If your digital camera is not a DSLR, it has a smaller sensor with a higher noise level, making it unsuitable for deep-sky work. More importantly, it doesn't have interchangeable lenses.

Those facts don't make it useless. Non-SLR digital cameras have one great advantage – their shutters are almost totally vibration-free. This enables them to get sharp images of the sun, moon, and planets (Figures A.1–A.3). In fact, a Nikon Coolpix 990 is my usual camera for photographing lunar eclipses.

The coupling of camera to telescope has to be afocal; that is, the camera is aimed into the telescope's eyepiece. Numerous adapters to hold the camera in place are available; my favorite, which only works with older Nikon cameras, is a ScopeTronix eyepiece that threads directly into the filter threads of the camera. With a bright image of the Moon, the camera can be hand-held.

Set the camera to infinity focus (the mountain symbol), turn off the flash, and focus the telescope while viewing the display on the camera. If the Moon fills the field, the camera can autoexpose; otherwise, you'll have to set the exposure manually by trial and error. Set the self-timer (delayed shutter release) so that the picture will not be taken until you have let go of the the camera and it has stopped vibrating.

The examples shown here are single images, but you can of course take many exposures of the same object and combine them with RegiStax (p. 206).

In what follows, I'm going to assume that you have learned how to use your DSLR for daytime photography and that you have its instruction manual handy. No two cameras work exactly alike. Most DSLRs have enough in common that I can guide you through the key points of how to use them, but you should be on the lookout for exceptions.

Taking a picture manually

Shutter speed and aperture

To take a picture with full manual control, turn the camera's mode dial to M (Figure 3.2). Set the shutter speed with the thumbwheel.

To set the aperture, some cameras have a second thumbwheel, and others have you turn the one and only thumbwheel while holding down the +/− button. Note that on Canon lenses there is no aperture ring on the lens; you can only set the aperture by electronic control from within the camera. Most Nikon lenses have an aperture ring, for compatibility with older manual cameras, but with a DSLR, you should set the aperture ring on the lens to the smallest stop (highest number) and control the aperture electronically.

Naturally, if there is no lens attached, or if the camera is attached to something whose aperture it cannot control (such as a telescope), the camera will not let you set the aperture. You can still take pictures; the computer inside the camera just doesn't know what the aperture is.

Manual focusing

In astrophotography, you must always focus manually.

CCD and CMOS sensors

Electronic image sensors work because light can displace electrons in silicon. Every incoming photon causes a valence electron to jump into the conduction band. In that state, the electron is free to move around, and the image sensor traps it in a capacitive cell. The number of electrons in the cell, and hence the voltage on it, is an accurate indication of how many photons arrived during the exposure. Modern sensors achieve a quantum efficiency near 100%, which means they capture an electron for nearly every photon.

The difference between CCD and CMOS sensors has to do with how the electrons are read out. CCD stands for charge-coupled device, a circuit in which the electrons are shifted from cell to cell one by one until they arrive at the output (Figures 11.1, 11.2); then the voltage is amplified, digitized, and sent to the computer. The digital readout is not the electron count, of course, but is exactly proportional to it.

CMOS sensors do not shift the electrons from cell to cell. Instead, each cell has its own small amplifier, along with row-by-column connections so that each cell can be read out individually. There is of course a main amplifier along with other control circuitry at the output.

Which is better? Originally, CCDs had the advantage; CMOS image sensors were designed to be made more cheaply, with lower-grade silicon.

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.

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.

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).

This chapter will tell you how to start with raw image files from your camera, perform dark-frame correction, decode the color matrix, combine multiple images into one, and carry out final adjustments.

Vita brevis, ars longa. Digital image processing is a big subject, and I don't plan to cover all of it here. In particular, in this and the following chapters I'm going to skip almost all of the mathematics. To learn how the computations are actually done, see Astrophotography for the Amateur (1999), Chapter 12, and other reference books listed on p. 195.

