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: On The Water Trial of Digital Camera CN
    From: Marcel Tschudin
    Date: 2010 Jun 22, 10:56 +0300

    George,
    
    You are correct to be cautious. Your reflections are correct. Whenever
    you start to use a new measuring instrument, it is correct to question
    all the possible effects which might lead to errors. Some of those
    possible influences you might be able to estimate some others may be
    not. However, at one point you start to do measurements and calibrate
    it. If the instrument shows repeatedly useful results you start to
    gain confidence in it. By doing more and more measurements you learn
    more and more on its behaviour under different conditions. With
    sufficient experience one can then even adjust the initial calibration
    to those different conditions which, by experience, lead to slightly
    offset results.
    
    Like there are different quality sextants, there are also different
    quality camera-lens-systems. Greg's experience shows that his
    camera-lens-system and its calibration has a sufficient quality in
    order to be used for CN. It could well be that this would even be
    possible with a cheaper camera, provided it has been well calibrated.
    The only thing I can recommend: Try it, if it's of interest to you!
    
    Marcel
    
    
    On Tue, Jun 22, 2010 at 2:48 AM, George Huxtable  wrote:
    > I would like to add a few more cautionary words to those from Marcel about
    > the claims for precision "close to that of a metal sextant" made by Greg.
    >
    > That observation relies on the comparison with a calibration, made at a
    > different time and under different circumstances.
    >
    > What about thermal expansion, for example? The CCD array will expand with
    > the coefficient of Silicon. The lens-array spacing will expand according to
    > the combination of metals and plastics that it's constructed from. The
    > ratio between them will determine the pitch of the cells, in arc-minutes.
    > That will vary, if the temperature at calibration differs from the
    > temparature at observation. I do not know if the effect will be a
    > significant one, but it needs to be checked, before making claims for a
    > precision that approaches that of a sextant. A sextant has its component
    > materials chosen with some care, to ensure stability of calibration as the
    > temperature changes.
    >
    > Marcel asked about the horizon. How was the level of the horizon defined? I
    > presume that the upper edge of the horizontal thin streak of light
    > (actually, two streaks, with a darker zone between) has been taken to be
    > the horizon. But that upper streak is several pixels wide, with a somewhat
    > indeterminate upper edge. And can we be sure that this upper edge
    > represents the true horizon, and is not simply a ripple-line as sunlight
    > dances on the water? The problem here is that the horizon image is
    > necessarily very underexposed.
    >
    > The algorithm for correcting distortion has assumed that the angle being
    > measured has been equally split, above and below the centre point of the
    > array. I imagine that the correction is rather tolerant and forgiving about
    > small deviations, but how was that equal split obtained? Presumably, just
    > by eye-estimation in the viewframe of that SLR camera. It can, of course,
    > be checked after-the-event, and perhaps some correction might then be made
    > later, if found necessary.
    >
    > Measurements are very reliant on the optical quality (no prismacity) of the
    > polaroid filter pair. But if it was arranged that the orientation of these
    > filters had been marked, and then set up the same when calibrating and
    > observing, that could cancel out
    >
    > It would be interesting to deduce the Sun diameter, both vertically and
    > horizontally, to see how it compares with its almanac value, having made
    > allowances for tangent distortion, different in the two directions. If
    > there's agreement, it would provide confidence that it's the true edge of
    > the Sun disc that is being detected, and not one that's been expanded by
    > overexposure.
    >
    > Finally, the accuracy of the spacing of the elements of the CCD array is
    > being taken on trust, presuming a very high degree of precision. Are
    > specifications available to confirm that these elements are exactly where
    > they are supposed to be, with no position scatter or distortion? Perhaps
    > they are indeed placed to sufficient accuracy, but before relying on that,
    > perhaps some tests should be called for.
    >
    > All these matters should be at least considered, and weighed up, before
    > making claims about comparison with a sextant.
    >
    > 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.
    >
    >
    > ----- Original Message -----
    > From: "Greg Rudzinski" 
    > To: 
    > Sent: Saturday, June 19, 2010 5:22 AM
    > Subject: [NavList] On The Water Trial of Digital Camera CN
    >
    >
    > See the link to a morning Sun image taken while at anchor in Smugglers
    > Cove, Santa Cruz Island.
    >
    > Camera: 10 MP Canon Rebel SLR
    > � � �Lens: 50mm SMC Pentax 1:2
    > Polarizers: Vivitar linear
    >
    > Settings: ISO 200, F22, 1/4000s, Infinite Focus, Manual Mode
    > Date: 6/18/2010
    > Time: UT 14:30:23
    > Temp: 62�F
    > Press: 1013 mb
    > H.E. �: 7 ft.
    > GPS La: �34� 01.1'N
    > GPS Lo:119� 32.4'W
    >
    > Pixels: 3093 Horizon to the upper limb
    > Hs by graph: 3093 x .3749=1159.6'=19�19.6' (int 0.4'T)
    > Hs Tschudin Formula: 3093x.371+12.4=1159.9'=19�19.9' (int 0.7'T)
    > Hs Huxtable Formula: 120xARCTAN3093/18161.8=1159.8'=19�19.8' (int 0.6'T)
    > Az: 74.0�
    >
    > Digital camera CN summary
    >
    > Pros
    > 1. Accuracy close to that of a metal sextant.
    > 2. Automatic UT time stamp of image.
    > 3. Very quick observation time.
    > 4. Light weight and compact.
    > 5. Observations saved to laptop.
    >
    > Cons
    > 1. Not waterproof.
    > 2. Only good for day time Sun and Moon observations up to 24�.
    > 3. Precision 0.4' vs 0.1' of a sextant.
    > 4. Requires editing on a laptop.
    >
    > Conclusion
    >
    > Digital camera CN is an intensely interesting way to obtain a Sun or Moon
    > LOP with the bonus of improving photo editing skills.
    >
    > Greg Rudzinski
    >
    >
    > ----------------------------------------------------------------
    > NavList message boards and member settings: www.fer3.com/NavList
    > Members may optionally receive posts by email.
    > To cancel email delivery, send a message to NoMail[at]fer3.com
    > ----------------------------------------------------------------
    >
    > 
    File:


    > > > > >

       
    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