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    Question Telescope Eyepieces Apparent Field of View



    Telescope Eyepieces Apparent Field of View -


    There is a good post here about it however I am still confused:

    AFOV (Apparent Field of View)

    Is more Apparent Field of View (AFOV) better always?


    How to calculate it:

    Apparent field of view = True field of view X Magnification
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    How to calculate it:

    Apparent field of view = True field of view X Magnification
    Well Zorba that's fine, but before you can do that you need to calculate the TFOV (True Field of View)

    The AFOV is usually, well these days 99% of the time, stated by the manufacturer of the EP.

    Not wanting to contradict Mr David Knisely whom you have quoted, and he is a very knowledgeable amateur indeed, I will say that David often complicates issues that could easily be simplified.

    Here is my take on TFOV:

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


    Is more Apparent Field of View (AFOV) better always?
    So lets consider this: The AFOV (apparent field of view, but sometimes called angular field of view which is a term that I prefer) describes how the TFOV is distributed, if you like, over the EP.

    This therefore, and given the above, tends to suggest that the wider the AFOV of an EP the better.

    Well, yes this is usually the case, but there are exceptions.

    The actually quality and design of the EP will come into play. The human eye can only take in about 65° to 68° of AFOV at the EP. Any more than that will mean that you need to actually move your eye to take in the whole view. Even at 68° AFOV unless the EP is excellently made, the edges will lose sharpness and contrast. One of my favourite recommendations for a 68° AFOV EP, the Baader Hyperion, does lose a little sharpness at the edges in most scopes, but is way ahead of the pack.

    In poorly made EPs of a wider AFOV than about 52° we get a rather nasty effect called the "Kidney Bean" . In this case, if you try to pick up the edge definition, the centre of the view loses resolution and actually darkens into a bean shape.

    Finally, in very fast scopes, say large Newts of f/4.5 or less, the quality of wide AFOV EPs becomes very significant. Very short light cones require very high quality EPs if you want to go up to higher AFOVs

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