The first image Galileo saw was a tiny mottled spot of light surrounded by darkness way at the end of his telescope tube.  What little detail within this spot that he could see was  dancing wildly around with each tiny movement of his instrument.  Then slowly with concentration "?" the spot would seems to grow  into a larger   image with more detail becoming  more recognizable with time.

This is what you see minus of the white text! A huge black hole out to the periphery  of your eye   and  in the very center a little hole in which your object lives .  Other web sites show you just the magnified view of the object which can be very miss leading ----. So don't be disappointed when you look through one of his telescopes but marvel at the skill Galileo showed in all of his observational work.






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Above is the overall
  scene that was being used while we were collecting shots for the History Channels program on Galileo. The arrow points at the church carrying the steeple we were photographing through the replica of Galileo's telescope..


Why does the image grow and improve as the observer concentrates on it?



Much of the literature  reports that the optics of a Galilean telescope has  a calculated  angular  field of view of 15 minutes of arc. That is if  the object is a 1000 away from  the telescope one would only see what is included in a 4 foot diameter circle of the object.   However few authors have noted that the Galilean telescope's field of view is largely determined by the pupil of the observer's eye instead of  baffles in the  optics and that the size of the pupil changes with the amount of light interning the eye.

For example normally the pupil of the eye has adjusted to the ambient light of the surroundings before looking into the telescope. Thus if the sun is out the pupil of the viewers eye is quite small.   Thus what one first sees looking into the telescope is a  field of view that is just a small point of light surrounded by a sea of darkness. The darkness is a combination of the optics and partly because the rods and cones in the eye have not become dark adapted.  The field of view changes with time. As the observer looks into the telescope in this low light level  the pupil and the rods and cones adjusts themselves to the lower  light level and becomes  larger increasing the field off view therefore  one begins  to recognize more details in the  object.  See the picture below showing  the changes of the pupil diameter  for  two light levels. The upper picture shows a pupil in brighter light conditions than the eye below.




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Copy right Jim & Rhoda Morris 6/02/2006