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Measuring the Seeliger Effect on Saturn's rings

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#1 Lacaille

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Posted 26 July 2020 - 08:20 PM

Hi All,

Rain and clouds have closed in here in Canberra so, intrigued by various postings here on CN on the Seeliger Effect on Saturn’s rings, such as the nice one by Andrew (Tulloch), I thought I would review my own 2020 images to see what I could extract.

 

It hasn’t been a great year for Saturn so far for me; the seeing at the time I have been out has never really allowed me to attain the exquisite images that others have achieved. By the same token, I have often only imaged it briefly (typically three X three minute runs) as an adjunct to a heavy Jupiter or Mars session.  Increasingly, I realise that you need to be imaging for hours to get those prized moments of good seeing – altitude is just part of the picture.

 

On the Seeliger effect, I was interested in how long the perceived brightening would last, and what was the shape of the brightening curve.  On the internet, I found references to its lasting just a few days around opposition.   The effect is attributed to the make-up of the rings – lots of large chunks of material which show bright and shadowed portions for much of the time, until opposition, when the amount of shadow seen falls close to zero, with the sun shining on the rings from directly behind us – a bit like looking at the full moon rather than the terminator.

 

I discovered I had 13 images dating back to 12 March 2020 that would allow image measurements. Here are the images:

 

Seeliger effect images 2020.jpg

 

You can see the apparent brightening of the rings around opposition. Here is a split view of the planet a day after opposition on 21 July UT and just four days earlier, for example:

 

Seeliger divided view.jpg

 

You can also see, however, a lot of variability between images, due to seeing, processing and equipment variations, plus the well-known “klutz factor”, that might delude one into seeing what is not there. So I decided to try and use ratios to remove some of this variation from the picture, by comparing ring brightness to the brightness of the yellowish equatorial zone (EZ) of Saturn in each image.   My images generally show the A ring and the B ring separately, so I attempted to measure the trends in brightness for the two rings. 

 

I downloaded and taught myself the basics of ImageJ software (free!) from the NIH (generally used for measuring images of tissue samples, but with wider applications, including in astronomy – e.g. we have also been using it in a school project to measure the position of the terminator on Venus). 

 

I used the software to measure average grey value in the EZ, and the  A and B rings, for the 13 images above.  I then expressed the values of brightness in A and B as a proportion of the value for the EZ, in an attempt to reduce the uncontrolled variation between images. Of course, this assumes there is no great background variation in the brightness of the EZ, perhaps a reasonable assumption over the five months of these images?

 

Here are the results:

 

Seeliger effect graph 2020.jpg

 

Note the image of Saturn showing the areas marked where I was measuring grey value.

 

You can see that the values for both A and B rings reach a sharp peak around opposition and it does seem that the effect is very short-lived.  However, for the B ring in particular, there does seem to be a slow trend upwards in relative brightness as opposition approaches. For the A ring, this is also apparent, though there is much more variation in the trend, probably because of the smaller measurement area I had to use.   For the B ring, the peak value is reached at 112% brightness compared to the EZ, and for the generally dimmer A ring it is 60%. The values fall precipitously just 3 days after opposition.

 

I hope I haven't made some major blunder and that this is of some interest!

 

Regards

 

Mark

 

Edit: I did blunder in one annoying way - opposition was around 21 July Australian Eastern Standard Time, but 20 July UT, and all my image dates  above were on the basis of UT, so I was a day out in my earliest version of this post. The text and graphs above are now correct on the basis of 20 July opposition.  Apologies.


Edited by Lacaille, 27 July 2020 - 02:36 AM.

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#2 Kokatha man

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Posted 26 July 2020 - 08:33 PM

Very interesting Mark! waytogo.gif

 

I've only had a cursory read of your post as I'm about to publish some of my own data :lol: on the noise inherent in the ASI462MC camera...your own investigations probably hold much more than my initial (& clumsy!) attempts to quantify some of the fiddling around I've done on my subject however..! :rofl:

 

Anyway, will come back & read much more carefully...posting mine & then off for the day. ;)


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#3 Lacaille

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Posted 26 July 2020 - 11:20 PM

Thanks for the encouragement Darryl!  I have had a look at your Noise/462 post but have not posted anything as I am not sure if you have finished posting images!   I think you can use ImageJ for measuring noise and calculating SNR to compare cameras but it might make our heads explode!

