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Appendix A

Astronomical

Camera technology has developed remarkably quickly over the past two decades. Just as has been transformed by the arrival of digital —which are now so small and cheap that they can be embedded into laptops, and tablet computers—many of the same devices are also ideally suited for astronomical photography. Before their sudden arrival, photo- sensitive films had been the only technology available for astronomical photogra- phy, and had been in use for almost 150 . In the 1990s, digital cameras were crude devices, ideal for beginners who wanted the instant feedback that their displays could provide, or who liked the ease of not having to develop endless rolls of film. But they could not compete with the high-end film cameras used by more serious photographers, since their sensors lacked the very high resolution that could be reached by the best film cameras. Each in a digital is a tiny light-sensitive electronic component, which needs to be wired up. Making these small enough to rival the resolution that could be achieved by the traditional method of spreading light-sensitive chemicals across a film proved very challenging. Since then, however, digital cameras have sold in such vast numbers that it has become economic to put a vast development effort into building sensors with ever- larger numbers of ever-smaller pixels. At the of writing, cameras are routinely built with more than ten million pixels (megapixels), fabricated and sold at a cost of much less than a thousandth of a cent per pixel. The turning point came in around 2005, when the resolution of moderately-priced digital cameras had become high enough that even most professional photographers found their images no longer inferior to those produced by film cameras, but their instant feedback much more convenient to use.

D. Ford, The Observer’s Guide to Planetary : Explaining the Cycles of the , 215 The Patrick Moore Practical Series, DOI 10.1007/978-1-4939-0629-1, © Springer Science+Business Media, LLC 2014 216 A Astronomical Imaging

Simultaneous with the revolution that has occurred in daylight photography, astrophotographers have also almost universally hung up their film cameras in favor of digital devices. Digital sensors have turned out to perform substantially better than film in low-light conditions, meaning that the gap between the image quality achieved by terrestrial and astronomical photographers has closed considerably. Even though developing cameras specifically for astronomical use is highly expen- sive, it has been possible to merely repackage chips built for into cases that can be mounted onto the back of a . This is a fast-moving area, and so this appendix does not recommend particular camera models, but it provides an overview of some of the techniques used by astrophotographers which are likely to remain constant in years to come.

Historical

The art of astrophotography is almost as old as that of photography itself. The nineteenth century polymath John Herschel was not only an , but also a chemist who experimented with early photographic films, inventing the process in 1842. The intrinsic faintness of astronomical objects has always been a challenge for those trying to capture them on film. Indeed, the principal problem that almost all astrophotographers struggled to solve until very recently was to find ever-more sensitive films or sensors, which would reduce to within reasonable bounds. The first known successful astrophotograph was taken by John Draper (1811–1882) in March 1840, who caught the on film with a 20-min exposure. The improvement in technology that has occurred in the 170 years since then is demonstrated by the fact that a modern astrophotographer would typically use an exposure of around a tenth of a second or less to take the same image today of this, the ’s brightest object. By the mid nineteenth century, progress was being made in developing more sensitive films and the range of objects accessible to astrophotographers was grow- ing. In 1850, became the first to be captured on film, photographed by William Bond (1789–1859) and John Whipple (1822–1891). The following ’s solar was the first that is known to have been photographed. The first image of the spectrum of a star followed in 1863. Faint fuzzy objects—nebulae—proved considerably more difficult, and the first image of the (M42) was not captured until 1880, by (1837–1882) using a 51-min exposure through an 11-in. refractor. Many of the problems with which the pioneers had to grapple remain current today. Any telescope that is used for astrophotography must remain firmly centered on the same particular patch of sky for the whole duration of each exposure, to avoid forming a blurred image. The telescope must follow the sky’s diurnal rota- tion, and although a motorized drive can help, inevitably any device that simply rotates the telescope in once every 23 h and 56 min will have defects that cause it to drift over time. Among these, the telescope’s mount may not be A Astronomical Imaging 217 properly aligned to the , causing the drive to rotate the telescope about the wrong point; alternatively, the drive motor itself may not move at a perfectly steady rate; or the drive motor may not run at quite the right speed. Moreover, even the sturdiest mount may have some susceptibility to being shaken by the wind. To correct for any or all of the above, a device called a guider—which may be either or automated—is usually needed to monitor the alignment of long expo- sures and occasionally give the telescope a nudge. Three years after Henry Draper’s initial image of the , Andrew Common (1841–1903) and George Calver (1834–1927) pioneered a new mecha- nism for manually guiding over long periods, which allowed them to expose a single piece of film for 90 min in 1883 (see Fig. A.1) without appreciable drift in the telescope’s pointing. Later in the same decade, the Paris Observatory was confident enough about the new technology to establish a world-wide collab- orative project to photographically survey the whole sky—the Cartes du Ciel— though this over-ambitious project ended up taking decades to complete, and the last observations not taken until 1950. The full survey was eventually published, nearly 80 years later, in 1964.

The Advantages of Digital Sensors

Despite over a century of refinements to the chemistry of photographic films, digi- tal sensors have brought three immediate advantages over film photography. Firstly, the sensors themselves are intrinsically much more sensitive to light. Even the most sensitive films—often chemically modified by using a range of tech- niques collectively called hypersensitization—only respond to a few percent of the photons of light that fall on them. By contrast, modern charge-coupled devices (CCDs) typically have quantum efficiencies in excess of 70 %, meaning that more than 70 % of photons falling on them are detected, and correspondingly that expo- sures of a fraction of the length are needed to reveal comparable detail. Secondly, digital sensors have vastly superior dynamic range to film. Many astronomical objects have large contrasts between bright features and fainter struc- tures that are visible around them. This is especially true in the deep sky: the central regions of and globular clusters often vastly outshine their extremities. But it is also an issue that affects any planetary imager who wants to the Moon occulting , passing through the , or the of alongside their parent . Mars’s two moons Phobos and Deimos are so faint that they are almost impossible to separate from its glare, and even with modern sensors, anyone who manages to capture an image of either can count themselves an astrophotographer of the highest caliber. invariably has poor dynamic range, because individual pho- tons are more likely to trigger chemical reactions in the film in the presence of other photons. In other words, well illuminated areas of photographic film are more sensitive to light than less illuminated areas. This effect, known as reciprocity 218 A Astronomical Imaging

