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: SNO-T "night" sight tube?
    From: Bill Morris
    Date: 2018 Nov 22, 11:53 -0800

    Brad,

    You write that anti reflective coatings attenuate the light transmitted through a lens. You may like to reflect on the following.

    Around  8 to 10 percent of light may be lost by reflection at the surfaces of a lens, depending on the angle of incidence and the refractive index of the glass, and in a two lens system, the transmission losses may exceed 20 percent. The corresponding loss of brightness would not be perceptable (we can just about perceive a halving of brightness), but it  would result in a disproportionate loss of contrast, important for example when there is poor contrast at the horizon.

    Some of the light reflected by one surface may be reflected at a second, with a net result that it is returned in its original direction. But there is no guarantee that it will come to the same focus as the light that has not been reflected, resulting in ghost images. If it arrives nowhere near an image plane, it introduces diffuse illumination called flare.

    The coating on the SNO-T lenses appears to be purplish blue in reflected light and reduces the co-efficent of reflection to around one percent for visual work in daylight.

    Kind regards

    Bill Morris

    Pukenui

    New Zealand

       
    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