs, turn to the body's azimuth, and identify the star. Sun or moon shots took only one minute between sights, since you didn't need to make any changes to the sextant readings or find the body. All sights were precomputed so you could set up the sextant to the Hs and azimuth. Airborne celestial is a whole lot more difficult and not as accurate as marine celestial.

Anyway, I would not recommend using a bubble sextant at sea. A bubble was needed in a plane since the horizon was not usable in the day (way too much dip correction) and not available at night. A standard marine sextant is much more suited for marine celestial than a bubble one and far easier to use. You need only bring the body to the horizon and record the Hs. With a bubble sextant, you need to keep both the bubble and the body centered on the crosshairs—not an easy thing to do at sea. You do not need to use the averaging mechanism at sea because you’ll be taking instant shots. If you use the bubble sextant there is no need to apply a dip correction to the Hs. Without a calibration, there is no way to determine the index error. If there is an Air Force installation nearby, you can contact them to see if they could do it for you. Lacking that, you can always try setting the sextant on a table at a measured distance from a wall and check the sextant elevation against premeasured marks on the opposite wall. For example, using standard trig, if the sextant is five feet from the wall, then a mark five feet up the wall from the same height as the sextant eyepiece is above the floor should give an elevation of 45 degrees. If you read anything else then the difference is the index error for that elevation. You can then use trig to determine the height on the wall for sextant elevations of 10, 20,30, 50 and 60 degrees. The errors will be different, so you need to have an index correction table made up for the different Hs readings, just as you have different deviation corrections for the various compass readings. For an Hs between these calibration points you will need to interpolate.

The bottom line is that you will most likely get as accurate a reading from a plastic sextant as you will from this antique bubble sextant. My advice is to forget about the bubble sextant and get as good a marine sextant as you can afford, even if it is plastic.

If you insist on using the bubble sextant, you may want to buy a copy of AFM 51-40, Air Navigation, from Celestair, 416 S. Pershing, Wichita, KS, 67218 for $28. You can phone them at 1-800-727-9785. This book has a chapter on bubble sextants. Also ask to speak with Ken Gebhart (the owner) since he may be able to help you out with a calibration.

 


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