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27-04-2016, 04:56 PM
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Regulus - Couer de Leon
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Join Date: Nov 2011
Location: Devonport, Tasmania
Posts: 2,350
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a x b=
Can someone give me (or point me to) the equation for determining the increase (decrease) in light gathering power between lens apertures.
Thanks - Trev
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27-04-2016, 04:58 PM
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Registered User
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Join Date: Mar 2012
Location: Mornington Peninsula, Australia
Posts: 3,997
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hi trevor
do you mean the area of the aperture? - eg pi r squared?
rusty
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27-04-2016, 04:59 PM
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Aidan
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Join Date: Oct 2014
Location: Sydney
Posts: 1,669
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piR^2 (area of a circle)
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27-04-2016, 05:35 PM
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Regulus - Couer de Leon
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Join Date: Nov 2011
Location: Devonport, Tasmania
Posts: 2,350
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Hi Russell.
I want to know into the future what percentage of light-gathering increase there is in going larger. Eg. How much more light gathering power does a 130mm have over a 100mm (leaving aside aperture as a consideration).
Cheers - Trev
Last edited by Regulus; 29-04-2016 at 02:49 PM.
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27-04-2016, 05:38 PM
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Registered User
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Join Date: Jan 2005
Location: Sale, VIC
Posts: 6,033
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Light per unit area of focal plane is determined by the f-ratio alone. Amount of light hitting a sensor there is proportional to the square of the f-ratio. For example, take f/4 vs f/6: the f/4 lens collects (6/4)-squared = 1.5 x 1.5 = 2.25 times more light per detector pixel than an f/6.
Light per unit area (or solid angle) of sky OTOH is proportional to the square of the aperture (diameter) of the lens: double the aperture gives you 4x the light; increase/decrease the aperture by factor N, you'll increase/decrease the number of photons collected from a fixed portion of the sky by factor N x N.
The reason for this perhaps counter-intuitive discrepancy between these two ways of figuring it is that if you increase the aperture but leave the f-ratio unchanged you also increase the image scale (magnification) in proportion to the aperture. So you do get more photons from a fixed bit of sky but they are spread over a larger area in the focal plane.
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27-04-2016, 05:39 PM
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Registered User
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Join Date: Apr 2015
Location: Langwarrin Melbourne
Posts: 99
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Hi Trevor,
Simply square the diameter of the objective.
eg 4 inch scope squared is 16
6 inch scope squared is 36
6 inch scope therefore collects 36 divided by 16 times more light, ie 2 1/4.
Mamba
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27-04-2016, 05:41 PM
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Registered User
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Join Date: Jan 2005
Location: Sale, VIC
Posts: 6,033
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Re your last post, for plain common sense visual astronomy: the 130mm will collect (130/100)-squared or 1.3 x 1.3 ~= 1.7 times more light from any given patch of sky.
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29-04-2016, 02:51 PM
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Regulus - Couer de Leon
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Join Date: Nov 2011
Location: Devonport, Tasmania
Posts: 2,350
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Thank you David, and Steve. Pretty much what I needed.
Cheers - Trev
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