Sky & Telescope news
Friday, September 4
• The last-quarter Moon rises around 11 or midnight tonight very close to Aldebaran, depending on your location. The Moon occults Aldebaran soon after rising for parts of eastern North America, and in Saturday's dawn or daylight for Europe and western Asia. See the article in the September Sky & Telescope, page 51. Here are local timetables for the star's disappearance and for its reappearance.
• The Moon is exactly last quarter at 5:54 a.m. Saturday morning Eastern Daylight Time.
Saturday, September 5
• The wide W pattern of Cassiopeia is tilting up in the northeast after dark. Below the W's bottom segment, by a little farther than the segment's length, look for an enhanced spot of the Milky Way's glow (if you have a fairly dark sky). Binoculars will show this to be the Perseus Double Cluster, even through a fair amount of light pollution.
Sunday, September 6
• Sagittarius and the summer Milky Way, rich with deep-sky objects, stand highest on the meridian in the south right after dark this week. Work this area now, before moonlight returns next week. By the time the Moon leaves and we have a dark evening sky again, this area will be past the meridian.
Monday, September 7
• Even though it's not yet autumn, you can already greet Fomalhaut, the 1st-magnitude Autumn Star, twinkling low in the southeast by mid-evening. It shines highest in the south around midnight or 1 a.m.
Tuesday, September 8
• In the dawn of Wednesday the 9th, look for Venus and Mars below the waning crescent Moon in the east, as shown at right.
• And if you're out early enough before sunrise for the stars to still be visible, you get a winter preview! The sky displays the same starry panorama as it will at dusk next February. Orion stands high in the south, Sirius and Canis Major sparkle to its lower left, and Gemini occupies the high east.
Wednesday, September 9
• An hour before sunrise Thursday, look low in the east for the crescent Moon between bright Venus and faint Mars (in the time zones of the Americas).
Thursday, September 10
• Got light pollution? Cygnus overhead offers many sights for small telescopes regardless. See Ken Hewitt-White's "Cygnus in the City" in the September Sky & Telescope, page 58, with chart and photos.
Friday, September 11
• How soon after sunset can you identify the big Summer Triangle? Vega, its brightest star, is nearly straight overhead (for skywatchers at mid-northern latitudes). Deneb is the first bright star to Vega's east-northeast. Altair shines less high in the southeast.
• Thin-Moon challenge: Saturday's dawn offers a chance to try to set your record old-Moon sighting. Look just above the horizon low in the east as dawn grows bright, as shown here. Bring binoculars or a telescope. If you detect the Moon, note the time. Then see how close this is to the time of new Moon: 2:41 a.m. Sunday morning September 13th Eastern Daylight Time (6:41 September 13th UT).
Saturday, September 12
• The Great Square of Pegasus is high in the east after dark, balancing on one corner.
From the Great Square's left corner extends the big line of three stars, running to the lower left, that mark the head, backbone and leg of the constellation Andromeda. (The line of three includes the corner.)
Upper left from the end of this line, you'll find W-shaped Cassiopeia tilting up.
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 standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, 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, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.
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). 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 (magnitude +0.1) is sinking away very low in the west shortly after sunset. You might still try for it with binoculars or a wide-field telescope down in the murk. Don't confuse it with Spica to its upper left.
Venus (magnitude –4.7) climbs into clearer view in the east every morning. It a telescope it's a beautiful crescent, especially if you follow it rising higher into thinner air after sunrise. Venus is shrinking as it pulls farther away from Earth around the Sun, even as its crescent thickens.
Mars, more than 300 times fainter at magnitude +1.8, is with Venus in the dawn — left of it early in the week, lower left of it later in the week. They're about 10° apart: about a fist-width at arm's length.
Jupiter is still hidden deep in the sunrise.
Saturn (magnitude +0.5, in Libra) shines in the southwest at dusk, to the right of upper Scorpius. Orange Antares, less bright, twinkles 12° to Saturn's left. Between them is the near-vertical line of three fainter stars marking Scorpius's head. (The brightest of these is the middle one, Delta Scorpii.)
Uranus (magnitude +5.7, in Pisces) and Neptune (magnitude +7.8, in Aquarius) are up in the east and southeast, respectively, by 9 or 10 p.m. Finder charts for Uranus and Neptune.
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
After a journey lasting nine and a half years, NASA's New Horizons spacecraft finally reached the distant world of Pluto. The three-billion-mile expedition culminated with New Horizons sweeping a mere 7,800 miles above Pluto's surface.
During its flyby last month, the probe obtained a treasure trove of scientific data, snapping by far the most detailed photographs ever taken of this mysterious object and its several moons. Instead of a cratered, barren orb, as some scientists expected, Pluto appears to be a startlingly dynamic world with soaring mountains and smooth plains of exotic ices. More facts on Pluto will continue pour in from New Horizons well into 2016 as the spacecraft transmits all of its data back to Earth.
