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Taking dramatic pictures of the stars and Milky Way is easy if you have an equatorially mounted telescope (Chapter 4). Load your camera with color slide film that has low reciprocity failure and a strong response to deep red light; Kodak Elite Chrome 200 or E200 Professional is an excellent choice. Attach the camera to the telescope somehow, either on a piggyback bracket or by any convenient means. The camera and telescope need not point in exactly the same direction, though they should be reasonably close.

On a clear moonless night, take a 2- to 5-minute exposure of the sky with the camera lens wide open, while the telescope tracks the stars. With a 100-mm or shorter lens, you will probably not have to make guiding corrections. Use shorter exposures under a bright town sky, longer exposures in the country. Figures 7.5 and 7.6 show what you can achieve.

Figure 7.5. Dramatic view of the Milky Way in Sagittarius. No guiding corrections were needed during this 2-minute piggyback exposure on Kodak Elite Chrome 200 slide film with a 50-mm lens at f /2.8.

Figure 7.6. The Belt of Orion and Orion Nebula. One-minute piggyback exposure (without guiding corrections) on Elite Chrome 200 with a 90-mm lens at f /2.8, under a town sky. In the country, a longer exposure would bring out far more nebulosity.

If, on the finished slides, the stars are elongated north to south, your polar alignment wasn't accurate enough; if they are elongated east to west, your tracking wasn't smooth enough. Either way, guiding corrections during the exposure will alleviate the problem.

Can you do anything like this on an altazimuth mount? Maybe, if your tracking motors are fairly smooth. The trick is to choose a star field fairly low in the eastern or western sky, where field rotation is at a minimum (p. 39), and expose for no more than two or three minutes. That should be enough to whet your appetite for an equatorial wedge.

Why do I recommend slide film? So that you won't be at the mercy of the people or machines that make the prints. Most photo labs do not know how to print negatives of star fields; the pictures come out either too light or too dark. With slides, you can see what the camera actually recorded, and you can then order custom prints that match the slides.

7.4 Equipment for astrophotography

7.4.1 Telescope requirements

You can do lunar and planetary photography with almost any telescope as long as the mount is steady. Deep-sky work requires an equatorial mount (or a fork mount on an equatorial wedge) to prevent field rotation.

Schmidt-Cassegrain and Maksutov-Cassegrains are the easiest to couple to cameras because you can easily put the focal plane where you want it, right at the eyepiece or deep inside a camera body. Refractors are almost as easy to work with. As already noted, Newtonians are less versatile than other types because their focal plane cannot be placed very far beyond the end of the eyepiece tube; that means compression and direct coupling are likely to be impractical, but afocal and projection photography are no problem.

When coupled directly to a camera body, very few telescopes fill the field of a 35-mm camera; after all, they are designed to work with eyepieces a good bit smaller than 35-mm film. Two-inch eyepiece tubes help somewhat, but there is usually still some vignetting.

The 10-inch and larger Meade LX200s, and the 11-inch and larger Celestrons, are exceptions. These telescopes have a back that matches that of the 8-inch, but the central part of it can be removed to give a larger opening. Camera adapters that make use of the larger opening are available from Lumicon (2111 Research Drive, Livermore, CA 94550, U.S.A., http://www.lumicon.com) and other suppliers; they produce images that are free of vignetting in direct mode and only slightly vignetted when a compressor is used.

Smooth tracking and a steady mount are vital for deep-sky work. Many of the less expensive computerized telescopes, such as the Meade ETX-90, are not designed for photography and are usable only with difficulty. Others, such as the Meade LX200 and Celestron Ultima 2000, are fine photographic instruments. For smooth tracking, you need a well-made worm-gear drive, preferably with periodic-error correction (PEC, p. 53) so that the irregularities in the gears can be memorized by the computer and counteracted automatically.