 

Regards

 

Mark


Edited by Lacaille, 26 July 2020 - 11:20 PM.


#4 DMach

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Posted 27 July 2020 - 05:03 AM

Nice analysis Mark!


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#5 Dunkstar

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Posted 27 July 2020 - 05:19 AM

Good work Mark waytogo.gif

 

Most of my efforts at capturing Saturn this year have been short lived with apparent rough seeing, but Jupiter being especially close by is too tantalising...it’s so dynamic just watching it shocked.gif


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#6 MalVeauX

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Posted 27 July 2020 - 10:07 AM

Excellent observation, work and presentation! Very interesting!

 

Very best,


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#7 KTAZ

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Posted 27 July 2020 - 09:18 PM

You know, I didn't even realize it until I looked at a photo that I just happened to grab on the 19th...and I was wondering why the rings looked so bright (thought I'd made a processing error!).

 

WTt4vFS.jpg


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#8 sunnyday

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Posted 27 July 2020 - 09:22 PM

I will go to bed smarter tonight thanks to this wonderful report.
thanks.


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#9 Mike Phillips

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Posted 29 July 2020 - 09:24 AM

Lacaille, this is a fantastic write up, thanks for sharing!

 

Mike


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#10 Lacaille

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Posted 31 July 2020 - 06:37 AM

Thank you very much for the likes and kind comments. In a few weeks, I hope to post an update with images from the period after opposition and the revised grapH showing the return to background brightness.

Regards

Mark

#11 james7ca

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Posted 31 July 2020 - 09:37 AM

An interesting report with some nice graphics. Thanks for posting.


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#12 Lacaille

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Posted 13 August 2020 - 09:01 PM

It has been a fairly disruptive period for us here with work going on in the yard to make it more astronomy- (for which read C14-) friendly.  I have all the scope gear and mounts stored safely but rather inaccessibly.

 

Last weekend saw us nearly inundated, as the very clayey ground had been dug up, but the new surfaces and drains had not been started, and a long downpour began on Saturday through to Sunday and resulted in blocked storm drains and water rising to within a few cm of the door frames. 

 

Yard no 2.jpg

 

I borrowed wellington boots and wore these for the first time since boyhood.  Sticking a crowbar into a drain cleared a plug of clay and resulted in a subterranean belching noise followed by a whirlpool as 24 h of rain disappeared in 30 minutes, blowing the caps of next door's drains but leaving us tired but happy.  There was a period where we realised we no longer needed to be out there in the rain but we stayed out anyway, digging channels and making dams in the mud to allow the water to drain away more rapidly. All this took me back to my childhood.

 

Things are steadying up now as the yard takes shape, hopefully in good time for the Mars opposition.   Last night I was able to find a comparatively undisturbed corner of the yard to set up my trusty 8SE, with the ADC/EFW/1.3X Barlow/ASI462MC combo in the back.  I had to fashion a counterweight for all this using 50 cent coins in a ziplock bag velcroed to the OTA, and a simple solar system alignment did a good job of tracking Jupiter and Saturn, but the seeing was awful as it has been often this year.  I was able to do only a rather poor Saturn,  to continue collecting data on the return to background brightness from the peak at opposition.

 

Here is the updated graph showing that the relative brightness of the rings is returning to background quite quickly. 

 

Seeliger graph Aug 2020.jpg

 

I have been trying to work out why the peak should be so sharp. If the lumps that make up the rings were behaving like the moon, you might expect a smoother gentler rise to a peak (considering how the moon's phases and brightness change during the monthly cycle).  With a peak as sharp as this, I can think of the following possibilities that could explain it:

 

1.An artefact of the imaging process;

2. Mutual shading of the ring particles;

3. Ring particles deeply honeycombed with pits and caverns;

4. Some kind of polarisation of reflected light of the ring particles - but why would they all be similarly lined up?

 

I have looked for scientific material on-line but so far have had lean pickings.   I will keep looking and update this post in a couple of weeks, perhaps with more data.

