Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
    Re: A noon fix inland with an peri-sextant
    From: Brad Morris
    Date: 2018 Jan 9, 13:21 -0500
    David, you wrote
    Also, now the colder weather’s arrived, I was beginning to notice some misting inside the sextant and the mounting getting stiffer, so I decided to get the 28V heating to the mounting and sextant working.  The trouble is it was designed for use at high level, and on the ground it gets rather hot.  This might also be affecting the calibration.

    But of course.  The Thermal Coefficient of Expansion has certainly affected your instrument calibration.   Undoubtedly so.  A sextant left out in the sun will be affected, and so will yours, more so by the direct heating element.

    You can, of course, control that direct heating element.  It is very likely a resistive heating element.  That is, there are likely coils and/or resistors inside, positioned to heat the sextant.  

    A simple method would be to place a dimmer switch in series with the heating circuit.  Dimmer switches are readily available from any home hardware store.  In practice, it will work as an additional variable resistor in your heating circuit, reducing the heating efficiency!  So it wouldn't get so dang hot.

    With a bit more effort, you could add a circuit that senses the internal temperature of the sextant and applies the voltage to the heating element upon demand.

    In either event, knowing the temperature of the sextant should relate directly to the correction.  You could generate a table of sextant temperature vs correction.  Note that this will account for ambient temperature, as you will be using the actual temperature of the sextant, not the dimmer switch setting!

    Brad


    On Jan 9, 2018 12:45 PM, "David Pike" <NoReply_DavidPike@fer3.com> wrote:

    Antoine

    Sorry for leaving my reply to your question yesterday until last.  I had to jog my memory a bit.

    You said you applied no instrument corrections.  I used raw Hs values for the graph and applied -2 to the graphical Mer-Pass Hs to get Mer-Pass Ha.  If I hadn’t applied -2 my found latitude would have been 5307.7 which is the same as your calculated 5307.6, so we’re both in the same boat.  I’ll return to this later.

    I only applied one refraction correction at the very end to the Mer-Pass Ha.

    As far as I recall, I was attempting to put the Sun in the centre of the graticule of the pendulous reference.

    You don’t need to apply dome correction with a periscopic sextant, they have optical glass windows out in the slipstream with gold film heating to keep them free of hoar frost.  Therefore we can ignore the remainder of that paragraph except say that the way I remember how you should apply dome refraction if you needed to is to imagine the thinning of the dome towards the top as being like the top half of a convex lens.  A light beam will be bent down towards you, so the star will appear to be higher than it actually is.

    So before applying instrument error, we’re both about 2.6 miles south of where we should be.  Assuming I haven’t got a chronic personal error, it has to be the instrument error correction, and I can think of two possible reasons.  Firstly Hughes MkIX series aircraft sextants which led to the Smith’s Mk2 series have stop settings from -10 to about 80, i.e. -10 to 3, 0 to 13, 10 to 23 and so on, because of the limitations of the clockwork averaging mechanism, which can only count from about 0 to 13degrees.  You really need to calibrate for each stop setting, and I’m nowhere near that stage yet with this sextant.  I used -2 because that’s roughly what I’d been using for much higher altitudes earlier in the year.  Also, now the colder weather’s arrived, I was beginning to notice some misting inside the sextant and the mounting getting stiffer, so I decided to get the 28V heating to the mounting and sextant working.  The trouble is it was designed for use at high level, and on the ground it gets rather hot.  This might also be affecting the calibration.  What do they say “A bad workman ……….”?Hope this helps.  DaveP

     

    View and reply to this message

    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site