On August 26, New Horizons team members Richard Binzel (Kavli Institute for Astrophysics and Space Research at MIT) and Cathy Olkin (Southwest Research Institute), along with Kavli Prize Laureate Michael E. Brown (California Institute of Technology) joined The Kavli Foundation for a live discussion. These planetary scientists answered questions about the mechanisms that might be shaping Pluto's landscape and what this strange new world can tell us about the other bodies at the Solar System's fringes.About the participants (left to right):
- Richard Binzel is a Professor of Planetary Sciences and the MacVicar Faculty Fellow in the Massachusetts Institute of Technology's (MIT) department of Earth, Atmosphere and Planetary Sciences, and a member of the MIT Kavli Institute for Astrophysics and Space Research (MKI). He is a co-investigator on the New Horizons mission and has studied the Pluto-Charon system for 35 years.
- Cathy Olkin is a Principal Scientist at the Southwest Research Institute (SWRI) and a deputy project scientist for the New Horizons mission. Her planetary science interests include the study of the icy surfaces and tenuous atmospheres of outer Solar System worlds.
- Michael E. Brown is the Richard and Barbara Rosenberg Professor of Planetary Astronomy at the California Institute of Technology and the 2012 Kavli Prize Laureate in Astrophysics for his research on the Kuiper Belt. His research specialty is the discovery and study of bodies at the edge of the Solar System.
- ADAM HADHAZY (moderator) – is a freelance science writer who chiefly covers astrophysics and astrobiology. He has a Master's degree in science journalism from New York University.
- What has amazed each of you about the New Horizons data? (3:10)
- What could be replenishing Pluto's atmosphere? (5:50)
- Could internal heat from Pluto be contributing to its geological changes and if so, what is its source? (10:10)
- Why don't we see as many craters on Pluto? (11:25)
- What would it be like to ice-skate on Pluto? (13:25)
- Could Pluto and its moon Charon have formed at a later date than the rest of the solar system? (15:45)
- What will the New Horizons mission be able to tell us about other objects in the outer solar system? (17:30)
- How can studying Pluto tell us about the origins of Earth and the inner solar system? (20:55)
- Since Pluto is so seasonal is it possible that the Tombaugh Regio region would ever disappear? (22:15)
- What data about Pluto are you eagerly awaiting? (26:40)
- Just how different is Pluto's geology from earth? (28:50)
A small body known as 2014 MU69, found by the Hubble Space Telescope barely a year ago, will be the next destination for NASA's New Horizons spacecraft.
NASA's New Horizons team members are still basking in the afterglow of July's historic flyby of Pluto — and still awaiting most of the observations made there. But they're already anticipating the spacecraft's second and likely final encounter in the distant Kuiper Belt.
Last week the space agency announced the spacecraft's target. It's 2014 MU69, an object situated 43.3 astronomical units (6.49 billion km) from the Sun. Astronomers know little about 2014 MU69, other than it's an incredibly dim 25.6-magnitude blip. It's not big — assuming a surface that's 20% reflective (just a guess), astronomers estimate its diameter to be about 45 km (30 miles) across. That's roughly 10 times the size of a typical comet.
But New Horizons isn't zeroing in on this object based on its size or any other physical characteristic. Instead, it's all about location. The spacecraft should be able to reach 2014 MU69 in a reasonable amount of time and still have comfortable fuel reserves for its maneuvering engines once it gets there. Plans call for a series of four trajectory corrections in late October and early November to set course for a rendezvous on January 1, 2019.
It all seems so matter of fact. But finding a suitable post-Pluto target became an urgent problem in early 2014, after 3 years of intensive searching with ground-based telescopes failed to find one. Fortunately, last-ditch searches with the Hubble Space Telescope from July through September turned up three potential targets (PTs). Follow-up observations showed that 2014 MU69 — PT1 in Hubble's short list — was the best all-around choice. Runner-up PT3, designated 2014 PN70, is a slightly larger body, but it would have required more fuel to reach.
Despite its small size, New Horizons scientists are excited about what 2014 MU69 might teach them about the formation and evolution of the distant Kuiper Belt in which it lies. New Frontiers in the Solar System, a "decadal" blueprint for future planetary exploration released in 2003 by the National Academy of Sciences, strongly recommended that small Kuiper Belt objects (KBOs) be included in any mission to Pluto.
This object's orbit is nearly circular (eccentricity = 0.05) and inclined just 2½°. So it has not been strongly perturned or altered since the solar system's formation 4½ billion years ago. "2014 MU69 is a great choice because it is just the kind of ancient KBO, formed where it orbits now, that the Decadal Survey desired us to fly by,” notes Alan Stern, the mission's principal investigator, in a press release announcing the selection.
