|Geminid meteor by Robin Scagell using Ektachrome 1600 film, 3 min exposure.|
The Meteor Section's instruction booklet "Observing Meteors" provides some general notes on meteor imaging techniques and how to report those observations. This page gives some further notes and additional ideas on some of the more technical aspects of meteor imaging. Most of these notes will concentrate on digital photography, as it has particular advantages for meteor work, though some notable drawbacks as well. Bear in mind that meteors are notoriously reluctant to be photographed, and the success rate is usually very low, requiring dozens if not hundreds of frames per captured meteor. Patience and perseverance, as with most aspects of meteor work, are very helpful qualities for the meteor observer to cultivate!
Exposure limits and imaging sensitivities
In digital imaging, while the light from a star spends many seconds or, in the case of guided exposures, minutes, building up on an individual pixel in the camera, the light from a meteor spends only a small fraction of a second on each pixel. An identical process happens in a film camera, except that instead of pixels on an electronic sensor, the light falls on individual tiny crystals of the film's photographic emulsion. In either case, this means even a bright meteor may record very faintly, though stars below naked-eye visibility can appear clearly on the image, and can seem brighter than such a meteor. It is not uncommon to see a meteor flash through the area of sky you are photographing, only to discover that it has not been recorded at all, or only faintly. It is useful to carry out a visual watch of the same area as you are imaging, thus allowing you to get an idea of the true magnitude of any meteors that you capture.
Digital imaging in particular also collects stray background light from the sky very well, which will build-up and overwhelm whatever else you are trying to image in the night sky quite quickly, hence the need to keep exposures short and to experiment under your site's conditions to find the optimum for any given night. The 20-minute maximum limit for exposures mentioned in Observing Meteors is unachievable with most digital cameras, since even at a good, dark-sky, country site in the UK, exposure times at ISO 1600 may be no longer than a few minutes. This is where film photography can have the advantage over digital, as film's relative insensitivity to stray background light means light pollution records less well with it. Thus a similar ISO 1600 film at such an excellent site could allow exposures lasting many minutes.
Meteor capture rates
As mentioned already, the difficulty of imaging meteors usually means a large number of unsuccessful frames for each meteor caught, even on nights with good shower activity. Meteors can appear in any part of the sky, but the reasons for aiming your camera's field of view about 50° from the horizon and 30° to 40° from any active shower radiant, as suggested in Observing Meteors, are two-fold, and both involve improving your possible meteor capture rate.
First, at 50° or so elevation, your camera is imaging the optimum swathe of the atmospheric meteor layer at around 80–120 km altitude. At significantly higher angular elevations than this, you are viewing a progressively smaller volume of that layer, thus reducing the number of possible meteors crossing through your field of view substantially. At lower elevations, atmospheric extinction reduces the number of meteors that can be usefully imaged at a faster rate than the greater volume of the observed meteor layer increases them.
Second, meteor paths vary in length dependent on how far from their radiant they are. Close to the radiant, less than 15° or so from it, meteor paths are very short, which makes them extremely hard to find among the short star trails or points on a typical digital exposure. Farther away than 50°–60°, and paths often become too long to fit within the camera's field of view. Again, 30°–40° from the radiant gives the optimum chance of capturing most of a given meteor's trail on the image, and whose appearance means it can be quite readily identified.
In terms of captured meteor numbers, there is no real advantage to using a driven mounting as opposed to making undriven exposures. However, a guided image will give clearer and more pleasing shots of the constellations, as well as allowing more useful data to be derived, as described in Observing Meteors.