Projection method
As a method of projection in which is Astronomy and the other with optics involved areas of knowledge understood the method, by a telescope presented to the eye virtual image by extrafocal defocusing of the eyepiece on a screen or a photographic plate to be projected.
The method, which is also known as " ocular projection " in amateur astronomy , is suitable for, among other things
- the astrophotography bright celestial bodies (z. B. 7 large planet , sun and moon)
- for geodetic measurement of solar azimuths ,
- for finding bright stars, even if the eyepiece is not accessible to the eye,
The easiest method is with the sun :
The sun is projected onto a piece of white paper by adjusting the telescope eyepiece a little extra-focal (about a quarter turn counterclockwise). A convenient projection distance is around 15-20 cm, which makes the image of the sun a few centimeters large. With such a bright picture, the lines of the crosshairs on the paper are also visible (if not, you can shade annoying daylight ).
In the case of solar direction measurements (e.g. the edges of the sun or the sun's radius ), this method has a number of advantages over looking through the eyepiece (which is still dangerous even with a filter !) Or the risk of fire in photographic processes:
- It is completely safe (when looking for the sun , the telescope of the theodolite should be aligned with the sun with the help of its shadow so as not to look in its direction).
- It is easier than the direct measurement at a mostly rather steep angle of elevation .
- The projection - which optically corresponds to that of a slide projector - allows a convenient measurement of both sun and moon edges within a short time, which (after averaging the two directions and times) results in the center of the star in an almost perfect way.
literature
- Albert Schödlbauer: Geodetic Astronomy . De Gruyter, 2002.
- Wolfgang Schroeder: Practical Astronomy for Star Friends . Kosmos, Stuttgart 1958.