7.4.2 35-mm SLR cameras

Most amateur astrophotography is done with 35-mm single-lens reflex (SLR) cameras. As Figure 7.7 shows, an SLR has a built-in mirror and focusing screen so that you can see the actual image formed by the lens or telescope. The mirror flips up before the shutter, behind it, opens to make an exposure.

For astrophotography, older SLRs are better than the most modern ones. New features such as autofocus and auto exposure are not needed. Instead, what you need is full manual control, combined, if possible, with the ability to make long exposures without running down the batteries (though a supply of extra

Figure 7.7. Cross-section of a 35-mm SLR. The mirror intercepts light for focusing on the screen, then flips out of the way before the shutter opens. (Olympus, reproduced by permission.)

batteries may be cheaper than a new camera). The ability to change focusing screens is useful; if the screen is not interchangeable, you will have to make do by focusing on the smoothest part of it, not the central prism or split-image device.

To keep the camera from shaking when the mirror flips up, it is handy to have either mirror lock or mirror prefire. Mirror lock means there is a separate button to flip the mirror up in advance of the exposure. Mirror prefire means that when you make a delayed exposure with the self-timer, the mirror flips up at the beginning of the cycle, several seconds before the shutter opens.

People often ask me which SLR is best for astrophotography. The truth is that almost any SLR can be used to some extent, so if you already have a camera, see what you can do with it. Also look for older cameras (vintage about 1970) that may have been languishing in your relatives' or friends' closets; almost all SLRs from the 1970s are good choices. For a chart of many suitable camera models and their features, see Astrophotography for the Amateur. But before shopping for a new camera, do as much as possible with whatever you already have.

7.4.3 Other film cameras

Some amateurs do astrophotography, particularly piggybacking, with mediumformat SLRs such as the Hasselblad, Bronica, Mamiya, and Pentax. The lack of grain in larger-size negatives is helpful.

For piggybacking and afocal coupling, the camera need not be an SLR. It is sufficient that you be able to focus it to infinity and control the exposure manually. For afocal photography, the telescope can be focused with a separate hand-held telescope (p. 105). I have done both piggybacking and afocal photography with a scale-focusing Voigtlander Vito B that is more than 50 years old.

Unfortunately, most automated "point-and-shoot" cameras are completely unsuitable. The exposure cannot be controlled manually, and the lens is, in any case, tiny - often f /9 or slower. If you want a versatile camera at a low price, look for a fully adjustable one from the 1970s or earlier.

7.4.4 Digital and video cameras

You can get good images of the Moon and planets with an ordinary digital or video camera aimed into the eyepiece of the telescope. It is easy to focus and adjust the exposure because you can see immediately what you are getting. The camera focus should be locked at infinity so that all focusing can be done with the telescope.

Some digital cameras give you manual control of exposure. Others let you adjust exposure indirectly with a "lighten/darken" adjustment. In some cases, the camera may insist on overexposing a small planet image seen against a dark background; in that case your only resort may be to add a neutral density filter between the eyepiece and the camera.

Figure 7.8. Mars. Olympus 2.1-megapixel digital camera aimed into 9-mm eyepiece on an 8-inch Meade LX200 (222 x). This is the best of several images, and it was enhanced with Adobe Photoshop LE.

Digital cameras and camcorders are not suitable for deep-sky work. The reason is that their CCDs (charge-coupled device image sensors) are not cooled, and in exposures longer than about one second, there is excessive noise, which shows up as a speckled pattern.

7.4.5 Astronomical CCD cameras

CCD cameras designed specifically for astronomy use thermoelectric coolers to keep the image sensor at a low temperature, where it is less subject to noise (random leakage of electrons), so that long exposures are possible (Figure 7.9). These cameras perform impressively on both planets and deep-sky objects, and because the CCD's response to light is perfectly linear, it is relatively easy to subtract out the effects of city lights, making it possible to image faint nebulae and galaxies even in the suburbs.

