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    lalunanelpozzo's Avatar
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    Default f ratio vs pupil exit



    Hi,
    comeing from photography where the f ratio is really important to call a lens fast or slow, i feel myself a little bit disoriented about what brighter really means in the telescopes world where a quite new concept, the pupil exit, call my attention. It seems I have understood that f ratio is not that important in visual observation as the eyepiece is involved. A 30mm will give brighter view over a 15mm because the 30mm ep should grant a larger pupil exit. So, at the end, a faster f ratio does not automatically means a brighter view if it is no accompaigned by a large, within anyway 7mm pupil exit.
    If we have a 500mm f/6 and a 1200 f/10 , in the photographic field, 500mm is brighter for sure. In the visual observation we might need insted to know what eyepiece is on. So , provided we have the two above mentioned scopes, and provided we want a 100X magnification we need a 12mm ep for the 1200mm scope and a 5mm for the 500mm one. So, as the pupil exit is calculated dividing the eyepiec focal lenght by f ratio we should have 5mm/6 for the 480mm scope and 12mm / 10 for the 1200mm. So at the end the visual observation at 100X with the 1200 f/10 is brighter over the 500mm f6.
    Just to know if what i tryed to understand whit some astronmic formulaes i found aroun is correct.
    Thanks for looking / commenting

  2. #2
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    Default

    Here's how I see it. When it comes to visual observing with a scope, think of the f number as just a number, forget fast and slow. It's like shorthand for aperture/focal length. Aperture = brightness. The more aperture you have, the more light you let in, the more detail you see (you already know this). Focal length dictates magnification. A longer focal length scope produces a larger image than a smaller focal length of the same aperture.

    You can use a Barlow with a low mag eye piece in order to achieve higher power while keeping good eye relief and pupil size.

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    Her's a link to a telescope calculator that lets you input values and see how things change - it's not the last word, but gives a good idea of how the eyepiece+telescopescope performs as a system.

    Here's another link to a eyepiece calculator that provides a more verbose description and caveats.

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