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    Re: The new guy on the horizon...
    From: Gary LaPook
    Date: 2011 Oct 10, 15:17 -0700
    You can download "Bowditch" here:


    Follow the instructions in it for adjusting your sextant they are on page 267 (278 of the pdf) but Dutton should also show you how.

    Other navigational publications here:



    --- On Mon, 10/10/11, Gary LaPook <garylapook@pacbell.net> wrote:

    From: Gary LaPook <garylapook@pacbell.net>
    Subject: [NavList] Re: The new guy on the horizon...
    To: NavList@fer3.com
    Date: Monday, October 10, 2011, 3:03 PM

    Look at my prior post about this:



    --- On Mon, 10/10/11, Gary LaPook <glapook@pacbell.net> wrote:

    From: Gary LaPook <glapook@pacbell.net>
    Subject: [NavList] Re: The new guy on the horizon...
    To: NavList@fer3.com
    Date: Monday, October 10, 2011, 2:52 PM

    The size of the intercept as calculated from an AP has no meaning as to the accuracy of the shot, merely a coincidence of the error and the azimuth to the AP. The important measure is the distance from the plotted LOP and your actual position.

    First thing to try. From your description of having to turn the leveling screw back after swapping the level end for end indicates that there is an error in the level itself since it should be level in either orientation. So try this, only remove half the error when you turn the level end for end. The bubble should be off center by the same amount in either orientation.

    Since you are using an AP instead of your known position, any error in the measured altitude will be magnified when you plot it if the azimuth from your actual position to the AP is not at a right angle to the azimuth of the LOP of the lop itself, you want that azimuth to the AP to be close to the azimuth to the body (or 180° different.)

    You may want to try the law of cosines method again, I posted a keystroke by keystroke sequence for use with a simple calculator having only 3 memories.

    It is best not to fiddle with the mirror adjusment screws but to just allow for the IE. But you must make sure that the index mirror is perpendicular to the frame using the side adjustment screw.

    --- On Mon, 10/10/11, Randall.F.Morrow@kp.org <Randall.F.Morrow@kp.org> wrote:

    From: Randall.F.Morrow@kp.org <Randall.F.Morrow@kp.org>
    Subject: [NavList] Re: The new guy on the horizon...
    To: NavList@fer3.com
    Date: Monday, October 10, 2011, 12:54 PM

    On artificial horizons:
       Having been frustrated but both the lack of star reflections in liquids and the difficulty of leveling a mirror, I watched Nav-list for helpful comments and enjoyed the lively discussion that ensued.   Mercury seems to be ideal but very expensive and almost impossible to acquire due to environmental regulations.   I have made an aluminum, three screw leveling plate with a 6 ½ inch square first-surface mirror as my a.h..    It is convenient and wind proof but a not very accurate since my Craftsman digital level is only accurate to 1/10 degree.   A hyper accurate level such as a coincidence level costs over $1300.00.   A Freiberger a.h.  runs over $800.00.   Cheaper digital levels run $40.00 and $162.00 for 1/10 degree and 5/100 degree accuracy respectively.  I am stuck with the mirror system for stars but now I use molasses (treacle?) for sun and moon sights.  
    I level each of the three legs at least twice, reversing the level by 180 degrees at each leg to get the most accuracy.   The wing-nut head screws have ¼-20 threads with a 90 degree, (1/4) rotation giving 1/10 degree change.   When I rotate the level 180 degrees it takes about 1/8 turn to get back to zero.   The digital level is easier to see in the dark compared to the standard torpedo spirit levels I started with.  No flashlight required.
    On intercept distances:
       From 7/30 to 10/9 I have performed and reduced and plotted over 400 observations of the sun, moon, Jupiter and 22 of the navigational stars, but with mixed results.   Most of these were done with my leveled mirror.   I have had intercepts as small as 0.5 nm but more often than not they range from 5 to 25 miles.   The LOP accuracy does not seem to correlate well to intercept distance, as I have had LOP’s that run right over my location with “a” of 22 miles and some way off with much shorter intercepts.   (I don’t own a GPS unit so I got my Lat.-Lon. from a web site using my address.)

    On sight reduction methods:    
       I use pub. 229 for sight reductions.   My actual position is about 35* 20’ N and 119* 5’ W.   I will use 119* xx’ for 0-30 minutes GHA and 118* xx’ for 30-60 minutes GHA, to arrive at whole # LHA within 30 minutes of my AP, as suggested in Dutton’s.   Is this correct?    I have tried direct solution using law of cosines but found it much more tedious and error prone than just using the tables.   I use a cheap, very basic calculator.   I use a sight reduction form that I adapted from examples in Dutton’s and Hobbs texts.

    On index corrections:
       My sextant is a 25+ year old Davis Mark 15 with micrometer drum.   I am the sole owner; it just stayed in the box a long time.   Rather than doing index corrections using the sun and counting on that number to be the same after sunset, I always adjust out the IE with the screw on the horizon mirror at the start of each round of sights.   The outside screw takes out the IE and then I adjust the inside screw for side error.   Is this correct?

    On time:
       I love my watch, a cheap radio-resetting digital by Casio that cost less than $30.00.  My watch error entry is always zero.  

    My thanks to all for sharing your knowledge and experience.   I look forward to checking my e-mail for Nav-List posts.

    Randall F Morrow PT
    Senior Physical Therapist
    Ergonomics Safety Consultant
    Chronic Pain Program
    Department of Physical Medicine
    Kaiser Permanente - Kern County - Bakersfield
    Phone:  661-852-3677 (Tieline - 378)
    Fax:  661-852-3516 (Tieline - 378)

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