CCD cameras for amateur use are made by Meade; SBIG (Santa Barbara Instrument Group, 147-A Castilian Drive, Santa Barbara, CA 93117, U.S.A., http://www.sbig.com); Apogee (11760 Atwood Road, #4, Auburn, CA 95603 U.S.A., http://www.ccd.com); Starlight Xpress (Ascot Road, Holyport, Berkshire, SL6 3LA, U.K., http://www.starlight-xpress.co.uk); and other companies. CCD cameras generally have to be connected to a laptop computer during use, but the SBIG STV includes its own controller and video screen.

Professional astronomers switched from film and plates to CCDs in the 1980s, and amateur astrophotographers are following suit. Unfortunately, CCDs within reach of amateur budgets give rather small images, often as small as 320 x 200 pixels; if you like big prints, you should (at least at present) stick to film. Soon, however, 1000 x 1000-pixel or 2000 x 2000-pixel imaging will be within reach of amateur budgets, and then film will face serious competition from CCDs.

(42 X 0.75 mm) housing comPuter

Figure 7.9. An astronomical CCD camera includes a thermoelectric cooler to permit long exposures. (From Astro-photography for the Amateur)

(42 X 0.75 mm) housing comPuter

Figure 7.9. An astronomical CCD camera includes a thermoelectric cooler to permit long exposures. (From Astro-photography for the Amateur)

Figure 7.10. CCD image of the walled plain Janssen on the Moon, taken with an SBIG STV coupled directly to an 8-inch (20-cm) Meade LX200 telescope. The air was generally unsteady, but the camera was able to make the most of a moment of steadiness.

One important advantage of CCD cameras for lunar and planetary work is that it is easy to take large numbers of images and keep only the best. Some cameras can even select the best images automatically during the observing session. This makes it possible to seize brief moments of atmospheric steadiness just as an experienced visual observer does (Figure 7.10).

CCD cameras invariably plug into the telescope in place of the eyepiece; a Barlow lens or a compressor can be used ahead of the camera to change the image size. Almost all CCD cameras can also be used as autoguiders (p. 118).

7.5 Focal length, image size, and f -ratio

7.5.1 Finding the effective focal length

Just how much magnification does your setup give you? In a sense, the "magnification" of a picture is almost meaningless because any picture that you take of a planet will be far smaller than the planet itself. Instead, what you want to know is the field of view or the image size. To find that, you must first find the effective focal length (EFL):

• With an ordinary camera lens (as in piggybacking), the EFL is simply the focal length.

• With direct (prime focus) coupling, the EFL is the focal length of the telescope. For example, the focal length of a Meade ETX-90 is 1250 mm. (Throughout this book, I give all focal lengths in millimeters, no matter how large.)

• With afocal coupling:

EFL = Telescope magnification x Camera lens focal length

For example, a camera with a 50-mm lens looking into a 100 x telescope has an EFL of 5000 mm. With digital and video cameras, the focal length of the lens is often unknown, but the field of view is easy to determine by direct experimentation. With positive projection:

where F is the eyepiece focal length and S is the distance from the eyepiece to the film. For example, with a Meade ETX-90 (focal length 1250 mm) and a 25-mm eyepiece located 125 mm from the film, the EFL is 1250 x (125 — 25)/ 25 = 1250 x 4 = 6000 mm.

With negative projection, if you're using a teleconverter, you can rely on it to give exactly its rated magnification. Thus a telescope of 1250 mm focal length and a 2x teleconverter will have an EFL of 1250 x 2 = 2500 mm.

With a Barlow lens, the best thing to do is experiment, since you normally do not know the focal length of the Barlow lens. As a rule of thumb, most Barlows give 50% to 100% more magnification when used ahead of a camera than when used ahead of an eyepiece, since the image plane is further from the Barlow lens. For example, a 2x Barlow might multiply the telescope focal length by 3 or 4. To get a more exact value, photograph the Moon and measure the size of the image.

