Telescope Reviews: Your favorite eyepiece for planetary detail

Unfortunately, what will work good in your SCT won't even be a candidate for planetary in the NP101 because of the radically different focal lengths. A 5mm eyepiece in the SCT will get you around 500x and in the NP101 only 100x. So the planetary choices will be quite different.

Magnification and exit pupil should be considerations in the choice, as well as what your local seeing conditions will allow on most evenings, and on rarer evenings. For me, the local seeing usually allows 200x most all the time, and then more rarely 400x. So that is my working range.

I also find that a .75mm exit pupil is a good working max for maintaining good contrast. Not a hard-fast rule but just what my eye tends to like. In the NP101 that exit pupil would be produced by a 4mm eyepiece (135x). In your SCT that exit pupil is produced by a 7.5mm eyepiece (330x). But that magnification is a bit high so if you back it down to 200x then the SCT would be using a 12.5mm eyepiece. Bottom line is that the SCT, due to its larger aperture, will having working planetary eyepieces between 12.5mm to 7.5mm or even 6mm, whereas the NP101 will likely have a working range starting at 4mm then getting smaller, probably down to 2mm on the very rarest evenings under very dark conditions since the view will be dim using 2mm.

If it were me, I would be looking for 3 eyepieces for the SCT, probably 8mm, 10mm, and 12mm. Without getting exotic, an 8mm and 12mm Brandon, then need to find some suitable eyepiece near 10mm, so perhaps an 11mm TV Plossl or the older 10.5mm smoothsided version. Going exotic would opt for the 8-10-12 TMB Supermonocentrics or AP-SPLs since they both come in those focal lengths.

For the NP101 the 2-4 Nagler Zoom gets you the focal length range you need in one eyepiece, so something to consider. Another thing you could do would be to get the 8-10-12 trio for the SCT, then get a 3x Barlow to use those same three with the NP101. This way save some money by using the Barlow to get to the shorter focal lengths that the NP101 needs for productive planetary. Yes, not a purist approach for planetary mandating a Barlow, but it works good enough and IMO a higher class planetary, like the Brandon, when Barlowed will at least be the equal of the 2-4 Zoom in terms of how detailed the image is.

A final consideration is to think about operating ergonomics. The NP101 shines as a rich field telescope with its very short focal ratio. In that role you are likely to be using very wide field eyepieces with some good eye relief. So the switch to a tight eye relief narrow FOV planetary may not "feel" like a great transition. So the better tactic from this standpoint would be to perhaps get some wide fields with good reputations for planetary also and then Barlow them for the NP101. So in the 8-12mm focal length range you could assemble some Pentax XWs or Delos to fit that also.

Lots to consider...most of it driven really by personal preferences.