Saturn through the Webcam

Saturn is the most challenging and, arguably, the most rewarding target for the webcam imager. The biggest problem posed by the ringed planet is simply its low surface brightness. Saturn's globe is 16 times fainter than the globe of Mars and a third as bright as Jupiter. With the maximum available AVI video exposure being 1/5th of a second (5 frames per second at, confusingly, the 1/25th setting on the Philips driver settings) and 1/10th being more desirable (to freeze the seeing) the raw Saturn image is always going to be a faint and noisy affair at focal ratios of f/25, f/30, and higher. Of course, a larger aperture, permitting an f-ratio of 20 or smaller will improve matters, but larger is not always better. As we learned at the start of this book, telescopes over 25 cm in size have considerable thermal mass and some take hours to cool to the night air; especially those with mirrors more than about 45-mm thick. Large optics in amateur hands rarely have the accuracy of smaller quality optics and they are also harder to operate and set up, except when installed in a permanent observatory. In the latter case, massive observatory domes cause further thermal problems. In recent years, amateurs using Schmidt-Cassegrains, Newtonians, and Maksutovs in the 23- to 30-cm range have taken Saturn images that compare with any taken with much larger ground-based instruments. Telescopes above 35 cm aperture often seem to have insurmountable thermal problems unless they are equipped with fan cooling or are used at dawn,

Figure 14.3. The Great White Spot on Saturn discovered by stage and screen comedian Will Hay on August 3, 1933. This drawing by Hay is from Volume 44 of the BAA Journal.

when everything is in thermal equilibrium. While excellent deep red lunar images have been taken with amateur telescopes of 45-, 50-, or 60-cm aperture, the same cannot be said for color planetary images where the best 25 cm ones are rarely beaten. Another factor here of course is the maximum aperture that the atmosphere will allow. As an observer of over 30 years experience, using telescopes from 22 to 49 cm, I would say that, on 95% of clear nights, a 25-cm aperture will give you as good an image as you can ever expect from the U.K. and a larger aperture will frequently give you a myriad of overlapping images, in other words, a larger aperture can be worse! Of course, there are exceptions to every rule though, and the results of Florida's Don Parker and Japan's Isao Miyazaki, with 0.4-m reflectors at superb locations, prove that high-quality instruments, in the right hands, can deliver the goods, even if they do not outperform 25-cm apertures. As I mentioned much earlier, Florida's Maurizio Di Sciullo is, at the time of writing, constructing a 36-cm planetary Newtonian with a water-cooled primary to try to overcome the inherent cooling problems of such a large glass blank.

So, assuming we are using an aperture of around 25 cm, how can we salvage a result from faint Saturn? The standard color webcams like the ToUcam Pro can still produce spectacular Saturn images in the hands of an expert like Damian Peach, especially when the planet is at high altitude and color dispersion is minimized. One of Damian's ToUcam images is shown in Figure 14.4. One option is to use a sensitive B&W monochrome webcam such as the ATiK 1HS to acquire an unfltered luminance image and then to used red, green, and blue filters to acquire the color. Or, the red and green images can be added to create a very good luminance image. Another option is to increase the webcam frame rate to its slowest rate of 5 frames per second.

Figure 14.4. Saturn, imaged on October 28, 2003, by Damian Peach, using a 280-mm aperture Celestron 11 SCT at f/30 and a ToUcam Pro webcam. 3000 thousand frames taken at 1/5th-second exposure (5 frames per second on the misleading 1/25th setting) were used in the final image stack. The raw stacked image is at the top and the final processed image below. Image: D. Peach.

Figure 14.4. Saturn, imaged on October 28, 2003, by Damian Peach, using a 280-mm aperture Celestron 11 SCT at f/30 and a ToUcam Pro webcam. 3000 thousand frames taken at 1/5th-second exposure (5 frames per second on the misleading 1/25th setting) were used in the final image stack. The raw stacked image is at the top and the final processed image below. Image: D. Peach.

As we have already seen, at 5 frames per second, most webcams will be exposing for 1/5th of a second in manual mode, regardless of the actual exposure time registered. Of course, in poor seeing, 1/5th of a second will not freeze the view. It is easy to get carried away when an image on the PC screen is faint and increase the webcam gain to 100%. Unfortunately, this will result in extremely noisy frames. Usually, a dim (but clearly visible) image will have a much better signal-to-noise. So try to keep the webcam gain to about 85% maximum. A related issue in this context is that very low light levels can confuse the auto color-balance settings in a commercial color webcam. Yet another possible factor relating to noise is, of course, the ambient temperature. There is not much that can be done about this, short of fan cooling or Peltier cooling the webcam chip. Fan cooling the webcam electronics may reduce the chip temperature by 6 or 7°C, thus halving electronic noise, but serious Peltier cooling introduces potential condensation problems, which, in turn, can lead to artifacts when processing the image. Also, it is readout noise, not thermal noise that dominates in webcam frames. Peltier-cooled webcams are only really necessary in long exposures or in hot tropical climates. Another alternative approach for Saturn is not to use a webcam at all. Modern cooled CCD cameras have fast USB download times and are extremely quantum efficient. They can also take exposures of much longer than 1/5th of a second but, again, this reduces the chance of being able to freeze the seeing. Of course, a webcam can cost you well under $100, whereas dedicated CCD cameras cost well over $1,000. Software such as SBIG's planet master can be used to select the best frames and stack them. Using a cooled CCD camera on Saturn you will end up stacking 100 or 200 frames, rather than thousands of noisier webcam frames. However, let us not get too pessimistic here. Truly excellent webcam pictures of Saturn have been secured with f-ratios as high as 40, as long as there are thousands of frames to stack. One such image, with a 235-mm aperture, is shown in Figure 14.5.

Saturn Images Taken Philips Webcam

Figure 14.5. Amazingly, this superb image, taken on December 11, 2004, was made with a relatively modest aperture Celestron 9.25 (235-mm aperture) SCT working at f/40. Images taken through red, green, and blue filters with an ATiK 1HS webcam were used for this remarkable composite of several thousand frames. Image: Damian Peach.

Figure 14.5. Amazingly, this superb image, taken on December 11, 2004, was made with a relatively modest aperture Celestron 9.25 (235-mm aperture) SCT working at f/40. Images taken through red, green, and blue filters with an ATiK 1HS webcam were used for this remarkable composite of several thousand frames. Image: Damian Peach.

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