Exit Pupil

In cases where the diameter of your eye's pupil is the same or a greater than the exit pupil of the eyepiece you are using, you will receive all of the light from the eyepiece, and so get the brightest image you possibly can, this is particularly important in astronomical applications.

To ensure you take full advantage of your telecopes light gathering capability the eyepiece exit pupil diameter should not be any larger than 7mm, which corresponds to the average pupil size of your eyes when adapted to the dark.

Effective eyepiece designs will have an exit pupil diameter that will approximate the eye's apparent pupil diameter, and located around 20mm from the last surface of the eyepiece for the comfort of the observer.

Overtime as people age the eye does not dilate as well,and for those fifty or over it is frequently in the range of 5mm or less.

If you happen to fall into this category it's possible you could be wasting some of your scopes light gathering capability as some of the light will not enter your eye, falling on your iris instead.

It's a simple process to find the exit pupil diameter for any given telescope and eyepiece, simply divide the eyepiece focal ratio by the f/ratio. Or another way is to divide the objective lens by the magnification.

You will soon see that by using different combinations of eyepiece, f/ratio and objective lens the exit pupil will vary considerably. Work with eyepieces that fall within your own approximate pupil size and you will get the best out of your telescope viewing experience.

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