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    Re: Problem shooting index error
    From: Frank Reed
    Date: 2019 Oct 13, 16:17 -0700


    Finally getting to your question, yes, for some types of observation, the deeply-merged field of view of the 7x35 scope can be confusing. But for most observations, it's distinctly desirable. There is also one adjustment for a scope like this that you need to check.

    Some scopes, like your low-power model, create a view through the sextant that only slightly merges the left, horizon side, and right, reflected side, of the field of view. If you're using the sea horizon to test or eliminate index error, this is a good thing. You see the horizon on the left, clear and sharp, and the horizon on the right, also clear and sharp, with a relatively narrow overlap zone where the two view are superimposed. There is a clear "step" from left to right. 

    If you use a scope like the 7x35 monocular, or similarly if your horizon mirror is one of those "full-view" (not split) glasses that's semi-reflective all the way across, you will find that there is a much wider, or possibly full field of view, merger zone where the two sides, direct and reflected, overlap. If you're trying to assess or reduce index error, this is visually more problematic. You don't see that nice "step" from one side to the other. Instead you see two slightly-separated horizon lines. It can be visually confusing. With a 7x35 scope like that you will get a better estimate of index error, by nearly a factor of two, but the sea horizon will be a relatively difficult choice for the process. Instead, as I have noted in other posts, find a radio tower two or three miles away. Place your sextant on its side on a table and look at the tower as a "stand-in" for the sea horizon. You should find that it's easier to align the two sides. 

    Note also that the wide merger of the two sides of the field of view that you find with the 7x35 scope is much more convenient for nearly all other observations. If you're trying to place the Sun on the horizon, it can be much easier visually with that high power scope.

    There's an adjustment that you may need to consider. The field of view of the 7x35 scope is relatively small, and by design the scope can be moved in or out, towards the sextant frame or away. If it's either too close to the frame or too far, and if you have a standard split-horizon mirror, you may find that you're only looking at one side of the full field of view. You can test this relatively easily by setting the sextant to some moderate angle, like one degree. Next look at a landscape, like some hills and trees in the distance (this does not have to be as far as necessary for an index test... your trees could be a few hundred yards away for this). Do you see two distinct copies: one left and one right, displaced vertically with respect to each other? If you only see one copy, then there are two possibilities: 1) did you forget to flip the shades out? (happens to the best of us now and then), or 2) the scope is too high or too low relative to the frame. If so, adjust it on its slider (riser) and try again.

    As others have noted, you could just ignore the whole issue. Zero out the index error using the low-power scope.  Then swap in the 7x35 and shoot your sights. Index error is a function of the relative positions and orientations of the mirrors. The sextant scope observes the error, but it doesn't change or add to it. More magnification gives you a better view. The high-power scope can provide a better test of index error, but you don't really need that additional accuracy.

    Frank Reed

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