But just because New Horizons has a suitable target doesn't guarantee that it will be operating once it gets there. Technically the spacecraft is in tip-top shape and fully able to accomplish another close flyby. But first the team needs to make a compelling scientific case for doing so. That proposal, due next year, will be evaluated against other missions competing for the space agency's funds. NASA managers will then decide if it's worth extending the mission's funding for at least three more years.
Meanwhile, astronomers will continue studying this object to refine its orbit and perhaps to get a better handle on its diameter and shape. But don't expect a permanent number or name for it anytime soon. According to Gareth Williams (Minor Planet Center) it will be mid-2016 at the very earliest, and more likely mid-2017, before the orbit of 2014 MU69 is known well enough to justify a better moniker.
Demoted dwarf planet or "King of the Kuiper Belt"? Get to know Pluto (and the New Horizons mission) with the Discover Pluto Collection now available through ShopatSky.
They say it takes two to tango. Find out how two closely-orbiting stellar pairs create fireworks you can see in your own backyard telescope.
Nature invented fireworks and has evolved countless ways to light the fuse. Some of the most violent cataclysms occur in binary systems where a small, fantastically dense object like a white dwarf, neutron star, or black hole forms a close pair with a larger, cooler star. The massive object siphons gas from its hapless companion, surrounding itself in a swirling accretion disk.
As material from the cooler star streams into the disk or funnels from the disk to the surface of the compact object, several things can happen, all of which involve spectacular releases of energy.
With many white dwarfs, gas piles up in the accretion disk until it's dumped onto the white dwarf. As the material plunges toward the surface, the release of gravitational potential energy heats the disk to glowing, and visual observers witness a sudden brightening of the system called an outburst. Outbursts of 4 or 5 magnitudes are fairly common among these cataclysmic variable stars, better known as dwarf novae. They last about a week and occur at least several times a year. Frictional heating as the swirls of captured gas work their way from outer to inner edge can also cause a sudden brightening of the system.
Material that accumulates on the dwarf over thousands of years can become so compacted and heated that it ignites in a thermonuclear fusion called a nova. It's thought that many dwarf novae undergo nova outbursts over the long haul.
Similar events transpire at neutron stars. Gas robbed from the reluctant "donor" star is drawn into an accretion disk around the compact star. Friction between the gas in neighboring regions of the disk can drive up temperatures to millions of degrees, releasing copious amounts of X-rays as well as visual light.
As the densest form of matter imaginable, black holes harbor tremendous gravitational force. In a black hole, material is compressed and heated by friction as it spirals down toward the event horizon, the point of no return. Vast amounts of energy are released across the electromagnetic spectrum, especially in the powerful X-ray region. Sometimes we see flaring and flickering in real time as the rate of flow or amount of material streaming into the hole's gravitational maw varies from moment to moment. If a dwarf nova's a firecracker, a black hole binary's a hundred cherry bombs.
Thanks to observations made across the spectrum using both ground and orbiting telescopes, we know such violent events are common in our cosmos. The trick is finding targets bright enough to examine in amateur telescopes. Fortunately, there are several.
I've selected three examples visible in late summer and early fall that are bright enough to show in 6- to 10-inch telescopes and show easily detectable light fluctuations: the black hole X-ray binary in V4641 Sagittarii; the famed neutron star binary Scorpius X-1 (V818 Scorpii); and the dwarf nova SS Cygni.
V4641 Sgr, which normally hovers around magnitude +13.6, will require a 10-inch or larger telescope, unless you happen to catch in the midst of a powerful flare like astronomers did in 1999 when it shot up to magnitude +8.8! Astronomers think the tremendous outpouring of radiation was caused by material in the accretion disk plummeting to the fringe of the black hole's event horizon. Amazing that even a modest Dobsonian can see the flickerings of this matter muncher across a distance of 24,000 light-years.
Scorpius X-1 or V818 Scorpii, a neutron star paired with a low-mass (just 0.4 solar) star, outshines every X-ray source in the sky except the Sun and ranges from about magnitude +11.9 at maximum to +12.8 at minimum. Located 9,000 light-years away in the constellation Scorpius, it was the first X-ray source discovered outside of the Solar System. While you'll need time to catch an outburst of V4641 Sgr, V818 Sco variations occur more frequently, requiring only about a week of regular observation to spot a flare or fade.