 

Mark


Edited by Lacaille, 14 August 2020 - 12:22 AM.

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#13 KiwiRay

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Posted 14 August 2020 - 01:12 AM

My understanding was that it was mainly (2), with shadows of the ring particles disappearing from our perspective at opposition, but I've made no effort to look at scientific work to support that theory.  The brightness surge isn't just in imaging, though.  It's detectable to the naked eye, and results in a sharp spike in Saturn's visual magnitude, so I don't think (1) is part of the answer.  Interested if you turn up any more information.

 

Also, I had no idea of the commitment owning a C14 entailed - earthworks and flooding?!  Yikes.


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#14 Lacaille

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Posted 14 August 2020 - 01:54 AM

Also, I had no idea of the commitment owning a C14 entailed - earthworks and flooding?! Yikes.


And wellingtons- don’t forget the wellingtons!
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#15 troyt

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Posted 14 August 2020 - 05:14 AM

Not sure if you have come across this information Mark

This was reported around the late 1800's see here   

His is another report here  around 1963

 

This effect seems to go by different names even among amateurs and searching for these names might be helpful to unearth more information. 

 opposition surge

 opposition effect

 opposition spike

 Seeliger effect

 

Visually in images and your graph seems to support the greatest brightness difference comes from the B rings and I can only speculate at this point for the reason why this is. 


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#16 Tulloch

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Posted 14 August 2020 - 06:38 PM

And wellingtons- don’t forget the wellingtons!

I was thinking that at this point you might need to employ something like the "crawler" that NASA use to move the Saturn V rocket or Space Shuttle to launch pad 39 from the Assembly building lol.gif .

 

ksc-20180522-crawler.jpg?itok=XEjH5mxN


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#17 Lacaille

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Posted 14 August 2020 - 06:45 PM

Thanks for the likes and comments!  Troy, those references are useful thanks.  I will see what I can dig up over the next few weeks - not much astronomy wiull be happening here for a bit!

 

Andrew, here is my equivalent. My stately progress out to the mount with the OTA has a lot in common with the Saturn V's roll to the launch pad.

 

C14 on trolley.jpg


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#18 Lacaille

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Posted 09 August 2022 - 09:08 PM

As Saturn's opposition approaches (14 August), I thought I would resurrect my thread for those pursuing the ring brightening of Saturn at opposition.  As bad weather and bad seeing persists here in Canberra, we here are likely to make a contribution on the topic this year!

 

Here is the updated graph for 2020:

 

Seeliger effect 2020.jpg

 

Key factors are that it is mostly the B ring that shows the effect, and it is very pronounced in the few days around opposition. I did not make headway in understanding the phenomenon better, but the mutual shading idea of ring particles is quite prevalent as an explanation.

 

Best regards

 

Mark


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#19 Uwe Pilz

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Posted 15 August 2022 - 12:40 AM

I observed Saturn last night. I saw the Seeliger effect very clearly, and made a sketch.

The coherent back scattering causes most of the brightening. It is an effect of the electromagnetic field, which polarizes matter, so the particles become dipoles. The dipoles itself emit light. This light is strongly directed.
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#20 Lacaille

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Posted 15 August 2022 - 04:10 AM

Thanks for your comment Uwe- do you have a reference so I can read up on this mechanism?

Regards

Mark
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#21 Uwe Pilz

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Posted 15 August 2022 - 04:56 AM

Dear Mark, most instructive are the pages of Philip Laven
http://www.philiplaven.com/index1.html
The content is the optics of the water drop, but dust particles are similar.

I wrote a few programs (in C) which simulates most of the effects. With that, I can estimate the size range of the particles which cause the effect: a few micrometers to a few dozen micrometers.

 

It would be more accurate if I select the size range so that you very accurate measuring fit. I am on it, calculating that.


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#22 joachim.ong

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Posted 15 August 2022 - 09:54 AM

Thanks for bringing up the graphs again i find it fascinating. Curious that this phenomenon isn't more widespread, although it seems like more people have heard of it now (i only came across it last year).

I observed saturn yesterday night as well and the rings indeed looked really bright to my eye, almost glaring. The difference in brightness was more pronounced than I recall during last year's opposition, although I am wondering if it could be due to the fact that i was using a lower magnifcation this year.

