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CAMERA AND TRIPOD
One of the easiest ways to enter the field of astrophotography is to mount a camera and lens on a tripod and take a brief time exposure. The camera should have a Bulb (B) setting. This will allow you to keep the shutter open with a locking cable release. Most automatic cameras only allow for a maximum exposure of several seconds. Also, a cable release also gives you more stability. Directly pushing the shutter release will often shake the camera on your tripod.
With today’s selection of high speed films, you will be able to photograph many more stars than would be visible with the naked eye. Still, there are distinct limits to how long your exposure can be before the stars begin to look like streaks rather than points of light. This is due to the rotation of the earth on its axis. The length of time that you could expose the film before trails start to appear depends on the focal length of the lens you are using and the section of the sky you are photographing. Stars near the pole move a much shorter distance than do stars near the celestial equator.
Below is a table that lists the maximum exposure time (in seconds) before star trails begin to form when using a 35mm camera:
| FOCAL LENGTH OF LENS | DECLINATION OF STAR FIELD | --------------- | 0o | 30o | 45o | 60o | 75o |
| 18mm | 55 | 65 | 80 | 110 | 220 |
| 28mm | 35 | 40 | 50 | 75 | 140 |
| 35mm | 30 | 33 | 40 | 60 | 110 |
| 50mm | 20 | 23 | 28 | 40 | 75 |
| 100mm | 10 | 12 | 14 | 20 | 40 |
| 135mm | 7 | 8 | 11 | 15 | 30 |
| 200mm | 5 | 6 | 7 | 10 | 20 |
| 300mm | 3 | 4 | 5 | 6 | 13 |
| 400mm | 2 | 3 | 4 | 5 | 10 |
Of course, there may be times when you want to capture the effect of star trails. In this case, it is best to use a lower speed film and leave the shutter open for many minutes, even hours. Also, star trails are not a problem when you are attempting to capture meteors on film. In any case, do not be too quick to overlook or underestimate this method. You can obtain some nice shots with a minimum of effort or expense.
PIGGYBACK PHOTOGRAPHY
The piggyback method of astrophotography is so named because you attach your camera atop a telescope that is clock driven on an equatorial mount. This technique allows you to capture much fainter stars than would be possible with just a camera and tripod. It is still very easy to accomplish. Several types of mounting brackets are commercially available to fit most telescope tubes. Some telescope tube rings already come with a 1/4-20 threaded bolt to hold a camera body. You should not use this method with a lens that exceeds 300mm focal length. Beyond this, additional guiding would be necessary.
Notice the difference between the shots that were taken in this manner (Hyakutake2 & 3) as compared to those taken with a camera and tripod (Hyakutake4 to 6). As mentioned above, you can see many more stars in the frame. You may also notice that the foreground tends to become blurred during the exposure. While the telescope’s clock drive keeps the stars from trailing on the film, it causes any fixed objects to gradually distort. The longer the exposure, the more noticeable the distortion.
PRIME FOCUS
Most of the images contained in this site were obtained through the prime focus method. In this technique, the camera body is directly attached to the rear of telescope tube. Many types of telescopes are suited for prime focus shots and camera adapters are available for each of them. The telescope essentially acts as a camera lens. The image size and field of view are determined by the focal length of the telescope. The f/ratio of your setup is simply the diameter of your telescope’s objective lens divided by its focal length. Occasionally, one may use a focal reducer to provide a wider field of view as well as allow for shorter exposure times.
This method requires additional guiding of the telescope during long exposures. We cannot just trip the shutter and sit back because no clock drive is totally accurate. There is periodic error in the gears that drive the shaft. This causes the drive to run a bit slower or faster than ideal. Even if a perfect clock drive existed, atmospheric refraction would cause a shift in the apparent location of an object over time.
Accurate polar alignment is essential before any time exposure can begin. If the mount is not aligned properly, the star field would appear to rotate around the object you are photographing, regardless of how well you guide the scope. The results would look similar to those you would obtain by taking a long exposure of Polaris with a camera and tripod.
Once you have polar aligned your telescope mount, you are ready for prime focus astrophotography. There are basically two methods used to make fine tuning in our guiding: 1) a separate guidescope or, 2) an off-axis guider. Each has its merits and problems. A separate guidescope makes it much easier to locate and center a bright guide star. But the guidescope must be securely attached to the main telescope or mount. Otherwise, differential flexure will likely occur. This means that gravity will pull the various components on the mount unevenly. Although the guide star may not move as viewed by the guidescope, it would move in relation to the main scope. An off-axis guider avoids this problem. It is inserted between the telescope and the camera and deflects a small portion of light from the telescope’s actual field of view. Guiding is much more accurate with this approach but it does make finding and centering a guide star more difficult. Oftentimes, once you are fortunate to have centered a guidestar, the object you are photgraphing is located toward the side of the frame. This makes compostion of the shot difficult.
Both guiding methods typically use an illuminated reticle eyepiece that allows you to keep a guide star centered during the exposure. Your mount must have slow motion controls on both axes. A hand controller is highly desirable. You can align the reticle so that the guides star moves along each of the crosshairs on the reticle. It is even more helpful to hold the controller in such a way that the buttons on it correspond to the direction the guide star would move when each button is pushed. You may need to change the polarity of the drive in order to accomplish this. Fortunately, most hand controllers have switches that allow you to do this.