Fig. A.1 The development of astronomical photography: the Orion nebula (M42), as photo- graphed by Henry Draper (1880; top-left), Andrew Common and George Calver (1883; top- right ) and by the Hubble ( bottom ) A Astronomical Imaging 219

failure, makes it very easy to take images where bright areas of the frame are over- exposed, while darker areas are simultaneously under-exposed. By contrast, modern CCDs work by counting the number of individual photons that land on each individual detector element (pixel), and so their response is very nearly linear: a pixel which reports twice the illumination of another pixel received twice as many photons during the course of the exposure. Perhaps the greatest revolution brought by digital sensors, however, has been the immediate feedback that they provide. In the past, focusing was a highly approxi- mate art, with no direct means of assessing the sharpness of the image that was falling onto a piece of film until it came to be developed. Effects such as the thermal contraction of a telescope tube over the course of the night due to the falling ambient temperature were virtually impossible to mitigate. The result was that pho- tographic setups gradually drifted in and out of focus. By contrast, it is now possible to routinely take test images throughout the night, and the focal position of a camera can be periodically adjusted. The task remains one of the most tedious aspects of astrophotography, but the results are far superior to what was possible in the past. The immediacy with which images taken by digital cameras can be accessed has allowed them to feed data directly into telescope control software. In the past, the only way to ensure a telescope remained properly aligned on a target was to mount a separate guidescope on the back of the telescope. The alignment of this guides- cope would be adjusted, usually with thumbscrews, until a bright guide star lay on its cross-hair. Then, the telescope would be manually nudged by the photographer whenever the guide star drifted. Often this process would be continued for hours on end in freezing cold conditions. Lately, autoguiders have eased the boredom and thermal discomfort of this pro- cess. They work by mounting a second CCD onto the back of a telescope, which feeds its images directly into the telescope’s drive software. The software identifies in these images, and continuously monitors their positions, subtly adjusting the telescope’s drive rate to keep them in fixed places.

Imaging Setups

The quickest and simplest way to photograph the night sky is to mount a consumer onto a and to open its for around 30 s. This can pro- duce compelling images of the and, from a dark enough site, of the . However, to see fainter objects, and to obtain less grainy images, it is necessary to collect light over a longer period. Using a static tripod, this is difficult, since any single exposure of longer than 30 s will begin to blur due to the sky’s rotation. Two solutions are possible: either the camera can be mounted onto the back of a telescope which does track the sky, or multiple photos can be taken and subsequently co-added in a software package which can shift them into alignment—a process called stacking. The freely-available package RegiStax is commonly used to do this. 220 A Astronomical Imaging

Higher magnification can be achieved with an SLR camera by removing its front lens and mounting the camera body onto the back of a telescope with an adaptor called a T-ring. The image formed by the telescope’s mirror or lens can then be focused directly onto the camera’s sensor. This configuration is called prime focus imaging, since the image formed by the telescope’s primary mirror or lens is not subsequently magnified by an . It is generally used for deep-sky observing, since it achieves a moderate magnification over a wide field-of-view. Still higher magnification can be achieved by placing an additional lens between the camera’s sensor and the image formed by the primary lens, which acts as a magnifying glass. This lens may either be integral to the camera, or a standard eyepiece, in which case the configuration is called eyepiece projection. This is more often the technique of choice for planetary imagers, since the present very small disks which need to be highly magnified to reveal much surface detail.

Reducing Noise

Any camera records not only an image of the pattern of light it receives, but also unwanted noise which makes images appear grainy. Each pixel of a charge coupled device (CCD) counts the number of photons that have landed on it by storing elec- trical charge in proportion to the amount of light it has received. Noise can arise in this counting process from a variety of sources. For example, read-out noise stems from inaccuracies in the process of measuring how much charge has accumulated on each pixel at the end of the exposure, and transmitting these data back to the computer driving the camera. Pixel noise is associated with malfunctioning or over- sensitive pixels in an array. Thermal noise stems from a gradual accumulation of charge on each pixel from processes other than the detection of light. The last of these is an especial concern for astrophotographers, as it accumulates over time and affects long exposures especially badly. Much of the nuisance of thermal noise can be alleviated by the fact that it accumulates in a way that is roughly independent of the amount of light received by the detector. An image taken through a telescope with its lens cap on—called a dark frame—should be completely dark in the absence of thermal noise. In practice such an image will not be perfectly dark, and the amount of light that each pixel claims to have detected can be used to measure how much thermal noise it generates. It is subsequently possible to simply subtract such dark frames from images taken of the sky. For best results, dark frames should be taken in the field, and perhaps repeatedly at intervals through the night, since thermal noise can depend on ambient conditions—especially temperature, as its name suggests. Dark frames should also ideally be taken with the same exposure length as will be used to image the sky, since although it is usually a fair approximation to assume that thermal noise accumulates at a steady rate, and that a frame with twice the length of exposure will have exactly twice as much thermal noise, this may not be perfectly borne out in reality. A Astronomical Imaging 221

