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Oh I agree bojan. The number of stars recorded is determined by aperture not by FL. In other words it doesn't matter whether a 100mm scope has an FL of 500mm or 1000mm, both will record the same number of stars, though over a different field of view. The responsiveness to faint extended objects however is determined by f/ratio (FL/A). So to successfully image faint small extended objects you need a long focal length with wide aperture, ie M57 will look better though appear as "bright" imaged through 2000mm with 500mm aperture than through 200mm with 50mm aperture (both f/4).
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