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:
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:
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:
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!
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.