Interesting that Televue say on their website: "Tele Vue recommends maximum magnification of 60x / inch (2.5x / mm) of telescope aperture and 350x maximum regardless of aperture."
- and that is for premium refractors! A lot of commercial scopes simply don't have good enough optics to go that high.
On a practical note, once you start to get an exit pupil smaller than 0.5mm (ie. a magnification of 2x per mm or about 50x per inch) you are really starting to battle with "floaters" and what your eye is capable of (not to mention needing a better than "diffraction-limited" scope!).
I have taken my TSA102 to 270x-300x, but the seeing has to be just about perfect, and generally I get a sharper view with better contrast at around 200x.
Having said that, I remember using an ETX125 at Arkaroola, looking at Saturn at 500x
in sub-arcsecond conditions: and the view was crisp and contrasty. I suspect it was a freak scope in freak conditions!
- Dean
Footnote: I just conducted an experiment. The seeing is quite good tonight, and I just looked at the Trapezium in M42 with my Tak TSA102, and a 3-6mm Nagler zoom. This gives magnifications of 136x to 272x. The images were sharp at each step, with text-book-perfect star images all the way. I was able to easily see the "E" and "F" stars with direct vision at 136x and 163x. Not so easy at 204x, and difficult at 272x; better with averted vision at both 204x and 272x. This shows the effect of contrast reduction as the power is increased, with the best, highest contrast images at 136x and 163x, or 1.33x and 1.6x per mm (or 34x and 41x per inch) respectively...