lew (11/01/82)
The problem is not that I can't see as many stars as Galileo drew, it is that I can see many that are not drawn which are brighter than all the candidates for the drawn ones (this is in the Orion drawing.) I think the answer does lie in the limitations of Galileo's telescope. I found in "Galileo at Work" the statement that his telescope had a field of view of 12 minutes of arc. This is on the order of 1/1000 the angular area of the Orion drawing. His drawing was just not very thorough. I think he found a few stars around the brightest (naked eye) ones and drew them at disproportionately large distances from them, leaving out many in the intervening areas. There are two more drawings of the "Head of Orion" and the Praesepe cluster, in addition to the belt and sword of Orion (what I am calling the Orion drawing) and the Pleiades. I observed the Head of Orion and found it to correspond to the drawing about as well as the Pleiades did to theirs. Praesepe comes up too late so I'll see it in a month or so. Galileo's telescope had a concave eyepiece which has the field of view limited by a virtual exit pupil. The exit pupil is the image of the objective formed by the eyepiece. With a modern eyepiece, this image is real and is where the observer places the eye, so that all the light gathered by the objective enters the eye. In this position it does not limit the field of view, which is determined by other factors. In Galileo's telescope the exit pupil fell behind the eyepiece near the eyepiece focal point. One can gain an impression of this appearance by holding one's eye at a distance of five or ten centimeters from the eyepiece of a modern telescope. Lew Mammel, Jr. ihuxr!lew