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5 Degrees? That isn't very much. Less than 1/10 of an average eyepiece field of view. Or is there a typo?

Thomas

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No. Increasing magnfication lowers the brightness of the image, making an object less visibile. At the same time, increasing magnifcaiton increases the apparent size of the object you are observing, making it more visible. The best magnification is dictated by where the magnitudes of these effects become equal. My own personal experience at the eyepiece, as well as my own independent analysis of the Blackwell data (a study on contrast thresholds from 1947 or 48), seems to indicate that the best magnification at which to view a specific object or detail in an object is the lowest magnifcation at which that object or detail spans AT LEAST ~5 degrees, and does not exceed a 7mm exit pupil. For big object such as NGC 7000, that's your lowest power, since even at like 20X, NGC 7000 is like 40 degrees across in the eyepiece. For a tiny object like a galaxy 2 billion light years away that spans 10", that could be as much as a 1mm exit pupil. Analysis of EXTRAPOLATIONS of the Blackwell data seems to indicate that exit pupils of even smaller than 1mm may be benefitual for very small objects, but I have yet to test this observationally.

Also, please note that this ~5 degree size is somewhat arbitary. It could be 5 deg + or - a degree or two. The optimal size could be as small as 2 degrees! More analysis is required to tell exactly where it is. Also, exactly where this optimal magnified size for a specific object may depend on other factors such as the observer, star background, etc.

A good example of the star background making the optimal magnified size larger is where you're trying to see an extremely faint object next to a star or group of stars. In adverted vision, your spatial resolution goes WAY down and it becomes hard to tell if you're seeing a smudge of an object or a smudge of a star if the star and object are close together. Pumping up the power an additional amount gives you enough separation to tell if you're seeing the star(s) or the object.

Edited by tatarjj (07/21/08 01:34 PM)

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