Digital Photography 101: Low-light photography, part 3

How to photograph the moon and stars

 

If you live in the city or any area with a lot of light pollution, it can be easy to forget how beautiful the night sky is. But whether you're looking for the International Space Station or watching planets dancing close together, the planets, stars, and moon offer beautiful photographic opportunities if you can get to a dark enough location to see them.

Look for the International Space Station in the night sky this week

The ISS will be particularly visible as it orbits the Earth over the next few days

 

While there are plenty of natural celestial bodies to be admired in the night sky, this week you'll be able to see a man-made masterpiece among them. The International Space Station is visible in the night sky from time to time, but it will be particularly easy to spot over Europe and North America for the next few days.

The ISS should outshine everything else in the sky for a few fleeting 5-minute long intervals, so timing is critical if you want to catch a glimpse of the space home that houses a global crew of astronauts.

To find out when (and if) you'll be able to go space station spotting, NASA has a useful online tool that will help you track flybys based on your location. And if you'd rather have the info come straight to you, a convenient app for iPhone and Android can help you scan the skies too — you know, so your neck doesn't get tired.

There are definitely some tricks to getting lovely photographs of the night sky, requiring planning and strategy, but the results are worth it!

Choose your timing
The timing of your photographic expedition depends on your goals and your subject. If you're trying to photograph the full moon, aim for when the moon is rising, just as the sun is setting, for the most dramatic pictures. This will give you enough light to illuminate the setting in front of the moon (such as mountains or trees) without overexposing the moon. Websites such as the U.S. Navy's Complete Sun and Moon Data site give the exact times for sun and moon rises and sets.

If you're shooting for the stars, your best bet is a night when the moon is either new or a thin crescent. Since stars are so much dimmer than the moon and require much longer exposure times to photograph, even the slightest sources of light will be bright in your image. Wait until the sky is fully dark, and try to get as far away from sources of light pollution as possible; your best bets are the desert, mountains, or rural area. 

Bring the right gear
A tripod is an absolute must for night photography, and photographing the moon and stars is no exception. You'll want to use a telephoto lens with a focal length of at least 200mm (preferably more) for the moon, and you might want to try a wide angle or fish eye lens for shots of the stars. Since any movement of the camera will result in motion blur, either a self-timer or remote control is also essential.

Shoot the moon
It's a gorgeous evening, the moon rising full and lovely on the horizon. You pull out your camera and snap a picture… but to your dismay, that beautiful orb ends up looking like a small white blob in your image.

What's going on? This happens because your camera will automatically use everything except the moon to calculate its optimal exposure, thus overexposing the moon itself. Your eyes perceive a much wider range of light than your camera, which is why what looks beautiful to your eye won't look so great automatically in the camera.



After all, the moon is bright because it's reflecting all of the light of the sun from around the edge of the world. So despite the fact that it might be dark where you are, you can't use that as a basis for gauging your settings to capture all the detail on the moon. Use your camera's manual settings for focus — a low ISO setting of around 100 and a medium aperture of about f/11. Take a shot with a relatively fast shutter speed of about 1/100th of a second, then take a few going either direction, slower and faster. Experiment and see what works for your particular environs. Every shot is different.

Photographing the stars
The stars are much, much farther away than the moon, of course, and their light is much dimmer. You'll need a very long exposure to properly capture their beauty. You'll also need to ask yourself whether you want to embrace or avoid star trails, the paths the stars make across the sky as the earth rotates beneath them.

Depending where you are and how much light pollution you have to shoot through, try shutter speeds ranging from 2 seconds up to a minute or more. If you're lucky enough to be in a truly dark locale, you could even try leaving the shutter open for 5 to 10 minutes or more to see what happens. The longer the shutter speed, the longer your star trails will be. If you want to get a circular pattern around one star, aim your camera north toward Polaris. If you do not want any star trails at all, you'll need to use an equatorial mount that will move your camera to follow the earth's rotation.

