Saturns Rings

As well as Saturn's rings and the Cassini division providing an excellent sharp edge on which to focus, they also act as a great seeing indicator and can be used to check the color balance of an image. Let us look at the ring structure in more detail.

The Pioneer 11, Voyager 1 and 2, and Cassini spacecraft all showed that the Saturnian ring system is highly complex. Fortunately, the view through the amateur's telescope is a lot simpler and basically boils down to a system of three rings, called, not surprisingly, A, B and C. Saturn's A ring is the outer one and has a grayish or grayish-blue cast. The A ring, as we saw earlier, has a diameter of 274,000 kilometers and a width of 14,600 kilometers. Near the outer edge of Ring A, some 90% of the way from inside to outside edge is the black gap called the Encke division. It has a width of only 325 kilometers and so subtends only 1/20th of an arc-second as seen from the Earth! However, because it is such a dark feature, it can be detected, as a contrast drop in ring A, with instruments as small as 15 or 20-cm in aperture, despite being well below their stellar resolving power. The Encke division is, undoubtedly, the ultimate planetary resolution goal for any amateur imager and can only be seen or imaged when seeing conditions are close to perfection. However, under such situations it can even be detected, as the merest dark sliver, on single raw frames. Before the CCD/webcam imaging era, visual observers rarely, if ever, spotted the Encke division. Indeed, prior to Pioneer 11's flyby of the planet in September 1979 the exact position and even the existence/permanency of the feature was disputed, despite the fact that James Keeler had sketched the position accurately, using the 36-inch Lick Refractor, in January 1888. When Saturn's rings are wide open and conditions are close to perfection the Encke division can be imaged almost all around the rings with 25-cm apertures. However, in practice, even stunning amateur images rarely capture the Encke division far from the ring ansae (the east and west tips) and as the rings are now narrowing the Encke division will just become harder to resolve, even at those points. To my knowledge the first amateur image ever showing the Encke division was obtained in 1998 by the French imager Thierry Legault, using a 30-cm Meade LX200 and a Hi-Sis 22 CCD camera. Prior to that, the only images showing the feature clearly from Earth seem to be ones taken with the 1-meter Pic du Midi Cassegrain in the Pyrenees and 1.54-meter Catalina telescopes in Arizona. However, since the webcam era, the feature has been resolved annually in the very best amateur images. It is interesting to note that, quite often, the feature will only be recorded at one ansae and not both. This may well be because the feature is so thin that if the CCD chip is not perfectly flat in its base, one side may be in focus and the other side fractionally out-of-focus, rather than a physical difference in the Encke division width. Imaging the Moon with the same set-up can identify problems of this nature.

The Encke division, which is incredibly thin and dark, should not be confused with the Encke minimum, which is more of a perceived gradual contrast feature across the A ring thickness, making the outer half of the ring appear slightly darker than the inner and giving the illusion of a division in the A ring center.

It is important to note that spurious ring divisions can easily occur when stacking thousands of images of Saturn together. Think, for a moment about what is happening in less than perfect seeing. The atmosphere not only blurs the planet, it physically distorts it, too. The most popular stacking software, Registax, tries to cope with this by rejecting the most distorted images, but, the fact remains that the boundary between Saturn's A ring and the night sky is a high-contrast feature. If the position shifts too much, artificial ring divisions can appear.

Moving in from the gray outer ring we come to the much wider Cassini division. This chasm is 4,700 kilometers wide and so spans almost 0.8 arc-seconds at opposition. Even in poor seeing the Cassini division is usually easy to spot when the rings are wide open, but if seeing is appallingly bad, it too can disappear in small telescopes! These days, once thousands of webcam frames are stacked, on a reasonable night, the Cassini division can easily be traced all the way around the planet. However, by the time you purchase this book, the rings will be closing noticeably and, as 2009 approaches, even the Cassini division will become a challenge.

