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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Basics of computing sunrise/sunset
From: Paul Hirose
Date: 2009 Jun 19, 19:10 -0700
From: Paul Hirose
Date: 2009 Jun 19, 19:10 -0700
douglas.denny@btopenworld.com wrote: > > If this was not so - explain please why it is that stars invisible to the naked eye of aperture diam 6mm - because they are too faint (intensity too low to stimulate the retinal cells), become sudenly visible with a telescope of aperture say, 100 mm diam? That occurs because a star is a point source. Binoculars don't make it look larger, but they gather more light. Since more light is concentrated on the same area of retina, you can see a star which would be invisible to the naked eye. An extended object such as the Moon behaves differently. Its image on the retina responds to magnification. Suppose you're using 7x35 binoculars. They gather 36 times more light than a naked eye with a 6 mm pupil. But due to the 7x magnification, the image on the retina has 49 times more area than the naked eye image. Consequently, extended objects look dimmer! This is effect is minimized if the binocular exit pupil is at least as large as the eye pupil. For example, suppose the binoculars are 7x50s. A 6 mm eye pupil utilizes 42 mm of the objective lens. That diameter gathers 49 times more light that the naked eye. The light is distributed over an image with 49 times more area. Therefore, the Moon appears as bright as with the naked eye (less the small amount of light lost in the optics). Amateur astronomers are familiar with this phenomenon. Increased aperture doesn't make nebulae look brighter. They just look bigger. -- --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---