SS Cygni, a dwarf nova and favorite of variable star observers, is the easiest of the three, with a range from 8th magnitude in outburst to 12th in quiescence. Even a 4-inch telescope will suffice to follow its antics. I've watched this star for years; when caught in outburst, it always puts me in a good mood, like pulling that prize from a box of Cracker Jack. Expect an eruption about every 40 days, when the slumbering system leaps four magnitudes in one or two nights. What a sight!
Detailed maps for all three of our featured stars can be freely downloaded from the American Association of Variable Star Observers (AAVSO) website. Type the star name in the Pick a Star box and then click on the Create a Finder Chart link to create a customized chart. Or, you can use my direct links: V4641 Sagittarii (high magnification), V4641 Sagittarii (low magnification), V818 Scorpii, SS Cygni. All are oriented with south up.
AAVSO'ers love watching the ups and downs of these and hundreds of other variable stars. They say not much changes in the heavens, but the closer you look, the more alive the sky becomes.
Find your way through the night sky with Sky Atlas 2000.0!
Amateur and professional astronomers worked together to discover a rare eclipsing binary system — and a chance to study a supernova before it happens.
A collaboration of amateur and professional astronomers has uncovered a rare variety of eclipsing binaries. The European Space Agency’s Gaia satellite first imaged the eclipsing pair, named Gaia14aae, in August 2014. Researchers took notice of Gaia14aae when it suddenly flared five-fold within a single day.
The Gaia14aae system is composed of a white dwarf in a tight orbital embrace with a larger (by volume) companion. The tilt of orbit is along our line of sight, so observers on and near Earth — such as the Gaia mission in space — see an eclipse of the pair once every 50 minutes.
A worldwide pro-am collaboration carried out follow-up observations of Gaia14aae, cinching its nature as an eclipsing binary star. This effort included the Centre for Backyard Astrophysics (CBA), a group of amateurs who monitor cataclysmic variables using small telescopes in backyards around the world. CBA members kept eyes on the system after Gaia’s initial sighting of its outburst, as did a collaboration of 86 professionals based at facilities including the Catalina Real-time Transient Survey, PanSTARRS-1, and ASAS-SN based in Chile and Hawaii.
“Enrique de Miguel from CBA noticed that the system appeared to be eclipsing, based on a period of dips in the brightness of the system,” says Morgan Fraser (Cambridge Institute for Astronomy, UK). “From this, we realized that this could be quite an exciting system, and this led us to take further observations.”
This movie shows 30-second exposures from the Loiano Observatory over a span of 88 minutes (sped up by a factor of 250), revealing two eclipses of the Gaia14aae system.
Gaia14aae is located 730 light-years from Earth in the constellation Draco. The ‘Gaia14aae’ designation denotes the discovery year (2014) followed by the sequence, with ‘aaa’ being the first object of interest discovered in that particular year. Astronomers conducted spectroscopic analysis of the system using the William Herschel Telescope in the Canary Islands. They found that Gaia14aae is in fact a rare type of binary system that varies dramatically in brightness over short periods of time, known as an AM Canum Venaticorum (AM CVn) cataclysmic variable. This type is characterized by the absence of hydrogen and the abundance of helium in its spectrum.
Forty other such binary systems are known, but this one’s an eclipsing binary, each star passing in front of and blocking the light of its partner in turn. Eclipsing binaries are valuable because they reveal a key ingredient, the tilt of the system’s orbit — it has to be edge-on for the stars to eclipse each other. Knowing that one fact makes calculating other properties, like the mass of the two stars and the distance between them, easy.A Supernova in the Making
The discovery is important to researchers studying Type 1a supernovae, the apocalyptic explosions of white dwarfs that eat too much and whose detonations shine with a characteristic brightness. These “standard candles” are crucial for measuring extragalactic distances and serve as a cornerstone for the discovery of the acceleration of the expansion of the universe due to dark energy.
Here, we’re seeing the anatomy of a probable Type 1a supernova in the making: a star 125 times the volume of our Sun locked in a death spiral with a white dwarf 100 times more massive than it. Researchers are unsure whether the two stars will collide in a dramatic supernova explosion, or if the white dwarf will devour its tenuous companion first.
The eclipsing nature of the system gives researchers the unprecedented opportunity to measure the physical parameters of a Type 1a supernova before it occurs. A galactic supernova courtesy of Gaia14aae would be easily visible from Earth, though such a spectacle is probably still thousands of years in the future.
“The eclipse means we can measure exactly the mass of both stars and their separation and work out their evolution,” says Heather Campbell (Cambridge Institute of Astronomy, UK). “The system could also be an important laboratory for studying ultra-bright supernova explosions, which are a vital tool for measuring the expansion of the universe.”
