Space
The thing about space is when you think you have seen it all, something truly bizarre shows up.
Astrophotographers have many specialities to pursue but in the main, the images that adorn the multitudinous websites consist of stars, special events, planets and deep sky objects. This chapter and the next few give an astronomical grounding in the various objects in space and the systems we use to characterize, locate and measure them. It is not essential to understand astronomy to obtain good pictures, but I think it helps to decipher the lingo and adds to the enjoyment and appreciation of our own and others’ efforts.
Stars
The points of light that we see in the night sky are stars, well, almost. Our own Sun is a star, but the planets of our solar system are not, they merely reflect our own Sun’s light. Every star is a gravitationally bound luminous sphere of plasma; a thermonuclear light bulb. With the naked eye, on a dark night, you might see up to 3,000 after a period of dark adaptation. That number decreases rapidly as light pollution increases. A star may have its own solar system, but its distance and brightness is such that we cannot directly observe any orbiting planets, even with the help of space-borne telescopes. In recent years in the never-ending search for extraterrestrial life, the presence of planets has been detected outside our own solar system but only by the effect of their gravitational pull on their home star’s position. Not all stars are equal; they can be a range of masses, temperatures and brightnesses. Stars have a sequence of formation, life and decay, starting in a nebula and subsequently converting their mass into electromagnetic radiation, through a mechanism governed by their mass, composition and density. Hertzsprung and Russell realized that the color and intensity of stars were related and the diagram named after them shows this pictorially (fig.1). Most stars comply with the “main sequence” on the diagram, including our own Sun. Notable exceptions are the intensely dense white dwarfs and the huge red giants, some of which are so large, we could fit our entire solar system within their boundary. There are countless stars in a galaxy but at the end of a star’s life, if it explodes and briefly becomes a supernova, it can outshine its entire parent galaxy. In our own Milky Way galaxy, documentary evidence suggests on average, there are about three supernova events per century.
fig. 1 The Hertzsprung-Russell diagram, named after its developers, shows the relationship and observed trend between the brightness and color of stars. The color is directly related to temperature. Ninety percent of stars lie on a diagonal trend known as the main sequence. Other groups and some familiar stars are also shown. At one time, scientists thought that stars migrated along the main sequence as they age. More recent study suggests a number of different scenarios, depending on the makeup, mass and size of the star.
From a visual standpoint, although stars may be different physical sizes, they are so distant from Earth, they become singular points of light. The only star to be resolved as something other than a point of light, and only by the largest telescopes, is the red giant Betelgeuse in the constellation Orion. It is puzzling then that stars appear in photographs and through the eyepiece in varying sizes, in relation to their visual intensity. This is an optical effect which arises from light scatter and diffraction along the optical path through our atmosphere, telescope optics and the sensitivity cut-off of our eyes or imaging sensor. Stars as single objects are perhaps not the most interesting objects to photograph, although there is satisfaction from photographing double stars and specific colored stars, such as the beautiful Albireo double. Resolving double stars has a certain kudos; it is a classical test of your optics, seeing conditions, focus and tracking ability of your setup.
When imaging stars, the main consideration is to ensure that they all are circular points of light, all the way into the corners of the image, sharply focused and with good color. This is quite a challenge since the brightness range between the brightest and dimmest stars in the field of view may be several orders of magnitude. In these cases, the astrophotographer has to make a conscious decision on which stars will over-saturate the sensor and render as pure white blobs and whether to make a second, or even third reduced exposure set, for later selective combination. Very few images are “straight”.
Constellations
Since ancient times, astronomers have grouped the brighter stars as a means of identification and order. In nontechnical terms, we refer to them as constellations but strictly speaking, these star patterns are asterisms and the term constellation defines the bounded area around the asterism. These are irregular in shape and size and together they form a U.S. state-like jigsaw of the entire celestial sphere. This provides a convenient way of dividing the sky and referring to the general position of an object. The 12 constellations that lay on the path of our companion planets’ orbits (the ecliptic) have astrological significance and we know them as the constellations of the Zodiac.
Star Names
Over thousands of years, each culture has created its own version of the constellations and formed convenient join-the-dot depictions of animals, gods and sacred objects. It has to be said that some stretch the imagination more than others. Through international collaboration there are now 88 official constellations. The brightest stars have been named for nearly as long. Many, for instance “Arcturus” and “Algol”, are ancient Arabic in origin.
