Photographing meteors involves a great deal of luck for the most part. In turn, the results have the potential for producing the most accurate data. Photographing meteors can provide accurate radiant determinations, meteor durations, velocities and approximate magnitudes. Orbits can be determined for meteors that were photographed by two different cameras separated by at least 25 to 100 miles.
In order to increase your chances of capturing a meteor on film, there are a few basic items required. These include a camera with a 28mm to 50mm lens, appropriately fast film, a sturdy tripod with a cable release and as you might guess, a lot of patience.A. CAMERA
A standard 35mm camera is recommended, with either a 28mm f/2.8 lens or a 50mm f/1.8 lens. The camera should have a shutter that is not controlled electronically. Time exposures, especially out in the cold, can drain the battery quickly ending your photography session early.
A 50mm f/1.8 lens catches meteors of 0.0 magnitude or brighter quite well. But, the field of view of a 50mm lens is rather small. Quite often you may only get part of the meteor. On the other hand, a 28mm lens has a wider field of view, therefore increasing the odds of a meteor crossing the camera's field. But, the wider lens makes it necessary that a meteor be brighter to be captured on film, somewhere in the -2.0 range.
So what lens should you use? The choice of lens should be based upon the expected meteor showers population index (discussed in a later chapter). A useful rule of thumb is that if the population index is 2.5 or less, choose the 28mm wider angle lens, otherwise, try the 50mm normal lens.B. FILM
The choice of film hardly gets any easier. Often color is used because that's what the film makers try to sell and that's what we remember the most. However, most of the light from a meteor is from the blue to ultraviolet part of the spectrum (therefore never use any type of UV filter while photographing meteors). However, your lens absorbs most of the UV light anyway. The visible light that is recorded is white. If the meteor appeared green, blue or red, the image will still be white. So, the only color will be from the stars, which we all will agree is appealing in itself.
Black and white film is the preferred film, especially the T-max films. On moonless nights, T-max 3200 probably is the best choice. It's fast and the grain is not as bad as one would think. Prints up to 5"x7" come out quite pleasing. T-max 3200 can be used even when there is a quarter moon, just make sure not to aim the camera in a direction where moonlight will harm the exposure. Any other time, choose the slower T-max 400 film.
When deciding on your choice of film, keep in mind that for scientific purposes, one of the things that can be determined from photographs is the magnitude of the meteor. But this can only be done with black and white films.C. TRIPOD AND CABLE RELEASE
One thing that can't be over emphasized is the importance of a sturdy tripod and a good locking cable release. Part of the problem with a lot of tripods is that they are so light they don't rest on the ground firmly. One possible remedy is to suspend a heavy weight under the tripod's head. As to cable releases, make sure that it is the type that will lock and that it operates smoothly, even on those cold, frosty mornings.D. METHODS OF PHOTOGRAPHING METEORS
There are two methods of photographing meteors - unguided and guided. Many people simply place a camera onto a tripod, set their camera to the bulb or "B" setting and lock the cable release for the duration of the exposure. The camera equipment stays motionless, but the sky moves. This is unguided photography. For scientific purposes, unguided exposures can be used to determine right ascension and declination coordinates for both the beginning and end points of the meteor's trajectory. But accurate times are very important here. You must know, within three seconds, the beginning and end times of the exposure as well as the time of the meteor's appearance. Also, you need to know where the camera is pointed and be able to identify at least six stars. This is much easier if you are careful to frame a couple of easy to recognize stars in the field of the camera. Using this method, experiment with exposures of 15 minutes or less.