Pixel noise is a more difficult problem to alleviate. Individual pixels in digital cameras have different sensitivities to light. This may be intrinsic to the electronics of the pixels themselves, or may reflect the presence of dirt in the camera or else- where in the telescope tube. Even if the camera’s optics are perfectly clean, the edges of images are often less well exposed than the center of the frame—an effect known as . Vignetting can stem from a variety of causes in particular telescopes, including obstructions in the optical path that reduces the telescope’s effective for off-axis light. In wide-field imaging, a camera’s effective aperture is invariably reduced towards the edge of the frame, since its front aperture presents a smaller collecting area for incoming light when viewed obliquely (at an angle) as compared to when viewed straight-on—an effect known as natural vignetting. Such non-uniformity can be corrected by first taking an image of a uniformly illuminated white screen, called a flat frame. A perfectly calibrated detector should record a perfectly smooth image when pointed at such a screen. If there is any deviation from constant brightness in the image that a camera produces, this is a measure of its intrinsic non-uniform sensitivity, which can later be corrected in software. Because non-uniform sensitivity can stem from imperfections in both the camera itself, and the telescope it is attached to, such flat frames should ideally be taken through the entire optical chain that will later be used to image the sky. The camera should be mounted in approximately the same focal position on the tele- scope that will later be used to return a focused image of the sky. This ensures that any dirt in the telescope tube presents exactly the same obstruction to the flat frame as it will to any subsequent images of the sky. In practice, it is not always straightforward to find a suitably blank surface for taking flat frames. Any brightness gradient across the surface will lead to miscali- bration, and can easily lead to an end result that is far worse than would have been achieved if no flat frame had been used at all, and the camera had been assumed uniformly sensitive. Commonly, either the sky or an out-of-focus image of a flat observatory wall may be used. A more portable and easily reproducible solu- tion is to place an electroluminescent (EL) panel over the telescope’s aperture. Such panels are usually much brighter than is wanted for viewing through a telescope, but some opaque material can be used to cover it, providing it doesn’t introduce any pattern into the light. Noise concerns planetary and deep sky imagers in different ways, and this often means that the cameras of choice for the two groups differ. Specifically, deep sky objects are often desperately faint, and imaging them requires thermal noise to be kept to an absolute minimum over long time periods. High-end sensors are often fitted with heatsinks to help keep them cool. For planetary imaging, a much higher magnification is wanted, and, as later sections will show, a large number of short, fast exposures are usually taken. This means that thermal noise is less of a con- cern, but read-out noise must be kept to a minimum. In practice, it has often been found that cheap consumer webcams have been designed to meet almost exactly the same noise requirements. Many successful planetary imagers have used the Philips ToUcam range of webcams in recent years, with a little modification to its 222 A Astronomical Imaging mounting. More recently still, however, this trend has reversed somewhat. At the time of writing, the majority of consumer webcams are now built using cheaper CMOS sensors rather than CCDs. CMOS sensors have significantly worse noise characteristics, and so planetary imagers have once again had to return to using high-end sensor chips.

The

The atmosphere poses two problems for astrophotographers. Firstly, and other tiny in the atmosphere mean that it is never quite transparent. This is at its most obvious when the sky is cloudy, but even on supposedly clear , some haze typically remains, and depending how much there is, the sky is said to have good or bad transparency. This has two effects: it attenuates the light from astronomical objects and, more seriously, it reflects coming from below, usually giving the sky a faint uniform orange glow when observed from a built-up area. Haze also scatters when the Moon is above the horizon, making it very difficult to observe faint objects. Transparency is a serious issue for deep sky observers because of the faintness of most nebulae: only a very little haze is enough to entirely swamp their diffuse glow. The light of the Moon is a particular menace, being such a bright source of scattered light that even when the atmosphere is comparatively clear, it is virtually impossible to image deep sky objects when it is far above the horizon. For plane- tary imagers, transparency is much less of an issue. The planets are all quite bright, which means their light has little trouble in exceeding the sky background. Moreover, a diffuse haze that appears to cover large portions of the sky tends to decrease in brightness when it is magnified. Even if the haze appears bright to the , the eye is seeing the total amount of scattered light that comes from areas of sky measuring arcminutes across—the smallest angular scales that the eye can resolve. The total amount of scattered light coming from any given patch of sky which measures only a few arcseconds across will be much less. Yet this is the typical field-of-view of a telescope that is being used to view the planets, and so this is the amount of scattered light that will appear in the frame and that the light of the planet must compete against. On occasion, I have found it quite possible to observe the brighter planets through a telescope even when they have appeared to be entirely shrouded in cloud to the naked eye, though never with any particular clarity. The second problem that the atmosphere poses is of much greater nuisance to planetary imagers: seeing . When any of the planets are observed at high magnifica- tion, their surfaces appear to be shrouded in a rippling heat haze. To the eye, the effect is most apparent when looking at the Moon, on account of its brightness and the large contrasts that can be seen where crater rims cast sharply-defined along the . When long-exposure are taken, the camera records an average of this rippling motion over the duration of the exposure, and A Astronomical Imaging 223 the result is a blurred image. Typically all detail is lost on length scales smaller than an arcsecond or two. Since most of the planets only present disks a few arcseconds across, this seriously restricts the amount of detail which can be seen. The origin of atmospheric seeing is turbulence between of air in the ’s atmosphere which have different temperatures. Heat haze appears above the spout of a kettle, because boiling hot air is rising out of the spout, and just as light refracts (bends) when it passes through a glass lens, it also refracts when passing through pockets of air with different temperatures. The refractive index of air dif- fers depending on the air’s temperature. The turbulent of air above a boiling kettle is rather like a chaotically arranged lens, whose components are swirling around. The Earth’s atmosphere forms a very similar optical arrangement, since the air closer to the ground is much warmer than the air above it, but convection leads the warm air to rise and the cooler air to sink. Atmospheric seeing and transparency are typically much less of a problem at high-altitude sites, since much of the Earth’s weather arises at low altitudes in its troposphere, which mountain tops commonly lie above. This is why most profes- sional observatories are situated at the tops of some of the world’s highest moun- tains. However, relocation to such exotic locations is rarely an option for amateur observers, except perhaps for short trips, and so alternative solutions must be found.

Lucky Imaging

Until recently, visual observers could generally glimpse much higher resolution views of the planets than it was easily possible to capture with a camera. Staring at the rippling heat-haze-affected image of the disk of a planet, a visual observer will see occasional moments of clarity, when it is possible to see minute details which are normally blurred from view. In other words, the distorting effect of atmospheric seeing is not constant, but fluctuates on timescales of around a tenth of a second. Staring for long enough, the human eye is remarkably adept at putting together the fragments of detail seen in these moments of clarity, forming a mental picture of details which are lost to view most of the time. By contrast, a simple still photo- graph merely averages the swirling motion of the atmosphere over the full length of the exposure, to produce a blurred end result. In planetary imaging, the greatest revolution of digital astrophotography has undoubtedly been in enabling a technique that allows astrophotographers to com- pete on much more even terms with visual observers, using a technique called . Instead of taking a single still exposure of the sky, a is mounted onto the back of a telescope—typically a is used—and a series of very short exposures is recorded. The term lucky imaging itself originates from a 1978 paper by the atmospheric scientist David Fried (1933 –), though the tech- nique was first attempted around two decades earlier using early cine cameras. For the technique to work, each frame of the video must be exposed for no more than around a tenth of a second—a timescale short enough that individual frames can 224 A Astronomical Imaging catch moments of good seeing as conditions fluctuate. The simplest way to analyze the resulting video is to simply assess its individual frames for sharpness, and to keep only the best. These can then be added together to form a synthetic long expo- sure, based only on those moments of the best seeing. Historically this was a size- able computational task, done after the event, but now software packages commonly offer the facility to work in real time as the observation is being made. The limitation of lucky imaging is that it only works if individual frames, each exposed for only a fraction of a second, detect enough light for their sharpness to be meaningfully assessed. This means that it is much better suited to planetary imaging than to deep sky photography. While the planets are generally bright enough that such exposures show some limited detail, the same is not true for nebu- lae, unless they happen to share a field with an exceptionally bright star which can be used as a indicator of seeing. Nonetheless, lucky imaging has been applied to the deep sky: in the 2000s, a team at the Institute of Astronomy in Cambridge began exploring its use for bright nebulae, using research-grade telescopes with of 2 m and larger. With this much light-collecting area, it is possible to detect some structure in frames as short as a tenth of a second, and in many cases the team’s final processed images were able to achieve a resolution of better than 0.15 arcsec- onds—surpassing that of the (HST). Numerous software packages are available which allow amateur planetary imag- ers to use lucky imaging, and many of them are freely available. The best known and most widely used is RegiStax. Alternatives include AviStack, which at the time of writing has facilities to not only select the best frames from a video, but also to correct the distortion of poorer frames and extract useful information from them.

