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    Re: Star CN with DSLR Camera
    From: Paul Hirose
    Date: 2016 Jan 28, 14:18 -0800

    On 2016-01-27 10:29, Bob Goethe wrote:
    > Depending on the focal length you choose, you are going to get a different 
    number of pixels per unit of angle in the center of the field-of-view than 
    you are out at the edges of the picture.
    That's true. In fact, the second part of the sentence is correct
    regardless of the focal length. However, with a long enough lens or
    small enough area of the photo, the variation can be negligible.
    > My suspicion is that you will get the most consistent results if you choose 
    what used to be known as a "normal lens" i.e. 50 mm focal length for a 35 mm 
    camera.  What that is in a digital world, I have no idea.
    Traditionally, a "normal" lens has a focal length about equal to a
    diagonal on the negative, which is 43 mm for a 35 mm camera. On a DSLR
    it depends on sensor size. There are several common sizes and
    considerable difference between them. My camera has a sensor smaller
    than a 35 mm film frame, in almost an exact 1 to 1.5 ratio, so I set 35
    mm focal length on the zoom to get the view of a normal lens.
    Beginning in the 19th century, many methods have been proposed for
    determination of position or time by photograph. In the Monthly Notices
    of the Royal Astronomical Society, January 1895, Captain E. H. Hills
    explains his photographic longitude method.
    "The question of the practicability of some method for the determination
    of longitudes by means of photographs of the moon and stars is one that
    has engaged the attention of several experimenters.
    "It is of great importance for the explorer and surveyor, the inaccuracy
    of the methods at present available being so great that they are
    practically never employed in the field."
    First he mentions previous efforts in this direction, by one "Dr.
    Schlichter, of England," and Dr. Runge in Germany. The latter I noted a
    few years ago. Runge uses separate exposures of the Moon and reference
    stars on the same plate, the camera being kept carefully fixed in the
    same orientation in the interim. If desired, considerable time can
    elapse between exposures.
    On the other hand, Schlichter simply photographs the Moon and a bright
    star simultaneously and measures the lunar distance. Disadvantages are
    that you need a bright star near the Moon, and lens focal length must be
    known and stable. Nevertheless, Hills says, "There seems no doubt that
    results can be obtained by this method far exceeding in accuracy any
    that can be obtained by the use of a sextant or theodolite."
    In his own method Hills takes non-simultaneous exposures of the Moon and
    stars on the same plate. Any amount of time may elapse between
    exposures, as long as it's measured accurately and the camera maintains
    a fixed orientation. He makes no attempt to measure the separation
    angles between images of the bodies, that is, compute a "lunar". Rather,
    Hills determines the Moon right ascension, and from that the time. (It's
    assumed the observer has accurate local apparent time.)

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