Jupiter and its moon, Io. Credit: David Dickinson

The last half of 2020 will provide an amazing opportunity to observe and image planets. Already, Jupiter and Saturn rise to the east as the Sun sets, and Mars will join them shortly as it approaches a fine opposition in October.

One of the best ways to capture images of planets at the eyepiece is a method known as image stacking. This is simply carried out by running video with a camera connected to the telescope, and using a processing program to clean and stack images to tease out detail. This technique is known as ‘lucky imaging,’ and takes advantage of capturing fleeting moments of steady seeing while the turbulent atmosphere roils and shimmers in the view.

It’s strange to think: I’ve been image stacking with a planetary webcam for nearly two decades now. Early amateur pioneers were simply modifying off-the-shelf webcams by removing the existing lens and affixing an eyepiece barrel, to attach the camera at the prime focus of the telescope. I used an inexpensive 20$ Logitech webcam for years, before recently upgrading to a dedicated ZWO ASI 120MC-S camera (MSRP $149 US). This camera has a 1.2 megapixel (1280 x 960 array) color imaging sensor, allowing for fast 72 frames a second capture at full resolution.

Mars. Credit: David Dickinson

Such an upgrade is like going from driving a Pinto to a Lexus. Other new planetary and deep-sky imaging cameras on the market from the company include the ZWO ASI 178MC camera at $350 US, up to the high-end ZWO ASI 1600MM camera at $1,280 US.

How image stacking works

Image stacking increases the signal versus noise ratio, bringing out delicate planetary detail while cleaning out unwanted noise caused by poor seeing conditions, temperature changes, and even cosmic ray hits. The first stage any user must master is focusing and acquiring the planetary target. I like to crank the gain up, which overexposes the target planet, but makes it much easier to find and center on the screen. Sometimes, the slightly out-of-focus planet is lurking, just off to the side out of view. The field of view of many planetary cameras is often narrow, perhaps just a dozen arcminutes wide.

Then you’ll want a razor-sharp focus before you begin capturing frames. A focusing mask can help with attaining a fine focus, which is necessary for seeing planetary detail. Aperture focusing masks are commercially available, or you can build a simple one out of cardboard by cutting three equally spaced holes in a triangle-shaped pattern. Some image capturing programs even include a digital focus routine.

A tracking mount aids immensely in capturing quick video sequences, though it’s not essential; I’ve successfully tracked a target during image acquisition by hand, slowly guiding the telescope in azimuth and elevation. For years, we used K3CCDTools ($49 US) for camera control… now, we use Fire Capture (by donation) to control the camera and acquire images. One caveat: video capturing creates large files, and quickly eats up precious laptop hard-drive space. I use an external one terabyte hard-drive to save capture files (and save my laptop disk space!)

Post processing

The next workflow phase is image processing. For years, I used a program known as Registax; today, a common free program many observers use is called AutoStakkert! A stacking program will align, analyze and stack images automatically, and can be set to reject all but the very best images; I usually set the limit at 10 percent. I usually do a final cleanup in Adobe Photoshop or Lightroom, helping to remove the image noise, sharpen and adjust the brightness and contrast.

When not to stack

Ironically, there are limited situations were you may not want to stack images. Stacking, for example, will remove spurious or moving targets that may still be of interest. These can include impact flashes that can turn up in video review, or things that may ‘photobomb’ deep-sky images, to include asteroids or comets . . . of course, this sort of automatic removal is handy, when it comes to unwanted streaks of aircraft or satellites.

Clear skies for planetary observing season 2020!

About David Dickinson

David is a freelance science writer, frequent contributor to Sky & Telescope and Universe Today, author of several astronomy books and long-time amateur astronomer. He lives with his wife Myscha in Norfolk, Virginia.

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