I find that you can go beyond 50x(aperture, inches) or 2x(ap, mm),
but it gets dimmer and very sensitive to dust and floaters
(including stuff floating outside, on your cornea).
You also need a very, very solid tripod for the shakes.
I usually keep to 25x(aperture, inches) or 1x(ap, mm),
..it usually looks cleaner and more colorful, and my eyes can
see the little details. Dim makes for a sloppy image
for your cortex to process.
Like it says above, highest power might only be used for splitting stars,
Something like a simple eyepiece (Plossl, 4mm as you posit)..
can be dust-sensitive and like wearing a contact, relief-wise.
A more complex eyepiece, or a Barlowed longer-fl simple eyepiece
is less annoying.
Edited by MartinPond, 10 May 2021 - 11:05 PM.