Fraser adds that the masses are essential to testing theory. “This means we can start to understand how systems like Gaia14aae come about — and what it would have looked like billions of years ago when it formed,” he adds.More Discoveries to Come
And this could be the first of many exciting new discoveries. “This year, [the Gaia team has] been searching for new transients in a very manual way, but we are switching to doing things in a much more automated way,” says Campbell. “This means we will start finding lots more transients every day. Many of these will be supernova explosions, but it also opens up the potential for finding many more exciting objects.“
Launched in 2013, the Gaia observatory’s primary mission is astrometry, or the ultra-precise measurement of stars’ positions. As a spinoff, researchers expect Gaia to make serendipitous discoveries both near and far during its five-year mission, including new asteroids, comets, Kuiper Belt objects, variable stars, quasars, and much more. Gaia may spy transiting exoplanets as well.
The discovery of Gaia14aae is a great example of amateur and professional astronomers working together, and a sign of more exciting discoveries to come down the road.
Read the original paper of the discovery, "Total eclipse of the heart: the AM CVn Gaia14aae/ASSAN-14cn" in the Monthly Notices of the Royal Astronomical Society.
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This month's stargazing features pretty planetary treats in the eastern sky before dawn — and the last total lunar eclipse visible until 2018.
The equinox falls on September 23rd at 4:21 am Eastern Daylight Time. On this date the Sun momentarily shines directly down on Earth’s equator as it heads south in declination. Equinox comes from the Latin word aequinoctium, meaning “equal nights” — days and nights everywhere are both 12 hours long, and the Sun rises due east and sets due west no matter where you are.
Facing east before dawn, you'll see a really bright “star” — the planet Venus — lingering over the eastern horizon. Late in the month look for Jupiter to its lower left and, between them, Mars and Regulus.
If you're out stargazing after sunset, look for the Summer Triangle high overhead. Saturn, the evening's lone bright planet, is low in the southwest.
Meanwhile, the full Harvest Moon on the night of Sunday, September 27th, will be special. Watch for a total lunar eclipse centered on 10:48 p.m. Eastern Daylight Time.
There's lots more to see by eye in the September evening sky. To get a personally guided tour, download our 7-minute-long stargazing podcast below.
There's no better guide to what's going on in nighttime sky than the September issue of Sky & Telescope magazine.
Improve your deep-sky images with this innovative program.
By Bob Fera in the June 2013 issue of Sky & Telescope
As any experienced astrophotographer, and he or she will tell you that transforming a bunch of noisy sub-exposures into a colorful piece of art is no small feat. The process involves many steps using a variety of software packages, each with its own learning curve. For many imagers, the “art” happens in Adobe Photoshop. But before you can use a tool such as Photoshop to apply your personal touch to an image, your data must first go through a series of decidedly less sexy steps — calibration, alignment, and combination. And while these steps involve limited creative input, they are nonetheless critical to the final look of your picture.
Among the numerous programs for processing CCD data, I prefer CCDWare’s CCDStack 2 (www.ccdware.com) for PCs to calibrate, align, stack, and stretch my images into 16-bit TIFF files that are ready for the final tweaks in Photoshop. The program’s strength lies in its intuitive user interface, as well as some “live” stretching features. CCDStack 2 has worked well for me over the years and should provide you with a solid foundation for developing your own methods.Image Calibration
Let’s begin by preparing our calibration files. I always record several dark, bias, and flat-field images and combine these into “master” calibration frames to ensure that my final result is as clean as possible. This reduces any spurious artifacts in my calibration frames due to cosmic ray hits or other unwanted signals.
Start by opening the program and select Process/Create Calibration Master/make master Bias. The program will immediately open the last folder you used in CCDStack 2, so you may need to navigate to your calibration files folder. Once there, select all the bias frames that match the temperature you shot of your light frames. The Combine Settings window then opens, and allows you a few different ways to combine your biases into a “master” bias frame. I prefer to use the sigma reject mean method, and change the sigma multiplier to 2, and an iterations limit of 2.
In a few moments, your master bias frame is displayed. Simply save the result as a 16-bit FITS file, and repeat the same process to combine your dark frames by selecting the Process/Create Calibration Master/make master Dark.
Generating your master flat-field image is also similar, though the program will first ask you if you wish to dark/bias subtract each flat frame. If so, choose the master bias frame you’ve just created, and also the dark frame master that matches your flat-filed image. When you reach the Combine Method dialog, again choose sigma reject mean with a multiplier of 2 and an iteration value of 2. Make sure to repeat this routine for all your flats taken through various filters you shot through. Now that we have our calibration frames ready, let’s tackle our raw data.
