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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."

http://adsabs.harvard.edu/abs/1895MNRAS..55...89H

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.

http://fer3.com/arc/m2.aspx/Lunar-distance-camera-1893-Hirose-nov-2011-g17415
http://fer3.com/arc/m2.aspx/lunar-distance-camera-1893-Hirose-nov-2011-g17417

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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