With compression, the focal length obtained with a particular compressor lens is generally specified by the manufacturer. For example, Meade's f /6.3 compressor for f /10 telescopes multiplies the telescope focal length by 0.63.

7.5.2 Image size and field of view

The size of the image on the film depends on the apparent size of the celestial object and the EFL, thus:

Apparent size of celestial object (arc-seconds) ,

For example, the apparent diameter of Jupiter is typically 40", so with an EFL of 5000 mm, you get images of Jupiter that are (40/206 265) x 5000 = 0.9 mm in diameter. It follows that you need an EFL of at least 5000 mm to get usable images of the planets, and even then, the images are small.

There is a related formula for finding the field of view. Let's assume that the usable area of a piece of 35-mm film is about 19 mm in diameter; that is reasonable because there is usually some vignetting and edge blurring. Then the formula simplifies to:

Field of view (arc-seconds) =

For example, with an EFL of 1250 mm, the field of view is 3200", and the Moon, which is 1800" in diameter, fits comfortably into the picture. As a rule of thumb, use EFLs between 1000 and 2000 mm to photograph the full face of the Moon; use longer EFLs to enlarge selected areas.

7.5.3 Finding the f -ratio

To calculate exposures, you need to know the f -ratio of the complete system. Fortunately, that's easy:

Focal length (mm)

Telescope aperture (mm)

Note that the focal length and aperture must be given in the same units.

For example, consider again the Meade ETX-90 with eyepiece projection giving an EFL of 6000 mm. The aperture is 90 mm, so the f -ratio is 6000/90 = 66.7. That is, you have turned a telescope into a 6000-mm f /66.7 telephoto lens. High f-ratios, f/45 and higher, are common in lunar and planetary work.

7.5.4 Exposure, film, and development

In astrophotography, you have to buy your film by name, not just by speed. For example, Kodak Elite Chrome 200 and Kodachrome 200 are both 200-speed color slide films, but they are as different as night and day. Kodachrome 200 has severe reciprocity failure, which means that it loses speed at low light levels, in long exposures. In a ten-minute exposure it is a great deal slower than Elite Chrome 200, which has very little reciprocity failure.

The fastest films are not the best, for several reasons. Faster films generally have more reciprocity failure, so Elite Chrome 400 is actually worse than Elite Chrome 200 in long exposures. Faster films also have more grain, which is undesirable since it hides star images and planetary detail.

Films also differ in their response to the hydrogen-alpha wavelength, 656 nm, in the deep red. That is the main emission from the Lagoon Nebula, North America Nebula, and their kin. On some films, such as Elite Chrome 200, these nebulae are bright red; on other films, such as Tri-X Pan, they hardly show up at all. The Kodak Elite Chrome films have a strong response to hydrogen-alpha; so do the Kodak Supra (not Portra) color negative films. Fuji color films have a weaker response to hydrogen-alpha, and most black-and-white films have no response at all.

One exception is Kodak Technical Pan film, which responds strongly to hydrogen-alpha but requires treatment with hydrogen gas ("hypering") to increase its speed and reduce reciprocity failure. Hypered Technical Pan was a mainstay of astrophotography in the 1980s; today, the best color films pick up nebulae almost as well and are much easier to work with.

Table 7.1 Exposure table for Kodak Elite Chrome 200 (E200)film, roughly correct for other 200- and 400-speed films f- ratio

Table 7.1 Exposure table for Kodak Elite Chrome 200 (E200)film, roughly correct for other 200- and 400-speed films f- ratio

Object

2.8

4

5.6, 6.3

8

10,11

16

32

64

128

Moon (thin crescent)

1/125

1/60

1/30

1/15

1/8

1/4

1 sec

4 sec

Moon (half)

1/500

1/250

1/125

1/60

1/30

1/15

1/4

1 sec

4 sec

Moon (full)

