I've had a new telescope to try out and with (at last) some decent seeing was surprised how little magnification I needed to see that Saturn is ringed. I therefore decided a fortnight or so ago (15th Sept 2020) to test exactly what magnification I needed for this, and also to see the Cassini Division.
1. My new scope is an Altair 72mm f/6 EDF De-luxe with a test certificate showing a Stehl of 0.95.
2. I also set up an old Opticron 22x60 spotting scope I've used for birdwatching.
I observed for a total of 90 minutes, half before and half after Saturn reached the meridian at 20.46 local time. The maximum elevation for me in the UK was 15.3 degrees. I looked up that the rings are currently tilted at an angle of 21.5 degrees. Seeing was 4/5, occasionally slightly better.
Baader 8-24mm Mk IV zoom
Svbony 7-21mm zoom (reported to be identical to the new Orion Explorer E-series zoom)
Vixen 22mm LVW
With zoom eyepieces I could have any magnification within their range so could get more accurate observations for this experiment. I started at the lowest power as I felt that would minimise any preconceived ideas.
Baader zoom 24mm/18x magnification. Could see that Saturn had "ears", but couldn't see a ring, even in moments of better seeing.
Vixen LVW 22mm/19.6x magnification. Could see that Saturn had "ears", but couldn't see a ring, even in moments of better seeing.
Baader zoom 21mm/20.6x magnification. Could see a ring in moments of better seeing. This was the minimum magnification I could manage this.
Svbony zoom 21mm/20.6x magnification. Could see a ring in moments of better seeing. This was the minimum magnification I could manage this.
60mm Opticron spotter 22x magnification. Could see a ring in moments of better seeing.
Baader zoom 16mm/27x magnification. This is one of the click stops on the Baader zoom. I didn't try any focal lengths between 16 and 21mm. At 16mm/27x I could clearly see a ring all the time.
Baader zoom at 8mm with matching 2.25x Barlow. Magnification 121x. Couldn't see Cassini Division even in moments of better seeing. The image had begun to deteriorate at this magnification and, although I could add extension tubes to get an even higher magnification, I felt this would be a waste of time, especially as this was late in the session and Saturn was getting even lower.
I have very sharp eyesight, tested at 6/4 (=US 20/12 I believe). Although my eyesight is sharper than average, my dark-adapted pupil size is under average at only 4.5mm, meaning that I can't see as faint stars as my wife. With a spherical correction of -4.00 in my dominant eye I have a fairly high degree of short sight (myopia), and a moderate amount of astigmatism. I observed both with glasses, and also without but with a Televue Dioptrx astigmatism corrector. I also tested my level of astigmatism by using 2 different strengths of Dioptrx. My daytime astigmatism prescription is for 0.75 correction, but the 1.00 Dioptrx was clearly but not hugely better. The angle I turned the Dioptrx to made a marked difference.
Accurate focusing made a big difference to the amount of detail I could see. The Altair has a very good dual speed rack and pinion focusser, whilst the Opticron is also dual speed with two separate focusers. Both though required frequent small corrections as the seeing varied, perhaps more so because of the low altitude.
I was surprised and somewhat disappointed that the 22mm Vixen LVW wasn't quite as sharp as either the Baader or Svbony zooms at the same magnification. It was however very comfortable to use and had a much wider field of view. On the other hand I was was pleasantly surprised at the Svbony zoom. It was as good on axis at f/6 as the Baader zoom, but not quite as good off axis and had a smaller field of view. Neither the Baader nor Svbony zooms are parfocal for me, but that may be my lack of accommodation with advancing years. I was also pleased with the sharpness of the Opticron 22x60 spotter.
So for me, in that night's conditions I could see that Saturn had a ring at just over 20x magnification, despite the low altitude. Yet even with just over 120x I couldn't see Cassini's division, although Saturn was even lower then.