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Posted by David Arditti at 15:20 on 2010 Mar 25
The previous night was "opposition night" but I was occupied that night with a very successful public observing session run by West of London AS, on which we showed 250 people the Moon, Mars and Saturn. So these are my closest images to opposition this year. I had poor seeing and very poor transparency. The spot (or "storm") in the SSTrZ is on the CM in the R image, at a System III longitude of 6 degrees. It is visible in IR as well. I have left Titan in all the images for added interest, to the lower right – bright in IR, very faint in blue. When stretched, the IR image shows 4 more moons, but not Mimas which is too close to the rings on the preceding side.The colours of Saturn’s globe never look "normal" when imaged within a day or so of opposition. They always look dulled and blue-ened. I think this phenomenon, noted to me by several good imagers, remains unexplained.
Posted by Andrea Tasselli at 07:51 on 2010 Mar 26
Well, actually the explanation might be quite simple: The imager adjust his/her gain level so as not to over-expose any area of the planet. As Saturn closes in toward opposition (at very small phase angles) the rings brighten up considerably but not the globe. Keeping the same gain as not to overexpose the rings yields a lowered signal for the globe and hence more muted colours. By the way during this opposition I haven’t noticed any undue dulliness, since the rings brightness increase is very small (small rings angle).Andrea T.
Posted by David Arditti at 14:07 on 2010 Mar 26
That may be part of it Andrea, but I’m not convinced it is the complete explanation, because the colours of the globe, in most imagers’ images, around opposition, are really quite different to the colours away from opposition, as seen here. The effect is more than just an intensity change.I have done various observations to test the Opposition Effect of Saturn over the last 3 apparitions. In 2008 I concluded that the brightness of the rings increases most in the UV and IR, somewhat in the blue, and hardly at all in the red and green (I published a note on this, with images, in the Journal, April 2008). Now, assuming that the CCD filters that observers use are effectively blocked against UV and IR, this means that, in images, the rings should become blue-er near opposition. But I think actually imagers generally balance the colours so that the rings look white. This process would be expected to relatively suppress the blue in the globe, and make it redder or yellower. But what we are seeing is not quite this either. In fact I have found that no balance of the channels can make the globe colours look normal at opposition. So I am not decided whether this effect is something to do with the characteristics of filters, if if something more "real" is going on here.
Posted by Andrea Tasselli at 10:41 on 2010 Mar 27
Hi David,I don’t know about other imagers but I have imaged Saturn since 2001 and it never occurred to me that it changed colours at (around) oppositions. As for the current one (I missed the past apparition’s) I’m pretty sure it didn’t change colours, by and large. Fact is that *measuring* photometric changes in saturn’s globe is devilishly difficult and full of traps and catches. I tried to keep track of relative response of rings and globe in various colour bands (I think from 2004) but it is difficult to come down to hard facts. Certainly if such a thing does really happen any factual assessment can’t be based on the fancies of this or that imager. Besides, I can’t think of any reasonable physical explanation for the purported colour changes.Andrea T.
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