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    Re: Sextant calibration in the workshop
    From: Frank Reed
    Date: 2008 Jan 13, 19:21 -0500

    Two weeks ago, Bill Morris wrote about calibrating his sextants. Just
    fascinating stuff. Sorry I didn't reply earlier...
    Bill, you wrote:
    "Members may wonder about the precision of an autocollimator. A basic Hilger
    and Watt Microptic autocollimator's least graduation is 0.2 seconds and with
    a little practice and a mirror of good quality, readings can be repeated to
    within about 0.3 seconds. Even at first acquaintance, repeatability to
    within 1 second is easy. Using a photoelectrical readout, precision is about
    five times better. Its accuracy is of a similar order."
    Wow. I want one. :-)
    "Even so, in a series of 24 repeat readings with a SNO-T
    sextant, 95 percent of readings can be expected to fall within a range
    of 3.7 seconds."
    That surely is close to the practical limit for a sextant. Somebody wake up
    Alex E. (he seems to be busy with work again). I'm sure he would be
    interested in all of this.
    And you wrote:
    "In setting the sextant micrometer, it must always be rotated in the same
    direction to avoid backlash errors. In sextants like the SNO-T and
    Freiberger, it does not matter in which direction, as long as it is always
    the same, but in the BuShips Mark II and sextants of similar construction
    including Tamaya and clones, there is only one correct setting direction.
    As it has only one thrust bearing, preloaded by a rather weak spring, it
    must be rotated so as to load the thrust bearing, not the spring."
    Since I have a Tamaya clone, can you elaborate on this? I'm not clear which
    direction you're talking about.
    "My 1981 instrument meets Alex's specification but not mine: 15d. +1"; 30d
    +1"; 45d +4"; 60d +7"; 75d +8"; 90d +11"; 105d +9"; 120d +8"."
    That's excellent.
    "My Mk II s/n 14176, 1942 in near-new condition with a certificate from Long
    Beach Shipyard dated 7 January 1986, does well too, except above 90 degrees.
    I give the original certificate figures followed in brackets by my own,
    which are each based on the mean of three careful readings: 15d +3"(+1");
    30d -14"(-12"); 45d -17"(-16'); 60d -25"(-22"); 75d -30"(-31"); 90d
     -33"(-40"); 105d -12"(-31"); 120d 0"(-30"). For most practical purposes
    except lunars, the differences are insignificant."
    And even for lunars, that shows no practical deviation from the original
    certificate except for angles of 90 degrees and above. Considering that I've
    seen measurable, repeatable deviations of over 90" of arc, these are still
    very good numbers. And of course, with that kind of calibration, your
    sextants are effectively perfect (after correction) for all navigational
    purposes including lunars. I'm green with envy. ;-)
    You concluded:
    "If someone can tell me how to add attachments to the list I will post a
    diagram and a photograph to make the process plainer."
    I would enjoy seeing a photo. I gather that you post your messages from the
    google groups web page? If so, there's no means to include attachments
    directly from there. About two-thirds of the list members send their
    messages by email (and you can do this, too) and read the messages by email,
    too. So to send an attachment, just compose an email to
    NavList-AT-googlegroups.com. Some group members very much dislike
    attachments, so try to keep them small (avoid wrapping images up as "pdf"
    files, try resizing images and saving them small, etc.). If you prefer, you
    can email anything you want to attach to me (or any other cooperative member
    of your choice --ask first) and I can post the attachment to the group.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

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