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    Re: Graphs of Lunar Distances.
    From: Frank Reed
    Date: 2010 Sep 28, 17:01 -0700

    Douglas you wrote:
    "It is noticable that any off-centre light rays are refracted slightly by prismatic effect, being displaced slightly. The star aligned on the limb of the Moon is no longer on the limb if the image in the telescope is allowed to drift to the side of the field of view: move back to the centre again and it is back in alignment. The effect is small, but about one or two minute of arc which is serious with lunar distances."

    This is probably a normal effect of the collimation of the sextant's telescope. But you MAY have collimation error if this is large, and from your desscription, it is. To test it: line up the limb of the Moon and some other body at the largest angle you can manage (use Jupiter or the Sun for best accuracy) so that they are exactly touching at the center of the field. Now rock the sextant to the left so that objects are on the extreme left side of the field of view. They should separate slightly. Next shift them to the right side of the field of view. They should separate again slightly (by the same amount and NOT overlap). If you get this symmetrical separation on both sides of the field of view, then you probably do not have any issues with collimation. But if the bodies separate on one side and overlap on the other, or separate by noticeably different amounts, then you have collimation error, and this will throw off your observations in a systematic way. Most of the old navigation manuals described this and emphasized the importance of checking telescope collimation. Luckily, collimation is easy to test and adjust using a variety of simple tabletop techniques. You can also directly observe collimation problems using these methods. Details upon request.

    -FER


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