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: Venus
    From: George Huxtable
    Date: 2005 Dec 12, 15:59 -0000

    About the dangers of making telescope observations close to the Sun, I
    wrote.
    
    > "As we have  discussed before, the telescope doesn't make a Sun image
    > intrinsically  brighter; only bigger, on the retina. Nevertheless, the
    > brightness of any Sun  image on the retina is enough to quickly cause
    > irreverseable damage. Be  warned!"
    
    and Frank has responded-
    
    > I agree with that. Isn't that a bit of a change of heart for  you? I
    > thought
    > that you had previously argued that this fact that surface  brightness
    > can't
    > be changed by an optical system (which is very true) meant that  the
    > threat of
    > eye damage from looking at the Sun through a telescope  (specifically a
    > sextant telescope, in previous discussions) was no greater than  the
    > threat of eye
    > damage when looking at the Sun for the same period of time  with the naked
    > eye.
    
    . No change of heart. When that subject has cropped up before, I have been
    careful to state that
    looking at the Sun with the naked-eye is itself dangerous, and must be
    avoided. What I have pointed out is that a telescope can fry a bigger patch
    of the retina, in the same time that a naked-eye Sun fries a much smaller
    patch, though the intensity at the retina is no greater. I have also said
    that that argument applies to the retina only, not to the exterior surfaces
    such as the cornea. If that message hasn't come across, then I have failed
    in my intention.
    
    It is certainly true that when employing a large objective with great
    magnification, as in Frank's case, there are considerable dangers in
    overheating  the cornea, the iris and lens, also the eyelids (which will
    close as the blink-reflex occurs).
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    

       
    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)

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