We are all treasure hunters – storybook pirates searching for riches in the endless sea above. Taking the helms of our telescopes, we lay a course among the stars with the sails of our imaginations open. And what wonders await us as we make our way through the charted territories of the Milky Way: rich, open clusters of hot, young stars, some still swaddled in their nascent nebulosities; ancient globular clusters, the senior citizens of our galaxy, whose teeming suns are packed together like gold doubloons in a sea chest; there are galaxies too numerous to mention lurking beyond our forest of stars, living out their lives in various stages of evolution; and then there are the ghosts – the smoky shells of dying stars, whose very nature reminds us of the ultimate fate of our life-giving Sun.

These celestial treasures cannot be plundered. They can only make us feel, as Joseph Conrad writes in his 1902 adventure novel, Heart of Darkness, “meditative, and fit for nothing but placid staring.” And there are jewels in the night almost too numerous to mention, some of which rarely get viewed by amateur astronomers. That is why novel lists of neglected deep-sky objects are becoming increasingly popular among observers. And that is why I created this book.

Hidden Treasures is the third title in my Deep-Sky Companions series – the other two books are Deep-Sky Companions: The Messier Objects and Deep-Sky Companions: The Caldwell Objects. This latest work fills an important void.

Every moment spent under the stars is a treasure hunt – a visual journey that leads us to endless riches in the heavens above. And I've loved each adventure from the beginning. When I was young, perhaps age eight, I set out on one of my first deep-sky adventures – to hunt down the great globular cluster M13 in Hercules. I had seen a full-page photo of it in Planets, Stars, and Space (first published in 1957 by Creative Educational Society in cooperation with the American Museum of Natural History, New York), which my father kept on the lower shelf of a bookcase set up in the living room. The book's authors, Joseph Miles Chamberlain and Thomas D. Nicholson, described the cluster as a “huge ball of stars … so numerous that the center … resembled a brilliant mass of light.” My Golden Guide, The Sky Observer's Guide, went one step further, saying that this “globular may have 100,000 [stars].” It also said it may be seen with the naked eye.

It seemed incredible to me at the time that if I could first find the Keystone of Hercules among the multitude of stars overhead, I could then search for a citadel of 100,000 suns – one so distant and so tightly packed together (yet so bright) that I could see it with the naked eye as a hazy star.

Caroline lucretia herschel was born in Hanover, Germany, on March 16, 1750. She was the eighth of 10 children born to Isaac and Anna and nearly 12 years younger than her revered older brother, Friedrich Wilhelm (later William). She had, like her father and William, a penchant for music and was a talented soprano. In 1778 she was offered an engagement for the Birmingham Festival, but she declined, having resolved to sing in public only where her brother, William, was conductor.

The strong attachment and affection between Caroline and William began as soon as Caroline could show or express her feelings and continued throughout their lives.

If the messier catalog opens with a bang (M1, the Crab supernova remnant in Taurus) and the Caldwell catalog opens with a whisper (NGC 188, an old and dim open cluster in Cepheus), Hidden Treasures opens with a surprise: NGC 189, an 8th-magnitude open cluster in Cassiopeia. The surprise is that indefatigable Caroline Herschel discovered this dim collection of suns in 1783 – a fact that had gone unrecognized for more than two centuries until British astronomical historian Michael Hoskin introduced the world to this fact in the November 2005 Journal for the History of Astronomy.

While reviewing Caroline Herschel's original observing notes, Hoskin scrutinized the following description of an object Caroline discovered on September 27, 1783, shortly after she had discovered the open cluster NGC 225 (Hidden Treasure 2) on the same evening:

Hoskin realized that since NGC 225 precedes Gamma (γ) Cassiopeia, this new object must precede it also. The only deep-sky object in the vicinity of Caroline's description is the 8th-magnitude open cluster NGC 189, which lies nearly 1° southwest of NGC 225. “But it is William's VIII.64 (NGC 381) that is credited to Caroline,” Hoskin says, “even though this cluster follows Gamma Cas.”

One of the basic astronomical pursuits throughout history has been to determine the amount and temporal nature of the flux emitted by an object as a function of wavelength. This process, termed photometry, forms one of the fundamental branches of astronomy. Photometry is important for all types of objects from planets to stars to galaxies, each with their own intricacies, procedures, and problems. At times, we may be interested in only a single measurement of the flux of some object, while at other times we could want to obtain temporal measurements on time scales from seconds or less to years or longer. Some photometric output products, such as differential photometry, require fewer additional steps, whereas to obtain the absolute flux for an object, additional CCD frames of photometric standards are needed. These standard star frames are used to correct for the Earth's atmosphere, color terms, and other possible sources of extinction that may be peculiar to a given observing site or a certain time of year (Pecker, 1970).