Open all of your individual exposures taken through one of your filters (if you use a monochrome camera with color filters). Next, select the pull-down menu Process/Calibrate. The Calibration Manager window opens, which will automatically find your master dark, bias, and flat frames if they were saved to the same folder you were working in previously. If not, click the “Dark Manager” button and navigate to your master frames. Once all of your master frames are selected, simply click the “Apply to all” button at the bottom left and in a minute or so, all of your images in this group will be calibrated. Save each of these calibrated images by selecting File/Save data/ Included in the pull-down menu. A new window will open that allows you to add a suffix to your file title, to avoid overwriting your raw data. Select the 32-bit FIT float file option. Now you can repeat the same steps for each of your other filtered-image groups.
Now that all our images are calibrated, let’s align each frame. If you have plenty of RAM on your computer and a fast processor, you can open all your calibrated exposures and align them all at once. If you have limited memory, you can perform your alignment in groups, but remember to select one image to be the “base” image that all the others will be aligned to. Make sure your alignment frame is the image visible, then select the Stack/Register pull-down menu, and the Registration window opens. CCDStack 2 automatically detects multiple stars in your images, or allows you to select your own points to register if you so choose. Once you’ve selected the alignment points, click the “align all” button at the bottom left, and in a few moments, each of your sub-exposures should be aligned properly. Before applying the alignment permanently, pan through each of your images to make sure each one worked properly. If so, click the Apply tab at the top right. The program offers a few resampling options to compensate for the sub-pixel shifting of each frame. I prefer Bicubic B-spline, but you can experiment to see what works best for your images. After the alignment is applied, save the results with a new suffix.Data Rejection
At this point we have all our images calibrated, aligned, and ready to stack. Combining your sub-frames properly will dramatically increase the signal-to-noise ratio of your final image, while eliminating unwanted airplane and satellite trails and other random artifacts. In CCDStack 2 this involves three steps: Normalize, Data Reject, and Combine.
Normalizing your data mathematically compensates for variations in sky background and transparency, scaling all of your open sub-exposures to similar brightness values for corresponding pixels. This step is necessary to produce the best stacked result.
First open all the images taken with a single filter and Select Stack/Normalize/Control/Both. A small window opens that asks you to identify the background sky area. Simply click your mouse and pull a tiny rectangular selection around an area that will be a “neutral” background sky with no bright nebulosity, galaxies, or stars in your selection. For images where nebulosity permeates the entire image, try to find a region with the faintest nebulosity, or a dark nebula, as your background selection. After you’ve made your selection and clicked OK, the program will then ask you to select a highlight area. This will most likely be your main subject, whether it’s a galaxy, nebula, star cluster, or comet. Make a selection around the brightest area and click OK. The Information window pops up and will display the calculated offset for each of your open images.
Next, we need to choose which method of data rejection to use. Data rejection identifies and removes undesirable artifacts in each of your individual images, replacing the offending areas in your final stacked result with the corresponding region from multiple unaffected sub-frames.
Choose Stack/Data Reject/Procedures and another new command window opens. Here we’ll select the data rejection algorithm from the pull-down list. I prefer to use the STD sigma reject, but you can experiment again to find what works best for your images. Check the “top image %” box, and set the value to 2, then click the “Apply to All” button. This can take a few moments, but when complete, the program will display all the rejected pixels in each of your sub-exposures as bright red. Now simply close the window and move on to the next step.
Now we’re ready to combine our images into the final stacks. Once again, the program offers a number of ways to do this. Refer to the internal help file to determine which suits your images best. I prefer mean combine, so I’ll select Stack/Combine/Mean from the top pull-down menu. The software will then compute the mean value for each pixel in the stack of sub-exposures, while excluding the rejected pixels. This will give you the maximum signal-to-noise ratio in your final image. When completed, save the resulting image (File/Save Data/This), and again choose 32-bit FITS integer files. Close all files (File/Remove all images), and repeat the same steps for all like-filtered files.
Now we have master FITS files ready to combine into a color image. I prefer to process luminance images separately and then add them to the color result in Photoshop. Before combining any of the stacks, check them over carefully and address any gradients that may be affecting the individual stacks. CCDStack 2 has a gradient removal algorithm that can be found in the pull-down menu Process/Flatten Background, which requires you to click areas in your image until they appear evenly illuminated.Stretching and Deconvolution
Now let’s stretch our luminance file using the Digital Development Process (DDP) feature. One of the software’s most important features is its ability to do a “live” DDP on the displayed version of your file. First open your master luminance image, and select Window/Adjust Display, opening a window that displays sliders to adjust the Background, Maximum, Gamma, and DDP levels of the displayed image. You can now simply adjust each of the sliders until you’re happy with the displayed result. The lower the DDP value is (when moving the slider to the left), the brighter the image becomes. I suggest keeping the image appearing slightly darker than how you’d like it to eventually look. This performs the bulk of the required stretching, but still leaves room for final tweaks in Photoshop. Once you get the image looking the way you want it, lower the Background value by around 50 points to avoid clipping the black level in your final image. Apply the display settings to your image with the pull-down option File/Save scaled data/This, and select TIFF 16 bit.
