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Friday, September 2
• A twilight challenge: Shortly after sunset, use binoculars or a wide-field scope to start looking for the super-thin crescent Moon near Jupiter and Venus in the bright sky just above the west-southwest horizon, as shown at right. The Moon will be much easier tomorrow.
Saturday, September 3
• Shortly after sunset, the crescent Moon low in the west-southwest points the way lower right to bright Venus and tougher Jupiter, as shown at right.
Sunday, September 4
• The Moon has stepped farther to the upper left of Venus in the afterglow of sunset. Can you pick out little Spica twinkling below the Moon?
Monday, September 5
• Crisp nights of late summer are prime Milky Way time, as hot-weather humid hazes give way to dryer, clearer air (at least where a lot of us live). After dark, the Milky Way runs from Sagittarius in the south, up and left across Aquila, through the big Summer Triangle very high in the east, and on down through Cassiopeia to Perseus rising low in the north-northeast.
Tuesday, September 6
• As summer approaches its end, Venus becomes the zenith star around the end of twilight (for skywatchers at mid-northern latitudes). And Arcturus, its zero-magnitude equal for brightness, shines moderately low in the west.
Wednesday, September 7
• The asteroids 1 Ceres and 18 Melpomene, magnitudes 8 and 9, will both fit in a telescope's low-power eyepiece very late tonight and tomorrow night; they're 0.8° apart. See the September Sky & Telescope, page 51.
Thursday, September 8
• The Moon this evening forms a roughly vertical lineup with Saturn and Antares below it, while brighter Mars glows yellow-orange to their left, as shown here.
Friday, September 9
• Now the Moon shines over Mars at dusk. The Saturn-Mars-Antares triangle, the emblem of this summer's evening sky, continues to morph. Saturn and Antares will continue to move farther toward the lower right, while Mars hangs back and fades. By mid-autumn Saturn and Antares will be gone.
Saturday, September 10
• The Moon at nightfall shines over the Sagittarius Teapot. The Teapot is tipping and pouring to the right.
Want to become a better astronomer? Learn your way around the constellations! They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.
This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy.
Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or new Jumbo Edition), which shows stars to magnitude 7.6.
Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5, nearly three times as many. The next up, once you know your way around, is the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.
You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger Night Sky Observer's Guide by Kepple and Sanner.
Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (meaning heavy and expensive). And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."This Week's Planet Roundup
Mercury is hidden in the glare of the Sun.
Venus (magnitude –3.8) is very low in the west in bright twilight. Look for it 20 or 30 minutes after sunset from a spot with a good open western view.
Jupiter, following its conjunction with Venus on August 27th, has slid away out of sight down to Venus's lower right. It's passing around the far side of the Sun.
Mars (magnitude –0.2, in the feet of Ophiuchus) continues moving eastward away from Saturn (magnitude +0.5) and Antares (+1.0, below Saturn) in the south-southwest at nightfall. The triangle they make is widening.
Uranus (magnitude 5.8, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are well up by midnight in the southeast and south, respectively. Info and finder charts.
All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.
Eastern Daylight Time (EDT) is Universal Time (UT, UTC, or GMT) minus 4 hours.
"This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours."
— Neil deGrasse Tyson
Today's annular solar eclipse, visible over south-central African and parts of the Indian Ocean, was a dazzling crowd pleaser!
Today's annular eclipse has come and gone, but not before putting on a brilliant show for those fortunate enough to find themselves positioned along the center line of eclipse path. Annular solar eclipses are a close relative to total eclipses, in that we see them when the Moon passes directly in front of the Sun from our vantage point on Earth. However, if the Moon passage occurs before the Moon reaches its orbital apogee, the lunar disk appears just a bit smaller than average. So, unlike during a total solar eclipse, the Moon doesn't quite cover the entire the Sun. At what would otherwise be the "total" stage of the eclipse, brilliant sunlight beams out around the Moon's edges, creating an appearance of a "ring of fire."
Satellite images taken during the eclipse and shared by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) show that the clouds interfered with visibility for some viewers in the north, but didn't much bother the rest of those anticipating a great show. While the best views of the annular stage, which lasted three minutes, were from the region of Tanzania and Madagascar, observers in other locations around the African continent and Indian Ocean still witnessed a satisfying partial eclipse.
As of yet, no one has submitted images taken from the far edges of the eclipse path to our online gallery, but the partial phase was visible as far north as Aswan, Egypt. At 3% coverage, that's a tiny slice of darkness! On the other hand, we are seeing some dramatic images of the partial phases from elsewhere on the continent, like the moody view captured in Kenya shown here at the right.
Ahead of the event, most eclipse-chasers seem to be headed for Tanzania and Madagascar, locations where meteorologist Jay Anderson predicted the chance of sunny skies would be the greatest. And indeed, Tanzania had wonderfully clear skies.
Many observers watched the eclipse safely with eclipse glasses or by creating projections with pinholes or colanders. But we also received multiple reports of people in Kiglai (Rwanda) and the Jos Plateau (Nigeria) watching the eclipse by viewing the Sun's reflection in a basin or bucket of water.
