Overview

Photography gives you a way to record what you see through the telescope. Surprisingly, though, astrophotography does not reproduce what you see visually. In lunar and planetary work, it is very hard to get pictures as sharp as what the eye can see, because the eye can seize moments of atmospheric steadiness in a way that the camera cannot. In deep-sky work, on the other hand, the camera often records far more than the eye could see with the same instrument because film can accumulate light in a long exposure.

This chapter will tell you enough about astrophotography to get you started. It is not a complete guide; for that, see my other book, Astrophotography for the Amateur (Cambridge University Press, 1999).

One word of advice: astrophotography is a matter of skill, not just equipment. It is definitely not a matter of “You press the button, we do the rest” – it is a stiff test of how well you understand your equipment and the principles on which it operates. Never buy a piece of equipment until you know exactly what you'll use it for.

There is also an element of luck. The pictures that you see published in magazines are the work of experienced astrophotographers and are generally the best of many, many tries. Do not expect to equal them immediately.

But some techniques do yield very good pictures even in the hands of a beginner.

The lure of the deep sky

The “deep sky” is the sky beyond the Solar System, and while it theoretically includes all stars except the Sun, in practice “deep-sky observing” means observing star clusters, nebulae, and galaxies.

For many amateur astronomers, including myself, deep-sky observing is the most interesting specialty, the most far-ranging form of celestial sightseeing. We bought our telescopes in order to see the universe. For the variety of sights and the variety of astrophysical processes behind them, deep-sky observing is unsurpassed.

Critics point out that deep-sky observers are unlikely to contribute anything to science, since most of the objects are near the limit of visibility in amateur telescopes, and apart from occasional supernovae in distant galaxies, there is nothing that amateur equipment can discover. That doesn't deter us. Seeing the sights is enough.

Deep-sky objects

Asterisms

An asterism is any small group of stars that catches the eye, whether or not the stars form a cluster in space. M73, for instance, is an asterism of four stars in Aquarius.

In recent years, amateur astronomers have given colorful names to numerous telescopic asterisms, many of which were enumerated by Philip S. Harrington in The Deep Sky: An Introduction (Sky Publishing, 1998). That book is actually an excellent overall guide to deep-sky observing and includes a small but useful star atlas. I must confess to having subsequently named two asterisms myself, Webb's Horseshoe (p. 207) and the Perfect Right Angle (p. 245).

A brief history of the sky

You do not need to be told that day and night are not the same. But in what does their difference consist? That the one is light and the other dark? Surely there is more to them than this? There is, and it is this: the absence of airlight allows us to see stars and planets. Their nightly drift across the sky is much more than mere spectacle, though it certainly is that, for it is one of the few clues we have to the fact that we inhabit a spinning sphere of solid matter racing through a void.

The Solar System

For most people, mention of the Solar System probably conjures up that well-known image of nine concentric circles, each representing the orbit of a planet, all more-or-less centred on the Sun. In fact, this image is presented from a vantage-point that no human has ever occupied, somewhere far outside the Solar System. From such a distance, the only thing that you would see of the Solar System is the Sun, reduced to an extremely bright point of light. Without a telescope you would be unable to see Jupiter, the largest planet, let alone any of the other planets. The fact is, by any objective measure, such as mass, diameter or brightness, planets are insignificant motes compared with stars. Yet, what would the Solar System be without planets? Planets loom large in our view of the cosmos for several reasons, not the least of which is that we happen to inhabit one of them.

Many older maps still use epoch 1950 coordinates. To convert right ascensions and declinations from 1950 to 2000, find the entry in Table B.1 nearest the object's position in the sky and add the corrections indicated there.

To convert 2000 coordinates to 1900 for AAVSO variable-star designations and the like, double the tabulated correction and apply it in the opposite direction (subtracting instead of adding and vice versa).

