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Capturing the Lord of the Rings
Capturing the Lord of the Rings
Saturn image courtesy Alan Friedman

Riding the ecliptic near Regulus, Saturn is now prominent in the eastern evening sky as it heads to opposition February 10th. It will move slowly eastward, remaining the brightest star in Leo for the next two years. My first glimpse of the ringed planet through a friend's telescope many years ago got me hooked on this hobby. Decades later, an encounter with Saturn on a steady night still takes my breath away.

For the astrophotographer, a sharp and detailed image of Saturn is a prized and elusive catch. The ringed planet is a tough subject, revealing its treasures only in good conditions. The delicate disk bands and concentric minima in the B and C rings demand smooth, well-collimated optics and steady seeing. You also need to shoot at high magnification (a long effective focal length.) Saturn is small, spanning 45 arcseconds across the rings with a disk only 20 arcseconds in diameter. The Cassini division, a gap large enough for our Moon to pass through, measures a scant arcsecond at best. This represents a speck about half the width of a red blood cell at the focal plane of a 4" f6 telescope! And the holy grail of Saturn imaging, the razor thin Encke gap, is much smaller. Though beyond the theoretical resolution of amateur scopes at a twentieth of an arcsecond, the very best images will show its location in the outer region of the A ring.

You might want to start by getting to know the seeing patterns from your observing location. Although Saturn will be at its brightest and largest around opposition, my backyard often yields the best images closer to the beginning or end of an apparition, when the planets reach the meridian just before dawn or shortly after dusk. This transitional time can offer brief periods of very steady air. Pre-dawn imaging is my favorite, quiet and peaceful, with plenty of time for the telescope to adapt to ambient temperature. An optical tube shedding heat in falling temperatures will not reach its potential, even if seeing conditions are great.

Practice and experience will help you determine the best telescope set-up and camera settings. I use a B&W streaming camera (an industrial webcam) for planetary imaging. A Barlow extends the focal length of my 10" f 14.6 scope to 11 meters, placing a large Saturn on the 640x480 chip but still leaving some wiggle room for drift during the exposure. Sky transparency will determine the shutter and frame rate (between 15 and 30 frames per second). Camera settings are a juggling act, balancing image brightness with electronic amplification (more commonly called gain) — increasing the image brightness with a high gain setting will create a noisy image.

These B&W video streams provide the detail (luminance) for my final LRGB image. Color data is captured with the same camera using RGB filters. The filters consume light (hence the lower sensitivity of color cameras.) I often reduce the focal ratio to f30 for the filtered capture. The image will be smaller, but I can scale the color data to match the luminance data later on, combining both in Adobe Photoshop to construct a full color image.

Experimentation will determine the right Barlow and image scale for your telescope, mount and camera. If the image is too dim, focusing too difficult, or if tracking makes it hard to keep the image on the chip, move to a lower magnification. Saturn will become brighter and more stable on the monitor. The moons might also become visible. A wide field image of Saturn showing five or six pinpoint moons is a beautiful alternative to a high resolution close-up.

Sensitive image processing will reveal the detail in your recorded data. Using software programs like MaxIm DL and Registax, you can select and stack just the sharpest frames from your video streams. This magical process will increase the signal, average out the noise, and display features not visible in the individual frames. These programs allow you to select an alignment position for the frames in your stack. This can be a powerful advantage. On most nights, even the best frames in your stream will not be uniformly sharp throughout the image. One side of the rings might be sharper than the other. By processing the image multiple times using different alignment positions and then combining these optimized regions with mosaic techniques in Photoshop, you can create a seamless composite that records maximum detail from your session.

As a final step, image processing with software tools such as wavelets and unsharp mask can be used to increase contrast in the image. Your personal taste and end purpose will guide you here. I prefer a natural feel and so I use a restrained hand with these tools. Overdoing it will introduce artifacts in high contrast areas of the image - along the edge of the disk and in the rings and ring gaps.

It will take practice, but a detailed image of Saturn with its glorious ring system etched against the velvet blackness of space is a prize worth the effort.

February 2007

By day Alan Friedman is a designer and greeting card publisher — by night he captures high resolution images of our solar system neighbors from his Buffalo, NY backyard.

Date Taken: 05/04/2011
Author: Alan Friedman
Category: Archives

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