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Seeing the Encke division February 6, 2007

Posted by dorigo in astronomy, games, internet, italian blogs, personal, science.

The other night I spent three hours looking at Saturn. The planet is at opposition these days, and if you get the right weather and a still atmosphere, you can see an awful lot of details on this little jewel of a planet.

The seeing (a word that stands for an empirical measurement of the amount of turbulence in the air) was quite good, something that is not so rare in Venice: a fact that compensates for the horrible light pollution around my terrace – which prevents me from even dreaming of observing galaxies.

But of course, years of research and the digital revolution have made it clear to us that the seeing is a quite variable feature of the air column which separates your optical instrument from the celestial bodies you aim it at. So much so that modern-day telescopes now feature adaptive optics to compensate for the Hertz-frequency variations of the deforming effect of the atmosphere.

In fact, it is precisely that advancement that has made it desirable again to build larger and larger telescopes. After the construction of the 200″ mirror of the telescope in Mount Palomar observatory, only russian had gone higher (to 240″), but without adaptive optics those instruments only paid back the investment once or twice a year.

Nowadays, adaptive optics being off-limits for amateurs, web-cams have taken their place. If you film an object and offline select only the best frames, you can reach astounding levels of resolution with little more than home-made equipment.

Anyway. I stared at Saturn with my 16″ dobsonian scope after spending twenty full minutes on the alignment. I am a perfectionist, and by Jove, at the end I had managed to get all those darn photons to meet at one single point! And it paid off: I could push the magnification to 400x, 500x, and even 800x. Saturn was beautiful. The Cassini division ran all the way through the rings around the ball of the planet. Delicate hues of green and gold  in the globe allowed to clearly distinguish several nice features, the polar cap, the equatorial zone, and the rings’ shadow.

But the real treat was during a couple of moments of perfect seeing – literally, a couple of seconds in the whole observing session – when I believe I glimpsed the Encke division. This is a feature which is really hard to see with amateur instruments: it lies almost to the edge of the A ring, outside of the Cassini division (see picture above). Its width is of only 500km or so, which translates in an angular size of less than two tenths of an arcsecond.  That is beyond the diffraction limit of even a 16″ instrument, were it not for the fact that it is a very dark, line feature on a bright area. Studies indicate that such features can be seen even if they have a width as little as a fourth of the Raileigh limit.

My observation spurred some discussion in a forum of visual observers I contribute to. It transpires that observing the Encke is some sort of a Graal for amateur instruments. Discussions in the internet range from explanations that what people pictured with their instruments is not the Encke but a processing artifact of the stacking procedures, to arguments about whether a given instrument can possibly deliver the view.

Now my goal this month is to replicate the observation -to fortify my belief that I indeed saw that feature. But the weather has to collaborate!



1. erikrau - February 9, 2007

I can’t seem to find any descriptions on the interweb (or at least not in a trivial amount of time searching) but your description of the changeable nature of seeing certainly reminds me of a description I read in Stephen O’Meara’s excellent book about the Messier Objects. [amaz]

My own experience with planetary observing has been the same–ten minutes of observing for ten seconds of a really clear image.

2. dorigo - February 9, 2007

Hi Erik,

I have a link to an italian web page, maybe you can get it automatically translated and make some use of it. Here it is:



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