We start this chapter with a brief discussion of the basic methods of performing photometry when using digital data from 2-D arrays. It will be assumed here that the CCD images being operated on have already been reduced and calibrated as described in detail in the previous chapter. We will see that photometric measurements require that we accomplish only a few steps to provide output flux values. Additional steps are then required to produce light curves or absolute fluxes.

Although imaging and photometry have been and continue to be mainstays of astronomical observations, spectroscopy is indeed the premier method by which we can learn the physics that occurs within or near the object under study. Photographic plates obtained the first astronomical spectra of bright stars in the late nineteenth century, while the early twentieth century saw the hand-in-hand development of astronomical spectroscopy and atomic physics. Astronomical spectroscopy with photographic plates, or with some method of image enhancement placed in front of a photographic plate, has led to numerous discoveries and formed the basis for modern astrophysics. Astronomical spectra have also had a profound influence on the development of the fields of quantum mechanics and the physics of extreme environments. The low quantum efficiency and nonlinear response of photographic plates placed the ultimate limiting factors on their use.

During the 1970s and early 1980s, astronomy saw the introduction of numerous electronic imaging devices, most of which were applied as detectors for spectroscopic observations. Television- type devices, diode arrays, and various silicon arrays such as Reticons were called into use. They were a step up from plates in a number of respects, one of which was their ability to image not only a spectrum of an object of interest, but, simultaneously, the nearby sky background spectrum as well – a feat not always possible with photographic plates.

The current high level of understanding of CCDs in terms of their manufacture, inherent characteristics, instrumental capabilities, and data analysis techniques make these devices desirable for use in spacecraft and satellite observatories and at wavelengths other than the optical. Silicon provides at least some response to photons over the large wavelength range from about 1 to 10 000 Å. Figure 7.1 shows this response by presenting the absorption depth of silicon over an expanded wavelength range. Unless aided in some manner, the intrinsic properties of silicon over the UV and EUV spectral range (1000–3000 Å) are such that the QE of the device at these wavelengths is typically only a few percent or less. This low QE value is due to the fact that for these very short wavelengths, the absorption depth of silicon is near 30–50 Å, far less than the wavelength of the incident light itself. Thus, the majority of the light (~ 70%) is reflected with the remaining percentage passing directly through the CCD unhindered.

Observations at wavelengths shorter than about 3000 Å involve additional complexities not encountered with ground-based optical observations. Access to these short wavelengths can only be obtained via space-based telescopes or high altitude rocket and balloon flights. The latter are of short duration from only a few hours up to possibly hundreds of days and use newly developing high-altitude ultra-long duration balloon flight technologies.

Before we begin our discussion of the physical and intrinsic characteristics of charge-coupled devices (Chapter 3), we want to spend a brief moment looking into how CCDs are manufactured and some of the basic, important properties of their electrical operation.

The method of storage and information retrieval within a CCD is dependent on the containment and manipulation of electrons (negative charge) and holes (positive charge) produced within the device when exposed to light. The produced photoelectrons are stored in the depletion region of a metal insulator semiconductor (MIS) capacitor, and CCD arrays simply consist of many of these capacitors placed in close proximity. Voltages, which are static during collection, are manipulated during readout in such as way as to cause the stored charges to flow from one capacitor to another, providing the reason for the name of these devices. These charge packets, one for each pixel, are passed through readout electronics that detect and measure each charge in a serial fashion. An estimate of the numerical value of each packet is sent to the next step in this process, which takes the input analog signal and assigns a digital number to be output and stored in computer memory.

Thus, originally designed as a memory storage device, CCDs have swept the market as replacements for video tubes of all kinds owing to their many advantages in weight, power consumption, noise characteristics, linearity, spectral response, and others.

Even casual users of CCDs have run across the terms read noise, signal-to-noise ratio, linearity, and many other possibly mysterious sounding bits of CCD jargon. This chapter will discuss the meanings of the terms used to characterize the properties of CCD detectors. Techniques and methods by which the reader can determine some of these properties on their own and why certain CCDs are better or worse for a particular application are discussed in the following chapters. Within the discussions, mention will be made of older types of CCDs. While these are generally not available or used anymore, there is a certain historical perspective to such a presentation and it will likely provide some amusement for the reader along the way.

One item to keep in mind throughout this chapter and in the rest of the book is that all electrons look alike. When a specific amount of charge is collected within a pixel during an integration, one can no longer know the exact source of each electron (e.g., was it due to a stellar photon or is it an electron generated by thermal motions within the CCD itself?). We have to be clever to separate the signal from the noise. There are two notable quotes to cogitate on while reading this text. The first is from an early review article on CCDs by Craig Mackay (1986), who states: “The only uniform CCD is a dead CCD.”