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Luminosity of Telescopes

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#1 Olivier Biot

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Posted 06 December 2011 - 05:36 PM

Luminosity of Telescopes

By Rafael Chamón Cobos.

#2 mcoren



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Posted 07 December 2011 - 08:34 PM

Interesting article, hopefully it will get some good discussion going. I've never formally studied optics or visual physiology, but some of the statements made in the article seem to be missing some important considerations. For example, in the discussion of case 2-A, you state "...the eye pupil is small, say 2mm in diameter. By observing terrestrial targets trough a 10x50 binocular, which has an exit pupil of 5mm, the light flux entering the eye is not restricted by the instrument, therefore the luminosity of the observed field is the same as that of the observation with naked eye." This analysis ignores the fact that the 5mm binocular exit pupil has the light from a 50mm objective is concentrated in the 5mm exit pupil. In other words, the light flux density in the exit pupil is 100x greater than at the objective, even if not all of that flux reaches the optic nerve.

#3 Rafael


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Posted 08 December 2011 - 06:47 PM

Hello, Mike, thank you for reading the article and for your comment. I am no expert in optics by no means, but I will try to answer to the objection you raise.

In observations with telescope we have to consider the system "telescope-eye" as a whole. In this system the flux of light entring the eye is limited by the smallest stop that rays find along the complete light path. In the example referred in paragraph 2-A, this stop is the iris itself, which is closed to to 2mm, according to the example.

Optic systems have the property that the distribution of rays in the system is exactly the same for both directions of propagation: forwards and backwards. This means that rays passing the 2mm iris can be projected backwards till the objectiv lens, covering here a central zone of the lens with a diameter of 20mm (= 10x magnification X 2mm eye pupil). The rest of the objective lens is wasted. Then, the 10x50 binocular becomes a 10x20 binocular. It would be the same situation as if the objective lens was masked with a stop of 20mm. In other words, in this case, the light flux entering the 2mm iris of the eye is independent of the diameter of the objective, provided that this one is greater than 20mm, and the binocular does not amplify the light flux at the eye.


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