This appendix provides a reading list covering the aspects of CCD development, research, and astronomical usage. There are so many articles, books, and journal papers covering the innumerable aspects of information on CCDs that the material presented in a book this size or any size can only cover a small fraction of the details of such work. Even the list presented here does not cover all aspects of interest concerning the use of CCDs in astronomy, but it does provide a very good starting point. The growth of information on CCDs has risen sharply over the past ten years and will, no doubt, continue to do so. Thus the student of CCD science must constantly try to keep up with the latest developments both in astronomy and within the field of opto-electronics, both areas where progess is being made. The internet is a powerful tool to help in this pursuit. Using a good search engine (e.g. Google) type in items such as “deep depletion,” or “L3CCD,” or “MIT/LL” and you'll get back many items of interest.

Much of the information on CCDs is contained in books devoted to the subject. Numerous SPIE, IEEE, and other conferences publish their proceedings in books as well. Detailed information is available in the scientific literature some of which we reference in this volume. Many refereed articles of interest are not listed here as they are easily searched for via web-based interfaces such as the Astrophysics Data System (ADS).

Seven years ago, Cambridge University Press began a new series of books called Handbooks. I was fortunate enough to be asked to author the one on CCDs. Little did I realize how wonderful of an undertaking that writing this book would be. I have learned and relearned a number of details about CCDs and had cause to read many scientific and popular papers and articles I otherwise would have overlooked. The greatest benefit, however, has been the many gracious colleagues and students who have provided comments, revisions, suggestions, support, and simply said thanks. The first edition of the Handbook of CCD Astronomy was written for you and you have truly made it your own through this volume.

When I was first asked to write a second edition, I have to admit I was skeptical that enough had changed to warrant it. I am happy to say I was completely wrong. Upon going back and reading the original volume, I had no problem seeing its many pages of outdated material. There are, however, some fundamental discussions and properties of CCDs that are timeless, and remain in the present volume. New areas of CCD development abound and to highlight a few this second edition is a bit longer and has a few more illustrations. The areas of faster and higher performance electronics to control and read out a CCD, better analog-to-digital circuitry, and better manufactured CCDs are some of the additions discussed within.

Silicon. This semiconductor material certainly has large implications on our life. Its uses are many, including silicon oil lubricants, implants to change our bodies' outward appearance, electric circuitry of all kinds, nonstick frying pans, and, of course, charge-coupled devices.

Charge-coupled devices (CCDs) and their use in astronomy will be the topic of this book. We will only briefly discuss the use of CCDs in commercial digital cameras and video cameras but not their many other industrial and scientific applications. As we will see, there are four main methods of employing CCD imagers in astronomical work: imaging, astrometry, photometry, and spectroscopy. Each of these topics will be discussed in turn. Since the intrinsic physical properties of silicon, and thus CCDs, are most useful at optical wavelengths (about 3000 to 11 000 Å), the majority of our discussion will be concerned with visible light applications. Additional specialty or lesser-used techniques and CCD applications outside the optical bandwidth will be mentioned only briefly. The newest advances in CCD systems in the past five years lies in the areas of (1) manufacturing standards that provide higher tolerances in the CCD process leading directly to a reduction in their noise output, (2) increased quantum efficiency, especially in the far red spectral regions, (3) new generation control electronics with the ability for faster readout, low noise performance, and more complex control functions, and (4) new types of scientific grade CCDs with some special properties.

We are all aware of the amazing astronomical images produced with telescopes these days, particularly those displayed as color representations and shown off on websites and in magazines. For those of us who are observers, we deal with our own amazing images produced during each observing run. Just as spectacular are photometric, astrometric, and spectroscopic results generally receiving less fanfare but often of more astrophysical interest. What all of these results have in common is the fact that behind every good optical image lies a good charge-coupled device.

Charge-coupled devices, or CCDs as we know them, are involved in many aspects of everyday life. Examples include video cameras for home use and those set up to automatically trap speeders on British highways, hospital X-ray imagers and high-speed oscilloscopes, and digital cameras used as quality control monitors. This book discusses these remarkable semiconductor devices and their many applications in modern day astronomy.

Written as an introduction to CCDs for observers using professional or off-the-shelf CCD cameras as well as a reference guide, this volume is aimed at students, novice users, and all the rest of us who wish to learn more of the details of how a CCD operates. Topics include the various types of CCD; the process of data taking and reduction; photometric, astrometric, and spectroscopic methods; and CCD applications outside of the optical band-pass.

Most computer screens and image displays in use are 8-bit devices. This means that the displays can represent data projected on them with 28 = 256 different greyscale levels or data values of resolution. These greyscale levels can represent numeric values from 0 to 255 and it is common to only have about 200 levels actually available to the image display for representing data values with the remaining 50 or so values reserved for graphical overlays, annotation, etc. If displaying in color (actually pseudo-color), then one has available about 200 separate colors, each with a possible grey value of 0–255, or the famous “16 million possible colors” listed in many computer ads (see below).

