Parts of this chapter have been updated from S. J. Edberg and D. H. Levy, Observing Comets, Asteroids, Meteors, and the Zodiacal Light (Cambridge: Cambridge University Press, 1994).

How to record showers

The easiest way to record a meteor shower is to use a counter of some kind. I use a simple metal counter – I push it once whenever I see a meteor. The numbers range from zero – just didn't see anything – to 2403 meteors, which my wife Wendee and I saw on the night of November 18/19, 2001. Personally, I enjoy observing in this manner most of all. It requires absolutely no auxiliary equipment except the counter.

Counter

If you are observing alone and all you want is simple hourly rates, you can use a gate counter, or a golfer's stroke counter that you press every time you see a meteor. You will get hourly rates this simple way, but not the other information which is also useful. The advantage of the counter, when pressed for each shower meteor you see, is that it allows you to observe with the specific goal of determining shower rates.

It was the 2006 Quadrantid shower that inspired me to write this book. It is not a famous shower, but it should be: it is one of the strongest showers of the year, offering up to 200 meteors per hour (190 in 1965) per observer at maximum, in competition with the August Perseids. (The rates quoted are called zenithal hourly rates, and assume the ideal circumstance of a single observer observing under a dark, moonless sky, limiting magnitude 6.5, while the radiant is at the zenith. Since the radiant is never at the zenith during dark hours, the actual numbers are a lot lower.)

Late in the evening of January 2, I took The Beagle, our little dog, for his evening walk. The sky was dark and cloudy, and the forecast was not good. Through small breaks in the cloud cover I looked for meteors coming from the north, from a point in space between the head of Draco and the top of the kite-shaped figure of Bootes. From this little spot in the sky, not far from the star 47 Bootis, would emanate one of the year's best meteor showers.

But not this year, and this is why so few people pay attention to “the Quads.” There are two reasons.

Isn't it strange that of all the many pieces of our Solar System, it is the smallest ones – comets, meteors, and an eclipse of the Moon, that attracted the attention of civilizations right up to Shakespeare's time 400 years ago? While the great worlds orbit the Sun, their motions are predictable and are not a cause for alarm. But a meteor shower did attract notice, along with an increasing interest in the workings of our Solar System. Our solar neighborhood is a big place, with several different types of object in its domain, ranging in size from one star, the Sun, to tiny micrometeoroids that are even smaller than the meteors this book is about. These objects include the Sun, planets, moons, comets, asteroids, meteoroids, and micrometeoroids. It seems a simple division, but recent debates tell us that the boundaries among these types of objects are fuzzy at best. See Figure 4.1.

Categorizing objects in our Solar System

The best known debate concerns the definition of a planet. In 2006 the International Astronomical Union defined a planet in a most controversial way.

This chapter is a crash course in the principles of digital image processing. For more about most of these concepts, see Astrophotography for the Amateur (1999), Chapter 12.

What is a digital image?

A digital image is fundamentally an array of numbers that represent levels of brightness (Figure 13.1).

Bit depth

Depending on the bit depth of the image, the numbers may range from 0 to 255 (8 bits), 0 to 65 535 (16 bits), or some other range.

The eye cannot distinguish even 256 levels, so 8-bit graphics are sufficient for finished pictures. The reason for wanting more levels during manipulation is that we may not be using the full range at all stages of processing. For instance, a badly underexposed 16-bit image might use only levels 0 to 1000, which are still enough distinct levels to provide smooth tones. An 8-bit image underexposed to the same degree would be unusable.

For greatest versatility, some software supports floating-point data, so that levels can be scaled with no loss of precision; in a floating-point system, you can divide 65 535 by 100 and get 655.35. You can also use large numbers without going out of range; if you do something that produces a value greater than 65 535, it will not be clipped to maximum white.

Note that Photoshop always reports brightness levels on a scale of 0 to 255, regardless of the actual bit depth of the image. This is to help artists match colors.