Regardless of which method you employ, an autoguider is highly recommended. Autoguiders such as the SBIG ST-4 or STV use a CCD chip to track any movement in the guide star and send signals to the drive to make the necessary adjustments. Of course, the relief from manual guiding is not without its costs (besides the obvious financial one). There are several steps required before you can actually begin your exposure. Focusing on a guidestar can be cumbersome. You typically have to locate and center a guidestar using an illuminated reticle eyepiece (an indispensible accessory, you may have noticed). You would then need to remove the focusing eyepiece and replace it with the CCD head. It is all too easy to knock the guidestar off center in the process. If you are going to be investing in an autoguider, you should seriously consider purchasing a flip-mirror. This item eliminates the need to shift between the viewing eyepiece and CCD head. You simply flip a lever that moves a mirror to direct the light to either the eyepiece or the CCD. If you possess the Deluxe STV, use can use the video screen to help you select a guide star, eliminating the need for flip-mirror.
Once a guidestar is found and focused, the autoguider must be calibrated before tracking is possible. However, once you learn the nuances of this tool, the results are impressive. You are also free to look up at the sky rather than down at a guiding eyepiece. This allows you to keep a better watch out for airplanes or other intrusions that may interfere with your exposure. You can also just grab a pair of binoculars and enjoy the view.
There will still be times when manual guiding is necessary. One particular instance is when photographing comets. Since they are moving in relation to the stars, guiding on a star will give you pinpoint images of the stars but less sharp images of the comet. Guiding directly on the comet’s coma is necessary if you are doing a lengthy exposure. Notice the difference (albeit subtle) between the HaleBopp1 and HaleBopp2 images.
EYEPIECE PROJECTION
There will be times when you will want to obtain a larger image size than is possible through prime focus photography. This is most often the case in planetary or lunar photography. Eyepiece projection is the preferred method in these situations. This method requires a fairly inexpensive camera adapter that allows you to place an eyepiece (or barlow lens) between the telescope and camera. The image is projected onto the film plane. This drastically increases the f/ratio of the system and is therefore not suitable for deep sky photography. To determine the effective f/ratio or overall focal length of your system, use the following formulae:
| Effective f/ratio = f/ratio of telescope x [(d - FL of eyepiece)/ FL of eyepiece] |
| Effective FL = FL of telescope x [(d - FL of eyepiece)/ FL of eyepiece] |
where d is the distance between the field stop of the eyepiece and the film plane.
For example, using an f/8 1200mm FL scope with a 12mm eyepiece that is 70mm from the film plane:
| f/ratio = 8 x [(70 - 12)/12] = 8 x (58 /12) |
| = 8 x 4.83 |
| = 38.64 |
| The effective FL would therefore be 1200 x 4.83 = 5796mm |
The following table lists the proper exposure time (in seconds) for shooting various objects using ISO 200 film:
| F/RATIO | THIN CRESCENT MOON | WIDE CRESCENT MOON | QUARTER MOON | GIBBOUS MOON | FULL MOON | MARS | JUPITER | SATURN |
| 5.6 | 1/60 | 1/125 | 1/250 | 1/500 | 1/1000 | 1/500 | 1/250 | 1/60 |
| 8 | 1/30 | 1/60 | 1/125 | 1/250 | 1/500 | 1/250 | 1/125 | 1/30 |
| 11 | 1/15 | 1/30 | 1/60 | 1/125 | 1/250 | 1/125 | 1/60 | 1/15 |
| 16 | 1/8 | 1/15 | 1/30 | 1/60 | 1/125 | 1/60 | 1/30 | 1/8 |
| 22 | 1/4 | 1/8 | 1/15 | 1/30 | 1/60 | 1/30 | 1/15 | 1/4 |
| 32 | 1/2 | 1/4 | 1/8 | 1/15 | 1/30 | 1/15 | 1/8 | 1/2 |
| 45 | 1 | 1/2 | 1/4 | 1/8 | 1/15 | 1/8 | 1/4 | 2 |
| 64 | 2 | 1 | 1/2 | 1/4 | 1/8 | 1/4 | 1/2 | 4 |
| 88 | 4 | 2 | 1 | 1/2 | 1/4 | 1/2 | 1.5 | 6 |
| 100 | 6 | 3 | 1.5 | 1 | 1/2 | 1 | 3 | 11 |
| 130 | 15 | 9 | 4 | 2 | 1/2 | 2 | 6 | 20 |
| 160 | 20 | 15 | 6 | 3 | 1 | 4 | 9 | 40 |
| 200 | 70 | 30 | 11 | 5 | 2 | 7 | 16 | 70 |
AFTER THE SHOT
Once you have made great efforts to photograph your celestial objects, you will want to process the film. Assuming that most of us do not have our own darkrooms, we will most likely bring the film to a commercial lab. Actually, most labs do a good job with standard processing and printing of the film. It’s reprints or enlargements that present the biggest challenge to them. I would recommend that you provide the lab with some examples of how you would like your prints to appear.
Of course, it would be ideal to take charge of the entire process and do it yourself. One way to accomplish this is by scanning your negatives into a personal computer and enhancing the images with software such as Adobe Photoshop. You can easily perform darkroom techniques such as cropping, dodging, and burning the image; adjusting the contrast, brightness, hue, saturation; removing eneven background illumination; and even digitally stacking your negatives. I would refer the reader to the web sites of Jerry Lodriguss and Chuck Vaughn for more detailed information.
As a postscript, always make a point of recording the details of your shot: the instruments used, f/ratio, exposure time, film type, etc. It is equally important to record any post processing manipulation you did to the image on the computer. You never know when you may wish to reproduce your efforts.
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