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5haan_A

Some eye pieces are dimmer than others

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Hi,

I notice that when I'm observing and switching between my different eye pieces some give a dimmer image than others. In particular my 9mm eye piece gives a much dimmer image when compared to my 21 mm. 

Why does this happen? 

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The dimming is caused by the reduction in the exit pupil size as the magnification increases. Brightness  goes down by the square of the magnification.

Edited by johninderby
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Yes, the object is still emitting the same amount of light, but the increased magnification is spreading that light out over a bigger area. Less light per unit area => dimmer image. This is one reason why the obsession of "department store" telescopes of massive magnification figures is so wrong. It leads the novice to believe that more magnification = better, which it most certainly ain't.

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Cheers for the responses to that guys makes a lot of sense.

Just a follow up question to this. If there was a 8" aperture scope vs a 6" aperture scope and they both magnified on an object by the same amount would the bigger aperture show a brighter image? 

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1 minute ago, 5haan_A said:

Cheers for the responses to that guys makes a lot of sense.

Just a follow up question to this. If there was a 8" aperture scope vs a 6" aperture scope and they both magnified on an object by the same amount would the bigger aperture show a brighter image? 

Simple answer is yes.

Exit pupil is calculated as Aperture/magnification so at say x100 a 6" (150mm) scope will have an exit pupil of 1.5mm and an 8" (200mm) will have an exit pupil of 2mm giving a brighter image.

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