Image Brightness and Magnification

Any telescope or other optical aid for imaging the night sky serves two purposes. Firstly, it magnifies small structures to allow fine detail to be seen. Secondly, it collects as much light as possible, using a large aperture, to make faint structures appear brighter. The relative importance of these two functions depends on the object being observed. Deep sky objects are typically very faint and diffuse, but not necessarily very small. For example, our closest large companion , the (M31), measures five or six times the diameter of a from side to side. However, it is almost invisible to the unaided eye because its dim light is diffused over such a large patch of the sky. More distant deep sky objects are typically much smaller, but also much fainter still, and their faintness remains the principal prob- lem in any attempt to observe them. By contrast, all of the planets are fairly bright. With the exceptions and , they are bright enough to be visible to the naked eye, and even those two exceptions are within reach of binoculars. However, they present another challenge in that their disks are very small and they require substantial magnification to show any detail at all; when they are viewed visually, at least 100× magnification is needed. A Astronomical Imaging 225

The total amount of light collected by any telescope depends only on the size of its front aperture. The pupil of a human eyeball typically measures up to 9 mm across in dark conditions, while a telescope might have an aperture that measures 8-in. (200-mm) across. By collecting light over an area that is 500 times larger, the telescope can collect 500 times more light from the sky than the human eye. However, this does not necessarily correspond to making the sky appear 500 times brighter, since the telescope also magnifies objects. Magnifying objects means that their light appears to be spread over a larger area. Taking the example of a telescope with a camera attached, as the telescope’s magnification is increased, the light from any resolved object falls over a larger number of pixels in the camera. Since the total amount of light gathered by the telescope is fixed by how large the telescope’s aperture is, and the object’s light is being spread between a larger number of pixels, each must register a lower of illumination when higher magnifications are used. A convenient measure of the effective light-gathering power of a telescope, which takes into account its magnification, is the telescope’s focal ratio. This is defined as the ratio of the telescope’s to the diameter of its aperture. A telescope with a large focal ratio is said to be slow —objects appear highly- magnified and faint, and photographic exposures need to be long—while telescopes with smaller focal ratios are said to be fast, since their magnifications are lower and shorter photographic exposures are needed with comparable cameras. The exception to this rule is objects which are not resolved by the telescope. A point-like star or a small moon of Jupiter looks equally point-like when viewed through a telescope. That is to say, if a camera were attached to the back of the telescope, all of the star’s light would be directed towards a single pixel of the detector. For such objects, the spreading-out effect of the telescope’s magnification is imperceptible and can be ignored. To continue the example given above, the star would appear 500 times brighter when through an 8-in. telescope as compared to the naked eye, assuming it was sharply focused. This means that images that contain a mixture of stars and resolved objects— either planets surrounded by nearby stars, or nebulae with embedded stars—appear differently depending on the magnification used. Resolved objects get fainter as magnification is increased; stars do not. The will appear brighter in comparison to the planet’s disk when higher magnifications are used.

Color Imaging

CCDs are intrinsically sensitive to all of visible light. They also have sensi- tivity to some wavelengths beyond those that are humanly visible, including most near-infrared light and some near-ultraviolet light. To make images, filters which only transmit particular colors of light must be placed in front of the sensor. In consumer cameras, individual pixels have tiny red, green and blue filters mounted in front of them, usually arranged in a pattern called a Bayer filter in which there are two green-sensitive pixels for every one red and one blue pixel. 226 A Astronomical Imaging

This roughly mimics the performance of the human eye, which is considerably more sensitive to green light than to red or blue. The camera itself is wired to combine the information from the variously colored pixels to produce an RGB color image. High-end CCDs used by amateur astronomers are more usually monochrome. These chips are often designed for use in security cameras, and are well-suited for the needs of astronomers because they are optimized to work well in low-light conditions. Usually they are either sensitive to the full range of colors to which CCDs are naturally responsive, or they have an in-built filter which blocks infrared light and restricts their sensitivity only to visible light. With such sensors, color images can be built up by taking multiple exposures with a sequence of different color filters placed in front of the sensor, a process that can be automated with a motorized filter wheel. The resulting images can then be combining in software. These sensors offer much greater flexibility, because they do not restrict the observer to using only three colors of filter that are pre-built into the camera. For the planets, it means that any choice of color filters can be used, to maximize the color contrasts of the planet’s features. For deep-sky observers, it opens up the pos- sibility of using narrow-band filters such a hydrogen-alpha and -III, that only collect light from specific spectral lines that appear bright in particular envi- ronments within nebulae. Some may claim that taking such false-color images is slightly dishonest, since color contrasts are enhanced for artistic effect beyond what would be visible to the human eye—assuming, that is, that it had the sensitivity to see these objects in the first place. However, it must be remembered that the features revealed by false- color images are no less real than those that are apparent to the eye. By using color channels other than those that have evolved in the human eye, it is possible to see structures to which the eye is not intrinsically very sensitive. Color vision is not the same among all animals—many have a much better ability to distinguish colors than , which allows them to distinguish different types of berries which may be good or bad to eat. No doubt, if the survival of our ancestors had depended upon being able to see structures within astronomical objects, we would have evolved with very different eyes. Visual observers today would be able to see astronomical objects with much more vivid color contrasts than the subtle shades of yellow that often disappoint those newcomers who have grown used to viewing the through the eyes of the Hubble Space Telescope. Invariably, taking color images means reducing the amount of light collected by a CCD. Whenever a color filter is placed in front of a camera’s sensor, it throws away any light that is of the wrong color. In turn, this means that longer exposure times are needed. Given that exposure times for astronomical photos are usually already painfully long, this can be a problem. The very deepest images of the sky are invariably attained by using no color filter at all, making use of as much light as can possibly be recorded. Where cameras are supplied with infrared-blocking filters, these are often removed, to roughly double the range of wavelengths over which their sensors can collect light. A Astronomical Imaging 227