On the display, the color black is represented by a value of zero (or in color by a value of zero for each of the three color guns, red (R), green (G), and blue (B)). White has R = G = B = 255, and various grey levels are produced by a combination of R = G = B = N, where N is a value from 0 to 255. Colors are made by having R ≠ G ≠ B or any combination thereof in which all three color guns are not operated at the same intensity. A pure color, say blue, is made with R = G = 0 and B = 255 and so on.

While the Dipper may rule the sky of spring and early summer, different stars and new opportunities await the sky's watchers in late summer and fall.

High in the western sky is the “Summer Triangle” of Vega, Deneb, and Altair, leading their respective constellations of Lyra the Harp, Cygnus the Swan, and Aquila the Eagle. To the east of the Triangle flies the square of Pegasus, the winged horse. I have never thought this asterism looks much like a horse, let alone the kind that flies, but its four bright stars do resemble a baseball diamond. You can easily spot home plate and the bases, while the Milky Way to the north represents the fans in the stadium. A little further to the east is Andromeda the Princess, and Perseus.

We now focus on Beta Persei. In 1603, Johann Bayer published a star atlas in which stars in the different constellations were given Greek letters in approximate order of brightness. Many stars are numbered instead, according to John Flamsteed's 1725 catalog, which used increasing numbers from west to east in each constellation.

Beta Persei, the demon star of mythology, is more commonly known as Algol, the star that winks! Every 2.9 days, or 69 hours, Algol drops in brightness by a full magnitude for about ten hours. It is one of the finest examples of a celestial event: a star that is not just there, but is visibly doing something for all to see.

Through the magic of variable stars you now have traveled through a looking glass. You have discovered stars passing through the eruptions and inconsistencies of youth and you have visited and drawn comfort from the cosmic wisdom of the aged Mira stars. You have watched the dwarf novae perform and you have looked on as old novae sleep peacefully as the eons after their outbursts pass by.

You passed through the looking glass in a mood of curiosity and I hope you return unsatisfied, demanding more. Through the bibliography, I have suggested certain sources that may help, but the most important source of all is that of the glass of your telescope, coupled to the retina of your eye, and connected by optic nerve to an inquiring mind searching for answers. Every year, new variables are discovered and much more is learned about how these stars behave.

Quite possibly your own observations may add to this knowledge. Possibly also, you will want to pass this on to someone younger.

In the nature of variable stars lies a key to introducing children to astronomy. Stars are individual, they behave in definite ways. They are born, they grow up, they mature, they grow old, and they die. Stars also have moods – sometimes their light output changes. When a star is young, it might flicker with the intensity of a rebellious child, unable to decide on its future course.

Probably: and particularly if by now you are an enthusiastic observer enthralled with the antics of the stars. So let's take advantage of the situation and perambulate amongst two of the most unusual objects in our galaxy, 041619 T Tauri, and 155037 RU Lupi, two stars that represent the carefree conduct of cosmic youth. Both stars are irregular variables and represent stars that are observed in clouds of gas called diffuse nebulae.

T Tauri

Let's begin with T Tauri, the easier of the two, a star that can be watched easily from northern latitudes in the frigid evenings of winter.

If you can find the Hyades, you can find T Tauri. Moving from Delta to 64 Tauri to 4th magnitude 68 Tauri, you proceed to the northeast until you reach Epsilon Tauri, a 3.6 magnitude star (Fig. 22.1). Slightly to the southwest of Epsilon is a group of four stars in the shape of a malformed kite. I use this group as a guide to T Tauri.

Notice the magnitude range of 9.3 to 13.5; I have observed T Tauri for several years and I have never once seen it go fainter than magnitude 10.6 nor brighter than the mid 9s. Most observers have reported it hovering just brighter than 10th magnitude.

But that's only part of the story. T Tauri occasionally can flicker.

In winter, we draw inward as the frigid weather and short days beckon us away from the stars and towards the armchair. This is an unfortunate loss for northern hemisphere observers who forego the unparalleled richness and diversity of the sky at this time of year, a sky that dares us to defy the inside comforts and go outside and watch. This is a time of challenge.

With its stunning belt and sword regions, Orion is the first area we would look to for possible variables, and we will not be disappointed! Orion's Great Nebula harbors some of the most fascinating variables of the entire sky. Lurking within the nebula are some 50 variable stars, 10 of which are bright enough to be observed with a 15 cm (6-inch) telescope.

With a telescope, winter offers a host of unparalleled, delightful stars that are infrequently observed because of the clouds and the cold. U Geminorum, which can rocket from 14.2 to 8.8 in a few hours, is a highlight of winter observing.