Twin Lake Camp

As a spindly asthmatic eight-year-old Canadian boy, I was not familiar with the Fourth of July, 1956, as a national holiday. Four days earlier, I had boarded a train in Montreal with my two brothers Richard and Gerry and my sister Joyce. It was the first time in my life I would be away from Mom and Dad, and I was not happy about it. I was homesick before the train pulled to a stop by the small station at Whitehall in New York state. We boarded buses – one for Joyce who would attend nearby Camp Awanee, and another for the rest of us. Richard was looking forward to his elite experience on Senior Hill; Gerry was wondering about life in Freshman House, and I was headed for Bunk B on Junior Row. With these thoughts in our minds, our buses crossed the border into Vermont, near the historic Fort Ticonderoga where American soldiers fought the British during their war of independence. We reached Vermont Route 30 and headed south through beautiful undulating mountain scenery. Finally we passed a restaurant on the right called The White House, descended a hill, slowed down, then turned left. We had reached Twin Lake.

As we read in Chapter 3, the Leonid shower has inspired sky watchers way back into the distant past. But it appears that the orbit of the stream changes, probably because of continued influence from Jupiter's gravity. For long periods the densest portion of the stream does not come close enough to the Earth, and then returns for a better strike. Thus, the 1833 and 1866 storms were marvels to behold, but the showers around 1899 were mild, and there was very little in 1933. And so the great Leonid meteor storms, where one meteor per second or more could be seen, entered the realm of history.

The storm of 1966

Thus, the Leonids joined other showers in the weak to moderate category. I recall, however, in one of the first Sky and Telescope magazines I ever received as a child, that the November 1963 shower was a little stronger than expected. The article reminded us of the great storms of the past, and wondered if 1966 might see a return to those levels of history.

The Leonids of 1964 and 1965 were even stronger, and so astronomers awaited the night of November 17, 1966, with considerable anticipation. That Wednesday evening was completely cloudy over most of the northeastern United States.

The Delta Aquarids go back a long way for me. During the summer of 1963 I was a patient at the Jewish National Home for Asthmatic Children. Although visits home were not generally permitted, I was allowed to return home briefly to see the July 20, 1963 total eclipse of the Sun. While in Montreal I took advantage of the clear, hazy sky to enjoy my first Delta Aquarid observing session on July 26. In a little over two hours of observation I saw three meteors from the light polluted sky of a big metropolitan area. The next year at the Adirondack Science Camp, our team observed 13 meteors in about six hours of observation. Our night was a casual one. The count was so low because we were doing other forms of observing at the same time.

July and August are especially good months for meteor observing. As you can see in Chapter 19, several minor showers compete for attention. While the chances that, while outdoors on a July evening, you might see a meteor from a particular minor shower are low, the chances that you'll see one from any shower are substantial. Toward the end of July, two major showers awaken at almost the same time.

Why you need another lens

You will have gathered that I think piggybacking is one of the best astronomical uses for a DSLR. But the “kit” lens that probably came with your DSLR is not very suitable for piggyback astrophotography. It has at least three disadvantages:

• It is slow (about f/4 or f/5.6).

• It is a zoom lens, and optical quality has been sacrificed in order to make zooming possible.

• It is plastic-bodied and not very sturdy; the zoom mechanism and the autofocus mechanism are both likely to move during a long exposure.

Fortunately, you have many alternatives, some of which are quite inexpensive. One is to buy your camera maker's 50-mm f/1.8 “normal” lens; despite its low price, this is likely to be the sharpest lens they make, especially when stopped down to f/4. Another alternative is to use an inexpensive manual-focus telephoto lens from the old days. There are several ways of doing this; Nikon DSLRs take Nikon manual-focus lenses (though the autofocus and light meter don't work), and Canons accept several types of older lenses via adapters.

Big lens or small telescope?

But wait a minute–instead of a lens, should you be looking for a small telescope, perhaps an f/6 “short-tube” refractor? Some of these have ED glass or even three-element lenses and perform very well for astrophotography.

In my opinion and experience, good telephoto lenses perform even better. After all, they have more elements and more sophisticated optical designs.