To calculate precession, let α and δ stand for the object's initial right ascension and declination respectively, both expressed in degrees. (Recall that 1h = 15°.) Let N stand for the number of years between epochs (negative if you are converting from a later epoch to an earlier one). Then compute:

R.A. correction (in seconds) = (3.073 + 1.336 sin α tan δ) × N

Declination correction (in arc-seconds) = (20.042 cos α) × N

and add the corrections to the original R.A. and declination.

These formulae are for dates within a couple of centuries of 2000 and positions at least 0.1° away from the celestial poles. For more accurate formulae see the Astronomical Almanac published by the U.S. and British governments.

The Moon

Phases of the Moon

We often make jokes about activities that depend on the phases of the Moon, but amateur astronomy really does. When the Moon is high in the sky, especially if it is full or nearly full, you can't see faint stars, nebulae, or galaxies. Conversely, if you want to observe the Moon you need to know when and where it is going to appear.

Accordingly, all astronomical observers need to keep track of the phases of the moon. Figure 3.1 summarizes the whole cycle. The Casio “Forester” wristwatch, marketed to hikers and fishermen, keeps track of the cycle for you.

The Moon stays close to the ecliptic, though not precisely on it, and moves eastward, making a full circle relative to the stars every 27.32 days (one sidereal month). This means that it moves its own apparent width (half a degree) in slightly less than an hour. You can watch this happen when the Moon passes near a bright star.

The cycle of phases, or synodic month, takes 29.53 days. This is longer than the sidereal month because the Sun and Moon are moving in the same direction; the Moon has to move more than a full circle in order to catch up.

Introduction

This chapter describes the original Meade LX200 (Figure 10.1), which was made from 1992 to 2001. The newer LX200 GPS is optically and mechanically similar but has an enhanced version of the Autostar computer described in Chapter 12.

The information here is based on my experiences with an 8-inch LX200 purchased in 2000. I assume that you also have the Meade manual available for reference. This is not a complete guide to all the LX200's features.

This chapter is more detailed than the next two, for several reasons. The original LX200 was on the market for nine years, so there was plenty of time for the amateur community to learn all about it. All LX200s use similar firmware, and the total number of LX200s in use is very large, so this detailed information is useful to a large number of people. Finally, the original LX200 is at the end of its product life cycle, so there will be no further changes.

Even after it is discontinued, the LX200 will remain in wide use for many years. It is to computerized telescopes what the Nikon F is to cameras: an army of loyal users complain about its quirks but continue to trust it for serious work.

Evaluation of the LX200

Two useful features are conspicuously missing from the LX200: the ability to download software updates, and the ability to do a two-star alignment in equatorial mode to keep pointing accuracy from depending on polar alignment.

No light without shadow

A shadow is a volume of space not directly illuminated when light is intercepted by an object. Usually we are aware of these shafts of darkness only when they fall upon an illuminated surface, where they are seen as dim, distorted outlines. But if the medium through which the shaft of the shadow passes is filled with particles able to scatter light, such as dust-laden or hazy air, fog, or turbid water, the shaft itself becomes visible.

We are apt to overlook the effect of shadows on the way the world looks to us. If we notice them at all, it is probably as a diversion.

This book is about things that can be seen in the sky. We all look at the sky from time to time, though usually it is to check the weather. By and large we don't look at it for enjoyment, in part because we don't know what to look for. Very few people who are unfamiliar with the many wonderful sights to be seen in the sky accidentally notice halos or sundogs, two of the most common optical phenomena. To be sure of seeing these and other sights, you must know what to look for and when to look. This is where I hope this book will come in useful. It has been written to help you find your way around the sky, and see for yourself the many wonderful things that it has to offer.

My earliest memory of looking at the sky is of having the three stars that make up Orion's Belt pointed out to me. I can't recall what I made of them; I remember being told that they are distant suns, though that didn't mean much to me at the time. I was, I think, six or seven years old.

It was, nevertheless, a defining moment, the start of a lifelong fascination with the sky.