For deep sky imaging, a common compromise is to take an LRGB image. To do this, four exposures are taken—often themselves composites of many shorter expo- sures that have been stacked together. The first is a luminance frame, taken in unfiltered light. This is often exposed for a full half of the observing time available, to attain the deepest and sharpest possible image of the brightness features in an object. This is then followed by shorter exposures through red, green and blue fil- ters, typically each lasting for one sixth of the total observing time. The luminance frame is used to set the brightness of the final image, while the filtered images are used only to set the colors of the pixels, which can tolerate being much noisier. For imaging the planets, however, RGB imaging is still generally preferred. Features on the surfaces of the planets tend to manifest themselves as color con- trasts rather than as particularly sharp brightness contrasts, and so it is more impor- tant to have good color discrimination than a good response to low light levels.

Which Telescope?

In summary, the most pressing requirement on any telescope which is to be used for astrophotography is that it must be able to track the sky’s diurnal rotation as accurately as possible, using a reliable drive mechanism. This generally also means that the telescope’s mount must be accurately polar aligned, and that it must be as sturdy as possible to minimize wind shake. The need for accurate tracking might intuitively seem to be less of a priority for planetary imagers than for deep sky observers, since the exposures they take are relatively short. However, accurate tracking is important to planetary imagers too, since the magnifications they use are much higher, and so their images are much more sensitive to any slight drift of the field. The need for a large aperture is less immediately pressing for astrophotogra- phers than for visual observers, since it is always possible to compensate for a smaller aperture by exposing for longer. Nonetheless, small-aperture telescopes suffer from which compromises image quality; 4-in. telescopes are lim- ited to a resolution of around an arcsecond, while an 8-in. telescope can achieve twice that resolution. While both these examples would be perfectly adequate in the absence of any correction for atmospheric seeing, planetary imagers who hope to use lucky imaging to achieve sub-arcsecond resolution would benefit from using at least an 8- or 12-in. telescope. For deep-sky objects, for which lucky imaging is presently not feasible on an amateur budget, the only reason to use telescopes with apertures larger than 5 in. is to reduce exposure times. While there are strong incentives for doing so when col- lecting light from the faintest objects, a recent trend has been the use of small high- quality refractors for deep-sky imaging. Though it is prohibitively expensive to build refractors much larger then 4 in. in diameter, the best refractors produce sharper images than reflectors, since they do not have a secondary mirror obstruct- ing their aperture. Appendix B

Sources

Most of the tables of data in this book were computed by the author from models of the motion of the planets published by the Jet Propulsion Laboratory (JPL) in California. Since the 1960s, JPL has published a series of Development Ephemerides (DEs) which are used to support NASA’s space program by providing the best pos- sible information on the positions of the planets at any given moment in time. This book is based on DE405, which was published in 1998 and traces the positions of the planets to better than arcsecond resolution over the period 1600–2200. DE405 was computed on the basis of a wide range of information about the current and historical positions of the planets, ranging from direct telescopic observation, radar distance estimates, and data from visiting . The motion of each planet according to Newton’s laws of was then simulated forwards and backwards in time to produce a table of planetary positions over the 600 years period. The resulting is freely available from the JPL website at ftp://ssd.jpl..gov/pub/eph/ planets/ascii/ although these files do not contain readable ephemerides, but rather daily lists of coefficients which may be used to compute ephemerides from Chebyslev poly- nomials. The data in this book was derived from DE405 using custom software written by the author which searches the ephemeris for alignments between astronomical bod- ies which correspond to events of interest. This same software is also used by the author’s website, In-The-Sky.org, to present live information about what it in the sky. The dates of transits and were not computed from DE405, but taken from the following NASA publications: Five Millennium Canon of Solar Eclipses , NASA/TP-2006-214141 Five Millennium Canon of Lunar Eclipses , NASA/TP-2009-214172 Seven Century Catalog of Transits: 1601 CE to 2300 CE, Fred Espenak, NASA Six Millennium Catalog of Transits: 2000 BCE to 4000 CE, Fred Espenak, NASA

D. Ford, The Observer’s Guide to Planetary Motion: Explaining the Cycles of the Night Sky, 229 The Patrick Moore Practical Astronomy Series, DOI 10.1007/978-1-4939-0629-1, © Springer Science+Business Media, LLC 2014 Glossary

Aphelion The point along a planet’s where it recedes to its greatest distance from the . Arcminute One sixtieth part of a degree. Arcsecond One sixtieth part of an arcminute, equal to one 3,600th of a degree. A grouping of stars which are not one of the internationally-agreed constellations. A complex instrument, most commonly used to tell the time or to forecast when particular objects will rise and set. One side of the instrument has a sight which can be used to approximately measure the altitudes of objects in the sky. The other side has a crude brass . The astrolabe’s origin is obscure— some people attribute its invention to the Greek astronomer —but it remained in widespread use until the invention of the telescope. Astronomical unit A unit of distance, approximately equal to the Earth’s average distance from the Sun, 150 million km. Babylon A city 50 miles south of modern Baghdad, where astronomical observa- tions were made by astrologers throughout much of the fi rst and second millen- niums BC . Bode’s law A pattern observed by Johann Bode in 1772—and by Johann Titius several years earlier—that each of the planets orbit the Sun at roughly twice the distance of their inner neighbours, while the terrestrial planets are a little more closely spaced. The pattern is now thought to have little theoretical basis other than pure coincidence. Circumpolar In the , an object is circumpolar if it is close enough to the north celestial pole that it never sinks beneath the horizon. In the , the same term is used for objects that are close to the south celestial pole.