Polaris

Alpha Ursae Minoris

Polaris as seen by the Hubble Space Telescope.
Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Minor
Right ascension	02h 31m 49.09s
Declination	+89° 15′ 50.8″
Apparent magnitude (V)	1.97
Characteristics
Spectral type	F7 Ib-II SB
U−B color index	0.38
B−V color index	0.60
Variable type	Cepheid variable
Astrometry
Radial velocity (Rv)	-17 km/s
Proper motion (μ)	RA: 44.48 ± 0.11 [1] mas/yr Dec.: -11.85 ± 0.13 [1] mas/yr
Parallax (π)	7.54 ± 0.11 mas
Distance	433 ± 6 ly (133 ± 2 pc)
Absolute magnitude (MV)	-3.63±0.14[2]
Details
Mass	7.54 ± 0.6 M☉
Radius	46 ± 3[4] R☉
Luminosity	2200 L☉
Temperature	7200 K
Metallicity	112% solar
Rotation	~17 km/s
Age	? years
Other designations
Polaris, North Star, 1 Ursae Minoris, HR 424, BD +88°8, HD 8890, SAO 308, FK5 907, GC 2243, ADS 1477, CCDM 02319+8915, HIP 11767, Cynosura, Alruccabah, Phoenice, Navigatoria, Star of Arcady, Yilduz, Mismar

Polaris (α UMi, α Ursae Minoris, Alpha Ursae Minoris, commonly North Star, Northern Star or Pole Star, also Lodestar, sometimes Guiding star) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star.

Based on measurements from the Hipparcos astrometry satellite,Polaris is estimated to be at a distance of about 434 light-years from Earth. It is a multiple star, consisting of the main star α UMi A, two smaller companions, α UMi B and α UMi Ab, and two distant components α UMi C and α UMi D. α UMi B was discovered in 1780 by William Herschel.

Star system

α UMi A is a six solar mass F7 bright giant (II) or supergiant (Ib). The two smaller companions are: α UMi B, a 1.5 solar mass F3V main sequence star orbiting at a distance of 2400 AU, and α UMi Ab, a very close dwarf with an 18.5 AU radius orbit. There are also two distant components α UMi C and α UMi D.

Polaris B can be seen even with a modest telescope and was first noticed by William Herschel in 1780. In 1929, it was discovered by examining the spectrum of Polaris A that it had another very close dwarf companion (variously α UMi P, α UMi a or α UMi Ab), which had been theorized in earlier observations (Moore, J.H and Kholodovsky, E. A.). In January 2006, NASA released images from the Hubble telescope, directly showing all three members of the Polaris ternary system. The nearer dwarf star is in an orbit of only 18.5 AU (2.8 billion km, about the distance from our Sun to Uranus) from Polaris A, explaining why its light is swamped by its close and much brighter companion.

Polaris is a classic Population I Cepheid variable (although, it was once thought to be Population II due to its high galactic latitude). Since Cepheids are an important standard candle for determining distance, Polaris (as the closest such star) is heavily studied. The variability of Polaris had been suspected since 1852; this variation was confirmed by Ejnar Hertzsprung in 1911.Around 1900, the star luminosity varied ±8% from its average (0.15 magnitudes in total) with a 3.97 day period. Since then, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.

Research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it, changing from third to its current second magnitude.Astronomer Edward Guinan considers this to be a remarkable rate of change and is on record as saying that "If they are real, these changes are 100 times larger than [those] predicted by current theories of stellar evolution."

Names

Because of its importance in celestial navigation, Polaris is known by numerous names.

One ancient name for Polaris was Cynosūra, from the Greek κυνόσουρα "the dog’s tail" (reflecting a time when the constellation of Ursa Minor "Little Bear" was taken to represent a dog), whence the English word cynosure. Most other names are directly tied to its role as pole star.

