Saturns Time Window

Fortunately, Saturn does have one big advantage compared to Jupiter. The visible globe, at 20 arc-seconds in diameter, is less than half that of Jupiter at opposition. Thus, the planet takes longer to rotate before features exceed our 0.5 arc-second equatorial drift limit. The rotation period, at 10 hours, 14 minutes, is slightly longer too. Another factor here is that the Saturnian globe contains no high-contrast features. It will not really matter if features do drift a bit. Also, most Saturnian features are at relatively high latitudes, too; a spot at 60 degrees north or south will travel at half the speed across the planet's meridian as a spot at the equator. Subtle color changes take place in the polar regions every year and can be recorded by amateurs, as shown in Figure 14.6. With regard to image smearing with the planet's rotation, the simple fact is that, unless thousands of webcam frames are stacked together, the subtle, low-contrast Saturnian spots will never emerge from the noise anyway, at f/30, unless only red images are used, where the CCD is more sensitive and seeing generally better. To record Saturn's subtle features it may well be necessary to drift beyond the self-imposed limit of 0.5 arc-seconds. To record the subtle color changes in Saturn's polar regions it is perfectly OK to image for 15 minutes or so. If the equator drifts by over 1 arc-second, but there are no features in it, who cares!

Applying our well-used formula, and inserting 20 arc-seconds for the opposition diameter of the planet and 614 minutes for the rotation period gives us:

In practice, increasing this value to around 6 minutes will not produce any noticeable smearing on high-latitude spots. A 6-minute time window is obviously a far more manageable time span for changing filters than the 2-minute time span we suffer with Jupiter. At 10 frames per second, up to 3,600 frames can be exposed and even if almost half of these are rejected for poor quality, an image consisting of 2,000 frames will have quite a smooth appearance. As I have said, when there are no spots on the disc, or when seeing is not excellent, there is no reason why a 10- or 15-minute run should not be stacked. Such a composite will produce a very smooth result, giving a high-quality, if largely featureless, image. However, for images subjected to scientific analysis the precise exposure details should always

2002-2003 2003-2004 2004-2005

Figure 14.6. Subtle changes in the color of Saturn's south polar regions over three observing seasons can be recorded by amateur observers. Image: Damian Peach.

2002-2003 2003-2004 2004-2005

Figure 14.6. Subtle changes in the color of Saturn's south polar regions over three observing seasons can be recorded by amateur observers. Image: Damian Peach.

be recorded and supplied to the analysing body. Although Saturn appears sharpest in red and green frames when using filtered imaging, the blue frames are the highest contrast as regards revealing the darker belts. Reducing the blue content of the image will increase the sharpness but also decrease the contrast of the globe, which will tend to become featureless except at the poles.

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