1/2000

1/1000

1/500

1/250

1/125

1/30

1/8

1/2

Moon (partial eclipse)

1/125

1/60

1/30

1/15

1/8

1/4

1 sec

4 sec

Moon (total eclipse)

1 sec

2 sec

5 sec

10 sec

20 sec

Sun (through Baader visual filter)

1/2000

1/1000

1/500

1/125

1/30

1/8

Sun (total eclipse, no filter)

1/60

1/30

1/15

1/8

1/4

1/2

Mercury

1/125

1/60

1/30

1/4

1 sec

4 sec

Venus

1/2000

1/1000

1/500

1/125

1/30

1/4

Mars

1/250

1/125

1/60

1/8

1/2

2 sec

Jupiter

1/125

1/60

1/30

1/4

1 sec

4 sec

Saturn

1/15

1/8

1/4

1 sec

4 sec

Comets, bright nebulae (typical)

2 min

4 min

8 min

15 min

30 min

Galaxies, faint nebulae (typical)

6 min

12 min

24 min

Ihr

Table 7.1 gives exposures for a variety of celestial objects on Elite Chrome 200. The table is approximately valid for most other 200- and 400-speed films. When in doubt, vary your exposures, using the table only as a rough guide. There is no specific "correct" exposure for most astronomical photographs; the best exposure depends on what you want the picture to look like. For more exposure tables and explanations of how exposures are calculated, see Astrophotography for the Amateur.

7.6 Focusing and sharpness

Good lunar and planetary photography requires trying the same thing over and over until you finally get a picture in which the air is steady at the right moment. It also requires accurate focusing and freedom from vibration.

Consider focusing first. A camera attached to a telescope is much harder to focus than the same camera with an f /1.8 lens. The image does not snap into focus at any particular point. Instead, finding the point of best focus requires careful attention. The focus knob may have backlash; that is, the correct position may depend on whether you were last turning it clockwise or anticlockwise. The image through the telescope at a long EFL is sure to be somewhat blurry no matter how carefully you focus.

The central split-image or microprism area in an SLR focusing screen is useless with telescopes. Instead, focus on the smoothest matte area of the screen, typically a ring surrounding the central spot. If at all possible, change to a focusing screen that has a fine matte surface all over, such as a Nikon B screen or a Beattie Intenscreen.

Other types of focusing screens, such as clear screens with crosshairs, are popular with advanced astrophotographers, but their usage is tricky. Remember that the purpose of a focusing screen is not to make the image look sharp; it is to make it look blurred when it is out of focus. Images on clear-crosshairs screens tend to look like they are in focus when they're not.

If you find focusing difficult, check whether you can see the focusing screen clearly. Perhaps the camera eyepiece is not in focus for your eyes; a corrective lens can be added, or you can use an adjustable magnifier. The Olympus Varimagni Finder fits not only Olympuses, but also most Minoltas, Pentaxes, and Yashicas, among others (but no Nikons).

Finally, remember that not all blurriness is caused by incorrect focusing. Shutter vibration is a serious challenge to the lunar and planetary photographer. I usually get around it by choosing a film and f -ratio so that the exposure is 4 second or more, and then doing a hat trick. That means holding my hat or a large black card in front the telescope; opening the shutter; then carefully moving the hat or card away and back again. The hat or card functions as a vibrationfree shutter. In deep-sky work, shutter vibration is less of a problem because when the exposure lasts several minutes, a millisecond or two of vibration is only an infinitesimal part of it.

7.7 Deep-sky techniques

Deep-sky photography is at once easier and harder than lunar and planetary work. It's easier because you have a better chance of getting a good picture on the first try. It's harder because additional equipment and skills are needed.

As already noted, you need an equatorial mount or wedge. You also need an eyepiece with crosshairs for guiding. No matter how good your mount is, the drive motors in it are not perfect, and you will need to make manual corrections in the east-west direction to smooth out the motion. Corrections in the north-south direction will counteract small errors in polar alignment.