Ring B is Saturn's widest ring, at 25,500 kilometers from inner to outer edge. Unlike the A ring it appears to be almost colorless (a pure white or gray) and is, therefore, an excellent check on how good the color balance in the final image is. Ring B has nothing like the Encke division within it but, it does gradually darken as you move in toward Saturn. At the point where it almost disappears it merges into the illusive C or "Crepe" ring.

The Crepe ring is a feature that even today's CCD technology struggles to record. Not because it is narrow, it is not. The problem lies in the faint, ghostly nature of the C ring. Increasing the brightness and contrast of a webcam or CCD image will reveal the transparent Crepe ring through which the globe of Saturn is clearly visible. However, the dynamic range of even today's electronic detectors cannot quite match the human eye's ability to stare at the globe of Saturn and the Crepe ring and see them both together. Increasing the brightness and contrast of a webcam image to show the Crepe ring will saturate the planet's globe, but reducing the brightness and contrast to normal levels will sink the Crepe ring down into the blackness. In most amateur webcam images, the Crepe ring is only well seen where it crosses in front of the globe, and the planet can clearly be seen through it. On January 13, 2005, I was lucky enough to observe Saturn within a few minutes of opposition (i.e., Saturn opposite the Sun in the sky). The brightness of the rings was dramatic, as always happens that close to opposition. My colleagues Damian Peach and Dave Tyler were also imaging that night and we were all amazed at the increase in the ring brightness and the apparent dullness of the globe by comparison (see Figure 14.7). This is called the Seeliger effect and occurs partly because the individual ring particles are not casting shadows on top of each other. But, most noticeable of all was the obvious presence of the Crepe ring; normally ghostly, it was very obvious. Damian and Dave produced a stunning composite image from opposition night, consisting of 9,500 stacked frames.

Figure 14.7. Possibly the best amateur image of Saturn ever taken? Damian Peach and Dave Tyler, using Celestron 9.25 and Celestron 11 SCTs (respectively) and living only a mile apart, secured excellent images of the ringed planet on January 13, 2005, within a few minutes of precise opposition! Note the extreme brightness of the rings and the relatively dull globe at precise opposition. Both observers exposed thousands of red, green, and blue frames in 15-minute time windows and the resulting composite consists of 9,500 webcam frames! Image: D. Peach and D. Tyler.

Figure 14.7. Possibly the best amateur image of Saturn ever taken? Damian Peach and Dave Tyler, using Celestron 9.25 and Celestron 11 SCTs (respectively) and living only a mile apart, secured excellent images of the ringed planet on January 13, 2005, within a few minutes of precise opposition! Note the extreme brightness of the rings and the relatively dull globe at precise opposition. Both observers exposed thousands of red, green, and blue frames in 15-minute time windows and the resulting composite consists of 9,500 webcam frames! Image: D. Peach and D. Tyler.

Despite the faintness of Saturn's globe it can, remarkably, be imaged in morning and evening twilight when conditions are often very stable as the atmospheric cooling is at a minimum. The planet can be located, with the aid of a GO TO system, or setting circles, shortly after sunset and imaging can take place with the Sun less than 10 degrees below the horizon. In such situations, in the evening, the red images are best acquired first and the blue last, as the twilight sky is mainly blue and will interfere with the blue frames if twilight is too bright. At dawn the blue images are best acquired first, and the red images last. Twilight images are often inevitable when the planet is far from opposition. On these occasions the shadow of the Saturnian globe is quite evident as it spreads over the rings behind the planet and off to one side. A twilight image almost three months after opposition is shown in Figure 14.8.

Finally, before we leave Saturn, let us remind ourselves once more, that at low f-ratios, the ringed planet's brighter moons can all be captured in a webcam frame, even with 0.1 second exposures. Figure 14.9, by Dave Tyler, is a clever composite of a low-gain imaging run to capture the planet and a high-gain imaging run to record the moons. Both AVIs were taken within a few minutes and then superimposed. As the rings close up in the next few years, the moons will tend to appear more frequently in our webcam images.

