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July 4 - Deep Impact
July 4 - Deep Impact

Deep Fireworks
NASA's historic Deep Impact mission is set to climax on July 4th with the planned collision of a man-made projectile with a comet called 9P/Tempel 1. The 820-pound, washing-machine-sized "impactor" launched from the Deep Impact spacecraft will smash into the comet's nucleus at 23,000 miles per hour, expelling ice and dust debris and gouging out a crater a couple of football fields wide and some 50 yards deep. By analyzing the blown-out debris and the freshly excavated crater itself, scientists expect to learn more about what comets are made of. And from that they hope to gain new insights into how the solar system was formed.

While all manner of professional observatory telescopes and orbiting instruments such as the Hubble Space Telescope will be trained on Comet Tempel 1 before, during, and after the big crash, backyard skywatchers can get a first-hand look for themselves.

As for what gear you'll need to witness the event, there are basically three options: eyes alone, binoculars, or a telescope. Let's explore at each of these options.

Will the Eyes Have It?
If you don't have a pair of binoculars stashed in a closet somewhere and don't want to spring for a telescope, you can try going "naked"—taking a naked-eye viewing approach, that is. Unfortunately, with just your eyes you will not be able to see the comet before impact, because at 10th magnitude it's about 40 times dimmer than they can detect. But after impact, when the comet brightens to the expected 6th magnitude or so, it may become visible without optical aid, barely. But you would have to be an experienced observer to pick it out from among the thicket of stars surrounding it. Can you say needle in a haystack? So, no, the eyes really won?t have it. This is an event best observed with the aid of an optical instrument.

Is It a Sight for Four Eyes?
With a good pair of full- or giant-sized (60mm and larger) binoculars, you should be able to spot the comet. We'll define full-size binoculars as those having front lenses of 40mm to 50mm in diameter. Giant-size binocs sport lenses measuring 60mm or larger. The bigger the lenses the better, because bigger lenses take in more light, so you get a brighter image.

Before impact Tempel 1 may be hard to discern in binoculars, but upon brightening after the impact it should be easy to spot. Your best bet is to mount the binoculars on a tripod to steady the image and reduce arm fatigue. Most binoculars have a threaded socket in between the optical barrels that accepts an optional "L-adapter" (available from Orion), which couples the binocular to a standard camera tripod. If you don't have a tripod, brace the binocular against something solid like a wall or a car door to hold it steady.

Observe from as dark a site as possible and make sure your eyes are fully dark-adapted (see Tips and Tricks to the right). Using a planisphere or our all-sky map, find the constellation Virgo in the west-southwest part of the sky. Locate the bright star Spica and sweep the area immediately around it. When you see a fuzzy patch, you've spotted Tempel 1! On the date of impact, it will be about 3-1/2 degrees (about a third of a fist-width at arm's length) to the east-northeast (left) of Spica. Refer to the detailed star map to see the comet's exact position relative to Spica.

A Telescope's the Ticket
To get a good look at Comet Tempel 1, a telescope is the way to go. Any telescope design will do?refractor, reflector, or Cassegrain, but the bigger its optics, the more vivid the image will be. From a dark-sky site, a telescope with 4" optics should begin to reveal the comet's hazy glow. (if you have a smaller telescope, go ahead and give it a try. You just might get lucky.) You'll have better luck with an 8" or larger telescope, which will be a reflector or Schmidt-Cassegrain. Remember that moonlight will wash out the comet from June 8 to 23, so plan on starting your observations on the 24th, if you haven't already, when the Moon comes up later in the evening.

Your telescope should be equipped with a finder scope to help locate the comet, and a sturdy mount. An "equatorial" mount is desirable, since it allows you easily to follow the east-to-west motion of celestial objects in the sky by making manual adjustments with one slow-motion control, or by use of an optional electronic tracking drive. If your telescope is coupled to a simpler, "altazimuth" mount, not to worry. You can still track the comet by making occasional tweaks to both the vertical and horizontal positions of the scope. Plan on having at least a couple of eyepieces at the ready, one for low-power viewing and another for higher magnification.

Start by making sure the finder scope is aligned with the main telescope. The procedure for this should be covered in the telescope's instruction manual. Then do a quick polar alignment of the equatorial mount (if that's what you have); that's also covered in the manual. Put a low-power eyepiece in the telescope's focuser. Low power (less than 50x) is best for locating objects and for getting maximum contrast between objects and the background sky. Now you're ready to go comet-catching!

To get the telescope pointed in the right direction, locate Spica in the finder scope and use your telescope's slow-motion controls to center it on the finder's crosshairs. Remember that the view in a typical finder scope will be upside-down compared to a normal view. If you're using a star chart, rotate it 180 degrees to match the view in the finder scope. It's also important to know how the image in the main telescope's eyepiece compares to the star chart. In a reflector telescope the image will be upside-down. In a refractor or Cassegrain used with a "star diagonal" in front of the eyepiece, the view will be mirror-reversed, so you will have to mentally flip it back or else turn your star chart over and read it from behind to match the eyepiece view!

From Spica, move the telescope a few degrees in the direction of the comet using the mount's slow-motion controls. Refer to the detailed star chart to determine where the comet should be relative to Spica. With a low-power eyepiece in the telescope's focuser, see if you can identify the comet's fuzzy glow. If you can't, sweep the area a little with the slow-motion controls until you find it.

Once you have Tempel 1 in the eyepiece's field of view, study its appearance for a while. Can you detect any shape to its diffuse tail? Now insert a higher-power eyepiece, one that provides 100x magnification or so. The view will be dimmer but you may resolve more structure. Try other magnifications using other eyepieces, too, if you have them.

Of course, after the impactor slams into the comet on July 3/4, it should become much easier to see. Will the blown-off debris cloud cause the comet to look any bigger, or just brighter? How soon after impact will its appearance start to change? And how long will the "extreme makeover" persist? Nobody has the answers to these questions right now. But with a good telescope, a good eye, and patient observation, you can have fun finding out!

Download More Help
You can get more detailed observing information from our Deep Impact PDF.

June 2005

Date Taken: 05/26/2011
Author: Steve Peters
Category: Archives

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