© Brooke Clarke, 2001 - 2014
3207025 Optical System for Periscopic Sextant
This aircraft sextant was used for celestial navigation. I think it has been replaced by GPS and/or inertial navigation systems.
The patent is dated 1945 and the sticker on the sextant is June 1986. This instrument is a masterpiece of mechanical and optical engineering.
The bubble is there and can be controlled by the knob.
I think the idea is that the person sighting a star or the Sun, keeps it in the cross hair and at the same time keeps the bubble centered.
As the elevation control is moved up and/or down the clockwork mechanism is averaging the elevation over a period of time.
When the elevation dial is at 90 you are looking straight up and with it set for 0 you are looking at the horizon. The smallest division on the elevation counter is 1 arc minute ( 1/60 of a degree).
Note you can only see out the periscope after the clock lever is pressed, winding the clock AND pushing the button to start the averaging.
How you get the averaged answer is a mystery.
I have not wanted to break the seal, but if you did I expect that the clockwork inside would be very interesting.
There are two GE 327 lamps easily accessed behind metal covers. One illuminates the bubble and the other the azimuth scale on the mount.
The Filter Wheel (Fig 5) has 8 positions: 1= no filter, 2 = green, 3 = red, 4 = green+ND1, 5 = ND1+red, 6 = ND2+green, 7 = ND3+red, 8 = ND3+green. The Neutral Density filters starting at no. 4 are strong enough that you can look at the Sun. It's image is about the same size as the bubble.
To hold the sextant some type of support would be helpful, like a 2x4 with a hole to clear the 1.375" diameter periscope tube and a way to keep it from falling out (maybe use the pin on the side of the tube?). Unlike the sextants used on ships that are hand held, this one needs an external support.
I've heard that the MA-2 sextant was the precursor to the periscopic sextant. The MA-2 required a glass dome which required good optical properties and the dome was not supposed to break or blow out, but that did happen, so the periscopic solved those two problems. See anon. external web page Air Navigation Sextants based on AFM 51-40 (U.S. Air Force manual "Air Navigation") section pertaining to bubble sextants, from the 1955 and 1960 editions.
US Patent 2516187 -Calculating Instrument application Feb. 24, 1945 covers some of the operation of this sextant. (see Fig 6 above)
The key feature is the "averager" that mechanically averages the elevation setting over time.
MIL-S-5897C(ASG) contains operational specifications such as:3.3.1 Sextant. Hereinafter, the term "sextant" shall be construed to mean the sextant proper, Periscopic tube, connecting cable, and watch clip. The sextant shall be a bubble-type sextant built in the form of a periscopic telescope with the periscope projecting above the skin of the aircraft. Provision shall be made for 360 degree rotation of the instrument around the vertical axis and tilt of the sextant up to 14-1/2 degrees minimum from the vertical axis. The sextant shall conform essentially to Standard MS28011.MS28011(AS) consists of three drawings:
A.F. Manual 51-40 Air Navigation has a little information on this sextant. Vol 1 has descriptive info and Vol 3 operational info
- sheet one (shown above) has the layout of the sextant
- sheet two has information about the receptacle that mounts to the ceiling of the aircraft
- sheet three shows the shipping case
ID PlateThe ID palte reads:
Sextant, Aircraft, Periscopic
MS Part No. MS 28011-1
28 Volts A.C. or D.C.
Mfr's Part No. 1471B-01
Mfr's Serial No. 3870
Order No. NOas 52-535
Stock No. R88S0400-050-000
Kollsman Instrument Corporation
U.S. Property 35050-1
And the paper sticker says:
Aerospace Guidance and Metrology Center (AGMC)
Repair Date JUN 16 1986
Newark Air Force Station
Newark, Ohio 43057
The label on the periscope tube says:
Minutes -10 -1 -1 0 0 -.5 45 +.5 0 90 -2 -2
May 2014 - When combining the mount and sextant the crank on the mount does turn the Veeder-root counter but does not rotate the sextant in azimuth. There is a ring that's the lowest part of the mount that says "LOCK --->" but I can not rotate the ring any more to the right. There's an open threaded hole, maybe there's supposed to be some sort of bolt there to lock the azimuth?