After all this, you're probably itching to take a picture with your DSLR. This chapter outlines four simple ways to take an astronomical photograph. Each of them will result in an image that requires only the simplest subsequent processing by computer.

All of the projects in this chapter can be carried out with your camera set to output JPEG images (not raw), as in daytime photography. The images can be viewed and further processed with any picture processing program.

Telephoto Moon

Even though the Moon is not ultimately the most rewarding object to photograph with a DSLR, it's a good first target.

Put your camera on a sturdy tripod and attach a telephoto lens with a focal length of at least 200 and preferably 300 mm. Take aim at the Moon. Initial exposure settings are ISO 400, f/5.6, 1/125 second (crescent), 1/500 second (quarter moon), 1/1000 (gibbous), or 1/2000 (full); or simply take a spot meter reading of the illuminated face of the Moon. An averaging meter will overexpose the picture because of the dark background.

If the camera has mirror lock (Canon) or exposure delay (Nikon), turn that feature on. Let the camera autofocus and take a picture using the self-timer or cable release. View the picture at maximum magnification on the LCD display and evaluate its sharpness. Switch to manual focus and try again, varying the focus slightly until you find the best setting. Also adjust the exposure for best results.

To take exposures longer than a few seconds, you must track the stars. That is, the telescope must compensate for the earth's rotation so that the image stays in the same place on the sensor while the earth turns.

This book is not the place to give a complete survey of the art of tracking and guiding; see Astrophotography for the Amateur and How to Use a Computerized Telescope. In this chapter I'll review the essentials, with an emphasis on recent developments.

Two ways to track the stars

Figure 9.1 shows the two major kinds of telescope mounts, altazimuth and equatorial. Until the 1980s, only an equatorial mount could track the stars; it does so with a single motor that rotates the telescope around the polar axis, which is parallel with the axis of the earth. In order to use an equatorial mount, you have to make the polar axis point in the right direction, a process known as polar alignment and long considered somewhat mysterious, although actually, correct polar alignment can be achieved quickly (see p. 102).

Computerized telescopes can track the stars with an altazimuth mount, or, indeed, a mount whose main axis points in any direction. During setup, the computer has to be told the exact positions of at least two stars. It then calculates how far to move along each axis, moment by moment, to compensate for the earth's rotation.

Digital SLR cameras have revolutionized astrophotography and made it easier than ever before. The revolution is still going on, and writing this book has been like shooting at a moving target. New cameras and new software are sure to become available while the book is at the factory being printed. But don't let that dismay you. All it means is that we'll have better equipment next year than we do now.

This book is not a complete guide to DSLR astrophotography; the time is not yet ripe for that. Nor does space permit me to repeat all the background information from my other books. For a complete guide to optical configurations and imaging techniques, see Astrophotography for the Amateur (1999). To get started with a telescope, see How to Use a Computerized Telescope and Celestial Objects for Modern Telescopes (both 2002). All these books are published by Cambridge University Press.

What I most want to emphasize is that DSLR astrophotography can be easy, easier than any earlier way of photographing the stars. It's easy to lose track of this fact because of the flurry of technical enthusiasm that DSLRs are generating. New techniques and new software tools appear almost daily, and the resulting discussion, in perhaps a dozen online forums, thrills experts and bewilders beginners.

My goal is to save you from bewilderment.

Meteor societies and books

The International Meteor Organization. Founded in 1988, the IMO seeks to bring together meteor enthusiasts from all over the world. www.imo.net.

The American Meteor Society. Founded in 1911, this organization brings together serious meteor observers. www.amsmeteors.org.

Royal Astronomical Society of New Zealand. For southern hemisphere observers, the RASNZ has an active comet and meteor section headed by John Drummond. www.rasnz.org.nz/Sections.htm#comet.

The Dutch Meteor Society is one of the most active of the European meteor organizations. home.wxs.nl/~terkuile/.

Another excellent website is Gary Kronk's Meteor Calendar. It offers detailed histories and observing information for major and minor showers each month. comets.amsmeteors.org/meteors/calendar.html.