Figure 7.11. The Orion Nebula (M42). Five-minute exposure with an 8-inch Meade LX200 and a compressor giving effective f /5.6, autoguided with an SBIG STV CCD camera at the eyepiece position of an off-axis guider. Kodak Elite Chrome 200 film pushed one stop.

Figure 7.12. The North America Nebula and other nebulosities in Cygnus. Twenty-minute piggyback exposure on Ektachrome Elite II 100 film pushed two stops, using a 90-mm lens at f /2.8 and a broadband nebula filter. The slide was scanned and processed digitally to increase contrast.

For piggybacking, guiding is easy: mount the camera on top of the telescope and watch a star through the telescope itself, keeping it on or near the crosshairs. Guiding tolerances are discussed in detail in Astrophotography for the Amateur; suffice it to say that when piggybacking with a medium telephoto lens, you do not have to keep the star precisely on the crosshairs, merely near them.

I have had good results piggybacking with a NexStar 5, a lightweight telescope not designed for photography. The secret to good guiding is to make sure the tracking is set to "EQ North" so that only one motor is running; set the slewing rate to the lowest value; and turn off backlash compensation so there will be no sudden jerks. It is better for guiding corrections to be delayed than for them to be too sudden or irregular. On more advanced telescopes such as the Meade LX200, guiding goes very smoothly.

When you are photographing through the telescope, guiding is more of a challenge. For one thing, there is no room for error; the star must stay on the crosshairs. But the bigger question is, if you are using the main telescope for photography, how do you guide?

One solution is to use a separate guidescope. This tactic works well with refractors and reflectors, provided the guidescope has a high enough magnification

Figure 7.13. An off-axis guider intercepts a portion of the main image that would not fall on the film. Finding a suitable guide star is often difficult.

(200x is sufficient; 500x is better, though it's far too high for real observing). A Barlow lens ahead of the guiding eyepiece helps improve precision.

Separate guidescopes do not work so well with Schmidt-Cassegrains and Maksutov-Cassegrains because the mirror tends to shift slowly as the telescope tilts while following a star. Thus the image in the telescope undergoes a movement that the guidescope does not see.

The solution is to use an off-axis guider (sometimes abbreviated OAG), intercepting a small part of the image that would not fall on the film, so that you can guide on the same image that the camera is photographing (Figure 7.13). The hard part about using an OAG is finding a suitable guide star; all too often it seems that there is nothing brighter than twelfth magnitude in the right place. The more ways you can adjust the OAG to change the view, the better. Finding guide stars is much easier when you are photographing star clusters and nebulae in rich portions of the Milky Way than when you are photographing galaxies.

Guiding is tedious, but, fortunately, most CCD cameras can do it for you; a CCD camera used for this purpose is called an autoguider. Meade makes a low-end CCD camera, the 201XT, that can only guide on relatively bright stars; it's small and inexpensive. I use an STV from SBIG (the Santa Barbara Instrument Group), which autoguides very well and reports its accuracy as it goes. Either of these goes in place of the crosshairs eyepiece that you would otherwise use. An exception is the Apogee Lisaa, whose housing can be set up to work as an off-axis guider assembly in front of a 35-mm camera or another CCD.

But when you are taking a CCD image, how do you guide? Unfortunately, the same CCD cannot make guiding corrections and expose an image at the same time. One solution is to use a second CCD in an off-axis guider - a time-honored but expensive practice. More recently, several vendors have found ways to put a single CCD to two uses. Starlight Xpress cameras can track and record at the

Figure 7.14. 320 x 200-pixel image of the globular cluster M13. SBIG STV CCD camera, 8-inch telescope with f /5.6 compressor, combination of three 15-second exposures in track-and-accumulate mode.

same time, using alternate rows of pixels on the same CCD. The SBIG STV can "track and accumulate", which means that it makes a short exposure, checks for image shift, makes another short exposure, shifts it as needed to match, and adds the two together, over and over.

