Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    Name or NavList Code:
    Email:
       
    Reply
    Re: Back sights
    From: Brad Morris
    Date: 2010 Mar 24, 11:36 -0400

    Hi Bill

     

    Angle of incidence = angle of reflection for planar mirrors. 

     

    If the paths are not parallel from either of the horizon mirrors to the index mirror, then the angle of the beam out of the index mirror will be different for the non-parallel paths from horizon mirrors to index mirror.  Since non-parallel paths will not place the object on the horizon (or the star on the limb of the moon) for the same indication of the nonius of the vernier to the arc, a different reading will be had once the contact is made.  There is only one index arm, one index mirror and one arc.  The angular setting must therefore be the same (excluding index error), to achieve the same reading for the same angle, with both foresight and backsight observations. 

     

    In other words, set the nonius of the vernier to 90 degrees.  Assume index error for both the foresight and backsight to be zero.  Now observe a zenith star using the backsight and foresight observation.  For both observations, the index arm does not move.  Yet how can this be read as 90 degrees, if the optical path from the foresight horizon mirror is not parallel to the optical path of the backsight horizon mirror.

     

    I think the paths to be parallel, with an offset between them.  This means that the optical paths strike the index mirror in different locations.  However, since the index mirror is a planar surface, then the angle of incidence and angle of reflection are the same.

     

    Best Regards

    Brad

     

     

    From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] On Behalf Of Bill Noyce
    Sent: Wednesday, March 24, 2010 11:21 AM
    To: NavList@fer3.com
    Subject: [NavList] Re: Back sights

     

    I don't think the optical paths need to be parallel for the fore-sight and the back-sight. The only requirement for the scale to read right is for the back-horizon mirror to be at an angle of 90 degrees from the fore-horizon mirror. It can then be placed almost anywhere, as long as the back peephole then allows the index mirror to be seen in the back-horizon mirror. Remember that the genius of the octant (and sextant) is that it measures the angular difference between the horizon mirror and the index mirror, regardless of how the instrument is oriented.
    -- Bill N.
    ----------------------------------------------------------------
    NavList message boards and member settings: www.fer3.com/NavList
    Members may optionally receive posts by email.
    To cancel email delivery, send a message to NoMail[at]fer3.com
    ----------------------------------------------------------------



    "Confidentiality and Privilege Notice
    The information transmitted by this electronic mail (and any attachments) is being sent by or on behalf of Tactronics; it is intended for the exclusive use of the addressee named above and may constitute information that is privileged or confidential or otherwise legally exempt from disclosure. If you are not the addressee or an employee or agent responsible for delivering this message to same, you are not authorized to retain, read, copy or disseminate this electronic mail (or any attachments) or any part thereof. If you have received this electronic mail (and any attachments) in error, please call us immediately and send written confirmation that same has been deleted from your system. Thank you."
       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Get a NavList ID Code

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

    A NavList ID Code guarantees your identity in NavList posts and allows faster posting of messages.

    Retrieve a NavList ID Code

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

    Email Settings

    NavList ID 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