Eclipse specialist Jay Pasachoff (Williams College) joined a group of travelers for some more high-tech viewing on the French island of Réunion, east of Madagascar. He reports that they enjoyed clear skies with an occasional passing cloud.
Viewing for tourists and island residents was facilitated by the Observatory of Makes (Observatoire Astronomique des Makes). The observatory reserved space with a HD projection screen at a football stadium in Saint-Louis for the public and astronomers alike. Réunion was also selected by the Paris Observatory (Meudon) for its official observation site. If you didn't see the eclipse in person, you can watch this high-definition video of the eclipse recorded by astronomers from the Paris Observatory to get an idea of annularity.
Today's annular solar eclipse will be followed by a lunar eclipse two weeks from now, on September 16, 2016, when the Moon reaches full. However, this one will hardly be visible, a "barely there" penumbral event. Observers in Europe, Africa, and Asia should see a slight darkening of the Moon's northern half. Looking up? Let us know what you see.
And don't forget next year's big event. On August 21, 2017, observers in North America will witness a total solar eclipse, a coast-to-coast event. Total solar eclipses somewhere on Earth about every 18 months, but there hasn't been one in the contiguous United States since February 26, 1979. It's been a long wait for another one, but the good news is, this one will be observable, at least in its partial phase, across the entire country. The path of totality is about 70 miles wide and stretches across 14 states, beginning with Oregon and ending with South Carolina. If you're anywhere along the center line of the eclipse, you'll see at least 2 minutes of totality. That's worth making a trip north, south, east, or west! Check out the S&T Guide to the August 2017 Total Solar Eclipse so you can start making plans for the big day.
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You can't get rid of noise in your astrophotographs, but you can reduce its impact with more signal.
Taking good images is all about the relationship between signals — the light we’re interested in capturing — and noise. We quantify that relationship with what is called, appropriately enough, the signal-to-noise (S/N) ratio.
Sources of light in our photographs include photons from the astronomical object that we are interested in, but also from other things that we are not so interested in such as light pollution and airglow. Electrons generated by heat in the camera can also produce a thermal signature called dark current, which we can reduce with cooling and dark-frame calibration.
Because the signal from our chosen targets always appear in the same place in an image, it’s easy to subtract unwanted sources of light such as skyglow. And you can greatly reduce the impact of certain types of noise by increasing the amount of signal gathered.Noise
Noise can have different origins, but the main one that you’ll need to worry about is shot noise. Shot noise comes from the statistical variation of a source’s photon emissions over time. It’s this variation that leads to uncertainty (or noise) in the photon count.
In an image, shot noise looks like speckles, splotches, and sandpaper-like grittiness in otherwise smooth-toned areas. It’s more apparent in dark areas and makes it hard to distinguish faint details in deep-sky objects.
Shot noise is inescapable in our images. It’s part of the signal itself and due to the quantum nature of light. Even though it cannot be completely removed, its effects can be reduced.Signals and Noise
Whenever we measure a signal, there is always noise associated with it. That noise is equal to the square root of the signal. This is a good thing because if we gather more photons, the signal will accumulate faster than the noise.
So the only way to deal with this noise is to reduce it by averaging more signal in a process known as stacking, where multiple exposures are taken of a target and then combined into a single result. When combined together, the signal of our target stays the same, but the noise is averaged out. This is because the signal is in the same place in each image whereas the noise is random. The term stacking is a relic from the days of film photography, when two individual pieces of film were physically stacked on top of each other.
Here is an example of how we improve the S/N in digital images. If we count 100 photons, we have a count of 10 for the noise (10 is the square root of 100). So the signal-to-noise ratio is 100/10 = 10.
If we collect 10,000 photons, the noise is 100 (100 is the square root of 10,000), and the signal-to-noise ratio is now 10,000/100 = 100. This higher signal-to-noise ratio will produce a much better image even though, technically, it has more noise.
We can collect more photons with either longer individual exposures or a series of shorter exposures, which can be added or averaged together to equal a longer exposure.
When we average exposures, we reduce the noise by the square root of the total number of exposures. For example, 4 exposures will have twice the S/N of a single exposure, 9 exposures will have 3× the S/N, 100 exposures will have 10× the S/N of a single exposure, and so on. In averaging exposures, the signal stays the same in the final image, but the noise is reduced.
An image with a high signal-to-noise ratio can be more aggressively processed to bring out faint details, something we usually want to do with faint deep-sky objects.
At this point the question is often asked, can we just stack a single exposure on top of itself and get the same benefits? Unfortunately, the answer is no. This violates the “no free lunch” policy. Stacking the same image doesn’t work because the signal and the noise are in the exact same place. In different frames, noise will be in different places because it is random, so it can be averaged out.
After mastering the basics such as focusing and tracking, the single biggest thing that astrophotographers can do to improve their images is to gather more photons — either by taking longer exposures or recording more frames that can later be stacked to improve the S/N ratio.
Increasing the signal in the signal-to-noise ratio is how we beat noise.
Throw open the door and welcome back Orion at dawn. The Hunter's return brings relief from the heat and gives us a fresh shot at exploring winter deep-sky objects in comfort.
