Director: Tony Markham
Imaging Meteors - Advanced Notes
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| Geminid meteor by Robin Scagell using Ektachrome 1600 film, 3 min exposure. |
Contents
- Introduction
- Exposure limits and imaging sensitivities
- Meteor capture rates
- Equipment issues
- More advanced work
- Further information
Introduction
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.
Equipment issues
- Film While film may allow longer exposure times than digital, it is easy to get through several films in a night's photography. Any films should either be home-processed, or the processing company warned that the exposures will be lacking in obvious images, so that they don't cut through the middle of frames, or fail to make prints from them at all.
- Power sources Film cameras may rapidly use up film in a night's observing, but powered cameras can use up batteries almost as quickly sometimes. To prevent having to cut your meteor imaging short, a better alternative to having plenty of charged battery packs on-hand is to use an external power supply. This often requires AC power, however, which in turn means long extension cables or the use of a power inverter if you are out in the field – all additional complications and with the potential risk of AC power cables on dew-covered grass.
- Focusing As mentioned in Observing Meteors, focusing at night if you have no accurate focusing-scale on your camera can be a major problem, because autofocus systems rely on picking out some distinctive feature of the imaged scene, and stars are not generally bright enough to permit this to work. Some cameras have "live view" displays so that you can see the actual image on the viewing screen, which is an enormous help when focusing. Alternatively, use one of the older manual focus lenses (such as the ubiquitous M42 screw-thread lenses) if your camera can be adapted to use them. These stop at infinity focus, though do be aware that sometimes the lens may not focus at infinity when used with an adapter.
- Dewing Dewing up of the lens is a constant problem for all astronomical imaging. As with telescope lenses, the simplest solution is to use a dew shield – a cylinder on the end of the lens which restricts the amount of sky that the glass is exposed to, thus preventing it from cooling so quickly. However, a camera lens has a very much wider field of view than a telescope, so the cylinder can be only fairly short before the actual field of view is restricted. