Resolution Limit

Hello all. I have a question regarding observational resolution (not CCD), objective size, and the Encke Division of Saturn.

I am considering the purchase of a refractor (already have a 10" Newt). I very often read forums where refractor users claim they observe (not image) the Encke Division, some saying so with only a 6" Tak.

Well, if we assume Saturn is at its closest to Earth (1,195,000,000km) and that the rings are square on to the Earth, then the Enke Division (325km gap) will only subtend an arc of 0.06 arc-seconds.

Now, using the approx center of the visible spectrum (550A), it would take an 80 inch objective to achieve that resolution - forgetting about the limiting resolution of the atmosphere which is supposed to put Earthbound resolution limits in the 10-20" objective range.

So I guess my question is...what gives??!! Or is it just that "Believing is seeing!"

btw, Cassini is 4700km so at close approach that is a very easy 0.81 arc-seconds target for a 6" objective.

-Bill

Seeing (detecting) a thin line is a special condition for resolution. It is resonable to consider that a thin black line against a white background will be seen at R/5 to R/10. So it may be possible to see the thin line of Encke with a scope that has a resolution limit of 0.6 arcseconds. That's a 9" scope.

That will not resolve the line. But it may allow you to detect the line.

edz

William,
There is some confusion over the Encke Division and the Encke Minima. Most see the Encke Minima. But the Encke Division is possible to see one a great night with good contrast and excellent optics.
There are two well-known features visible in Saturn's A-Ring, one called the Encke Minima, and the other called the Encke Division. The Encke Minima is a broad, low contrast feature that is located about halfway out in the middle of the A-Ring. The Encke Division is a narrow, high contrast feature located near the outer edge of the A-Ring, and unlike the Encke Minima is an actual division within the ring.

It has been generally accepted that Johann Franz Encke first reported seeing what is now called the Encke Minima in 1837, while James Keeler first reported seeing what is now called the Encke Division in 1888. However, there is evidence that other observers saw both the Encke Minima and Encke Division before this. This article will discuss these observations in greater detail. It will discuss also some of the important factors involved in seeing these two features, including how wide the rings were as seen from Earth, Saturn's altitude, its diameter, seeing conditions, aperture of the telescope used, and magnification.