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Minimum magnification to see Saturn is ringed/Cassini division

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#1 Second Time Around

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Posted 01 October 2020 - 11:40 AM

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.

 

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 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

 

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).  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.


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#2 coopman

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Posted 01 October 2020 - 11:44 AM

Galileo noted that Saturn had "ears".  His scope was not sufficient to recognize the protrusions as rings.  20X sounds about right to me.  Of course, you also have the fore-knowledge that the rings are there.  


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#3 Michael Covington

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Posted 01 October 2020 - 12:00 PM

You've made some very interesting observations.  You should compare them to the historic Saturn observations (e.g., in Sheehan's Planets and Perception) and write an article or something about it.  You've bridged the gap between early telescopes and ours.


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#4 SiriusLooker

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Posted 01 October 2020 - 12:24 PM

Steve.

 Nice detail review..  Sad that you so north bound for viewing the planets at present, other than Mars at higher level. I have a few Svbony lens, and I am also impressed with their quality.. I do not have their zoom lens. May I ask if it has a click stop at each of the focal lengths like the Baader zoom lens has?

    

 Dennis


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#5 howardcano

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Posted 01 October 2020 - 12:25 PM

I'm at about 39deg north.  Just a few days ago the Cassini division was easy and obvious at 100x in my 102mm ED refractor.  If I get a chance, I'll see how low I can go.


Edited by howardcano, 01 October 2020 - 12:27 PM.

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#6 sg6

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Posted 01 October 2020 - 01:12 PM

My only question/concern would be getting a good 120x with a 72mm ED.

Yes I know the assorted "rules" on magnification, and also know most are not exactly a rule nor accurate.

The "Deluxe" aspect seems meaningless, it is mechanical and nothing to do with the optics - a camera rotator, focuser rotator, higher rings or a dovetail are not going to improve the optics.

 

Just have the idea that had you tried 120x on a scope easily capable of say 160x to 180x you may have seen Cassini at 120x.

 

In a way it is like my analogy to cars: If you want one that can do 100mph you buy one that is capable of 140mph, not one that says 105mph, or even 110mph. A 105-110mph car will get to 90mph, the other bit will be harder.


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#7 Redbetter

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Posted 01 October 2020 - 06:15 PM

This isn't going to get close to the limits at 15 degrees elevation.  Even if the seeing was somehow perfect that low, one still has to contend with a lot of atmospheric chromatic dispersion.  Considering seeing and chromatic dispersion, even 30 degrees is not that great for this sort of thing.  

 

Cassini's division can be seen down to ~40mm of aperture.  On a decent night I have done it with a simple 80 f/5 achro stopped down to ~43mm.  Others have reported seeing it at 40mm.  I consider it straightforward for the AT72EDII in good seeing, but I am not observing Saturn at 15 degrees elevation.

 

I remember doing some testing of aperture stopping down a 70mm achro with poor optics to see what it took to see Jupiter's SEB and NEB.  For that scope, stopping down to 20mm at ~17x was about the minimum effective combo from what I recall.  Detecting saturn's rings as having some form of pitcher handle shape had similar limitations.

 

A big limitation for any observer when it comes to low magnification of bright objects is any astigmatism in the eye, even very mild.  Why?  Because the astigmatism creates increasing glare/blur as exit pupil increases.  I prefer keeping the pupil below about 3mm for bright objects, others will have their own limits.   It becomes a bit of an optimization to find the best combination of aperture (resolution), exit pupil, and magnification for an individual's eye and the target itself. 


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#8 phillip

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Posted 02 October 2020 - 04:08 AM

Unfortunately Saturn is lower in the atmosphere. So won't get the needed decent view. My larger scopes division pops in with good sky rather well. 

 

Your scope has a slight shorter focal length as f/8 and above have the advantage.

 

Years ago had a Swift 65mm, but was closer to f/10. Recall seeing the division tho not pristine, it had very decent magnification in 130x range. Swift was known to have very decent optics which I can verify. 

