Hack31.Photograph the Stars with Basic Equipment


Hack 31. Photograph the Stars with Basic Equipment

Shoot star trail images.

If you have been interested enough in astronomy to read up on the subject, you've no doubt run into a number of stunning photographs of celestial objects. Many have been taken by professionalsno one with the astronomy bug can fail to fall in love with Hubble photographs. But, a large number of them are credited to amateurs, like you, with amateur telescopes, like yours, perhaps. It is then inevitable for the new (or not so new) amateur astronomer to decide that she, too, should take beautiful photographs to hang above her fireplace or to publish in one of the many fine astronomy magazines.

There is only one problem. Taking decent astrophotos is a tedious, difficult, time-consuming endeavor. It's also very expensivesuperb astrophotos increase the demands in all categories by an order of magnitude and require a pinch of luck.

I don't write this to discourage you. You will want to take astrophotos. Many experienced amateur astronomers scoff at the newbies' urge to take stunning photographs. But what they won't tell you is that they, too, long to take such photos and only through a superhuman effort of will (or the budget balancing efforts of an astrospouse) do they avoid temptation.

Astrophotography, like observing or equipment building, is a simple concept plagued with confusing details. The beautiful photo you admire in Sky & Telescope or Astronomy represents hundreds of hours and thousands of dollars invested by the photographer. You may wish to dive into astrophotography and, with perseverance, you will someday take good photos. There are a lot of methods and tips you'll need to know and you'll have to find most of them elsewhere, as none of the authors of this book have expertise in the area.

However, we can get you started.

The simplest astrophoto is known as a star trail. As Earth rotates on its axis, the stars appear to move in the sky, rising in the east and setting in the west. In so doing, they describe a circle around the celestial pole. If you photograph a group of stars for several minutes with a stationary camera, you'll capture a portion of this circle and you'll see an arca star trail.

In order to have stars appear as points, rather than trails, it is necessary to have the camera and telescope rotate in such fashion as to compensate for Earth's rotation. (An exposure long enough to show stars is long enough to cause trailing unless the camera and scope track.) This requires an equatorial mount and clock drive. Because clock drives aren't perfect, it is also necessary to guide the photograph. That is, you must watch through another telescope on the mount and keep a guide star centered on a crosshair. This sounds difficult enough as it is. Imagine trying to do it in subfreezing weather with a stiff wind. Fortunately, automated guiders are now available. If you have the money, this greatly eases the process of taking astrophotos through a telescope. In this writer's opinion, it also take a good deal of the romance out of the process, as well.


Star trail photographs come in great variety. An ever popular one is to aim a stationary camera at the celestial pole and open the shutter. In the northern hemisphere, you'll capture Polaris making a tight circle about the celestial pole (but, coolly, you will see that Polaris does, in fact, move in the sky) and the rest of the stars making larger circles about the pole. Photographers have used this as a backdrop for any number of interesting photos. I used to enjoy shooting my hometown with the stars wheeling overhead. Use your imagination here, you can come up with many neat foregrounds for star trails. Also, you need not limit yourself to the celestial pole. A shot of a constellation tracing arcs through the sky is usually interesting. Constellations with bright stars, such as Orion or Cygnus, make excellent targets in this regard (see Figure 2-22). Star-trail photography is also an excellent means of photographing bright comets and the only means of photographing meteors. Presented below are a few tips on star-trail photography. Take heart, it is very easy to do.

You'll need the following equipment to shoot star-trail images:

  • A 35mm SLR or digital camera capable of taking exposures on the Bulb (B) or Time Exposure (T) setting

  • A wide-field lens (a 28, 35, or 50mm lens is excellent for star-trail photography)

  • A stable camera tripod

  • A cable release with locking mechanism (unless your camera provides the "T" shutter setting)

  • Film (or the digital camera chip)


Lens

The lens is very important. You generally want a wide field in star-trail photography. Because the photo isn't guided, fine detail will not be captured. Therefore, zooming in will get you nothing except a larger blur rather than a smaller blur. I recommend a 35mm lens but star-trail photography also works well with a 28mm or 50mm lens.