In English, it was known as "pole star" or "north star", in Spenser also "steadfast star". An older English name, attested since the 14th century, is lodestar "guiding star", cognate with the Old Norse leiðarstjarna, Middle High German leitsterne. Use of the name Polaris in English dates to the 17th century. It is an ellipsis for the Latin stella polaris "pole star". Another Latin name is stella maris "sea-star", from an early time also used as a title of the Blessed Virgin Mary, popularized in the hymn Ave Maris Stella (8th century).In traditional Indian astronomy, its name in Sanskrit dhruva tāra, literally "fixed star". Its name in medieval Islamic astronomy was variously reported as Mismar "needle, nail", al-kutb al-shamaliyy "the northern axle/spindle", al-kaukab al-shamaliyy "north star". The name Alruccabah or Ruccabah reported in 16th century western sources was that of the constellation.

In the Old English rune poem, the T-rune is identified with tir "fame, honour", which is compared to the pole star, ᛏ [tir] biþ tacna sum, healdeð trywa wel "[fame] is a sign, it keeps faith well". Shakespeare's sonnet 116 is an example of the symbolism of the north star as a guiding principle: "[Love] is the star to every wandering bark / Whose worth's unknown, although his height be taken."

Role as pole star

Because in the current era α UMi lies nearly in a direct line with the axis of the Earth's rotation "above" the North Pole—the north celestial pole—Polaris stands almost motionless in the sky, and all the stars of the Northern sky appear to rotate around it. Therefore, it makes an excellent fixed point from which to draw measurements for celestial navigation and for astrometry. The moving of Polaris towards, and in the future away from, the celestial pole, is due to the precession of the equinoxes.The celestial pole will move away from α UMi after the 21st century, passing close by Gamma Cephei by about the 41st century. Historically, the celestial pole was close to Thuban around 2500 BC., and during Classical Antiquity, it was closer to Kochab (β UMi) than to α UMi. It was about the same angular distance from either β UMi than to α UMi by the end of Late Antiquity. The Greek navigator Pytheas in ca. 320 BC described the celestial pole as devoid of stars. However, as one of the brighter stars close to the celestial pole, Polaris was used for navigation at least from Late Antiquity, and described as αει φανης "always visible" by Stobaeus (5th century). α UMi could reasonably be described as stella polaris from about the High Middle Ages.

In more recent history it was referenced in Nathaniel Bowditch's 1802 book, The American Practical Navigator, where it is listed as one of the navigational stars. At present, Polaris is 0.7° away from the pole of rotation (1.4 times the Moon disc) and hence revolves around the pole in a small circle 1½° in diameter. Only twice during every sidereal day does Polaris accurately define the true north azimuth; the rest of the time it is slightly displaced to East or West, and to bearing must be corrected using tables or a rough rule of thumb. The best approximatewas made using the leading edge of the "Big Dipper" asterism in the constellation Ursa Major as a point of reference. The leading edge (defined by the stars Dubhe and Merak) was referenced to a clock face, and the true azimuth of Polaris worked out for different latitudes.

Equatorial mount

An equatorial mount is a mount for instruments that follows the rotation of the sky (celestial sphere) by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any object in the sky that has a diurnal motion by driving one axis at a constant speed. Such an arrangement is called a sidereal drive.

 

Astronomical telescope mounts

In astronomical telescope mounts, the equatorial axis (the right ascension) is paired with a second perpendicular axis of motion (known as the declination). The equatorial axis of the mount is often equipped with a motorized "clock drive", that rotates that axis one revolution every 23 hours and 56 minutes in exact sync with the apparent diurnal motion of the sky. They may also be equipped with setting circles to allow for the location of objects by their celestial coordinates. Equatorial mounts differ from mechanically simpler altazimuth mounts, which require variable speed motion around both axes to track a fixed object in the sky. Also, for astrophotography, the image does not rotate in the focal plane, as occurs with altazimuth mounts when they are guided to track the target's motion, unless a rotating erector prism or other field-derotator is installed.