For some time a simple naming system has been used to label the bright stars in a constellation: This comprises of two elements, a consecutive letter of the Greek alphabet and the possessive name of the constellation or its abbreviation. Each star, in order of brightness, takes the next letter of the alphabet: For instance, in the constellation Centaurus, the brightest star is Alpha Centauri or αCen, the next is Beta Centauri or βCen and so on. Beyond the limits of the Greek alphabet, the most reliable way to define a star is to use its coordinates. As the number of identifiable stars increases, various catalog systems are used to identify over 1 billion objects in the night sky.
fig. 2 The above illustration shows the constellation Ursa Major, of which the main asterism is commonly known as the Big Dipper, The Great Bear, The Plough and others. Many stars are named and take the successive letters of the Greek alphabet to designate their order of brightness. Several galaxies lie within or close to this constellation; the M-designation is an entry in the famous Messier catalog.
Deep Sky Objects
A deep sky object is a broad term referring to anything in the sky apart from singular stars and solar system objects. They form the basis of most astrophotography subjects and include nebulae, clusters and supernova remnants.
Star Clusters
As stars appear to be randomly scattered over the night sky, one would expect there to be groups of apparently closely packed stars. Clusters are strictly groups of stars in close proximity in three dimensions. They are characterized into two groups: Those with a loose sprinkling of approximately 100 to 1,000 younger stars, such as Pleiades, are termed an open cluster and often have ionized gas and dust associated with them. Those with 10,000 or more densely packed stars are older are referred to as globular clusters of which, in the Northern Hemisphere, M13 in the constellation Hercules is a wonderful example.
Although we can detect clusters in neighboring galaxies, they are too distant to resolve as individual stars. The clusters we commonly image are located in our own Milky Way galaxy. As well as being beautiful objects, clusters contain some of the oldest stars in the galaxy and are subject to intense scientific study too.
An image of star cluster is a showcase for good technique. It should have good star resolution and separation, extended to the dimmer stars at the periphery but without highlight clipping at the core. The stars should show good color too. This requires a combination of good tracking, focus, exposure and resolution and is the subject of one of the later case studies.
Star vistas can be wide-angle shots showing thousands of stars, the Milky Way or a landscape picture where the night sky plays an important part of the image. By their nature, they require lower magnifications and less demanding on pointing and tracking accuracy. They do, however, highlight any focus, vignetting or resolution issues, especially at the edges of an image.
Double and Binary Stars
A double star describes a distinguishable pair of stars that appear visually close to one another. In some cases they really are, with gravitational attraction, and these are termed visual binaries. Binary stars are one stage on, a pair of stars revolving around a common center of gravity but appear as one star. Amazingly, scientists believe that over 50% of Sun-like stars have orbiting companions. Most binary stars are indistinguishable but sometimes with eclipsing binaries the light output is variable, with defined periodicity.
Variable Stars
Variable stars have more scientific significance than pictorial. A class of variable star, the Cepheid Variables, unlocked a cosmic ruler through a chance discovery: In the early 20th century, scientists realized that the period of the pulsating light from many Cepheid Variables in our neighboring galaxy, the Small Magellanic Cloud, showed a strong correlation to their individual average brightness. By measuring other variable stars’ period and intensity in another galaxy, scientists can ascertain it’s relative distance. Supernova hunting and measuring variable stars require calibrated camera images rather than those manipulated for pictorial effect.
Nebula
A nebula is an interstellar cloud of dust, hydrogen, helium, oxygen, sulfur, cobalt or other ionized gas. In the beginning, before Edwin Hubble’s discovery, galaxies beyond the Milky Way were called nebulae. In older texts, the Andromeda Galaxy is referred to as the Andromeda Nebula. Nebulae are classified into several types; diffuse nebulae and planetary nebulae.
Diffuse Nebulae
Diffuse nebulae are the most common and have no distinct boundaries. They can emit, reflect or absorb light. Those that emit light are formed from ionized gas, which as we know from sodium, neon and xenon lamps radiate distinct colors. This is particularly significant for astrophotographers, since the common hydrogen, oxygen, sulfur and nitrogen emissions do not overlap with the common sodium and mercury vapor lamps used in city lighting. As a result, even in heavily light-polluted areas, it is possible to image a faint nebula through tuned narrowband filters with little interference. Diffuse nebula can also be very large and many fantastic images are possible with short focal-length optics. The Hubble Space Telescope has made many iconic false color images using “The Hubble Palette”, comprising narrowband filters tuned to ionized hydrogen, oxygen and sulfur emissions which are assigned to green, blue and red image channels.
Planetary Nebulae
These amazing objects are expanding glowing shells of ionized gas emitted from a dying star. They are faint and tiny in comparison to diffuse nebula and require high magnifications for satisfactory images. They are not visible to the naked eye and the most intricate details require space-telescopes operating in visible and non-visible electromagnetic spectrums. The first planetary nebula to be discovered was the Dumbbell Nebula in 1764 and its comparative brightness and large 1/8th degree diameter render it visible through binoculars. Only the Helix Nebula is bigger or brighter.