We might rise at dawn to see an eclipse, a bright new comet, or a meteor shower. But few of us would deliberately lose sleep to see a constellation. Unless that constellation was Orion. Every August, I attend a star party where seeing Orion rise in morning twilight is something of a badge of honor. If you're serious about making the most of the night, you'll stay up until Betelgeuse flits between the oak leaves.
After weeks of summer heat and humidity, we seek the cool breath of winter, finding it just before dawn in the guise of Orion. I love seeing him tilt upward above the trees in late summer while listening to the soft clatter of leaves in the breeze.
Exploring the Orion Nebula while wearing a short-sleeved shirt is a rare privilege, and there's no better time to grab a pencil and make a sketch. Or take a guided photo of the Belt or one of the constellation's many nebulae. Sure beats numb fingers and the agony of wind chill.
Mid-southern latitude skywatchers must associate their Orion with the coming of summer, a concept difficult to imagine for northerners but no less valid. So it is with associations we make between the stars and the seasons. Frost and the rich aroma of leaf decay are forever paired with the Pegasus Square as icy stillness is with Orion.
Earth's tour around the Sun recycles the constellations once a year. Every August at dawn, we stand with our backs to the galactic center, looking outward through the Orion Arm (or Spur) and into the Perseus Arm of the Milky Way galaxy. Nearby stars within the Spur align to form the striking belt and rectangular outline of the Hunter. Every day that passes, he's up four minutes earlier than the one before, "pushed" westward by Earth's revolution to the tune of 1.6 million miles (2.6 million km) a day. Come October, Orion rises at midnight; in late December, he appears at nightfall.
Each year's reacquaintance with the constellation provides another opportunity to dig out that deep-sky object or double star that weather or commitments prevented you from seeing the previous season. We all know the benefits of recycling; when it comes to skywatching, it means the opportunity for a fresh look with every yearly orbit.
You may do things differently, but I return at least 10 times a season to gaze at the Orion Nebula, which offers so much on so many levels.
A short list of things to look for inside this beautific bubble:
* Trace the extent of the nebula's outer "arms." Can you follow them all the way out until they touch to form a loop?
* Dig out the half dozen or so faint stars buried within the "curdled milk" of the nebula's bright central region near the Trapezium.
* Check the brightness and behavior of young variable stars such as T Orionis, which show continuous and erratic changes in brightness easily followed in small telescopes.
* Resolve the fifth and sixth members of the Trapezium multiple star on nights of good seeing.
* Examine how the appearance of the nebula changes when viewed through a variety of narrowband filters. You'll be surprised how different the it looks in OIII vs. H-beta filters.
* Visualize the nebula in three dimensions by deliberately viewing the dark patches of nebulosity in both M42 and neighboring M43 as in the foreground silhouetted against the bright material behind it.
And that's just one object in a constellations replete with every kind of deep-sky bonbon but a globular cluster. (For that you'll have to hop down to nearby Lepus to M79.) Sometimes a new telescope inspires taking a fresh look at a returning constellation. This season, I'm looking forward to wending my way along the brighter (northern) half of Barnard's Loop with a 10-inch short focus instrument and O III filter. Such a large object could mean several nights of adventuring. Maybe I'll put on a sweater and begin my pilgrimage in September.
Would that we could live lives more in line with the great cosmic cycles, say on the order of 100,000 years. That's about the time remaining for the Orion Nebula to process itself into a Pleiades-like star cluster, a dazzling heap of stellar gems with a few remaining wisps of lingering nebulosity. Would we still find more to see in Orion on that distant date as Earth traces yet another orbital loop? Of course!
Early evening features Mars and Saturn toward south, but keep an eye out for brilliant Venus climbing up from the west during twilight.
In this month's astronomy podcast, you'll learn about September's equinox, which marks the celestial end of summer and the beginning of autumn in the Northern Hemisphere. This year it occurs on the 22nd at 10:21 a.m. Eastern Daylight Time. At that moment the Sun shines directly overhead as seen from the equator.
Another celestial event is associated with this equinox: the Harvest Moon. Traditionally, it’s the name assigned to the full Moon that falls closest to the autumnal equinox, and in 2016 it falls on the night of September 16th. The Harvest Moon gets this name because around that date it never really gets dark between sunset and moonrise for several successive evenings, a boon to farmers working late to harvest their crops.
As darkness falls, look for the planets Mars and Saturn in the southwest. Over in the west, use the Moon's thin, rounded crescent on the 3rd or 4th to point to its lower right, very close to the horizon, to spot bright Venus glowing in the twilight. More challenging is finding Jupiter, to the lower right of Venus.
Meanwhile, look nearly overhead to spot the three widely-separated stars of the Summer Triangle. Vega is the one farthest west, Deneb is toward northeast, and Altair is farthest south. Watch week by week as this trio gradually migrates toward the western horizon.
For more skywatching tips — including how to find Draco, the Dragon, and the lovely Corona Borealis (Northern Crown) of stars — listen to or download our monthly astronomy podcast below.