[ngcles]

Hi Mbaddah & All,

In viewing Saturn which is a very small high surface-brightness object (as are all the planets) the question of exit pupil plays a minor role. Neither do you really have to dilate much --as you will see.

The telescope's ability to show detail on a planet (assuming diffraction limited or better optics) is governed by the diameter of the aperture but very much limited by the seeing at the time of observing.

The Dawes formula provides that the size of the airy disc a telescope produces in arc-seconds (R) is R=4.56/D where D is the diameter of the aperture in inches.

So for simplicity's sake, assuming a 10" 'scope, the smallest detail visible on a planet (or the moon for that matter) is about 0.46 arc-seconds across, but only in _perfect_ seeing.

The problem is that the seeing is rarely perfect, infrequently very good and commonly average or ... worse.

If you view at low power with a wide exit-pupil, all the detail that the telescope/seeing will permit to be seen is there in the image, but it is usually much too tiny to be seen comfortably. So to see it comfortably, we apply magnification. More magnification doesn't increase the resolution of the 'scope, it just makes what is there easier to see. Unfortunately, applying magnification also amplifies any imperfections in the seeing. The seeing will limit what magnification can be practically used while retaining good image quality.

As I see it, the trick is to use a magnification that enlarges the image of the planet so that it is big enough to see what can be seen, but no so much that the seeing degrades the quality of the image. The amount of magnification you can "get away with" while retaining image quality will change from day-to-day, if not hour-to-hour.

Higher magnifications produce a smaller exit-pupil and this can be an advantage -- For example: a 10" used at x250 produces an exit-pupil about 1.0mm. That means that the light from the 'scope will pass trough only the very centre of the cornea where there are likely to be less aberrations in the eye itself -- hopefully making for the best image possible.

Hope this helps!

Les D
Contributing Editor
AS&T