 

Keep trying at least 120X as sky can vary tremendously each night. 

 

You could experiment with additional eyepieces, I find most orthos perform well at very decent prices, the pinhole look can actually help with astigmatism issues. I had this year's ago, and noted better views in the smaller exit pupil eyepieces. 

 

Again don't give up, several outings might be all you need.

 

Clear Sky


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#9 Voyager 3

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Posted 02 October 2020 - 08:25 AM

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.

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 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

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). 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.

I observed Cassini for brief moments at 96× in my xt8 last week and Saturn was like 55° - this could be the factor . Also only during the brief moments had I observed it .
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#10 Second Time Around

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Posted 03 October 2020 - 08:09 AM

Galileo noted that Saturn had "ears".  His scope was not sufficient to recognize the protrusions as rings.  20X sounds about right to me.  Of course, you also have the fore-knowledge that the rings are there.  

Agreed, and not something I could forget.  The only thing I could do was to start at a low magnification and work upwards.



#11 Second Time Around

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Posted 03 October 2020 - 08:15 AM

You've made some very interesting observations.  You should compare them to the historic Saturn observations (e.g., in Sheehan's Planets and Perception) and write an article or something about it.  You've bridged the gap between early telescopes and ours.

Thanks, that book is now on my buy list.  

 

I've no plans to do an article though, as this is just one observation with Saturn being very low in UK skies.



#12 Second Time Around

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Posted 03 October 2020 - 08:22 AM

Steve.

 Nice detail review..  Sad that you so north bound for viewing the planets at present, other than Mars at higher level. I have a few Svbony lens, and I am also impressed with their quality.. I do not have their zoom lens. May I ask if it has a click stop at each of the focal lengths like the Baader zoom lens has?

    

 Dennis

No, there aren't any click stops on the SvBony 7-21mm.  I like these on the Baader, but don't like the fact that on the Baader the top of the eyepiece revolves when zoomed.  This makes changing magnification slower when using a Dioptrx, and could mean missing fleeting moments of good seeing.

 

BTW, a Dioptrx can be fitted to both the Baader and the SvBony with an O ring/elastic band if you take the eyecup off.  With the Baader it goes in the groove at the top just below the rim.  The eyecup is tightly glued on the SvBony 7-21mm but it does come off.  The ring then fits on the very top of the rim.



#13 Second Time Around

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Posted 03 October 2020 - 08:23 AM

I'm at about 39deg north.  Just a few days ago the Cassini division was easy and obvious at 100x in my 102mm ED refractor.  If I get a chance, I'll see how low I can go.

I'd be interested in what you find.



#14 davidc135

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Posted 03 October 2020 - 08:32 AM

During very fine seeing in 2018 I spotted Cassini's division with an F/15 3'' achromat, with Saturn as low as 13 degrees altitude and similarly with a 4'' sct. This year I've found it more difficult with the B&L 4000 but excellent seeing is the key.  David


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#15 Second Time Around

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Posted 03 October 2020 - 08:33 AM

My only question/concern would be getting a good 120x with a 72mm ED.

Yes I know the assorted "rules" on magnification, and also know most are not exactly a rule nor accurate.

The "Deluxe" aspect seems meaningless, it is mechanical and nothing to do with the optics - a camera rotator, focuser rotator, higher rings or a dovetail are not going to improve the optics.

 

Just have the idea that had you tried 120x on a scope easily capable of say 160x to 180x you may have seen Cassini at 120x.

 

In a way it is like my analogy to cars: If you want one that can do 100mph you buy one that is capable of 140mph, not one that says 105mph, or even 110mph. A 105-110mph car will get to 90mph, the other bit will be harder.

Altair test the optics and only those that have a Stehl of 0.95+ are used on the De-Luxe models.

 

However I do accept your point about the magnification.  It may well have been that had I used one of my Dobs at 120x I'd have seen the Cassini Division due to the greater resolution of the increased aperture.  On the other hand, the main limiting factor was probably the low altitude.  

