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Ken, the main reason f10 is better because flaws inherent in the design of these scopes are worse at shorter focal lengths. All else being equal, spherical and chromatic aberrations get worse as the f-number goes down. This becomes more important at high magnifications, i.e., planets.
BTW, at the same magnification both the short and long tube scopes will be just as bright as each another. It's just that you'll need different EPs to achieve the same magnification.
One thing to consider is just how wide you want to go. With a 30mm PL (which is probably the longest FL EP you'd use), the 90x600 will give you 20x, i.e. about 2.5 degree FOV, and the 90x900 will do 30x or about 1.7 degrees. So at its lowest magnification you'd get away with not having a finder on the short tube, but you'd need one with the f10.
Last edited by janoskiss; 08-08-2005 at 05:44 PM.
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