D. Ford, The Observer’s Guide to Planetary Motion: Explaining the Cycles of the Night Sky, 231 The Patrick Moore Practical Astronomy Series, DOI 10.1007/978-1-4939-0629-1, © Springer Science+Business Media, LLC 2014 232 Glossary

Conjunction Any occasion when two astronomical objects appear very close to each other in the sky. Specifi cally, planets are said to be “at ” when they make closest approach to the Sun. Planets at conjunction are invariably unobservable, unless they the Sun. One of 88 areas into which the sky was divided by the IAU in 1930. Prior to 1930, and stretching back into prehistory, constellations were group- ings of stars that came to be associated with mythical characters, technological inventions or animals, but which did not have well- defi ned boundaries. Decan One of 36 groupings of stars whose time of rising was used by astronomers in ancient Egypt to tell the time at night. Declination The celestial coordinate which is the counterpart to on the Earth’s surface; it measures the angle by which an object lies north or south of the celestial . Doppler effect A phenomenon which means that light from objects which are receding from the observer appears redder than usual, while light from objects which are travelling towards the observer appears bluer than usual. Draconic The time interval between successive occasions when the Moon passes through one of its two nodes; equal to 27.21 days. A term introduced by the IAU in 2006 for bodies which are slightly smaller than planets. A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has a nearly round shape, but in contrast to a planet (c) has not cleared all of the rocky debris from the neighbourhood of its orbit. Eccentricity A numerical measure of how much an elliptical orbit deviates from being circular. An ellipse with an eccentricity of zero is exactly circular. The path that the Sun traverses across the sky over the course of each year. One of the two occasions each year when the Sun crosses the , in March and September. Fusion The nuclear process by which stars are powered. In the cores of most stars, hydrogen atoms are joined together to form heavier helium atoms, a process which releases a vast amount of energy. Gas giant A planet whose mass is mostly in the form of gas, with no more than a small rocky core at its centre. A spacecraft operated by the European Space Agency, launched in 2013 to measure the of a billion stars to determine their distances. Giant impact hypothesis A theory proposed by Hartmann and Davis in 1975 that the Moon was formed when a large planet, similar in size to Mars, collided with the Earth 4.5 billion years ago. Giant A vast assemblage of interstellar gas, out of which stars may form. A tight condensation of hundreds of thousands of stars, which are usually extremely old. Their origin and eventual fate remain poorly understood. Great circle A circle in the sky which divides the sky into two equal halves. Hipparcos A spacecraft operated by the European Space Agency between 1989 and 1993, which measured the parallaxes of more than a hundred thousand stars in order to determine their distances. Glossary 233

Inferior planet A planet which the Sun more closely than the Earth. There are two inferior planets: Mercury and Venus. A collection a rocky bodies which lie around the outskirts of the , at a distance of 30–50 AU from the Sun. The is the best known Kuiper belt object. A violent episode of impacts which the Moon and Earth are believed to have experienced 4 billion years ago, evidenced by the number of Moon craters dating from this period. Lightyear A unit of distance equal to the distance that light travels in a year, around 9,500 billion km. A system of timekeeping in which the time of year is determined by the Moon’s cycle of phases, rather than the Sun’s cycle of and . The Muslim calendar is an example. Meridian The line across the sky which connects the cardinal points north and south on the horizon and passes through the . Metonic cycle A calendric cycle which is based on the almost exact match between the length of 19 years and 235 lunar . Micrometer A unit of distance equal to one millionth of a meter. Moving group A grouping of stars which are close to each other in space, but not bound together by gravity. Several of the stars of the Big Dipper are a moving group, which may once have been an . Mural quadrant A quadrant mounted on a north–south wall, used to measure the altitudes of objects when they transit. Neap Unusually weak which occur when the Moon’s phase is around fi rst quarter or last quarter. Nodes The points where an object’s orbit crosses the plane of the Earth’s orbit around the Sun. Occultation An event in which one passes behind another. A collection of icy bodies in the very far reaches of the solar system, at a distance of around 50,000 AU from the Sun. The Oort cloud has never been directly observed, but most are thought to have begun as Oort cloud objects. Open cluster A loose condensation of stars, close together in space and which cannot escape one another’s gravitational fi eld. The stars within such a cluster formed from a common molecular cloud, and have not yet dispersed through space. A planet is said to be at opposition when it lies in the opposite direction to the Sun in the sky. When it lies in that direction, the planet is at its closest to the Earth, and appears highest in the sky at around , making it ideally placed for observation. The amount by which a star’s position appears to wobble from side to side over the course of the year due to the Earth’s changing perspective on the Universe as it orbits the Sun. In practice, the parallaxes of stars are so minute that incredibly sensitive telescopes are needed to detect them. 234 Glossary

Penumbra The shadowed region behind an astronomical body, especially the Earth or Moon, within which the Sun’s disk is partially obscured. Perihelion The point along a planet’s orbit where it makes its closest approach to the Sun. Planet D e fi ned by the IAU in 2006 as a celestial body that (a) is in orbit around the Sun, (b) has a nearly round shape, and (c) has cleared all of the rocky debris from the neighbourhood of its orbit. A compact type of nebula which is formed when the outer layers of a star are expelled as their cores turn into white dwarfs. Planetary nebulae often have complex geometric shapes. The rate of a star’s movement across the sky due to its drifting motion through space relative to the Sun. A disk of gas and dust surrounding a young star, out of which planets may form. See protoplanetary disk. Quadrant An instrument used to sight the altitude of objects above the horizon, often with the naked eye. star A star which is near the end of its life, and whose outer layers have puffed up to many times their original size. Retrograde motion The westward movement of the superior planets across the sky for a few around the time that they are at opposition, in contrast to their usual eastward movement across the sky. Right ascension The celestial coordinate which is the counterpart to on the Earth’s surface. Sextant An instrument used to sight the angular distances between objects in the sky. Sidereal The time taken for the stars of the night sky to complete one revolu- tion around the celestial poles. Equal to 23 h and 56 min. Sidereal year The time taken for the Earth to complete one revolution around the Sun, such that the Sun returns to exactly the same point in the sky where it lay a year earlier. Equal to 365.2564 days. Solar calendar A system of timekeeping in which the time of year is determined by the Earth’s changing , and in which months are no longer tied to the Moon’s 29-day cycle of phases. One of the two occasions in each year when the Sun reaches its furthest points north or south of the celestial equator. Spring tide Unusually strong tides which occur around the time of and full moon, when the tidal pulls of the Moon and Sun are closely aligned. Superior planet A planet which orbits the Sun at a greater distance than the Earth. The only two planets which are not superior planets are Mercury and Venus. A catastrophic explosion which occurs at the ends of the of high- mass stars. The fi reball produced by a supernova can outshine an entire galaxy of stars for a few days. Glossary 235