You can also sharpen your image using deconvolution to tighten up the stars and sharpen small-scale features. CCDStack 2 has an excellent deconvolution routine called Positive Constraint that, when applied moderately, does a great job without introducing unwanted artifacts such as dark halos around stars. Select Process/Deconvolve. A new window opens, and a number of stars will appear with yellow + symbols over them. These are stars the program has selected to measure their point-spread function (PSF) to determine the strength of the deconvolution algorithm. You can also double-click on any stars you want the program to include in its calculations. Choose stars that are not saturated and are well defined (i.e. not embedded in nebulosity or within a visible galaxy). Next, select Positive Constraint at the bottom of the window, and set the number of iterations; I often use 30 to 50. Now click the “Deconvolve” button, and in a few minutes the process is complete; save the resulting FITS file. You can apply the same DDP settings to the deconvolved image as you did to the original by switching to the unprocessed version and clicking on “Apply to all” in the Display Manager window. Save the deconvolved version as a scaled 16-bit TIFF to be combined with the color image later in Photoshop.Color Combine
Finally, let’s combine our red, green, and blue files into an RGB image. In order to accomplish this best, you first need to know the correct RGB ratios for your particular CCD camera, filters, and sky conditions when the images were recorded. Although there are several ways to measure these values once for your system, each data set also requires adjustments to be made for atmospheric extinction caused by the target’s altitude when each series of color sub-exposures was taken. I prefer the free software eXcalibrator (http://bf-astro.com/excalibrator/excalibrator.htm) for determining an accurate color balance (see www.skypub.com/excalibrator). However, a simple method to get you started with approximate color balance in CCDStack 2 is to normalize your red, green, and blue files to one another, and then combine the images at a 1:1:1 ratio. As described earlier, select a neutral background area, then the highlights. After normalization, select Color/Create from the pull-down menu. The Create Color Imagewindow opens, where you can assign your filtered images to their respective channels. You can also incorporate your master luminance image here if desired, though make sure not to include the stretched luminance image. Click the “Create” button, and in a moment your combined color image will appear.
Immediately a small window called Set Background appears with your color file. If your image requires additional color adjustment, simply drag a box around a neutral background area and click “OK.” You can perform additional background and highlight corrections using the Color/Adjust command in the pull-down menu.
When you’re happy with the overall color image, you can stretch the result using the DDP slider and save the result for further adjustments in Photoshop, and include the stretched luminance image.
Performing these steps correctly provides a solid foundation upon which you can build and modify once you become familiar with all the tools available in CCDStack 2. Using the software’s sigma-based data-rejection algorithms, live DDP, and a mild application of Positive Constraint deconvolution will give you a head start on your way to producing images that may one day appear in Sky & Telescope.
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Celestron continues its partnership with high-end video camera manufacturer The Imaging Source, releasing the Skyris 132M and 132C (monochrome and color) cameras for $369.95 each. Both cameras feature the state-of-the-art Aptina AR0132AT CMOS sensor with a 1,280 x 960 array of 3.75-micron-square pixels. Each unit's high-speed USB 3.0 data transfer allows users to record up to 60 full-resolution frames per second (fps), or up to 200 fps when using a region-of-interest subframe, which is particularly useful when imaging the planets. Skyris incorporates a rolling electronic shutter and weighs only 3.6 ounces (102 grams). The camera is also capable of recording in 12-bit mode and comes with a C-to-1¼-inch nosepiece, a 10-foot USB 3.0 cable, and Celestrons iCap PC camera-control software.
SkyandTelescope.com's New Product Showcase is a reader service featuring innovative equipment and software of interest to amateur astronomers. The descriptions are based largely on information supplied by the manufacturers or distributors. Sky & Telescope assumes no responsibility for the accuracy of vendors statements. For further information contact the manufacturer or distributor. Announcements should be sent to nps@SkyandTelescope.com. Not all announcements will be listed.
Pictures are great, but there's nothing like holding another world in your hands to appreciate its unique characteristics.Sky & Telescope's planet globes are on sale! For a limited time, you can get them for 30% off the usual price. See ShopatSky's Maps & Globes page for details, or click on the individual links in the story below.
True confession: even from childhood, I have always been a map junkie. Sure, I use GPS these days, but my car is still stuffed with road maps. The National Geographic Atlas of the World is not far from my desk for fact-checking. I could spend hours (and have) playing with Google Earth.
It's the same with globes. In fact, one of my most prized possessions is a set of NASA globes from the 1970s. The Moon, Mars, and Mercury are accurately scaled to a 12-inch Earth globe, and to accomplish that feat the globes had to be specially crafted as solid wood spheres. Amazing.