 

I may do some further tests with my 8 inch Dob if I have time and if we have some more nights of good seeing before Saturn gets too low.  In the meantime I'm going to expand the comments in my report to take account of what you wrote.  Thanks.

 

P.S. It looks as though I've passed the time limit to edit the original post.


Edited by Second Time Around, 03 October 2020 - 08:50 AM.


#16 Second Time Around

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Posted 03 October 2020 - 08:44 AM

This isn't going to get close to the limits at 15 degrees elevation.  Even if the seeing was somehow perfect that low, one still has to contend with a lot of atmospheric chromatic dispersion.  Considering seeing and chromatic dispersion, even 30 degrees is not that great for this sort of thing.  

 

Cassini's division can be seen down to ~40mm of aperture.  On a decent night I have done it with a simple 80 f/5 achro stopped down to ~43mm.  Others have reported seeing it at 40mm.  I consider it straightforward for the AT72EDII in good seeing, but I am not observing Saturn at 15 degrees elevation.

 

I remember doing some testing of aperture stopping down a 70mm achro with poor optics to see what it took to see Jupiter's SEB and NEB.  For that scope, stopping down to 20mm at ~17x was about the minimum effective combo from what I recall.  Detecting saturn's rings as having some form of pitcher handle shape had similar limitations.

 

A big limitation for any observer when it comes to low magnification of bright objects is any astigmatism in the eye, even very mild.  Why?  Because the astigmatism creates increasing glare/blur as exit pupil increases.  I prefer keeping the pupil below about 3mm for bright objects, others will have their own limits.   It becomes a bit of an optimization to find the best combination of aperture (resolution), exit pupil, and magnification for an individual's eye and the target itself. 

 

One of the reasons for doing the observations was to get a better idea of what was needed by using zoom eyepieces.  I hadn't seen any reports other than with fixed magnifications, that are often quite a long way apart.

 

However, I agree that the low altitude doesn't make my observations in any way definitive.  There's always going to be a lot of factors involved including the scope, the seeing, the visual acuity of the observer, and of course the altitude.

 

Nonetheless, I hope they were of interest, especially for observers in more northern latitudes where Saturn won't be at a high altitude for quite a few years.

 

 

 

I'd be interested in hearing what others have found.



#17 Alex90900

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Posted 06 October 2020 - 03:44 AM

At 25 magnification, the rings are visible very confidently, so 20x is very close to the truth. Despite the fact that at 15x they are no longer distinguishable. As for the Cassini slit, its visibility strongly depends on the atmosphere, but even with a very humid sky, it can be seen at 130x, so 100x is quite reliable, in Atacama, I think, 70- 80x with sharp eyesight.


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#18 barbie

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Posted 06 October 2020 - 08:52 PM

Last night, I was able to see that Saturn is a ringed planet using 21x in my 72mm F5.6 apochromatic refractor and Cassini's division clearly and sharply defined at 105x in the aforementioned instrument. At 200x under excellent skies, I can usually see the polar hood and a cloud band or two along with a very dark shadow (when present)cast by the planet onto the rings.


Edited by barbie, 06 October 2020 - 08:58 PM.

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#19 Mark9473

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Posted 07 October 2020 - 06:04 PM

 

Baader zoom 24mm/18x magnification.  Could see that Saturn had "ears", but couldn't see a ring, even in moments of better seeing.

 

Try again, immediately after looking at a bright streetlight or the Moon.

If you get to Saturn before your eye pupils open up again, the sharper view might make a difference.


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#20 Second Time Around

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Posted 08 October 2020 - 06:00 AM

Thanks, Mark.



#21 gwlee

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Posted 08 October 2020 - 11:41 PM

At 62x I can easily see the cassini division with my 72mm f6 doublet. Have no idea what the minimum might be, but  have no reason to go lower. 




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