The colour of the daytime sky

Our atmosphere is a thin, transparent layer of air, not much more than 100 km deep, that separates us from the dark and starry void beyond. On cloudless nights we look out at this fathomless darkness, and only the twinkling of stars betrays the presence of the atmosphere through which we gaze. But when the Sun is up, the whole sky glows brightly, and the dark abyss is concealed from us by an almost tangible blue dome that seems forever out of reach, beyond even the highest clouds and the furthest horizon.

There is no blue dome, of course. Instead during daylight hours we are immersed in what is known as airlight, which is the glow of the atmosphere when it is illuminated by the Sun.

Unweaving the rainbow

How many rainbows have you seen recently, say in the last six months? Not many, I'll wager. The fact is that, unless you live somewhere where the conditions that favour their formation occur on an almost daily basis, such as an oceanic island like Hawaii, you probably don't often get the chance to see a rainbow.

Darkness and night vision

Dark adaptation

The human eye takes time to adapt to dim light. Although the pupil opens up almost immediately, that's not the whole story. Dark adaptation involves the release of the light-sensitive chemical rhodopsin (visual purple) in the retina. For astronomy, useful dark adaptation takes about ten minutes, and substantial improvement continues for half an hour or more.

The central part of the retina does not function in dim light; faint objects disappear if you look straight at them. Experienced observers use averted vision, which means that they view the faintest stars, nebulae, and galaxies by looking slightly to one side of the object rather than directly at it.

Visual perception of faint objects is not continuous. A star near the limit of vision may be evident only a third of the time; it will seem to pop into and out of view. As long as you keep seeing it in the same place, you can be sure that it's real even though you can't see it continuously.

Even a small amount of bright light prevents complete dark adaptation; that's why a distant streetlight or even an illuminated doorbell button can be so annoying. Red light does not do this as much as other colors, which is why astronomers use red flashlights. The light must be red (or orange), not just reddish; what matters is the absence of blue wavelengths, not the red color.

Around 1999 and 2000, when amateur astronomers adopted computerized telescope technology en masse, three telescopes led the revolution. They were the Meade LX200 (on the market since 1992), the Meade ETX-90 EC Autostar (introduced in 1999), and the Celestron NexStar 5 (2000).

The following chapters describe these telescopes in some detail. By now, none of them is still the manufacturer's latest and greatest. Technology is progressing so fast that new models appear almost every month.

But the older telescopes will still work as well as they ever did, and tens of thousands of them will remain in use for many years. The information in the next three chapters will help those who still use classic computerized telescopes, or are thinking of buying them, or simply want to know what they are like.

Light without form

The price we pay for city life is blank urban night skies, rendered almost starless by our addiction to light. Electric light is, without doubt, a ‘good thing’. But, like all good things, you can have too much of it. Many of our cities are so brightly lit that our eyes are perpetually dazzled, and we are unable to see any but the very brightest stars and planets.

Together with its companion volume, How to Use a Computerized Telescope, this book is a guide for a new generation of amateur astronomers. The two books began as a single project, originally a list of interesting objects that I put together for my own use at the telescope. Soon I added a concise summary of the Meade LX200 operating manual. Simon Mitton of Cambridge University Press saw my notes and encouraged me to turn them into a book. As the project grew, it became two books instead of one – a book about telescopes and a book about the sky. This volume is the latter.

While I was writing the two books, Scott Roberts of Meade Instruments lent me equipment to try out. The technical support departments at Meade, Celestron, Software Bisque, and Starry Night Software answered technical questions. Daniel Bisque supplied software for testing. Howard Lester, Dennis Persyk, Lenny Abbey, Rich Jakiel, T. Wesley Erickson, Robert Leyland, R. A. Greiner, Richard Seymour, Ralph Pass, Phil Chambers, Ells Dutton, Michael Forsyth, and John Barnes critiqued drafts of parts of the text. Tom Sanford let me try out his Meade LX90 at length. Earlier, Jim Dillard first got me interested in computer-aided astronomy by buying my old Meade LX3 from me and outfitting it with digital setting circles. There are probably others whose names I've forgotten to list, and I beg their indulgence.

Back in the 1850s, T. W. Webb noticed that amateur astronomy had become competitive – his contemporaries were trying to outdo each other by building larger and larger reflectors. He decided to break out of this trend and write a book for owners of 3-inch refractors and the like. His book, Celestial Objects for Common Telescopes, has become a classic and is still available.

In a similar spirit, I want to break away from the present-day tendency to equate deep-sky observing with star-hopping to the faintest possible nebulae and galaxies, which are visible only under extremely dark skies. There's more to the stellar universe than just “faint fuzzies.”

The objects in this list are visible with an 8-inch (20-cm) telescope under suburban skies with a naked-eye magnitude limit of 5. Most of them are fine sights even with considerably smaller telescopes under worse conditions. Many of them are stars (variable, double, multiple, or unusually colored) or bright star clusters.

This list is based on my own observations. It is organized by zones of right ascension, and within each zone, in sequence from north to south. Chapter titles such as “The January–February sky” refer to the time of year when the objects are highest at 10 or 11 p.m. local time. Many of the objects, especially the more northerly ones, can be viewed for much longer than just the specified two-month period. Those in the extreme south, however, are in the sky only briefly.

