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: Digital camera: stars in daylight
    From: Marcel Tschudin
    Date: 2010 Sep 14, 12:14 +0300

    George, regarding your last post. We are at the very beginning. We
    only just started to see what can be measured with a camera. Greg had
    his 50mm calibrated for the centre line only. Frank showed that a
    photo can be taken from Venus during daytime. There is still a lot to
    be explored. Let's advance step by step and see what we arrive at.
    
    Marcel
    
    
    On Tue, Sep 14, 2010 at 10:08 AM, George Huxtable  wrote:
    > Marcel wrote-
    >
    > "George,
    >
    > IMHO, the question should not be whether a camera can replace a
    > sextant one to one, but rather what can be done with a camera, and
    > only then you may eventually find what this can be useful to."
    >
    > ==============
    >
    > Presumably, that relates to my recent postings on the subject of whether a
    > camera can make a useful observing instrument at sea, and specifically
    > about Franks notion that it could be used to obtain 3-star fixes at sea in
    > daylight.
    >
    > I was doing my best to bend over backwards in relaxing the standards that
    > would normally be expected from a sextant, in these respects-
    >
    > 1. Angular range. "Ideally, an altitude instrument would have an angular
    > span of 90�, as an octant does. But that's asking a lot of a wide-angle
    > lens system, and perhaps a limit of, say, 50� might be acceptable; to take
    > in most, though not the upper part, of the sky, together with the horizon."
    >
    > Giving up a sextant's ability to measure altitudes of objects above 50� is
    > quite a sacrifice, but one that seemed worth making if it might allow a
    > camera to be used. If the altitude range was reduced much below 50�, that
    > would make Frank's proposal for 3-star daylight fixes even less plausible.
    > But if Marcel considers that I'm being unreasonable, perhaps he will
    > suggest a practical limit on angular span, which would still allow such a
    > sea-camera to be an instrument of general use, rather than one that only
    > comes out of its box in special circumstances.
    >
    > 2. Angular resolution. In an earlier posting, on 11 September, I had
    > written-
    > �"Is Frank, here, discussing what's presently possible, in a navigational
    > context, from the deck of a vessel, in ordinary sea-conditions? Is he
    > claiming that daylight shots are possible, in such conditions, that show
    > such stars or planets when at a respectable altitude, with a clear horizon
    > below in the same shot? With angular accuracy and resolution of, say, a
    > (very) few arc-minutes, in the angle between them? ..." Indeed, Frank's
    > answer was "yes", so he appeared to consider those as reasonable
    > requirements.
    >
    > Marcel may note that the suggested precision, of "a (very) few
    > arc-minutes", is relaxed, considerably, from the fraction of a minute that
    > we all expect of a sextant. Is even that asking too much? If he thinks it
    > is, then perhaps he will give us a notion of what we should expect from
    > such a camera.
    >
    > To perform the task that Frank was proposing, a camera would need many of
    > the properties of a sextant. However, that proposal seems to me fanciful.
    > If Marcel sees a more limited role for a sea-camera (which might well
    > apply), perhaps he will tell us what it is.
    >
    > George.
    >
    > contact George Huxtable, at �george@hux.me.uk
    > 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?

    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