Observing hints for cold weather

A winter night can be a devastating experience, which under no circumstances should be taken lightly. During his search for trans-Neptunian planets, one quiet, wind-free night, Pluto discoverer Clyde Tombaugh opened the shutter of the 13-inch telescopic camera and began an exposure. He had been out already for some time and looked forward to the chance to sit back and watch the telescope do his work for him.

While variable-star observing is a specialized branch of observational astronomy, the basic procedures of patience and care that apply to all observing also work with variables.

Plan your program in advance, but be flexible, since the sky often offers surprises. Choose your variable carefully. Is the star likely to be visible through your telescope, or is it obviously too faint? At the other extreme, is your star so bright that observing it is a waste of your precious telescope time?

Telescope

Telescope size

This is more of a consideration than most observers realize. In a sense, each variable star has its own best combination of telescope and eyepiece. The general rule is to use only enough power and magnification to see the variable clearly but not have it so bright that it is hard to estimate. Ideally, the variable should be about two magnitudes brighter than the faintest star you can see with your telescope. If it is much fainter than that, you will have a problem seeing the star, and if the variable is several magnitudes brighter, so many photons will enter your eye that its sensitivity to subtle magnitude variations will be affected.

At minimum, a star might be fair game for most telescopes smaller than 30 cm (12 inches), but as the star brightens you could use a smaller telescope. (When discussing a telescope size, I refer to the size of the mirror or objective lens.)

What else does the sky have that varies? Besides stars, there are other objects worth noting. Some are easily visible, and others are beyond the reach of all but the most powerful telescopes and detectors. They range in distance from a few million kilometers to billions of light years.

Variable nebulae

In the constellation of Monoceros is a variable star called R. It varies irregularly by about half a magnitude around 11. However, the star is usually very hard to see. The reason is that it is embedded in a nebula which also varies in brightness! This object is known as Hubble's variable nebula, NGC 2261, after the Mount Wilson astronomer who, in 1916, discovered that it varies in brightness, size, and even shape. The variation does not seem to follow the brightness changes in R Mon, and they do not occur with any regularity.

R Monocerotis and its nebula probably represent a planetary system in an early stage of formation. At least two other variable nebulae are known, NGC 1555 in Taurus, and a tiny wisp in Corona Austrina, NGC 6729, the home of R Coronae Austrinae (see Section 29.8).

Active galaxies

Innocently displaying some irregular brightness changes are a number of objects that have recently been identified as the cores of galaxies. The Seyfert galaxies are spiral galaxies with starlike nuclei that are very bright and slightly variable.

This season is variable time, with a cast of variables probably better than at any other time during the year. We still have the fine variables of the Milky Way, while toward the east, a different group of variables is gaining prominence.

This is also the time to get your fellow astronomy-club members excited about the challenging field of variables. Fall is the time for renewal in many northern-hemisphere astronomy clubs, where after the summer break, monthly programs and dark-of-the-Moon star parties are taking place once again. If you are fanatical about variables, you may be aware that this field of observing is not the most popular among the amateurs who attend astronomy club meetings. Observations of the changing light output of these distant suns are perceived to lack the luster of the Messier hunt or the glossy galaxy photo, and even the thrill of the meteor watch. Now is the time to insist that variables are fun.

Now we can observe Algol in all its glory, and use it as a motivation to start observing other eclipsing binaries. Two other easily found, easily observed stars are Delta Cephei, and its neighbor Mu Cephei, a huge red-giant sun with totally irregular and unpredictable variations.

Another exciting star is RU Pegasi, a dwarf nova. You never know exactly when the next outburst will take place! While RU Peg may be one of the most exciting stars of fall, it surely is not the most famous.

Delta Cephei

To get an idea of what variable-star observing is all about, here are two active, easily found stars that we will observe informally. The first is Delta Cephei, an ideal star to begin with for several reasons. It is part of a bright and compact star group, is usually around magnitude 4, and is an active star, always offering something interesting to watch.

Delta Cephei's variation was discovered by John Goodricke in 1784 (see Chapter 32), and it is the star for which all the Cepheid variables are named. The variation in this giant star is small but extremely regular with a period of several days. It is also typical that this star enjoys a leisurely decline to minimum that is followed by a rapid rise to maximum.

At maximum, Delta Cephei is easily visible at magnitude 3.5, and at minimum it shines at 4.4. If you observe every night or two you will soon see how it falls, then rises, week after week.

Notice the two stars Zeta and Epsilon in Fig. 6.1 and in the sky. Which is brighter? Let us give Zeta Cephei an arbitrary value of “1” and Epsilon a value of “5”. Each night estimate the brightness of Delta Cephei as follows:

1. – as bright as Zeta

2. – slightly fainter than Zeta

3. – halfway between Zeta and Epsilon

4. – slightly brighter than Epsilon

5. – as faint as Epsilon

Even though we are not using actual magnitude values here, we still estimate to better than 0.2 magnitude.