The 5mm Exit Pupil

Most binoculars for astronomy will give an exit pupil in the region of 3 to 5 mm. There are obvious exceptions to this. There are occasional "fashions" for using exit pupils of up to 7mm in both medium (e.g., 7x50) and giant (e.g., 15x110,25x150) binoculars, but there are very good reasons not to do so, as only a few objects can benefit from this even if our eyes' pupils do dilate that much. Similarly, there are some larger astronomical binoculars, usually with interchangeable eyepieces, where the exit pupil is smaller than 3 mm.

There are distinct advantages in using an exit pupil in the 3 to 5 mm range. In no particular order they are:

• Most observers' pupils do not dilate much beyond 6 mm. The eye's pupil therefore vignettes the light from the binocular.

• There is sufficient brightness to see most of the extended objects that are visible with a larger exit pupil. (Notable exceptions are the Andromeda Galaxy (M31) and the North American Nebula (NGC 7000), both of which are better with a larger exit pupil, if our eyes can accommodate it.

• It is easier to position the eyes so that the entire exit pupil is contained by the eye's pupil.

• Aberrations in the eye's lens and cornea tend, as they do in the lenses of optical instruments, to be more severe toward the periphery of the pupil than they are at the center. Many normally bespectacled observers find that they can, with smaller exit pupils, observe satisfactorily without spectacles.

• Larger exit pupils imply lower magnification. Most binocular objects are easier to resolve with greater magnification and many are easier to identify. An object is fully resolved on the retina when the exit pupil is about 1 mm, although this is impracticably small for binoculars.

• The higher magnification results in greater contrast because the sky itself is an extended object and consequently dimmed by greater magnification.

• Smaller exit pupils imply smaller real fields of view, so lateral chromatic aberration is reduced.

The obvious disadvantages are:

• Extended objects are fainter than they are with a larger exit pupil, assuming the eye can accommodate the larger pupil.

• Larger exit pupils imply lower magnifications, with consequently more relaxed tolerances for collimation between the tubes.

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