MA-1 bubble sextant
From: Gary LaPook
Date: 2007 Feb 19, 18:59 -0800

I have 6 Kollsman periscopic sextants, 1 MA-2 (just like the
periscopic but without the periscope), 1 A-7 and 1 A-10 all of which
use a bubble for reference. I just purchased on E-Bay an MA-1 which is
very similar to the MA-2 but uses a pendulous mirror instead of a
bubble. Does anybody have any information about this type of sextant
or the similar periscopic ones with a pendulous mirror? Since the
bubble ones are much more common is the reason for this that the
bubble ones work better or are they less expensive to manufacture? Any
source for a manual for the MA-1?

I removed the modern type of electronic averager from one of my
periscopic sextants and installed it on the MA-1 and it works very
well except the averager eats batteries ( 3 nine volt batteries in
series) since it uses 150 ma. just for the averager. The illumination
on the MA-2 and the other Kollsman sextants for night observations
only draw 75 ma. I thought of using this averager on the MA-2 but
decided against it because you can use the MA-2 during the day without
any electricity since the averager is mechanical. The MA-1 always
needs power to light up the reference line (65 ma) so it made sense to
put the electronic averager on it.

I am taking a series of sights with both of them to compare their
accuracy. I especially like the reference line in the MA-1 and use it
for lower limb sights of the sun. With the bubble sextants you have to
center the sun in the bubble which may make it less exact. I plan to
go flying next weekend and will take several sights with the MA-1 and
the MA-2 to see how they compare. I have taken many shots with the
MA-2 and many many shots with the A-10 in flight and have been happy
with the results. I will let you know how these shots work out.

A link to a site with description of these sextants is:


http://home.earthlink.net/%7Es543t-24dst/airnav/index.html#PERISCOPIC

The following was copied from this web site.

MA-2 Sextant

The MA-2 is a hand held bubble sextant for use in aircraft where space
will not permit installing an automatic or periscopic sextant. The
main body of the sextant, including the controls, averager, and bubble
control, are identical to the periscopic sextant. The MA-2, here
illustrated, is also provided with a hook for use in astrodomes of
older aircraft. The sextant has a field of vision of 12½° and is
capable of measuring altitudes from -10° to +92°. The averager
provides an average altitude accurate to two minutes of arc over any
period of time from 30 seconds to 2 minutes.

MA-2 exterior
COMPONENTS

The MA-2 sextant has four main systems: the horizon, the optical, the
altitude recording and averaging, and the electrical system. All
systems, other than the optical system, are identical to the
periscopic sextant. For information on these systems refer to the
section on the periscopic sextant.

MA-2 optical path

Optical System. Light entering the objective window passes through the
index prism, objective lens system, and filters, as shown in
accompanying diagram. It is then directed to the eye by a pentaroof
fixed prism and forms a real image at the focal plane of the field
lens. On the field lens is a reticle, consisting of a vertical and
horizontal line, which indicates the center of the field. The focal
plane of the eyepiece lens system coincides with that of the field
lens and reticle. The eyepiece may be adjusted for focus by rotating
the knob directly behind the rubber eyepIece.

Daylight for the illumination of the bubble enters through the
diffuser at the top of the bubble chamber. The image of the bubble
passes downward through an objective lens and through the pellicle to
a retro-reflector. I t is then reflected to the pellicle which in turn
reflects it to the focal plane of the main optical system. If the
diffuser prevents the entrance of sufficient daylight for the
illumination of the bubble, artificial illumination is provided by a
28-volt lamp controlled by a rheostat.

Checking the Sextant. Preflight, inflight, and postflight procedures
are identical to those given for the periscopic sextant.
MA-1 Sextant

The MA-l sextant, as illustrated, is very similar to the MA-2. The
major difference is the artificial horizon system. Where the MA-2 uses
the conventional bubble artificial horizon, the MA-l utilizes a
pendulous mirror to reflect the image of a horizontal line that is
used as an artificial horizon.

MA-1 exterior

As shown in the optics diagram of the MA-l, illumination for the
horizon system is supplied by a lamp located at the top of the sextant
body. The intensity of the light is controlled by a rheostat on the
front of the sextant. Light is projected downward through a condenser
lens and a reticle pattern. This pattern is projected through the
pellicle and continues to the pendulous mirror chamber. The mirror
pattern is reflected back to the pellicle which in turn reflects it to
the plane of the main optical system.

MA-1 optical path

Preflight, in-flight, and postflight checking procedures are identical
to those given for the periscopic sextant, except for the bubble
adjustment.

Gary LaPook


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