Since this book is intended to encourage you to use your eyes, unaided by a telescope, I should begin by saying a few words about the eye. Many people assume that the eye is like a camera, and that therefore we must see whatever is before our eyes. But, to see, you must look. Seeing is a conscious act and, unless you look actively, and have some idea of what you are looking for, you probably won't see many of the things mentioned in the subsequent pages.

Computerized telescopes are revolutionizing amateur astronomy. Even the least expensive entry-level telescopes are now available with computer-controlled motors to find and track objects in the sky. No longer do you have to search for NGC 1999 or Neptune by carefully comparing the view with a star map – you just tell the telescope what to point at, and it does it.

Do computer controls take all the fun out of astronomy? No more than paved highways take the fun out of the Arizona desert. Professional astronomers have used setting circles to find objects since the time of Tycho Brahe and have always tried to make them as accurate as possible. Amateurs have long had setting circles, but they weren't very accurate. Now, with the advent of computers, professional-level accuracy is within the reach of the amateur, and the computer actually controls the telescope rather than just telling you where it's pointed.

After 30 years of finding celestial objects the old way, I bought my first computerized telescope in 2000 and immediately found myself doing a new kind of amateur astronomy. Suddenly I was spending my time looking at objects instead of for them. No longer preoccupied with “star-hopping”, I could spare the time and attention to study the celestial objects themselves.

In fact I realized for the first time that, for all those years, my observing program had been skewed by the fact that some objects are easier to find than others.

This chapter gives quick solutions to a number of common problems that are likely to puzzle a new telescope owner. This list is far from complete. For more information, see your instruction book, as well as Chapters 10–12 and the websites mentioned there.

Electrical and computer problems

Telescope is electrically dead

Check all fuses. On the LX200, there is a fuse inside the connector panel as well as in the power cable.

Check keypad and declination cables too. If deprived of any of its necessary connections, the computer will not initialize normally and the telescope may seem to be electrically dead.

Check for a coaxial power connector that is the wrong size. The diameter of the central pin is smaller on Celestron than on Meade telescopes. The Meade connector fits Celestron telescopes but gives a loose connection.

Computer hangs or resets at random moments

This is often caused by a loose connection somewhere in the power system; inadequate battery voltage; or electrical noise on the power line, such as poor filtering of an AC-operated power supply.

If using internal batteries, check the battery contacts and try using an external power supply. Rechargeable lead-acid batteries are the gold standard for power supplies; they produce very clean, well regulated DC.

Make sure coaxial DC connectors are the right type and are firmly plugged in. On the NexStar 5 and some others, the central pin of the socket consists of two tines that you can spread apart with a screwdriver to get a better connection.

Small objects in the Solar System

A century ago, astronomers' picture of the Solar System was neat and clear. There were eight planets and an asteroid belt, a set of small, rocky bodies between the orbits of Mars and Jupiter, presumably the remains of a planet that had exploded or never formed.

By 1950 the picture was a bit less tidy. One planet, Pluto, was abnormally small and had an odd orbit. A few asteroids had been discovered with orbits outside the asteroid belt.

Today the situation is even less neat, and we are having to rethink old categories. Though the belt between Mars and Jupiter is predominant, there are asteroids all over the Solar System, including a second belt, the Kuiper (KHOY-per) Belt, outside the orbit of Neptune.

There is no longer a clear distinction between asteroids and comets; the asteroid 2060 Chiron has played both roles. It now appears that a comet is merely an icy asteroid from the outer Solar System that has been deflected close enough to the Sun to vaporize the ice.

The distinction between asteroids and planets is also becoming blurred; some scientists want to reclassify Pluto as the largest Kuiper-belt asteroid. Meanwhile, some planetary satellites, such as Mars' potato-shaped companions Phobos and Deimos, are physically indistinguishable from asteroids.

Overview

All stars are variable – it's just that some of them have not varied appreciably during human history. Every star changes brightness as it ages, and many stars pulsate – that is, they get brighter and dimmer in a regular cycle.

Besides being interesting to watch, variable stars provide opportunities for amateurs to contribute to scientific knowledge. Much of the year-by-year monitoring of variable stars is done by amateurs, and amateurs discover many novae and supernovae. A large telescope is not required; some of the most productive observers use small, wide-field instruments or even binoculars.

Amateur variable-star work is coordinated by the B.A.A. (p. 30) and the American Association of Variable Star Observers (AAVSO, 25 Birch St., Cambridge, MA 02138, U.S.A., http://www.aavso.org). The AAVSO maintains a web site with current information on thousands of stars and issues bulletins about unexpected phenomena.

Besides AAVSO and B.A.A. training materials, two very good guides to variable-star observing are the book Observing Variable Stars, by David H. Levy (Cambridge University Press, 1998), and the chapter on variable stars by M. Dumont and J. Gunther in Patrick Martinez' The Observer's Guide to Astronomy, vol. 2, pp. 775–846. Levy's book is designed for beginners and casual skygazers; Dumont and Gunther's treatment of the subject is more technical.