Comets

There is a long association between comets and disastrous events on Earth. Over the ages, they have been variously blamed for floods, droughts and pestilence. As far as the ancients were able to tell, comets appear unpredictably, seemingly from nowhere, and after a few weeks vanish, never to be seen again. They were seen as a challenge to the harmony and predictability that normally reigns in the skies, and were excluded from the heavens on the authority of Aristotle. He argued that the fact that comets appear and disappear without warning, means that they must be due to events close to the Earth, the only part of the universe where he believed change was possible. According to Aristotle, comets were atmospheric phenomena involving friction between layers of the atmosphere furthest from Earth.

Amateur astronomy for a new generation

This is a handbook for the modern amateur astronomer. As far as possible, I've tried to write the book that I'd like to have in my own hands while at the telescope – along with a star atlas and the Handbook of the B.A.A., of course.

Amateur astronomy isn't what it used to be. A generation ago, most serious amateurs observed from their homes with large Newtonians; one star atlas and two or three reference books were the amateur's complete guide to the sky; the latest news, arriving by magazine, was two months old; and most of the stars visible in the telescope were absent from even the largest catalogues and atlases.

Those days are gone, thank goodness. Telescopes have changed – they are nearly all portable, and compact designs such as the Schmidt–Cassegrain are popular. As often as not, the telescope is computer-controlled.

More importantly, computers have brought high-quality data sources within the amateur's reach. Alongside star atlases, we use software that plots the star positions measured by the Hipparcos satellite. We can compute the positions of comets, asteroids, and artificial satellites at the touch of a button. We can even track clouds by satellite to see if we're going to have clear weather.

Accordingly, a major theme of this book is the effective use of astronomical data, especially the Internet. Web addresses are given throughout, as well as detailed information about classic and modern catalogues of celestial objects.

Magnitude

The magnitude system

The magnitude of a star is its brightness measured in a somewhat peculiar way. In ancient times, Ptolemy and Hipparchos classified stars as “first class” (brightest) to “sixth class” (barely visible). These brightness classes were termed magnitudes, but there was no provision for exact measurement.

In 1856, Norman Pogson proposed the logarithmic magnitude scale that is now standard. The advantage of a logarithmic scale is that it can span a tremendous brightness range without using very large or very small numbers (Figure 8.1). Each difference of five magnitudes corresponds to a factor-of-100 difference in brightness. One magnitude corresponds to a brightness ratio of 2.512.

In this system, most stars still have roughly the magnitude that Ptolemy assigned them, but some of the brightest stars have negative magnitudes. The full moon is magnitude – 12 and the Sun is –27. This 15-magnitude difference means that the Sun is a million times as bright as the Moon.

The star Vega is defined to be magnitude 0.0, but in practice, the average of several stars is used as a standard for measurement.

The human eye's response to light is not actually logarithmic, but it is close enough for practical purposes. If a star appears to be halfway between two other stars in brightness, it will also be halfway between them in magnitude.

The importance of double stars

More than half of all stars belong to double- or multiple-star systems. That is, they are gravitationally bound to other stars and orbit around the system's common center of gravity.

Amateur astronomers today have forgotten the excitement that accompanied the gradual discovery of this fact during the nineteenth century. Double stars provided the first opportunity to measure the mass of stars and to test the laws of physics outside the Solar System.

Many double stars show measurable orbital motion over just a few years. Many others need to be measured now, since determination of an orbit requires observations many years or centuries apart, and double-star work fell out of fashion in the twentieth century. The Hipparcos satellite made accurate measurements of numerous double stars in early 1991; even these observations are now old enough that it is worth while looking for subsequent changes.

It is often unknown whether a particular double is really a gravitationally bound binary star or merely an optical double comprising two unrelated stars in the same line of sight. In between are common proper motion pairs, stars that appear to be the same distance from Earth and have roughly the same proper motion, but in which orbital motion has not been observed.

A visual binary is a binary star whose orbit can be observed with telescopes. Less than a thousand orbits are known in any detail, and the available information about them is often inaccurate.