The incoming Comet ISON is undergoing dramatic brightening as it heads toward its Thanksgiving Day rendezvous with the sun, but if you are hoping to snap photos of the icy wanderer, you'll need to be prepared.
Officially known as C/2012 S1 (ISON), Comet ISON was discovered in September 2012 and has been billed by some as a potential "comet of the century" because of its potential to flare up into a brilliant night sky object. It is already visible to the naked eye for observers in dark-sky sites and is now sporting a long tail.
If this icy cosmic visitor survives its fiery solar encounter on Thursday (Nov. 28), when it swings within 730,000 miles (1.2 million kilometers) of the sun's surface, it could reappear in the dawn sky in early December as a potentially spectacular naked-eye object for skywatchers in the Northern Hemisphere.
But comets are notorious for their unpredictability, as shown by Comet Kohoutek C/1973 E1 in the early 1970s, which failed to meet predictions. As our good friend and famed comet discoverer David H. Levy likes to say, "Comets are like cats; they have tails, and they do precisely what they want."
This uncertainty, however, has failed to dampen the interest and excitement of the public and the media in Comet ISON. So if you want to capture your own souvenir portrait of this first-time visitor to our solar system, here are some tips to keep in mind to increase your chances of success:
- Look for a site away from city lights that offers a clear, unobstructed view of the southeastern horizon about an hour or so before sunrise. With each passing day, the comet will plunge deeper and deeper into the brightening dawn sky as it makes its closest approach to the sun, called "perihelion," on Nov. 28. After it emerges from perihelion, the comet will climb higher and higher in the dawn sky, but at the same it will start to fade as it recedes from the sun and Earth.
WARNING:Do not attempt to observe or photograph Comet ISON when it is very close to the sun or is at perihelion. Staring directly at the sun, especially through a camera, telescope or binoculars without a proper, safe filter can result in serious eye injury or permanent blindness. No comet is worth losing your eyesight, so please be very careful!
Use a digital single-lens reflex DSLR camera to photograph the comet since it gives you more control on your focus and exposure settings. It also allows you to change lenses, from wide-angle to medium telephoto, to match the length of the comet’s tail.
- Since the comet would require exposures lasting a few seconds or more, you would need a sturdy tripod or mount to steady your camera setup.
- Switch your camera mode from Auto (A) to Manual (M) so you’ll be able to control its focus as well as lens aperture, shutter speed and white-balance settings. Set the camera to its highest resolution (RAW mode) so you can capture as much fine details and color information as possible. Consult your camera manual on how to change settings.
- Comets are inherently fuzzy objects so don’t rely on them for focusing your camera. To ensure sharp images, pre-focus the camera on Mars, Mercury or Saturn, or the bright star Spica. Use your camera's Live View feature, if it has one, to achieve accurate focus.
- You can minimize vibrations that can blur your images by using your camera’s mirror lock-up feature before each shot. You should also operate the shutter with an electronic cable release to eliminate camera shake.
- Boost your camera’s sensitivity to ISO 800, or higher. You would want to keep your exposures relatively short, especially if you are using your DSLR on a plain, fixed tripod. This will not only prevent the images of the comet and background stars from trailing due to Earth’s rotation, but it will also keep the brightening dawn sky from washing out the scene.
- Don’t forget to "bracket" your exposures — that is, take a series of shots of the comet at various shutter speeds and/or apertures. This will increase your chances of getting the correct exposure.
- Make sure your camera battery is fully charged, and put your spare battery in your jacket's inside pocket to keep it warm — cold temperatures at dawn can cause your battery to quickly lose its charge.
- When composing your shot, try to include some interesting visual elements in your foreground, such as a person, a tree or a monument silhouetted against the horizon.
If you want to capture detailed shots of the comet and record the full extent of its tail, you will need to use a more elaborate tracking system than the simple camera-on-tripod setup we just described, and may need binoculars or a telescope:
- Use a zoom or telephoto lens with a focal length of 400 millimeters or more to give you a fairly large image of the comet and its tail in the camera frame.
- The long-focus lens will magnify the comet's movement in the sky so you will have to mount your setup on a polar-aligned, motor-driven equatorial mount. The motor drive will “track” the comet and keep it centered in your camera’s field, allowing you to take exposures lasting several minutes.
- Alternatively, you can use a ball-head mount or a bracket to mount your camera/lens setup securely to a hefty telescope on a heavy-duty mount. In this "piggyback" mode, the telescope serves as a stable tracking platform for the camera.
- Use the camera’s Bulb (B) setting to take exposures longer than 30 seconds.
- Depending on your sky brightness, you can shoot a single long exposure or take a series of short exposures that you can later “stack” together digitally using an image-editing program, such as Adobe Photoshop, to create effectively one long exposure.
- For really close-up views of the comet’s head and tail structure, you need to shoot through a long-focus telescope on an equatorial drive. Attach the DSLR camera body securely to the telescope using a T-threaded adapter and a T-ring that is appropriate for your camera brand. Check your local camera supply store for these handy accessories.
- At such high magnification, any inaccuracies in the telescope’s drive system or polar alignment will be evident. That is why deep-sky astrophotographers use an "autoguider," an automated electronic guiding system that attaches to the telescope’s off-axis guider or guidescope. The autoguider can "lock" onto the comet’s head or selected guide star, and it will make automatic adjustments to the telescope’s drive system continuously throughout the exposure to ensure accurate tracking.
Assuming Comet ISON doesn't disintegrate before reaching the sun, the comet will continue on its outbound journey into deep space, never to return again for a long, long time. So don't miss this once-in-a-lifetime opportunity to catch Comet ISON now.
Source of Article: Space.com