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
    or...
       
    Reply
    Re: Off Center Sextant Scope Observations
    From: Greg R_
    Date: 2007 Nov 22, 20:51 -0800
    I think you just handed me my next "experiment" for whenever I get out to do some sights next time...  ;-)
     
    Seems like I always move the sun (or whatever celestial body) into the center of the scope's field of view before taking the final measurement, but now that I think about it I can't really be sure... so I think I'll try some "off-center" sights just to see what happens (Astra metal and Davis plastic sextants, though if our theory is correct it shouldn't make any difference for either one).
     
    > It shouldn't make any difference where they are in
    > the field of view since both rays of light, from the
    > sun and from the horizon, will travel through the
    > same spot in the optics of the telescope
     
    Agreed - since they're both going to be affected by the same error (refraction, or otherwise) when both index and horizon light rays go through the scope, then they should both appear in the same relative position of the scope. The only other variables would be the mirrors and the shade(s), hopefully both are acceptably "flat" to where any difference between the light paths where they enter the scope should be minimal. At least the way I see it...
     
    --
    GregR
     
     
     
    ----- Original Message -----
    Sent: Thursday, November 22, 2007 8:01 PM
    Subject: [NavList 4018] Re: Off Center Sextant Scope Observations

    Gary wrote:
    Or, put the lower limb on the horizon near the bottom of the sextant field, as you usually do, then quickly move the sextant so you are looking through the top of the field of view and the sun should still be on the horizon. It shouldn't make any difference where they are in the field of view since both rays of light, from the sun and from the horizon, will travel through the same spot in the optics of the telescope.

    gl

    Greg R. wrote:
    I haven't researched this in depth, but my instinct is would it really matter? Assuming you're talking about some sort of refraction error (through the sextant scope optics, not the atmosphere), since you're using the sextant's mirrors to make the lower limb tangent to the horizon wouldn't both be refracted by pretty much the same amount if viewed at that point (or really any point) of the scope?  
     
    But you can probably answer you own question - how close do your observations come to a known GPS position for the same location? If you can get to within a mile or so consistently from a shore location (no matter what part of the scope you use for the sight) I'd say not to worry about it.  :-)
     
    --
    GregR
     
     
     
    ----- Original Message -----
    From: "Greg Rudzinski" <gregrudzinski---.com>
    To: "NavList" <NavList@fer3.com>
    Sent: Thursday, November 22, 2007 4:39 PM
    Subject: [NavList 4015] Off Center Sextant Scope Observations

    >
    > What happens to an image of a celestial body when viewed off center in
    > a sextant scope? I have a bad habit of using the lower half of the
    > field of view when taking lower limb observations of the sun. I
    > suspect that this increases the observed altitude slightly.
    >
    > Greg Rudzinski




    --~--~---------~--~----~------------~-------~--~----~
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com
    -~----------~----~----~----~------~----~------~--~---

       
    Reply
    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)