Figure 14.8. Saturn imaged nearly three months after opposition by Dave Tyler on April 2, 2005. The image was taken in a very calm seeing period in evening nautical twilight. Celestron 11 at f/40. ATiK 1HS webcam. Image: D. Tyler.

Xetliys

Iapetus Tit 11 IT

Rhea t

Mimas • Encelaihis

Figure 14.9. Saturn's moons can easily be captured with a webcam. In this f/10 composite by Dave Tyler (Celestron 11), seven of Saturn's moons and two field stars have been captured with individual exposures of only 0.1 seconds. The faintest Moon, Mimas, is only magnitude 12.9. Image: D. Tyler.

CHAPTER FIFTEEN

Imaging Uranus and Neptune

This chapter could not have been envisaged before the webcam era; it would have been considered ludicrous! Uranus and Neptune are in a different category to all the other planets I have mentioned (with the possible exception of Mercury) because they are just so tiny even through a quality amateur telescope. Not only that, but even compared to distant Saturn they are a very long way away from the Sun, and therefore very dim. The critical parameters are detailed in Table 15.1.

At a glance, we can see that even at an image scale of 0.2 arc-seconds per webcam pixel, Uranus currently spans less than 19 pixels and Neptune only spans 12! Surely, little can be done with these planets. Well, we must be realistic here. There is, indeed, little prospect of revealing much more than major, global weather upheavals, but who is to say that such events do not occur? The image by Christophe Pellier in Figure 15.1 hints that major atmospheric events would be detectable, even with a small aperture. Only one spacecraft has passed Uranus and Neptune at close range: Voyager 2 in January 1986 and August 1989, respectively. The Hubble Space Telescope has imaged both planets from time to time and, in recent years, the giant Keck telescope on Hawaii has obtained Hubble-quality images too, using advanced adaptive optics techniques.

As we have seen with Mars, a small planetary disc does have some "silver lining" advantages for the webcam imager. With the Earth's atmosphere imposing a resolution of 0.5 arc-seconds on the overwhelming majority of nights, if a planetary disc appears tiny, it can be allowed to rotate for tens of minutes before the rotation smear exceeds the atmospheric/telescopic resolution. This is a big advantage with planets as faint as Uranus and Neptune where a webcam will be working right on its limit. Uranus rotates in 17 hours, 14 minutes and Neptune rotates in 16 hours, 7 minutes. However, there is an interesting twist with Uranus. Neptune has an axial tilt of 28.8°, similar to Saturn, but Uranus spins virtually on its side! The axial tilt of Uranus is almost 98°, so, as this is more than a right angle, it is technically rotating

Table 15.1 Critical Parameters of Uranus and Neptune

Mean Distance

Orbital Period

Diameter

2006 Opposition

from Sun

(years)

(equatorial kms)

Diameter

(millions km.)

(arc-seconds)

Uranus 2,870

84

51,118

3.7"

Neptune 4,500

165

50,538

2.4"

Figure 15.1. Uranus imaged with a modest 180-mm Newtonian and ATiK 1HS webcam on July 6, 2004. Image: Christophe Pellier.

backwards (like Venus, except much faster). So, at times, as Uranus moves around in its 84-year orbit, it will point its north pole towards us, then, 20 or so years later, its equator, then its south pole, then its equator again! Uranus' south pole was pointed at us in 1985. In 2030 we will see the north pole. So around the publication date of this book we are seeing both Uranus' hemispheres, much like for any other planet except, its axis will be horizontal as we look at it.

Applying our favorite formula for a maximum drift window to Uranus and Neptune gives us:

0.5"/((3.14 x 3.7")/1034 minutes) = 44.5 minutes and

0.5/((3.14 x 2.4")/967 minutes) = 64.2 minutes In other words, we have 45 and 62 minutes, respectively, in which to collect the frames before Uranus and Neptune's central features have drifted by half an arc-second. Imaging these planets will not be a rushed affair! Being realistic, Neptune is, perhaps, an object best left to the Hubble Space Telescope (see Figure 15.2). As for Pluto, well, that is without doubt, purely a Hubble target (Figure 15.3), even though it is possible to view it visually, as a dot, through a large amateur telescope.

