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    Re: CHO sun-scope collimation collar -- what about vertical?
    From: Bill Morris
    Date: 2010 Apr 3, 13:15 -0700


    There are two adjustments you can make to the telescope, other than focussing.

    The first is collimating the telescope, which means, literally, putting it in line (a medieval mis-transcription of "collineare" is responsible for the spelling). In our context, it means putting the optical axis of the telescope parallel to the plane of the arc. If the two are not parallel, all readings will be too large, by an amount that depends on the reading and the amount of displacement of the axis. The error is not great. For example, if the telescope axis is out by one degree, an amount easily visible to most, the reading will be 36 seconds too large at 60 degrees. You must start by adjusting out perpendicularity error of the index mirror and side error of the horizon mirror.

    The easiest way to collimate the 'scope on land is, with the sextant on its side, to place the two lttle right-angled vanes that should have come with your SNO-T/M on each end of the arc, parallel to the telescope and sight over their tops at some distant object (see attachment). The view of the object through the telescope should be in the centre of the field in the vertical plane. If it is not, make it so by slackening one of the adjusting screws in the collar that holds the 'scope and tightening the other. Don't over-tighten the screws, as it is possible to bend the collar or strip the fine screw threads. There are other ways involving stars and marks on walls, but this is the easiest. If you haven't got the sighting vanes, any two objects of the same height will do, such as a pair of dominoes or hexagon nuts.

    Many modern sextants were supplied with only a Galilean "star" telescope which were not generally provided with means to collimate them. They can still be checked by looking through the eyepiece from a few centimetres away and centring the object in the field of view when the internal pupil of the scope is concentric with the body of the eyepiece.

    The up and down ("rising") movement of the telescope is for adjusting the relative amounts of light entering the scope from the horizon and observed object. This can be very useful when horizon contrast is poor. The eye is not particularly sensitive to light intensity changes, but contrast is, so that when the horizon is poorly defined, raising the scope to see more of the horizon may help to improve the view. The ability to raise or lower the scope is also helpful when determining index error by horizon or star, to get the two images of roughly the same intensity in the centre of the field of view.

    Bill Morris
    New Zealand
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