Guiding a camera (where the camera's field moves across the sky at the same rate as the stars) is a lot more difficult, but is well worth the effort. It requires some method of allowing the camera to move at the same rate as the sky, so the equipment must be polar aligned to prevent star drift. It is still necessary to note shutter start/stop and meteor appearance times, but they can now be within thirty seconds of accuracy instead of three. It is still prudent to be as accurate as possible though. For guided exposures, it's possible to piggyback a camera onto a properly aligned telescope or use a device such as the "Vista Star Stepper". Then attach this to a sturdy tripod and make your polar alignments from there.E. EXPOSURES
How long should you expose the film? There are several things to consider. First, what is the purpose? If you want to be creative, there is no concrete rule for exposure durations. A lot of people find longer exposures to be more pleasing so experiment with exposure times. For unguided scientific purposes however, try to keep the exposures between 5 and 15 minutes to reduce star trailing. If there is an exceptional rate of meteoric activity, keep exposures at 5 minute increments.
For guided cameras, you will have to judge first how accurate your polar alignment is. One rule of thumb is this - the wider the angle of the lens, the longer the exposure can be before the stars start to trail off. With a 28mm lens, try 25 minutes as an initial exposure (20 minutes with a 50mm lens) and go from there. If a possible bright meteor cross the camera's field of view prior to the end of any 15 minute exposure, let it run for a whole 15 minutes to give the negative some star detail. If a meteor crosses after 15 minutes but before your scheduled time to end the exposure, stop the camera as soon as possible and prepare for the next exposure. Use a similar exposure timing for a 50mm lens.F. WHERE TO AIM THE CAMERA
Photographing meteors does involve some luck, but you can dramatically improve your chances by paying attention to where the center of the camera's field of view is positioned. Aiming the camera between 50 and 70 degrees above the horizon will insure the largest portion of sky is within the field of view.
If possible, also aim your camera about 30 to 40 degrees from the radiant itself. This zone represents a very compressed area from which a meteor will be coming from. If you aim at the radiant itself, it's even more compressed, and most meteors don't become visible until they have traveled somewhat from the radiant. Therefore, more meteors should become visible to your camera.
One last thing to consider is how you align the long axis of your film plane to the radiant. You will be able to cover more potential meteor crossing area if you align the long axis of the viewfinder so that a meteor coming from the radiant toward the center will be perpendicular to it. If the long axis is parallel, you will have less potential meteors.G. PRINTING
If you capture a meteor, be sure to make your print at least 5"x7" in size. Whether you have the film processed commercially or do it yourself, try to include all edges of the negative in your print and make the print lighter than it normally would be. This may not look the most appealing, but is best for scientific measurements. On the back of your print, write the following information:
-- Your Name
-- Day, Month, and Year (use double date)
-- Time of Meteor Appearance (use universal time or local time with the time zone specified)
-- Apparent Magnitude
-- Apparent Velocity (Very Slow, Slow, Medium, Fast, Very Fast)
-- Location: Latitude, longitude and elevation of the site along with a verbal location (for example: Sequoia National Park; Yuma, Arizona etc.)
-- Camera Lens (ex. 28mm lens, f/2.8)
-- Film (ex. T-max 3200, Developed in D-76, for 13 minutes at 70 deg F)
-- Estimated Print Center (ex. RA: 8h 30m, Dec: -8 deg)
-- Identify any bright stars by taping a thin sheet of paper over the print so labeling can be done (note that this is not necessary for guided images)
It's also a good idea to keep an accurate log for each photographic night. Include all the times for each exposure. Also include where the camera was aimed and a meteor number for any captured meteors that were simultaneously observed visually. This will allow you to associate pertinent information with the photo. Keep a record of what lens and film was used and whether you had a rotating shutter in operation.H. ROTATING SHUTTERS
These are relatively simple devices that resemble a fan. When a bright meteor passes through your camera's field, the end result will be a meteor image that is chopped up. From this, it is possible to compute the meteor's duration within ten thousandths of a second. To do this, divide shutter revolutions per second times two shutter blades, into the number of meteor breaks. An ideal RPM for a motor is probably in the 1,000 rpm range. If you happen to be working as a team with cameras separated by 25 - 100 miles and accurate times are being recorded, meteor altitudes, velocities, entry angles, and orbits can then be determined.
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