7.8 Digital image processing

The CCD revolution is almost insignificant compared with the digital image processing revolution. Whether they originated on film or on CCD, astronomical images can now be processed by computer to enhance contrast, correct color, and bring out faint details. Although there is plenty of special software for the purpose, such as Software Bisque's CCDSOFT, you can also do very good work with Adobe Photoshop Elements (formerly Photoshop LE) and similar products.

The most important parameter of a digital image is the number of pixels. As a rule of thumb, a 200 x 300-pixel image looks good when filling a small part of a computer screen. For a whole computer screen or a sharp postcard-sized image, you need at least 900 x 1200 pixels. For large prints, you need even more.

Most CCD images are on the order of 200 x 300 or 400 x 600 pixels. Thus, one of the first steps in processing them is to resample (enlarge) the image to increase the number of pixels. This does not bring out additional detail, but it does decrease the stairstep or boxlike appearance of fine detail in the image.

Film images, on the other hand, already have plenty of pixels. A 35-mm slide or negative, scanned with a good film scanner, can easily comprise 2400 x 3600 pixels (8.6 million pixels, 8.6 megapixels). It is often better to scan at half that resolution, producing a 1200 x 1800-pixel (2.1-megapixel) image that does not show film grain.

Film should always be scanned on a film scanner, not on a flatbed scanner with an adapter. Although some flatbed scanners are now almost good enough, the first generation of them did not have nearly enough resolution to do justice to film. Flatbed scanners usually have a true resolution of 300 to 600 pixels per inch (12 to 14 pixels/mm). (Interpolated resolution doesn't count; it is created by resampling the image and does not pick up additional detail.) Film scanners resolve 2400 pixels per inch (90 pixels/mm) or more and pick up all the detail that the film records. A film scanner also needs enough dynamic range (brightness range) to capture both the brightest and the darkest areas of the picture. Negatives have less dynamic range than slides and are easier to scan.

Once you have the image in the computer, what do you do with it? Figures 7.15 and 7.16 show two basic operations. You can sharpen the image by unsharp masking - that is, by exaggerating the differences between adjacent pixels. Originally an "unsharp mask" was a blurred negative, sandwiched with a sharp positive to smooth out large gradients without hiding fine detail; today unsharp masking is done by averaging and subtraction. It brings out detail in a dramatic way, particularly on lunar images. It can also bring out film grain.

Meade Etx Piggy Back

Figure 7.15. Digital image enhancement at work. Left: Original image, taken on Fuji Sensia 100 color slide film with a 5-inch (12.5-cm) f/10 Schmidt-Cassegrain, then scanned and converted to monochrome. Right: Same, after unsharp masking and Gaussian sharpening. (From Astrophotography for the Amateur.)

Figure 7.15. Digital image enhancement at work. Left: Original image, taken on Fuji Sensia 100 color slide film with a 5-inch (12.5-cm) f/10 Schmidt-Cassegrain, then scanned and converted to monochrome. Right: Same, after unsharp masking and Gaussian sharpening. (From Astrophotography for the Amateur.)

Figure 7.16. Improving a picture of Comet Hyakutake by digital image processing. The original slide (top) was scanned, a dark spot was retouched out, unsharp masking was performed to bring out detail, and contrast was adjusted.

You can also adjust the contrast of the picture to set the black and white levels where you want them and to control the gradation in between.

Once you have created a digital image, not only can you print it out - often making a far better print than could be made in a darkroom - but you can also share it with others on the World Wide Web. Some of my pictures are on display at http://www.covingtoninnovations.com; I also maintain web links to many other astrophotographers' work.

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Reasonable care has been taken to ensure that the information presented in this book is  accurate. However, the reader should understand that the information provided does not constitute legal, medical or professional advice of any kind.

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