Supernova Remnants
One other fascinating nebula type forms when a star collapses and explodes at the end of its life. The subsequent outburst of ionized gas into the surrounding vacuum, emits highly energetic radiation including X-rays, gamma waves, radio waves, visible light and infrared. The Crab Nebula is a notable example, originating from a stellar explosion (supernova), recorded by astronomers around the world in 1054. Amazingly, by comparing recent images with photographic evidence from the last century, astronomers have shown the nebula is expanding at the rate of about 1,500 kilometers per second. After certain classes of supernova events there is a gravitational collapse into an extremely dense, hot neutron star. Astronomers have detected a neutron star at the heart of the Crab Nebula. They often give off gamma and radio waves but also have been detected visibly too.
fig. 3 When Edwin Hubble discovered that galaxies exist outside our own, he went about classifying their types from their appearance. The original scheme above is the most famous, the “Hubble Sequence” and was added to later by other astronomers. The elliptical galaxies are designated with an “E” followed by an index x, spiral galaxies normally with a central bulge and two or more arms are designated “Sx” of which some have a center barred structure, designated “SBx”. The remaining class (“S0”) is known as lenticular and although they feature a central bulge in the middle of a disk-like shape, they have no observable spiral structure.
Galaxies
As mentioned already, the existence of other galaxies outside our own was a late realization in 1925 that fundamentally changed our view of the universe. Galaxies are gravitationally bound collections of millions or trillions of stars, planets, dust and gas and other particles. At the center of most galaxies, scientists believe there is a super massive black hole. There are billions of galaxies in the observable universe but terrestrial astrophotography concerns itself with the brighter ones. There are approximately 200 brighter than magnitude 12, but at magnitude 14 the number rises to over 10,000. The brightest is the Large Magellanic Cloud, a neighbor to our Milky Way and easily visible to the naked eye by observers in the Southern Hemisphere. Charles Messier in the 18th century cataloged many other notable examples and are a ready-made who’s who.
Galaxies come in all shapes and sizes, making them beautiful and fascinating. Many common types are classified in fig.3. Most of the imaging light from galaxies comes from their stars, though there is some contribution from ionized gases too, as in nebulae. Imaging galaxies requires good seeing conditions and low light pollution since they are in general, less luminous than stars or clusters and have less distinct boundaries. In general, a good quality image of a galaxy is a balance of good surrounding star color, galaxy color and extension of the faint galaxy periphery, without sharp cut-offs into the background or over exposing the brighter core. This requires careful exposure and sensitive manipulation and quite possibly an additional shorter exposure sequence for the surrounding brighter stars. Supplementary exposures through narrowband filters (tuned to ionized gases, or infrared) can enhance and image but in general, since these filters pass very little light, the exposure times quickly become inconveniently long and only practical when applied to the brightest galaxies.
Quasars may appear as stars, but in fact are the bright cores of very distant galaxies and are the most luminous things in the universe. They were first identified through their radio wave emissions and only later linked to a faint, visible, heavily red-shifted dot. The extreme energies involved with their emissions is linked to the interaction of gas and dust spiralling into a black hole. A few quasars are visible from Earth and within the reach of amateur astrophotographer’s equipment.
Solar System Objects
The prominent planets in our solar system were identified thousands of years ago. The clue to how is in the name. Derived from the ancient Greek, “planet” means wanderer, and in relation to the background of wheeling stars, Mercury, Venus, Mars, Jupiter, and Saturn appeared in different positions each night. Unlike the continual annual stately cycle of star movement, these planets performed U-turns at certain times in the calendar. Those planets closer to the Sun than the Earth are called inferior planets (Venus and Mercury) and correspondingly Mars, Jupiter, Saturn, Uranus and Neptune are called superior planets. By definition, planets orbit a sun and need to be a significant distinct ball-like shape of rock, ice and gas. The definition is a bit hazy and as such Pluto was demoted in the 20th century, after much debate, to a minor planet (of which there are many).
The Keplerian and Newtonian laws of motion amazingly predict the precise position of our planets in the night sky. Within planetarium programs, their position has to be individually calculated but from an imaging standpoint, for the short duration of an exposure, their overriding apparent motion is from the earth’s rotation, which is adjusted for by the standard (sidereal) tracking rate of a telescope.
From Earth, some planets change appearance: Planets appear larger when they are close to “opposition” and closest to Earth. Mercury and Venus, being closer to the Sun than the Earth, show phases just as our Moon does, and Jupiter, Saturn and Mars change their appearance from rotation and tilt.