In addition, almost every national astronomy society has a section, or a program, involving meteor observing. These sections can usually be found in the web pages of the various national societies.

There are several good books on meteors and meteorites. Mike Reynolds' Falling Stars: A Guide to Meteors and Meteorites (Mechanicsburg, PA: Stackpole Books, 2001) offers advice from an experienced meteorite collector and meteor observer.

Finally, more advanced readers will enjoy and benefit from Peter Jenniskens' Meteor Showers and their Parent Comets (Cambridge: Cambridge University Press, 2006).

With so much attention being paid to the Geminids peaking on December 13, we often forget the last major shower of the year, the Ursids. Discovered by the famous British astronomer William Denning around 1900, the shower was considered minor until the extraordinary night of December 22, 1945. Antonin Becvar was observing at the Skalnate Pleso Observatory in Czechoslovakia, and it was not hard to notice rising numbers of meteors coming from the north. He arranged for photographs to be taken and proper counts to be made of meteors coming from a radiant near Beta Ursae Minoris, the second brightest star in the Little Dipper after Polaris. A few years later Becvar became famous as the author of several star atlases, including the Skalnate Pleso Atlas of the Heavens, which served for most of the second half of the twentieth century as the leading star atlas for serious astronomers. Becvar also headed a most serious comet search program from Skalnate Pleso, which resulted in the discoveries of several comets from that site.

After his rediscovery of the Ursids, observations visual, photographic, and radar came within a few years. They are now considered a respectable meteor shower, the most recent of the major showers to be detected. Activity varies from year to year. Some years the rates may be below five per hour, but some years are far more favorable.

How does one develop a passion for the sky? Is it through careful mathematical study of the physical properties of the planets or cosmic distances throughout space? Or can it be through a simple, unintended look at the night sky thanks to a meteor?

You can, of course, process film images with the same software that you use for DSLR images. The grain in each image is different; there is no fixed-pattern noise. Stacking multiple images builds contrast and reduces grain.

First you have to get the film images into digital form. There are many methods. The best is a film scanner with a resolution of at least 2400 dpi (about 100 pixels/mm). This scans each 35-mm slide or negative into a file containing eight or more megapixels, comparable to the resolution of a good DSLR. I use a Nikon Coolscan III (LS-30) and get excellent results. It even has the ability to detect and electronically remove dust and scratches, which, unlike film, are opaque to infrared light.

I have not had good results with flatbed scanners that claim to scan film. In my experience, the flatbed scanner acquires a rather blurred image and then applies a strong sharpening filter to it. It's much better to scan the image faithfully in the first place.

You can use your DSLR to digitize film images. Any slide duplicator attachment that fits a film SLR will also work with a DSLR, except that it may not cover the whole slide because the DSLR sensor is smaller than a film frame. The alternative is to use the DSLR with a macro lens and light box, and simply photograph the slide or negative (Figure C.1).

After such a long interval – almost four months – since the last major shower, I never want to miss the Lyrids. As the night of maximum came along in 2006, I spent night after night observing the sky. Even though the darkness of the sky remained undimished by moonlight, the patterns of stars were mostly undisturbed by meteor trails. There were a few, and some were bright, but this year's shower was somewhat disappointing.

After the Quadrantids, we look forward to the Lyrids opening its curtain on the sky. These meteors can reach a theoretical rate of about 15 per hour per observer on the night of maximum, April 20/21, but I have never seen that many. In 1803 observers in eastern North America fared differently: they observed some seven hundred meteors in one hour. In 1982, some people noted activity reaching about 80 per hour, a dramatic increase characterized by fainter than usual meteors.

Even so, the 2006 display exceeded my experience from the year before. In 2005 I saw but one meteor. A single, solitary flash in the night was my sole experience with the Lyrids. But it was a wonderful meteor. Brighter than Vega, it sped through a few degrees of sky to the north of the radiant and left a short trail. It was greenish-white. And that was the Lyrids for 2005.