Equatorial telescope mounts come in many designs. In the last twenty years motorized tracking has increasingly been supplemented with computerized object location. There are two main types. Digital setting circles take a small computer with an object database that is attached to encoders. The computer monitors the telescope's position in the sky. The operator must push the telescope. Go-to systems use (in most cases) servo motors and the operator need not touch the instrument at all to change its position in the sky. The computers in these systems are typically either hand-held in a control "paddle" or supplied through an adjacent laptop computer which is also used to capture images from an electronic camera. The electronics of modern telescope systems often include a port for auto guiding. A special instrument tracks a star and makes adjustment in the telescope's position while photographing the sky. To do so the autoguider must be able to issue commands through the telescope's control system. These commands can compensate for very slight errors in the tracking performance, such as periodic error caused by the worm drive that makes the telescope move.

In new observatory designs, equatorial mounts have been out of favor for decades in large-scale professional applications. Massive new instruments are most stable when mounted in an alt-azimuth (up down, side-to-side) configuration. Computerized tracking and field-derotation are not difficult to implement at the professional level. At the amateur level, however, equatorial mounts remain popular, particularly for astrophotography.

German equatorial mount

In the German equatorial mount,(sometimes called a "GEM" for short) the primary structure is a T-shape, where the lower bar is the right ascension axis (lower diagonal axis in image at right), and the upper bar is the declination axis (upper diagonal axis in image). The telescope is placed on one end of the declination axis (top left in image), and a suitable counterweight on other end of it (bottom right). The right ascension axis has bearings below the T-joint, that is, it is not supported above the declination axis.

Open fork mount

The Open Fork mount has a Fork attached to a right ascension axis at its base. The telescope is attached to two pivot points at the other end of the fork so it can swing in declination. Most modern mass-produced catadioptric reflecting telescopes (200 mm or larger diameter) tend to be of this type. The mount resembles an Altazimuth mount, but with the azimuth axis tilted and lined up to match earth rotation axis with a piece of hardware usually called a "wedge."

Many mid-size professional telescopes also have equatorial forks, these are usually in range of 0.5-2.0 meter diameter.

English or Yoke mount

 

The English mount or Yoke mount has a frame or "yoke" with right ascension axis bearings at the top and the bottom ends, and a telescope attached inside the midpoint of the yoke allowing it to swing on the declination axis. The telescope is usually fitted entirely inside the fork, although there are exceptions such as the Mt. Wilson 2.5 m reflector, and there are no counterweights like German mount has.

The original English fork design has the disadvantage of not allowing the telescope to point too near the north or south celestial pole.

Horseshoe mount

 

The Horseshoe mount overcomes the design disadvantage of English or Yoke mounts by replacing the polar bearing with an open "horseshoe" structure to allow the telescope to access Polaris and stars near it. The Hale telescope is the most prominent example of a Horseshoe mount in use.

Cross-axis mount

 

The Cross-axis or English cross axis mount is like a big "plus" sign (+). The right ascension axis is supported at both ends, and the declination axis is attached to it at approximately mid point with the telescope on one end of the declination axis and a counter weight on the other.

Equatorial platform

An equatorial platform is a specially designed platform that allows any device sitting on it to track on an equatorial axis. It achieves this by having a surface that pivots about a "virtual polar axis". This gives equatorial tracking to anything sitting on the platform, from small cameras up to entire observatory buildings. These platforms are often used with altazimuth mounted amateur astronomical telescopes, such as the common Dobsonian telescope type, to overcome that type of mount's inability to track the night sky.

 
Every year, there are fewer and fewer places on the Earth where you can look up and capture the full beauty of a star-filled sky. Despite many communities' beginning to enact light pollution ordinances to combat the problem, humans have encroached on so much of the planet that it's hard to find somewhere dark enough to see the Milky Way the way our ancestors saw it. Still, even in the United States, you can find places for trying your hand at beautiful photos of the night sky.

thanks to

by Katherine Gray