The massive Jupiter spins very quickly and completes a revolution in about 10 hours. This sets a limit on the exposure time of a photograph to about 90 seconds at medium magnifications and less with more. Above this time, its moons and the surface features, most notable of which is the giant red spot, may become blurred.
Saturn, whose iconic ring structure has inspired astronomers since the first telescopic examination, has an interesting cycle of activity. These rings, which in cosmic terms are unbelievably thin at less than 1 kilometer, have an inclination that changes over a 30-year cycle. In 2009, the rings were edge-on and were almost invisible to Earth but will reach a maximum 30° inclination during 2016–17.
When we observe a distant galaxy, we are only seeing the stars and none of the planets. Even taking into consideration the extra mass of planets, dust and space debris, the rotational speed of the observed galaxies can only be explained if their overall mass is considerably higher. The hypothesized solution is to include “dark matter” into the mass calculation. Dark matter defies detection but its presence is inferred from its gravitational effect. In 2012 the Hadron particle collider in Switzerland identified a new elementary particle, the Higgs Boson, with a mass 125x that of a proton. It is an important step along the way to explaining where the missing mass is in the observable universe.
Mars rotates at a similar rate to Earth. Terrestrial photographs of Mars show some surface details as it rotates. In addition, there are seasonal changes caused by the axial tilt and its highly eccentric orbit. From an imaging standpoint, this affects the size of its white polar ice cap of frozen carbon dioxide during the Martian year (lasting about two Earth-years). Its size is under 1/120th degree and requires a high magnification and stable atmospherics for good results. It is a challenging object to image well.
Asteroids, Satellites and Meteorites
At various times, these too become subject to photographic record. Of these, asteroids are perhaps the least interesting to the pictorialist until they fall to Earth. These lumps of rock or ice are normally confined to one of our solar system’s asteroid belts, but in our prehistory, may have been knocked out of orbit by collisions or gravitational interactions of the planets. One of the largest, Vesta, has been subject to special up-close scrutiny by the Dawn spacecraft. Indeed, debris from a Vesta collision in space fell to Earth as meteorites. On rare occasions asteroids pass closer to Earth than the Moon.
Satellites, especially when they pass in front of the Moon or Sun in silhouette, are visually interesting and require forward planning. More commonly, satellite images are indistinct reflections of sunlight against a dark sky. There are thousands of man-made satellites circling the Earth. The most well known have published orbital data which can be used within planetarium programs to indicate their position or line up a computer-controlled telescope. They orbit from 180 km or more away at a variety of speeds, depending on their altitude and purpose.
Meteorites are not in themselves special objects, merely the name we give natural objects when they make it to the Earth’s crust. They are mostly comprised of rock, silicates or iron. Specimens are of important scientific value, for locked inside, there can be traces of organic material or of their source atmosphere. During their entry into our atmosphere, their extreme speed and the friction with the air heats them to extreme temperatures, leading to their characteristic blazing light trail and occasional mid-air explosions. The larger ones are random events, but there are regular occurrences of meteor showers that beckon to the astrophotographer.
Meteor showers occur when the Earth interacts with a stream of debris from a comet. This debris is usually very fine, smaller than a grain of sand and burns up in our atmosphere. These events are regular and predictable and produce a celestial firework display for a few successive nights each year. The events are named after the constellation from which the streaks appear to emanate. Famous meteor showers are the Perseids in August and the Leonids in November, which produce many streaks per hour. Often the most spectacular photographs make use of a wide-angle lens on a static camera and repeated exposures on a time-lapse for later selection of the best ones.
Special Events
Over thousands of years, astrologers have attached significance to special astronomical events. The most well known, yet strangely unproven, is the “Star of Bethlehem” announcing Jesus’ birth, which may have been a supernova explosion. These events include special causes, like supernova, where an individual star can achieve sufficient short-lived intensity to be visible during the day, or the sudden appearance of a bright comet. Many other events consider the relative positions of a planet and the Sun, the Moon and the Sun, the phases of the Moon or the longest day or night. Modern society has disassociated itself from Astrology, but the rarity of some events encourages astronomers and physicists to travel the world to study eclipses, transits or another one-off event. The good news for astronomers is that, apart from supernova, everything else is predictable. (Edmond Halley realized that most comets too have a predictable orbit and appearance.) For an imaging standpoint, the luck and skill of capturing a rare event adds to the satisfaction of the image. As they say, “chance favors the prepared mind” and is no different.