Synodic period The time taken for an astronomical object to return to the same position in the sky relative to the Sun. In the case of the Moon, this is the time interval between successive new moons. In the case of the planets, this is the time interval between successive oppositions. A planet with a rocky surface, like the Earth and unlike Jupiter or Saturn. Transit (1) An event in which one astronomical object passes in front of another. Transits of Mercury or Venus are occasions when these planets pass in front of the Sun, appearing as small dark disks blocking part of the Sun’s light. (2) The moment when the sky’s daily rotation carries a star or planet across the observer’s meridian. Tropical year The period of time between successive summer solstices, equal to 365.2422 days. Umbra The shadowed region behind an astronomical body, especially the Earth or Moon, within which the Sun’s disk is entirely obscured. Vernal equinox Another name for the . The compact remnant of a Sun-like star which has run out of fuel. White dwarf stars are typically of similar size to the Earth, yet have similar masses to the Sun. Zenith The point in the sky directly above the observer’s head. Zodiacal constellation One of the constellations through which the Sun passes on its annual path across the sky. There are 12 traditional zodiacal constellations, each spanning an equal 30° length of the ecliptic. Today, the Sun passes through 13 modern constellations; the additional constellation is . A faint glowing band across the sky which follows the line of the ecliptic. The glow is produced by tiny interplanetary dust grains in the plane of the solar system. Index

A Carbon dioxide , 155, 158, 160, 170, 180, Alcock, George , 37 181, 182 , 68, 69 Cassini, Giovanni , 118 Andromeda galaxy (M31) , 9, 14, 58, 199, , 30–32, 34, 37, 39, 58, 64, 70, 204, 224 83, 85, 97, 126 Apollo landings , 82 Challis, James , 19 Arthur, President Chester , 37 Chaos theory , 17, 160 , 14, 17, 25, 49, 59, 60, 70, 82, 139, Charge-coupled device (CCDs) , 178, 209, 217, 142, 145, 160, 171, 172 219, 220, 222, 225, 226 Astronomical unit (AU) , 25, 184, 210, 231 Christmas, date of , 62, 63 Atacama Large Millimetre Array (ALMA) , Clavius, Christopher , 62 60, 214 Climate change , 160, 180 Atomic clock , 71, 73, 206 Color fi lter , 147, 169, 226 Autoguider , 219 Columbus, Christopher , 91, 99 , 10, 12, 15–16, 25, 37, 70, 144, 145, 233 Common, Andrew , 217, 218 B Constellations , 3, 5, 6, 8, 9, 14, 34–42, 45, 48, Babylon , 1, 73, 231 63, 64, 89, 99, 125, 127, 199, 201, 203, Barnard's star , 3, 4 204, 219, 231, 232, 235 Bayer fi lter , 225 Coordinated universal time (UTC) , 72, 94, Bessel, Friedrich , 31 144, 194 , 88–90 Corot spacecraft , 209 Bond, William , 216 Couch Adams, John , 19 Brahe, Tycho , 10, 158, 161, 163, 164 Brocchi's cluster , 6 D Dark frame , 220 C Deimos , 79, 142, 155, 170–172, 217 Calver, George , 217, 218 Delta T , 72, 73 Canals, Martian , 156 Doppler effect , 31, 208, 232 Carbonates, on Mars , 182 Draconic month , 104, 232

D. Ford, The Observer’s Guide to Planetary Motion: Explaining the Cycles of the Night Sky, 237 The Patrick Moore Practical Astronomy Series, DOI 10.1007/978-1-4939-0629-1, © Springer Science+Business Media, LLC 2014 238 Index

Draper, Henry , 216, 217, 218 Globular cluster , 199, 201, 203, 204, Draper, John , 216 217, 232 Gould’s belt , 203 Great circle , 232 E Greatest , 186, 189–193 , 53, 54 Great Red Spot , 118 Easter, date of , 89, 90, 198 Greenhouse effect , 180, 182, 184 Eccentricity, orbital , 25, 67, 232 Gregory XIII, Pope , 62 Eclipse , 72–74, 85, 87, 90–102, 104, 105, 113, Gyroscope , 43–45, 102 148, 193, 195, 216, 229 Ecliptic , 23, 39, 40, 41, 43, 45–48, 58, 59, 63–67, 71, 84, 85, 99, 101–104, 122, H 125, 126, 127, 146, 147, 162, 185, 191, Halley, Edmund , 16 193, 195, 196, 197, 199, 232, 235 Halley's comet , 15–16 Egyptian pyramids, 1, 6 Harriot, Thomas , 13, 79 Einstein, Albert , 19 Harrison, John , 18 El Nino , 9 , 6, 9 Encyclopedia Brittanica , 157 Hellas basin , 169 Ephemeris time (ET) , 71 Herschel, John , 7, 62, 216 , 66, 67–69 Herschel, William , 14, 19 Equinox , 26, 38, 41, 42–48, 62–65, 85, Hipparcos space observatory , 31, 232 89, 102, 149, 150–152, 198, 232, Hooke, Robert , 14–16 233, 235 , 209–211 Evaporating gaseous globule (EGG) , 54 Hubble's law , 31 . See Extrasolar planet Hubble Space Telescope , 31, 53, 116, 156, Extrasolar planet , 2, 3, 205–214 183, 214, 218, 224, 226 Eyepiece projection , 220 Hven, island of , 11 Hypersensitization , 217

F First point of , 42, 48, 122, 123, 124 I , 3–5 Intercalary month , 90 Flat frame , 221 International Astronomical Union (IAU) , 35, Focal ratio , 225 232, 234 Fusion, nuclear , 9, 50, 51, 54, 121 Iron, on Mars , 168 Irregular moon , 142 Islam , 6, 90 G Isotope , 79, 80 Gaia observatory , 31 Galaxy , 3, 4, 5, 6, 9, 14, 31, 57, 58, 199, 201–204, 205, 211, 212, 213, J 224, 234 J2000.0 coordinates , 48 Galaxy cluster , 199, 204 Jeans, James , 52 Galaxy Zoo , 213 Jet Propulsion Laboratory (JPL) , 17, 20, 229 , Galilei , 13, 14 Jewish calendar , 89 Gas giants , 25, 28, 49, 59, 60, 79, 115–122, Julian calendar , 62 127–129, 139–143, 147, 161, 164, 205, Julius Caesar , 62, 63 210, 211, 214, 231, 232 Gegenschein , 145, 146 Giant impact hypothesis , 80–81, 232 K Giant molecular cloud (GMC) , 50, 52, 53, Kepler, Johannes , 10, 12–15, 28, 66, 158, 161, 54, 232 163, 164, 173, 209 Gilbert, William , 79 Kepler spacecraft , 213 Index 239

