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Beginner's Guide to Astrophotography

Beginner's Guide to Astrophotography
The Moon is an easy target for simple astrophotography, with or without a telescope. (Image credit: Bill Bradley)

You may have seen some of the beautiful photographs of sky objects that grace and fill the pages of astronomy magazines. Perhaps you've even wondered whether it might be possible for you to take photos of the night sky. The answer is most definitely "yes."

In fact, making the transition from normal photography to astrophotography is relatively easy. Astrophotography is a special adaptation of ordinary photography. You can make some interesting photographs of stars and planets even with inexpensive equipment.

Film vs. digital

Over the last decade, digital photography has overtaken film photography. Among the reasons: 

Digital is far more convenient and offers great quality for photojournalism and portraits; digital replaced film by the late 1990s for most big-city newspapers. Most people get better results with digital cameras, although some hard-core photographers still prefer the look of film. Admittedly, film takes much more work, but it can surpass digital for large prints and reproduction where textures in nature and landscapes are important. 

If you're an "old timer" like me, you may still own an "old reliable" 35 mm single-lens reflex camera, but taking astrophotos can be a bit of a problem if you can't readily obtain the film that you desire.  While less than a decade ago, you might have paid a visit to your local drug store to pick up a box of high-speed film, today you probably have to make a concerted effort to hunt down that film at a large camera store or order it online through a professional photography outlet. This is probably the main reason that most folks today are using general purpose digital cameras.  While film is not likely to go completely away anytime soon, just purchasing it has become a real chore!

Digital photography on the other hand, is a process where pictures are taken to a computer disk or memory card rather than film. The image is focused on a computer chip inside the camera and is instantly converted to an image that you can see on the screen on the back of your camera.

Unlike film cameras, digital cameras have no film to develop. If you take a picture you don't like, you see it right away and you can delete it and take a new one. You save only the best pictures and get rid of the ones you're not happy with.  To this end, I could probably fill two large waste baskets of all the sky images I've taken over these many years, of "puff ball" stars (camera out of focus), overexposed images of the moon, or simply a case of accidentally jostling the camera or kicking a tripod leg in the dark while taking a time exposure.  Nobody is perfect and we've all done it at one time or another.  But with a digital camera it's as if those mistakes never happened.       

Bill Bradley, Corresponding Secretary of New York's Amateur Observers' Society agrees:

"What I enjoy most is taking Wide Field Images with a DSLR or point-and-shoot digital camera. I always use a tripod and there is instant gratification because your image is on the screen immediately. With the DSLR I usually open the shutter all the way, bracket, focus on infinity, use the lowest f-stop and the cable release and click away. It doesn't matter how many images I take because it doesn't cost me anything. I can continue shooting until my flash card or memory card is full, which has almost never happened especially now that memory cards have gotten so cheap and come in sizes larger than 16 Gigabytes."

The basics

A fixed 35 mm camera should be mounted on a sturdy tripod to hold it rock steady. Try also to use a tripod with a three-way pan head. This makes it easier to aim your camera and frame the desired field. If you don't have a tripod, you might want to try a beanbag and place it on something solid that won't move. 

For photographing stars and constellations, you'll need to take a long exposure. 

The camera lens' focal length determines how big the star field will be on the film. This is called the photographic scale. Typically, a standard 50 mm lens can cover a swath of sky roughly 30 by 50 degrees: large enough to capture star patterns like the Big Dipper or Orion. I would also suggest a cable release or wireless remote or self timer to allow tripping the shutter and locking it open without physically touching the shutter button, which can cause vibration. Don't try to hold the shutter button down with your finger ? jiggling of the camera is almost inevitable.

The camera shutter is opened to the widest aperture at which sufficiently good definition is obtained. Remember that the smaller the number, the larger the hole in the diaphragm.  Usually, the widest opening is f/2.8. Some photographers advise going not to the widest aperture, but rather down one f-stop (to f/4) for slightly improved sharpness. Note however that a stopped-down lens passes less light.

The lens should be set at infinity (when very distant objects appear sharp in the viewfinder).  On most film cameras, you can manually make this adjustment by setting the ring of the lens to infinity, but with some digital cameras this procedure can be trickier.  For some cameras, you might have to go through the manual or menu and see if you can override the autofocus feature and manually focus on infinity.  If you are using a point and shoot camera, the infinity setting might be indicated by some kind of icon resembling mountains; try shooting something far off in the distance to verify that this setting works.

Exposure length

Many point and shoot cameras can go up to 15 to 30 seconds.  With longer exposures, the turning motion of Earth will cause the star images to appear as short streaks instead of points. Star trailing is greatest for stars on or near the celestial equator, where the sky appears to move the fastest. Thus, a 40-second exposure of Orion, an equatorial group, shows about twice as much trailing as one of the Big Dipper.

To this end, it's preferable that you use a unit which has a BULB setting, that will allow you to leave the camera shutter open for an indefinite period of time. Here, however, is where an old fashioned film camera has an advantage over digital, because so long as the shutter on a digital camera remains open its CCD will need to be powered.  In contrast, if you're shooting with film you can leave the shutter open all night and consume no battery power. So make sure if you are using a digital camera to have a fresh battery (especially if it's a cold night) or use some sort of continuous power.

Experiment with exposure length. Astronomers prevent star trailing by using "clock drives" to compensate for the Earth's motion. Mounting your camera on a clock drive is the next big step in astrophotography, should you decide to get more involved.

In big cities, longer exposures unfortunately bring out haze from nearby bright lights, which obliterate faint star images after only a few minutes. Also watch out for clouds drifting toward your camera field.

How fast?

If you're old fashioned and can find a photography shop that still sells film, you should look for an ISO rating above 400 to record the greatest number of stars. Keep in mind that faster color films usually give less vivid star colors. If you try an extra-fast color film, like ISO 1600, you'll get brighter but paler stars. On a digital camera you can adjust the ISO setting. In simple cameras it generally runs from 100 to 400; for more expensive cameras

it can go as high as 1600 or even 3200. 

If you still prefer film . . .

Your night sky pictures will be unusual to a developer, so some tips are in order.

At first glance your sky pictures may look strange to a technician - like they're blank with nothing on them - and your film may be returned simply as negatives with a "better luck next time" insert.

So before taking any pictures of the night sky, start your roll by photographing something while it is still daylight. This first picture will show the technician where to begin cutting the film. Also, be sure to give instructions that you want every frame printed.

Using a telescope

You might wish to try to capture close-up images of the moon through a telescope. Digital cameras are good in this regard for they can record bright objects that can stand some magnification.

You can try pointing your camera at the eyepiece of the telescope and just click the shutter. The moon is bright enough so that you can get an image in this manner.  Likely your first attempts will streak a bit because your hands might not be steady enough.  This problem can be rectified with a T-mount. 

A T-mount is a two piece adapter system designed to hook cameras to telescopes and spotting scopes. The first piece is called a T-ring and attaches to the camera. For most removable lens film cameras the T-ring attaches to the camera in place of a lens, but for cameras with non-removable lens, which is virtually all amateur level digital cameras, the T- ring attaches in front of the lens where a filter would go.

Of course, to get the best image, try a variety of aperture settings and exposures. With a film camera, you might end up taking numerous exposures, most of which would end up in the garbage.  But with a digital camera, all you need do it examine your image on the screen; if you don't like how it came out, just delete it, change your setting and try again.

That kind of photography should work for anyone.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

Joe Rao
SPACE.COM SKYWATCHING COLUMNIST — Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications, and he is also an on-camera meteorologist for Verizon FiOS1 News in New York's lower Hudson Valley.

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