[Wavytone]

Not true, though a starting point for “average optics”, “average seeing” and “average eyesight” :

- x2 per mm of aperture for refractors,
- x1.5 per mm for newtonians,
- x1.2 per mm for SCTs.

The useful maximum varies according to:
- the type of object observed, and it’s brightness;
- the type of scope;
- the quality of the optics,
- the observers eyesight, and
- the seeing conditions !


The brightness of the object matters a lot because the distribution of rods and cones in your retina is not uniform. If it’s bright enough that the cones respond (colour) the spacing of the cones in the macula (centre spot in the retina you use for reading) is extremely dense, but but the spacing drips off dramatically outside the macula. The spacing of the rods (night vision) is the inverse, ie few in the macula and more outside the macula - hence the trick of averted vision for faint objects. This also comes into play when deciding what magnification is optimum for a particular object.

Regarding faint extended objects like DSOs there is a sweet spot at about 1X per mm where things appear reasonably bright and sharp and increasing the magnification beyond that just makes them harder to see and no more detail is to be had.

But on bright double stars, with good eyesight, to resolve Dawes criterion on double stars Dawes noted that 3X per mm is required. This is interesting because even with modern optics few refractors can usefully go to such extreme power. On your 120mm triplet that means 80X per inch - far more than you suggested.

The other aspect is the quality of the optics; for example on double stars, the moon and planets an Intes M715 and my MK91 can run to 2.5X per mm of aperture. Yet if you try that in the average SCT the image breaks down into a blurry mess at half that.

Similarly short achromatic refractors will run out around 1.5X per mm, beyond that everything is a blurry mess. The reason why achro refractors were f/15 to f/20 was to reduce aberrations to achieve the best possible performance, and to provide a focal length long enough to achieve 3X per mm with comfortable quality eyepieces.