Edited by joachim.ong, 15 August 2022 - 09:54 AM.

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#23 Uwe Pilz

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Posted 15 August 2022 - 10:44 AM

The effect is more pronounced it Saturn is close to the nodes of the orbit. 2020 was much better.

~

I tried to simulate the Mark's measuring. First, I found the phase angle, which my program gives. Then I played around with the size of the particles, which scatter light. Best fitting were particles between 4 and 10 µm. But the effect was much more pronounced as seen. So we have to assume, that the light from the ring mixes between this scattering and a more homogenous part, most probably from other particle sizes. Sub micrometer particles give nearly homogenous light. I tried simulation a continuum form sub µm to 10 µm, but this did not fit. It may be the population between sub micrometer and some micrometers is underrepresented.

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#24 Lacaille

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Posted 15 August 2022 - 06:34 PM

Thanks for bringing up the graphs again i find it fascinating. Curious that this phenomenon isn't more widespread, although it seems like more people have heard of it now (i only came across it last year).

I observed saturn yesterday night as well and the rings indeed looked really bright to my eye, almost glaring. The difference in brightness was more pronounced than I recall during last year's opposition, although I am wondering if it could be due to the fact that i was using a lower magnifcation this year.

Thanks Joachim!  Unfortunately we have had such bad weather this year that I have not been able to repeat my study, and I was not aware that the effect might be more pronounced at the nodes of the orbit, as Uwe indicates below.

 

The effect is more pronounced it Saturn is close to the nodes of the orbit. 2020 was much better.

~

I tried to simulate the Mark's measuring. First, I found the phase angle, which my program gives. Then I played around with the size of the particles, which scatter light. Best fitting were particles between 4 and 10 µm. But the effect was much more pronounced as seen. So we have to assume, that the light from the ring mixes between this scattering and a more homogenous part, most probably from other particle sizes. Sub micrometer particles give nearly homogenous light. I tried simulation a continuum form sub µm to 10 µm, but this did not fit. It may be the population between sub micrometer and some micrometers is underrepresented.

Thank you very much for following up Uwe.  I had been working on the assumption that it was a shadow effect as described at this link:

 

"The opposition effect exists because of two contributing factors. One is due to the fact that the shadows of ring particles directly opposite the Sun from Cassini -- the region of opposition -- fall completely behind the particles as seen from the spacecraft. These shadows are thus not visible to the spacecraft: all ring particle surfaces visible to the spacecraft in these two images are in sunlight and therefore bright. Much farther away from the region of opposition, the ring particle shadows become more visible and the scene becomes less bright. The brightness falls off in a circular fashion around the opposition point."

 

But they also talk about a second factor: coherent backscatter:

 

"Here, the electromagnetic signal from the rays of scattered sunlight making its way back to the spacecraft is enhanced near the region of opposition because, instead of canceling, the electric and magnetic fields comprising the scattered radiation fluctuate in unison."

 

Is coherent backscatter the same thing you are invoking? Sorry, I am an ecologist, and an amateur astronomer, not a physicist!

 

Regards

 

Mark


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#25 Uwe Pilz

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Posted 15 August 2022 - 11:23 PM

 

 

"Here, the electromagnetic signal from the rays of scattered sunlight making its way back to the spacecraft is enhanced near the region of opposition because, instead of canceling, the electric and magnetic fields comprising the scattered radiation fluctuate in unison."

 

 

Yes, this is backscatter, slightly simplified. 

 

Scattering is in general underestimated. Beside backscatter, we have forward scattering too. The silver line around a cloud 

https://en.wikipedia...lver_lining.jpg

is forward scattered. And it is real bright! We don't have any shadow effect here. And we don't have light refraction in the droplets, because this would lead to color effects.

 

If you want to observe forward scattering: Look at the shadow of your head if the sun is rather low. You have some kind of aureole around it. It is seen easiest in the morning in damp grass.

https://hjschlichtin...000rv.jpg?w=382

Again, one could assume refracting effects, and probably there is some. But you see the aureole slightly weaker even in the evening and even at the shadow at a dusty path! This is back scattering.


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