Minimum magnification to see Saturn's rings

[Second Time Around]

I've had a new telescope to try out and with (at last) some decent seeing was surprised earlier in the week how little magnification I needed to see that Saturn is ringed.  I therefore decided last night (15th Sept 2020) to test exactly what magnification I needed for this, and also to see Cassini's Division.

Telescopes used

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.

Other parameters

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 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.

Eyepieces used on 72mm

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

Observations

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.

Discussion

I have very sharp eyesight, tested at 6/4 (=US 20/12 I believe).  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.  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.

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 last 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.

 

[martinl]

Interesting. Galileo’s telescope, which he used to first observe that there was something strange going on with Saturn, supposedly gave around 20x magnification. I can see the ears with my 20x60 binoculars, but I haven’t tried low powers with my refractor. I’ll try my TV85+28mm=21.4x next time I have the chance!

[John]

Interesting report.

I think the smallest aperture that I've seen the Cassini Division with is 70mm. That was the Tele Vue Ranger. I think that was at around 120x as well.

My old Tasco 60mm refractor showed the rings very clearly but I never managed to see the Cassini Division with that scope. Mind you I was using that at the outset of my observing so my experience was somewhat lower even if my eyesight was better !

[SiriusB]

On 16/09/2020 at 17:52, John said:

Interesting report.

I think the smallest aperture that I've seen the Cassini Division with is 70mm. That was the Tele Vue Ranger. I think that was at around 120x as well.

My old Tasco 60mm refractor showed the rings very clearly but I never managed to see the Cassini Division with that scope. Mind you I was using that at the outset of my observing so my experience was somewhat lower even if my eyesight was better !

Sounds right to me.

    I  can see the rings @ about 25x

,I've not been able to see the Cassini at all this Summer in 70-150mm, down to the low altitude, though others may have had more luck/skill.

When Saturn was high in the sky 15 or so years ago, Cassini division seen ok in 70 & 80mm scope at around John's suggested magnification.

For a bonus point there's seeing the Cassini division & then there's seeing it all the way round the rings, somewhat trickier & most likely needing more aperture.

Anyone for Encke's division?

Never seen it myself, back when i had a large enough scope to stand a chance Saturn was low in the sky like now.

......One for the future.

 

[davidc135]

In very good seeing the Cassini division was easily seen in an 8'' Meade sct but not visible in my 4'' B&L 4000. In 2018 when, I think, Saturn was even lower the little sct could barely make it out. At the same time my long focus 75mm achro refractor performed just slightly better but it was still fleeting.  David

[martinl]

Living at 60 degrees north Saturn transits the meridian at only 9 degrees this year but the seeing was exceptionally steady tonight, with the Cassini division clearly visible in my 150mm Mak at 150x despite the low altitude. The Mak, of course, struggles with low powers so I was relegated to using my humble APM 50mm RACI finder (which didn’t come with a Strehl report (!), perhaps I’ll run it through WinRoddier at some point for fun).

12mm Delos / 17.3x: Saturn clearly had ears, but no separation between the planet and the rings was visible at any time. 
8mm Delos / 25.9x: The rings were visible as an object separate from the planet the entire time, with pitch black separations between the planet and the ring. 

So despite an inferior optic and Saturns lower altitude my results were completely in line with Steve’s, a fact I find surprising. Perhaps we are up against the limits of human vision, rather than of optics, when trying to discern these tiny details at low powers?

Next time I’ll bring my 20mm Plössl. With the Celestron Ultima Barlow that should give 21.4x and a bit more with a few mm of eyepiece extension. 

[Les Ewan]

Back in  1972 I first saw Saturn's rings with a 40mm refractor using a power of 50X ,not very well even though the planet was riding high in Taurus at the time. Between 1974 and 1978 I had a 60mm refractor and got a lot better view of the rings,but It wasn't until I got a 150mm Newtonian did I first glimpse the Cassini Division.