Exposure

Astrophotos work by collecting light over an extended period of time. Your eye can only collect light for a second or two while film or chip can collect light as long as the shutter is open. This allows objects too dim to see to be recorded vividly in photos. It also makes light pollution that orange glare lifted up from city streetsa huge problem. An exposure hours long will record skyglow, whether natural or man-made, and the background in the photo will wash out. In order to make nice star-trail photos, you'll have to limit exposures so that the sky remains black in the photo. This will likely require some trial and error on your part, but take the obvious precautions. Don't set up under a streetlight. Find as dark of a location as is practical. Get out in the country and shield your camera from local lights [Hack #12] as much as possible. Depending on the sky and the film you choose, exposures may range from a few seconds, which will record only bright stars as very short trails, to several hours, which will record fainter stars and much longer trails.

Most stars appear white in standard star trail images. Try defocusing the camera slightly during part of the exposure, being careful not to move the camera when you refocus. The star trails recorded during that time will be slightly blurred and will show a range of colors, from deep red to bluish white.



Film

Star-trail photography is generally done with slower film than is used for deep-sky, guided, astrophotography. For one, the stars do not stay on the same place on the film, so collecting more light, in this case, doesn't get you a lot. That is, you aren't after really faint stars as much as a good picture of the brighter ones. Two, a fast film will "fog," or washout from skyglow, faster than a slow film. The one exception to using a relatively slow film is in meteor photography where exposures will stay short and capturing faint streaks is the goal.

For the most part, exposure and film must be considered together. If your goal is to take a five-hour star trail around the North Celestial Pole, you'll need a fairly slow film to avoid washing out the sky from skyglow. You may start with ISO 100 film and close the shutter to f/4, 5.6, or even 8. If your goal is to take a 90-second exposure of a bright comet, you'd want a faster film and probably need to go with something like ISO 1600 at f/2.8. Play around and see what you can get away with from your various observing sites.

Be sure to keep careful records. You'd hate to find a nice film/camera setting/exposure combination that gives great photos but forget what the settings were. A normal log is fine for recording such data, but you can also use a Hollywood system. Before each photo, write down the details of the next photo and photograph that note. That way, the data is right there next to your photo. (This may not be necessary if you use a digital camera; many digital cameras store exposure information inside the image file itself.)



Mount

To take the photo, place the camera on a tripod, aim to the desired point in the sky, lock the camera in place, and trip the shutter (on the "B" setting) with a cable release that holds the shutter open and then walk away. The worst thing that can happen is to bump the tripod while the shutter is open. Stand clear until it is time to close the shutter. Avoid the temptation to "check" the setup once the shutter is open. If you've screwed up somehow, you can't fix it at this point and, if you haven't screwed up, you can definitely still screw it up. The idea is to open the shutter on a camera solidly fixed in one position. A good tripod is a necessity. Obviously, a windy night is not a good night for such photography (or any astrophotography, really).


Processing

If you have a digital SLR, processing is as simple as transferring the file from the camera to your computer. You can then play around with the exposure. Darkening the sky is probably the best thing you can do for your electronic image. If you use film and have the film commercially processed, speak to the lab tech about keeping the background dark on the prints. A darker print may cost you a few faint stars, but the image will be much more pleasing. Also, you should probably ask that they not cut the negatives. Often astrophotos will not have a clear boundary between exposures and occasionally a frame may be cut down the middle by an inattentive tech.

Alternatively, start each roll of film by shooting an ordinary day-time picture. This gives the processing lab an index position, from which they can cut the roll and print other frames correctly


If you've successfully taken star-trail photos and would like to "move up," you can take piggyback photos. In this technique, you mount the 35mm SLR on an equatorially mounted, clock-driven telescope. You'll have to polar align the scope accurately. You can use slightly narrower fields of view in a piggybacked photo than in a star-trail photo, but a wide-field (28, 35, or 50mm) lens is still appropriate. In this case, you should use relatively fast film (or equivalent setting on a digital camera) and open the shutter to f/2.8 or so. Polar align the scope, center a bright star in a high power eyepiece in the scope, aim the camera, and open the shutter. If you notice the bright star in the eyepiece drifting noticeably, stop the exposure. Because your telescope has a much narrower field of view than the wide-field camera lens, you'll see drifting well before the stars streak on the film. Again, you'll have to experiment with exposures but they will likely be in the range of 115 minutes. A successful photo will show stars as points (or circles) and many, many more faint stars than you can see with the unaided eye. You may also see wispy nebulosity, as well.

Dr. Paul B. Jones