Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    Re: Which diameter of the sun in digital photos ?
    From: Marcel Tschudin
    Date: 2009 Aug 21, 21:19 +0300

    George, you wrote:
    > A discrepancy of 8% is hard to accept. Something is wrong, as Marcel
    > suspects. And such a big discrepancy should be easy to check for.
    Yes, the discrepancy is big. I thought I would have a reliable scale
    from my sunset observations. This scale is now put in question. It
    turns out to be not so easy, that's why I start to ask around, hoping
    that someone may have done a similar experience and may have even
    found the reason for it.
    > I suspect the angular calibration of his camera Perhaps he should check that
    > more simply, in terms of the angle subtended by an object a few metres away,
    > rather than by relying on distant topography. Near enough to that he can
    > check dimensions with a tape, yet large enough so that the camera focus
    > doesn't need to change from infinity, and also so that uncertainty in the
    > position of the effective optical centre of the lens is unimportant (within
    > much better than 8%).
    One reason could be distortion, but I wouldn't attribute it to barrel
    or pincushion distortion for the following reason: These optical
    distortions are important when referring to all of the image. The 6x
    zoomed pictures of the sun or the geographical feature (an island with
    a well defined shape) use only a very small part of the full picture
    size and they are usually near the centre. With the resolution used
    for those images the full size of the pictures is 958 x 1278pixels
    compared to the size of the island about 60 x 210pixels or the
    diameter of the sun about 105pixels. Within this small range these
    distortions are likely to be negligible.
    It doesn't seem to be a distortion of the computer screen when
    measuring the size of the geographical feature with GoogleMap. In
    order to verify this I made the measurement also with GoogleEarth
    which has a tool for measuring distances; the result was the same.
    > Alternatively, even though Marcel considers that the Sun image is not
    > overexposed, perhaps it is, and he is looking at a bright halo of light
    > around it.
    The scale derived from the sun is a statistical result from over
    hundred measurements, having a standard deviation of a little bit less
    than +/-1.5% or pixels. Indeed, I do also have overexposed photos, but
    they don't enter this statistics. The colour of the sun's
    circumference is a good indicator.
    Any suggestions for possible reasons are welcome! In the meantime I
    try to get a better estimation for this difference.
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting Code:

    Custom Index

    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