This is also not a software manual. For concreteness, I'm going to give some specific procedures for using MaxDSLR (including its big brother MaxIm DL) and, in the next chapter, Adobe Photoshop, but in general, it's up the makers of software to tell you how to use it. My job is to help you understand what you're trying to accomplish. Many different software packages will do the same job equally well, and new software is coming out every day.

How to avoid all this work

Before proceeding I should tell you that you don't have to do all this work. A much simpler procedure is to let the camera do most of it for you. Here's how:

• Turn on long-exposure noise reduction in your camera. That way, whenever you take a celestial photograph, the camera will automatically take a dark frame and subtract it.

• Tell the camera to save the images as JPEG (not raw).

• […]

A few years ago, I said that if somebody would manufacture a digital SLR camera (DSLR) that would sell for under $1000 and would work as well as film for astrophotography, I'd have to buy one.

That happened in 2004. The Canon Digital Rebel and Nikon D70 took the world by storm, not only for daytime photography but also for astronomy. Within two years, many other low-cost DSLRs appeared on the market, and film astrophotographers switched to DSLRs en masse.

There had been DSLRs since 1995 or so, but Canon's and Nikon's 2004 models were the first that worked well for astronomical photography. Earlier digital cameras produced noisy, speckled images in long exposures of celestial objects. Current DSLRs work so well that, for non-critical work, you almost don't need any digital image processing at all – just use the picture as it comes out of the camera (Figure 1.1). The results aren't perfect, but they're better than we often got with film.

As you move past the beginner stage, you can do just as much computer control and image enhancement with a DSLR as with an astronomical CCD camera. Some hobbyists bring a laptop computer into the field and run their DSLR under continuous computer control. Others, including me, prefer to use the camera without a computer and do all the computer work indoors later.

The video astronomy revolution

The reason DSLRs are not used for high-resolution lunar and planetary work is that another, much cheaper, instrument works much better. Just before the DSLR revolution came the video astronomy revolution. By aligning the best frames from a video recording, it suddenly became possible for amateurs with modest telescopes to take pictures like Figure B.1, which were, until then, almost beyond the reach of any earth-based telescope.

The resolution of planetary images is limited by the turbulence of the atmosphere. By aligning hundreds or thousands of images, the video astronomer can see right through the turbulence. Air movements that are different in every frame cancel each other out, and what's left is what all the frames have in common, namely the true appearance of the planet.

What's more, imperfect tracking is actually an advantage. If the planet image moves all over the sensor during the recording, no single pixel or dust speck will have much of an effect.

Using a webcam or video imager

A webcam is a cheap CCD camera that delivers a continuous stream of video to the computer by USB or FireWire connection. These are commonly used for low-cost videoconferencing. For astronomy, the lens is unscrewed and replaced with an eyepiece tube adapter (Figure B.2). Because the IR-blocking filter is in the lens, which was removed, it is desirable to add a UV/IR-blocking filter at the end of the eyepiece tube.

Webcam adapters are sold by many telescope dealers.

Focal reducers are invaluable for deep-sky work with DSLRs because they make the image smaller and brighter. Since the DSLR sensor is smaller than 35-mm film, you can switch from film to a DSLR, add a focal reducer, and cover the same field with a brighter image.

The most popular Meade and Celestron focal reducers multiply the focal length and f-ratio by 0.63 (giving f/6.3 with an f/10 telescope). That's a handy reduction factor because it shrinks an image from the size of 35-mm film to the size of an APS-C sensor. What's more, the image comes out (1/0.63)2 = 2.52 times as bright, cutting the exposure time to 40% of what it would have been.

But focal reducers are sadly misunderstood, and they don't always work the way the users expect. In what follows, I'll try to clear up some misconceptions.

Key concepts

The first thing to understand is that a focal reducer makes the image smaller – it doesn't make the field wider. It doesn't turn your telescope into a wider-field instrument than it was originally.

It's true that the field of view increases when the image becomes smaller, because more of the image fits on the sensor. But that is only true of the image that the telescope captured in the first place. A focal reducer will not make the telescope see things that were outside its field altogether.

That is one reason some vignetting is almost inevitable with a focal reducer.