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Thread: AFOV question

  1. #1
    mitaccio's Avatar
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    Default AFOV question

    I want to understand the value of say a Naegler (sp?) eyepiece. I have a scope with a 1200mm focal lenght. I'd like to get an eyepiece that will give me a really wide view of the sky. I know that the higher the focal length of the eyepiece the higher the magnification. And that to find the magnification I divide the focal length of the scope by the focal length of the eyepiece. But the FOV. I divide the magnification by the FOV? So if I have a 1200 mm scope with a 25 mm ep, I get 48x. If the eyepiece is 50mm FOV, I roughly have 1 degree of Actual FOV? Seems narrow.

    With a 1200 mm FL scope, what would be a good eyepiece setting to get in a wide sky view?

  2. #2
    Vinnie's Avatar
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    So what you are looking at is the TFOV (True Field of View) which is the actual width of the section of sky that appears in your view as related to the AFOV (apparent field of view, or sometimes called angular field of view) which is the angular width of that section of sky as it is distributed across the EP.

    There are three different methods available to calculate TFOV

    In the first, you need to calculate the magnification of the EP which is the FL (Focal Length) of the scope divided by the EPFL (Eye Piece Focal Length), which also demonstrates that the longer the EPFL the lower the mag

    Now by dividing the AFOV as specified by the EP manufacturer by the mag you will get a value for the TFOV

    This method is not particularly accurate, and could have an error as much as 10%, but does serve as a quick comparison of EPs and does also serve to demonstrate that for any given EP, shorter scope FLs give wider fields, or if you like, for any given scope lower mags give wider fields.

    The second method is to bring the EPs field stop into play. The field stop is the diameter of the aperture of the bottom lens in the EP, or if you like, the aperture that lets the light into the EP. Some manufacturers specify this, but its easy enough to measure anyhow.

    In this case the TFOV is the (Diameter of the field stop/focal length of the scope) x 57.3 where 57.3 is the rounded off value of a radian in degrees ie 180/Pi

    This method is more accurate, say an error of no more than about 2%, but again demonstrates that for any given EP, shorter scope FLs give wider TFOVs, or also since the longer the EPFL the wider the field stop, also shows that lower mags give wider TFOVs

    The third, and most accurate method (and I include this mainly just out of interest) is by Star Drift.

    Here you need to focus your scope on a known star (you need to know its Dec) and then move the scope so that the star drifts from one edge of the FOV, through the centre and to the other edge (ie along the diameter of the field of view) with the scope stationary. By timing this drift say 5 or 10 times then taking an average time for its drift you can use this equation

    TFOV = [(drift time) x cos(dec of star) x 360] / 86,164 where 86,164 is the number of seconds in a day

    .................................................. ........................................

    To get the widest TFOV possible with a given FL of scope you need to go to the lowest practical mag with the widest AFOV EP available.

    I can't afford Naglers, not many of us can, but if your scope accepts 2" EPs there are various EPs available at around 70° AFOV at budget prices

    So say a 38mm 70° AFOV 2" ep will give a TFOV of roughly 2.1° in your scope, or a 32mm would give about 1.9°

  3. #3
    bilbowski77's Avatar
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    Very good information, Vinnie, thanks. I am looking into some more EP's soon, but no Naglers for me, too rich for my blood.
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