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Exit Pupil, Apparent Field, Acuity

Started by splathrop, 02/03/2003 02:23AM
Posted 02/03/2003 02:23AM Opening Post
Will somebody who knows more please check me on this line of reasoning. I particularly want to know if my premises are off the mark.

1. With a given magnification and a given aperture, exit pupil is determined: field size, apparent or actual makes no difference to exit pupil size.

2. In a comparison of two EPs, one with a larger field, and one with a smaller field, but both with the same focal length, and hence the same exit pupil size, any given resolvable detail will have a smaller image size in the wide-field EP, because the wide-field EP fits more stuff into the same-size exit pupil.

3. So maybe planetary observers can get larger image scales for tiny planetary details simply by using narrow-field EPs instead of Naglers and Panoptics.

Anything wrong with this?

Steve
Posted 02/03/2003 03:22AM #1
Yes, there are problems with it.

1. Correct.

2. No, any details will appear the same size in both eyepieces. However, the one with the wider field will show a larger area of the sky.

3. Wide versus narrow field isn't really the issue. Magnification determines image scale. If you need more magnification, you use a shorter focal length eyepiece or a Barlow.

The exit pupil is not a single column of light with the image packed in any cross section. If that were the case, you'd need to remove the lenses from your eyeball in order to focus on it. Rather, each point in the view emerges from the eyepiece as a separate column of light. Look in a different direction in the apparent field of view, and you are accepting a different column of light and using the lenses in your eyeball to focus it on the most sensitive area of your retina. Other columns focus in other places on the retina. Anyway, the exit pupil diameter is the diameter of each of these columns. With a wide-field eyepiece there simply exist columns of light coming from greater angles.
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Mike