So, not surprisingly, over the years I've eagerly spearheaded Sky & Telescope's efforts to develop globes of the Moon and terrestrial planets. We started in the 1980s with Mars, added Venus, and more recently completed the set with the Moon, Mercury, and Earth. For each, we worked with planetary scientists and the astrogeology team at the U.S. Geological Survey in Flagstaff, Arizona. Together with S&T's illustration specialists, most notably Gregg Dinderman, we fretted over colors and lavished attention on every label. We've been fortunate to have found a willing partner in Replogle Globes.
The result is a set of attractive and eye-catching orbs that attract lots of admiring gazes wherever we show them. But they're far more than pretty mini beach balls on the shelf. Honestly, I need to refer to one or more of these globes almost every day at the office. And the more I pore over them, the more I learn. Here's a sample of those insights.
Mercury: Because its surface is nearly gray and heavily cratered, you might initially confuse Mercury for the Moon. But the look is subtly different, due to vast stretches of lava-capped plains. And then there's the crater Hokusai. It's only 95 km (59 miles) across, but its formation created a spectacular splash of bright crater rays that extends more than a quarter of the way around Mercury. No single picture can convey this breadth, but the globe makes it easy.
Venus: Nothing says "cloudy" like Venus. Its atmosphere is so opaque that, except for a few snapshots from the Soviet Union's Venera landers, we have no photographic views of its hellish surface. Fortunately, in the early 1990s NASA's Magellan orbiter used synthetic aperture radar to record the surface details and measure the global topography. Creating a globe of all this data was tricky — the resulting maps portray the radar "brightness" of the surface. But we worked with NASA centers to portray both surface details and elevation in a clear way. Venus is a weird, unique world — its signature geologic features are big, round coronae that are bounded not by high rims but instead deep circular fractures.
Earth: We've all seen globes of Earth that show national boundaries and major cities. For its Earth globe, however, Sky & Telescope's editors wanted to show our home planet the way visiting aliens might see it — crisscrossed by mountains, valleys, and other major geologic structures. We got map data for the land portion from a mosaic of thousands of images, known as the Blue Marble, acquired by NASA's Terra and Aqua satellites. For undersea features, we relied on bathymetry gathered by the British Oceanographic Data Centre. If you're at all interested in plate tectonism — the gradual grinding and collision of Earth's interlocking crustal slabs — this is the globe for you!
Moon: Sky & Telescope contempolated making a Moon globe decades ago but held off until NASA's Lunar Reconnaissance Orbiter delivered the enough detailed mapping to cover the globe consistently. From LRO imagery we made a true-to-the-eye "visual" globe that's quite dark (as the Moon really is). More than 15,000 images went into making the base map! And we used LRO's laser altimetry — 4½ billion individual measurements! — to create a companion topographic Moon globe. (You can get these individually or as a pair.)
A big challenge was determining how many craters and other features to label — the International Astronomical Union has assigned names to more than 9,000 lunar features! We ended up with about 850 labels, including all of the well-known telescopic targets favored by amateur astronomers and the names and landing dates for Apollo, Surveyor, and Luna spacecraft.
One great use of the visual globe is identifying features situated around the limb of the Earth-facing lunar hemisphere. Thanks to the periodic back-and-forth nodding of disk due to libration, some of these marginal objects occasionally come into telescopic view. But they're always seen obliquely, and the globe clearly shows how they really appear. As for the topography globe, the most striking feature — hands-down — is a gigantic impact basin on the far side known as South Pole – Aitken. With a diameter of 2,500 km and a depth of 13 km, it is the largest, deepest, and oldest impact structure on the lunar surface.
Mars: This is the planet that propelled us into the globe business. We've been selling 12-inch replicas of the Red Planet since 1990.
Little-known fact: the Sky & Telescope Mars globe is based on Viking images acquired in the 1970s. So why not use more modern imagery, say, from NASA's Mars Odyssey or Mars Reconnaissance Orbiter? We've explored those, but realistically a 12-inch globe wouldn't do justice to all that finer-resolution detail. (Maybe, someday, we'll make a 6-foot-wide version!) We did tap a more recent spacecraft, Mars Global Surveyor, for the global altimetry to create a color-coded topographic globe of Mars.
If you're a fan of The Martian, either of these globes would be an excellent way to track the progress of astronaut Mark Watney as he tries to survive on — and escape from — the Red Planet. They're also available as a two-globe combo.
Rest assured: we're not finished with making planet globes! Several new ones are in the works, and we're always looking for ideas. So drop a suggestion (or offer thoughts on S&T globes you already have) in the comment section below.