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It’s the same light entering the scope, spread over a smaller area so the image is brighter but smaller and the exposure time for extended objects (eg nebulae, moon) is correspondingly reduced as this is inversely proportional to image brightness.
The focal reducer reduces the effective focal length of the telescope, but the aperture remains the same.
The effective focal ratio e = R x F/D where R is the reduction of the reducer (eg 0.8x), F is the focal lengrh of the telescope and D is the aperture. Or e = R x f, where f is the focal ratio of the telescope (ie f=F/D)
Image diameter is proportional to R x F.
Image brightness is proportional to DxD/(FxR) or D/e.
Last edited by Wavytone; 13-10-2017 at 08:17 AM.
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