Figure 15.2. Neptune and it's large Moon Triton imaged by the Hubble space telescope. Image: NASA/STScI.

When taking images of objects that are as small as Uranus and Neptune, almost featureless, and so rarely imaged, it is impossible to verify the details that emerge after extreme image processing. The effects of atmospheric dispersion in Earth's atmosphere alone can cover most of the planet, making any features resolved somewhat speculative! To really unequivocally resolve details on Uranus and Neptune you have to image them with the Hubble Space Telescope or with ground-based telescopes with adaptive optics that can compensate for the effects of the Earth's atmosphere and are sited at the best locations on Earth.

Figure 15.3. Two faces of Pluto and Charon imaged by the Hubble space telescope on March 7, 1996. Image: NASA/STScI.

CHAPTER SIXTEEN

Imaging the Sun

When I started writing this book I had no plans to include a chapter on solar imaging. Why? Because the Sun is a dangerous object that should never, ever be observed either with the naked eye or through a telescope unless, in the latter case, the amateur astronomer is highly experienced and equipped with the right filters. Personally, although I would class myself as an experienced amateur and I do have the right filters, I still feel very nervous about observing the Sun, even with expensive equipment. Even experienced amateurs have made mistakes in the past and damaged their eyesight. The most common error is to imagine that a filter that dims the solar disk to a pleasing level is actually safe. DO NOT BE FOOLED! It is the infrared (heat) radiation that can destroy the retina, and that is invisible. However, despite my fears about solar observing I decided that, in the webcam era totally safe solar observing was possible: hence this chapter. The safest way to observe sunspots on the solar disc, without webcam equipment, is to project the image onto card, using a small refractor. A large cardboard shield can be used to prevent direct sunlight washing out the projected image. It is interesting to note that the vast majority of eyesight injuries, following events like solar eclipses, are due to people staring at the Sun through half closed eyes, in the misguided belief that this must be harmless. The lack of pain-generating nerves in the retina gives the illusion that you are just being dazzled and all is well . . . WRONG! NEVER STARE AT THE SUN WITH THE NAKED EYE!

Fortunately, the webcam, yet again, offers a superb way of imaging sunspots at high resolution but with absolutely no risk to the eyesight. In addition, a webcam is cheap, and even if it is damaged, it is not a big problem. Damian Peach, who is one of the world's leading planetary imagers, had an interesting experience when using a webcam to image the Sun. While packing the equipment away, he took the solar filter off first and then started to pack his PC away. When he next emerged from the house the webcam was totally ablaze! By removing the special solar filter the amount of heat and light falling on the webcam had increased by 100,000 times, setting fire to the webcam. However, as I pointed out to him, only the day before he had been imaging the Sun with a new $2,000 digital SLR. Maybe the webcam accident was a timely reminder of the Sun's power. Even when you are not observing visually accidents can happen. Telescope finders should be capped as well as the main instrument. There has been at least one documented case of a beard igniting after being set ablaze by light from the finder! Also, when you are packing equipment away and removing solar filters, point the instrument away from the Sun first. The heat entering even a small telescope is enough to melt a plastic secondary mirror holder or to make the glue on a Maksutov baffle tube go soggy and melt!

Large telescopes are not necessary for solar observing, and for two reasons. Firstly, the Sun is extremely bright, even when filtered, so light grasp is not an issue. Secondly, when the Sun is above the horizon, atmospheric seeing is at its worst, so a large aperture rarely gives any resolution advantage. A 10- or 12-cm refractor is the most that is needed and even an 8-cm instrument can get stunning results. The solar brightness leads to one big advantage, exposure times can be very brief to freeze the moments of good seeing. In addition, as there is no planetary rotation problem and Registax' alignment software will lock onto the shape of a sunspot, there is no time window restriction for imaging.

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