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    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 12�� 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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