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    IE in bubble sextants: not so easy
    From: Tony Oz
    Date: 2021 Oct 6, 05:03 -0700


    Recent IE/IC discussions reminded me of my own experience with my ИМС-3 - an enhanced SOLD KM2 clone. Its' bubble chamber is very much like the SOLD's one.

    The ИМС-3 is capable to show the natural horizon - through the bubble chamber. This is done to make IE adjustment easier. My ИМС-3 has IE almost zeroed: the direct view of the horizon, the reflected image (of the horizon) and the bubble's center - all coinside (provided I sit as low as ~half-meter at the sea shore).

    I also have an ИАС-1 and ИАС-1М, - early clones of the same original sextant. Neither is capable of viewing the natural horizon through their bubble chambers.

    As an attempt to check the quality of these extra bubble chambers - I swapped the original ИМС-3's with those. Neither of them gave the above mentioned coinsidense in the ИМС-3. What's worst - it is not possible to adjust the main mirror to achieve the IE == 0 because the bubble itself does not lay on the directly seen natural horizon!

    This explains why the bubble chambers have serial numbers - to be able to install them only in those sextants where their sitting places and their studs were properly adjusted - to give the correct local vertical.

    Because one can see the the natural horizon through the ИМС-3, and because the reflected image rotates against the direction of sextant roll (I mean the tilt, yaw and roll here), it is possible to bring the sextant to verticality by rolling it untill the images become parallel (and coinside). The best bubble chamber - at that moment - has the bubble exactly at the center of the grid marks. This - on ИМС-3 - is happening only with its' original chambers. Rogue chambers have bubbles far aside.

    So, the idea I wanted to explain here - if the chamber may be swapped - always check if it was adjusted for the given sextant. A rogue/faulty chamber will make a sextant to deviate from the perfect verticality, - like when one did not swing an arc by rolling a marine sextant. A systematic error will occur. The mirror's rotation axis will not be horizontal.

    This also applies if someone tried to take the glass out. The re-assembled chamber needs to be checked if the initial vertical was not altered (by new washers sitting differently).

    Just my two-pence.

    Warm regards,
    60°N 30°E

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