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