[Wavytone]

The optimum eyepiece depends on your scope - and it depends on your visual acuity, too.

Ages ago I wrote a paper on this topic with Greg Thompson. The optimum magnification is that where, when you are looking closely, the image is reasonably bright and sharp and you are just beginning to discern airy disks round stars, or diffraction on sharp edges like the lunar disk limb. At this point you're seeing all the detail your scope can provide - more magnification only makes the image dimmer and enlarges the diffraction rings and you won't really see more details.

This magnification is a function of the spacing of the rods and cones in the fovea on your retina, vs the scale of the image projected on the retina by your scope.

For most people it works out to be between x1 to x1.2 per mm of aperture of the telescope. Thus for a typical 200mm aperture f/10 SCT, this sweet spot is around 200 to 220X, which means an eyepiece around 8 to 10mm focal length. Magnification of x1 to x1.2 per mm of aperture is pretty much the sweet spot for all scopes that produce very distinct Airy disks - SCT's, Newtonian reflectors and Maksutovs.

Some observers will wind the magnification up a tad beyond this - x1.5 is not uncommon - on a bright object like the moon it may be a bit more comfortable if the seeing is steady enough, but you won't really see more detail.

On the fast ED and APO refractors however, these don't produce such distinct Airy disks due to higher order achromatic abberations - though the image is sharp - very sharp - and will stand magnifications around x1.5 to x2 per mm of aperture before the image starts to deteriorate.

On my Maksutov (it's 180mm f/15) the 13mm Vixen LVW gives 208X - IMHO the optimum in most conditions. With a 10mm (270X) the image is starting to break down and with the 8mm (338X) its very soft, due to diffraction.

The same will be true regardless of whether its a Hyperion, TV or other eyepieces.

Greg and I had a very lengthy discussion about whether or not this same magnification is also the optimum for small faint extended objects such as planetaries and galaxies. We both suspected it is, though not exactly sure why.