L Noise, imaging , 221, 222 Lacaille, Louis de , 35 , 64–70, 75, 145 Lascaux caves , 1 Late heavy bombardment , 82, 233 Leap second , 71–75 O Leap year , 61–62 Obliquity of the ecliptic , 65, 67 Le Verrier, Urbain , 19 Open cluster , 7, 54, 199, 200–203, 233 Lilius, Aloysius , 62 Opposition , 111, 113, 123–129, 144–146, 148, Limestone , 160, 182 157, 162, 163, 164, 168, 173, 185, 186, Local Group , 204 192, 233–235 Local solar time , 66, 67, 148 Opposition surge , 144, 145, 146 Longitude , 18, 32, 34, 37, 38, 45, 46, 47, 67, Oresund straight , 11 73, 148, 168, 196, 197, 234 Orion nebula , 3, 54, 203, 216–218 Lowell, Percival , 7 Oval BA , 118 LRGB imaging , 227 Lucky imaging , 223–224, 227 Lunar Reconnaisance Orbiter , 82 P , 103 Parallax , 10, 12, 13, 31, 32, 91, 98, 194, 195, Lunation , 84, 88, 89, 99 232, 233 , 89, 90, 104 annual , 26, 31 Penumbra , 92, 93, 94, 97, 98, 234 Phases M of Mercury and Venus , 191–193 , 178 of the Moon , 83, 84, 89, 90, 91, Magnetic fi eld , 14, 80, 81, 140, 141, 160, 161, 105, 233 175, 181, 182, 185 Phobos , 79, 142, 155, 170, 171, 172, 217 , 79 , 59, 60 , 173, 180 Planet Hunters , 213 , 157, 158, 173, 174 , 50, 52 , 183–185 Plato , 8 Mayor, Michel , 208 Polynesia , 5 Mean time , 65–67, 69, 70, 71 Precession of the equinoxes , 42–48 Meridian , 18, 33, 37, 68, 233, 235 Prime meridian , 37 MESSENGER , 183, 184 Protoplanetary disk , 55, 59, 60, 80, 121, 122, Metonic cycle , 90, 104, 198, 233 141, 142, 214, 234 Microlensing , 212 , 5, 205 Migration, planetary , 210–211 , 206–207 Milky Way , 3, 4, 6, 7, 9, 13, 23, 31, 51, 54, 56, 57–59, 199–204, 211, 212, 219 Milky Way, center of , 4, 23, 57, 201, 202 Q Moon Quantum effi ciency , 217 formation of , 79–81 Queloz, Didier , 208 orbit of , 84–85 phases , 83 Moving group , 54, 212, 233 R Radar , 20, 30, 31, 178, 195, 229 Radial velocity method , 208, 209, 210 N Rayleigh , 99 NASA , 20, 53, 57, 78, 94, 96, 101, 109, 116, Reciprocity failure , 217 117, 120, 151, 156, 159, 170, 173, 174, Red , 51, 113, 234 179, 194, 229 RegiStax , 219, 224 Newton, Isaac , 14–17, 164 Regular moon , 141, 142, 143, 147 Node , 99–105, 195, 196–198, 232, 233 Retrograde motion , 125, 234 240 Index

Riccioli, Giovanni , 79 116, 117, 118, 120, 143, 148, 155, Ring-plane crossing event , 152 156, 168, 169, 173, 178, 183, 192, Rings, planetary , 57 206, 209, 212, 213, 214, 216, , 144, 172 217, 218, 219, 220, 221–227, Romer, Ole , 148 231, 233 Rovers, on Mars , 140, 160, 174, 175 Tharsis volcanoes , 169, 170 Royal Greenwich Observatory (RGO) , 18, 34, 37 , 80, 81 , 112–113, 142–143 Tides, origin of , 106, 111 S Time delay, communications , 174 Saros cycle , 90 Titan (moon of Saturn) , 79, 142, 143, 158 Schiaparelli, Giovanni , 155, 156 Transit , 19, 33, 38, 64, 66, 68, 69, 90, 148, Schwabe, Heinrich, 19 181, 193–198, 208–210, 212, 213, 214, Seeing , 99, 125, 127, 155, 156, 183, 222, 223, 229, 232, 233, 235 224, 227 of an extrasolar planet , 208–209 Seeliger effect , 145 of Mercury or Venus , 193 Sexagesimal system , 38 Transparency , 222, 223 Sidereal month , 84 Sidereal year , 63, 234 Snow line , 121, 210 U Solar conjunction , 85, 124, 125, 128, 148, Umbra , 92–94, 97–99, 101, 102, 235 162, 174, 186, 193 Uraniborg , 11, 12 inferior , 193 superior , 186 Solar wind , 51, 54, 121, 141, 160, 175, 181, V 182, 183 Venera program , 181 Solid tides , 113 Vignetting , 221 Sol, Martian day , 168 and 2 , 158, 174 Spiral arms , 31, 202, 203 cluster , 204 Sputnik I , 20, 173 , 161, 169, 170, 179, 182 Stacking , 219 von Braun, Wernher , 173 Star trails , 34, 35 and 2 , 120, 139, 140 Stephenson, Richard , 72, 73, 74 Vulcan , 19 Sunspot , 19 Supernova , 10, 12, 207, 234 SuperWASP , 209 W Synodic month , 84, 104 Wells, H.G. , 157 Synodic period , 26–28, 30, 122, 123, 173, Wesley, Anthony , 119 186, 196, 198, 235 Whipple, John , 216 Syrtis Major , 169 World War II , 20, 72

T Z Telescope , 2, 4, 7, 11, 13–14, 18, 30, 31, Zodiacal constellations , 41, 42, 235 33–35, 53, 60, 72, 78, 79, 82, 88, 115, Zodiacal light , 59, 145, 146, 235