Exoplanets
In recent years, amateurs have joined in the search for exoplanets, made feasible by low-noise CCD cameras and high quality equipment. With care, one can not only detect known exoplanets through their momentary lowering of their host star’s flux but potentially find new ones too by the same means. A highly specialized area but one which is introduced in a later chapter. The image in this case is not of the planet itself (it is too dim) but a graph of the host star’s light output with a characteristic and regular dip.
A photograph of stars close to the Sun, taken by Arthur Eddington during a total solar eclipse in 1919, when compared to a photograph of the same stars with the Sun not present, showed a tiny deflection. It was the first measurement to substantiate that light-beams could be bent by gravity, predicted in Einstein’s general theory of relativity.
Comets
Comets become interesting when they pass close to the Sun. In space, they are lumps of ice and rock circling in enormous orbits. As their orbit passes close to the Sun, the characteristic tail and tiny atmosphere (coma) develops. The tail points away from the Sun and arises from the effect of solar radiation and wind on the comet’s volatile contents. Short-period comets, with orbits of less than 200 years, are widely predicted, and with a little luck, can be photographed in good conditions. More occasional visitors are often detected by the various near-earth object telescopes long before they become more readily visible. A bright comet discovered in September 2012, name ISON, passed close to the Sun in January 2014 and many hoped it would provide an opportunity for unique images. A photograph of a comet is a wonderful thing, but to image it as it passes through another landmark site, such as a star cluster, makes it memorable. Since the stars and comet are moving in different directions and speed, one must decide whether to track the stars or not during the brief exposure. However, ISON was imaged by Damian Peach and others as it approached the Sun but it never made it past perihelion and the solar radiation was too much for the muddy snowball. It should have been renamed comet Icarus!
Lunar Eclipses
A lunar eclipse occurs when the Moon, Earth and Sun are in a direct line and the Moon is in Earth’s shadow. We can still see the Moon, which is illuminated from scattered light through our atmosphere, and it often takes on a reddish appearance. A time sequence of a lunar eclipse from 2007 is shown in fig.4.
Solar Eclipses
A solar eclipse occurs when the Earth, Moon and Sun are in a direct line and the Moon blocks our view of the Sun. By amazing coincidence, the Moon and Sun have the same apparent size, and eclipses may be partial, where the Moon clips the Sun, or total, which provides a unique opportunity to image the solar corona safely. A total solar eclipse will only be visible from a select 100-kilometer wide tract of the Earth’s surface, and avid observers will travel to far flung corners of the world to get the best view of a “totality”.
fig. 4 This lunar eclipse was captured in March 2007 and assembled from a sequence of still photographs, taken with a consumer digital SLR mounted on a tripod and fitted with a 210 mm zoom lens.
Planetary Transits
Mercury and Venus, the “inferior” planets, lie closer to the Sun than the Earth. On the rare occasions that they pass in front of the Sun, they are in transit. Man-made satellites also transit the Moon and Sun for a few seconds. Photographing the Sun during a transit requires the same mandatory precautions as any other form of solar photography. Transits occur when the nearer object is smaller than the more distant object. (Occultations occur when it is the other way around and it is possible to get transits and occultations between planets too.) In 2065, Venus transits Jupiter and in 2067, Mercury occults Neptune. I’ll pass on that one.
Superior and Inferior Conjunctions
These are general terms for line-ups of astronomical bodies from an observer’s standpoint. These may be between planets, a planet and the Moon or Sun or other combinations. From an imaging standpoint it is interesting when one can make an image of two close important bodies, though the brightness difference often makes it a challenge. Planetarium programs are very adept at predicting these events and can produce timetables for their occurrence.
Opposition
Another particular event, opposition, occurs when two bodies are on opposite sides of the sky from an observed position. This is of most significance to astrophotographers since when a superior planet is in opposition, it generally is at its closest point to earth and hence its apparent size will be a maximum. Jupiter increases its apparent size by 66%. Mars’ change is more extreme and its apparent diameter increases by more than 600%. It is good practice to image planets when they are close to their opposition.
Equinoxes and Solstices
These regular events occur when the Earth is at a specific point in its orbit around the Sun. In the case of the equinox, the tilt of the earth’s axis is tangential to the Sun and it has the unique characteristic that night and day are of equal length. It does not have any significant imaging significance, but it does for our celestial coordinate system. There are two equinoxes per year (spring and autumn) and the celestial coordinate system uses the Sun’s position at the spring equinox to define an absolute reference point for measuring right ascension. (We will discuss coordinate systems in more detail later on.) There are also two solstices each year, in winter and summer. These mark the shortest and longest day and occur when the tilt of the Earth’s axis is in line with the Sun. Their significance for photography mostly relates to the number of available hours for imaging!