[ngcles]

Hi Paddy & All,

The 2mm rule isn't so much a hard and fast rule -- more a rule of thumb that has existed since Moses played fullback for Jerusalem.

Most experienced observers agree that for small low-contrast objects, and to achieve maximum resolution of faint stars in things like G.C's, LMC O.Cs, LMC star-clouds etc etc, a telescope produces its maximum visual contrast when the exit pupil produced by the Aperture/Magnification/Eyepiece combo is about 2mm.

The exit pupil is the diameter of the beam of light that comes out of the eyepiece. A telescope used at low power with a longer focal length eyepiece will produce a wider exit pupil than higher power.

Beside directly measuring the diameter of the beam, the exit pupil for a particular Aperture/Magnification/Eyepiece can be approximated by dividing the telescope aperture in mm by the magnification a particular eyepiece produces.

Example: 200mm aperture used at x100 produces a 2mm exit pupil.

250mm aperture used at x125 produces a 2mm exit pupil.

456mm aperture used at x185 produces a 2.46mm exit pupil.

456mm aperture used at x247 produces a 1.85mm exit pupil.

So, to achieve maximum visual contrast on small low-contrast objects, experienced observers almost universally agree a 2mm exit pupil is best. However, large low-contrast objects are better observed with a 3.5-4.5mm exit pupil.

As I said it is not my rule, it isn't a hard and fast rule but a rule of thumb.


Hope this helps.


Best,

Les D