32 replies to this topic

[Lacaille]

Thank you Uwe for your very instructive posts!

 

Best regards

 

Mark

[russ75]

Hi Mark,

 

I've had very limited opportunities to get scope time this month, but did manage to capture Saturn a few days before opposition and on opposition day. This is my take on the Seeliger, or opposition surge effect. Two observations, especially so close together, don't definitively demonstrate the effect, but my results here are at least consistent with what might be expected: i.e., the rings are ever so slightly brighter at opposition than several days earlier.

 

The July 2022 issue of Sky and Telescope uses two of Damian Peach's 2018 Saturn images to do something very similar, so I can't claim to have devised this methodology.

 

The images were captured with a Celestron C8 (203mm f/10), ZWO ASI224MC + Orion 2x Shorty Barlow with 13mm spacer (f/22). Seeing was poor both nights with Saturn being at 32 and 36 degrees elevation.

 

 

[Lacaille]

Nicely done Russ! It’s great fun measuring things on our images isn’t it?

If you control for the peak pixel value of the disc (slightly lower in the 14th image), the peak value for the B ring on the 14th would be more elevated. You could do this by expressing the B ring peak as a ratio of the disk peak.

You do need to control for differences between days in conditions- in this case it is working against your demonstration of an effect.

[russ75]

Mark,

 

Yes, it is fun to actually do something with the pretty images. I hadn't thought of a good way to control for the differing conditions in the two imaging sessions, but I like your suggestion. I will take it and see what it gets.  I did, however, try to keep the capture and processing as similar as possible. Here's a chart showing some of the details:

 

[BillHarris]

Very interesting study, Mark. More than interesting, I'd say brilliant.
The Seeliger Effect is caused by the shadows of the ring particles "disappearing" at opposition. This is complicated by the fact that the rings, and especially the B-ring, are inhomogenous and show spokes caused by ring particles levitating out of the orbital plane. I can see that the Seeliger effect could be affected by that.
The Cassini spacecraft orbited Saturn for over a decade and returned thousands of images of the rings. I've heard of opposition brightening in discussions of Cassini images; let me ask around.

--Bill

[BillHarris]

Here are a couple of images from the Cassini spacecraft of the opposition surge (aka Seeliger effect) on Saturn's rings. Note that the color image shows an apparent rainbow effect on the bright spot. This is a false effect-- the image is made of red, blue, and green filtered images combined to make an RGB color image. The bright spot moved between exposures and acquired a red-blue fringe. The spot looks small because the spacecraft was only a few hundred thousand miles from Saturn.

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[Lacaille]

Thanks for this Bill and for your earlier kind comments.

Just to be sure I’ve understood, the bright spot is real, and is a result of reflection back to the spacecraft of light from ring particles where the shadows are occluded- a mini Seeliger effect? It is more of a focussed effect because of the proximity of the craft to the rings?

On the other hand the coloured fringing is an artefact of the imaging process?

Very interesting thanks!

Mark

[BillHarris]

That is it-- at zero phase angle, the line between the Sun, the camera and the object (that spot on the ring).is zero. It shows bright because we are looking past the shadows on the ring particles. Move your view a small distance away from that zero point, just a degree or two, and the brightness drops because the ring particles are not shadowed. Remember, this photo is taken with the wide angle lens, with a field of many degrees. Similar to the heilegeschein phenomenon with your shadow.

And yes, the color image is a combination of red, green and blue greyscale images through filters and the relatively rapid movement of that spot streaks the colors.