Answer: When a sight is to be made the sextant is removed
from the carry case in installed into the mount (while the door is
closed) and the ring at the bottom of the mount is turned.
The sextant is now "locked" into the mount and you can let go
without it falling (I'd rotate it to be sure the ring has really
locked it). At this point when the outer door is opened
there will only be a very small air not a large one like would
happen of the outer door was opened without the sextant being
Prior to making a sighting you set the azimuth using the crank
and Veeder Root counter ( dd_dd_m) which turns the azimuth scale
that's visible in the eyepiece relative to the aircraft centerline
and you preset the sextant elevation angle using the Elevation
knob and the Veeder Root counter that's adjacent (ddm).
When it's time to make the sighting the outer door is opened and
the periscope lifted. There are two pins that hold the
periscope in the up position and these are missing from my
unit. You can see the two pins clearly in the photo below of
a mount installed in a C-133. They are 180 degrees
apart. There's a ramp on the protrusion on the sextant body
that indicates that these are spring loaded and all that's
required is to raise the sextant and it snaps into place.
The pin is pulled out to allow lowering the sextant when the
sighting is complete.
The 2 contact socket is marked 12S-3S.
The other connector has 3 pins and a smaller shell size.
|Perisocpic Sextant D-1 Mount
Mount, Periscopic Sextant
28 Volts A.C. or D.C.
Kolsman Instrument Corporation
Top plate is on aircraft skin.
|Perisocpic Sextant D-1 Mount
Each turn of the crank rotates the periscope
5 degrees in azimuth. The Veeder root counter is in
0.1 degree increments. 000.0 to 359.9 deg range but
there is not stop so you can just keep going in a circle.
The knurled device next to the switch is an aircraft
Swash plate with 4 mounting hole bracket on top side.
When the knurled knob is
unscrewed it allows the
air pressure to be equalized.
Bottom plate has the
|The bottom plate (4 screws)
is level in the aircraft.
|The ramp with hole for spring loaded pin that
I do not have.
||On the opposite side from the ramp there's no
hole for the second
spring loaded pin in the mount. You can see vertical marks from pin.
Maybe there's a second hole on newer sextant models?
Note up arrow that matches arrow on mount (see to right).
Pin on sextant tube fits bayonet LOCK in mount.
|Allignment arrow on LOCK ring.
Bayonet path for pin on sextant.
Azimuth scale inside (o deg), reference mark on outside.
The Veeder Root counter is off by about 4 degrees,
probably an assembly error?
Stabilized optical system,Braddon Frederick D, Mccartney Earl J (Sperry Rand), Jun 6, 1961, 356/149, 359/556, 359/557 - Celestial navigation from submarine periscope - cold war nuke prior to Transit satellite
Mk II Astro Compass
MD-1 Automatic Astro Compass
History of the Sextant - page 2 with various aircraft sextantsCarp Industries - Kollsman out-of-production spare parts - Installation & Operation Instructions, Pub #S332C-300-961 dated 09/61 - Government type high prices
Celestaire - Navy Mark V without Averager - Navy Mark V with Averager -
Garcia Avation - pricey
Helmut Singer Elektronik - stock varies
JANS of London -Instruments - Significant events in the evolution of marine navigational instruments
Kollsman -has licensed Carp Industries. (CAGE 0D9X5) to mfg these systems.CARP INDUSTRIES INC, 4550 U S HWY 1, GRANT, FL 32949NavList: A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
CAGE CODE: 0D9X5, Status: A - Active
DUNS Number: 194909032
Voice Telephone: 321-952-1303
SICs: 3728 Current List of SIC Codes
Date CAGE Code Established: 09/26/1988
Last Updated: 01/04/2001
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page created 6 Aug 2001.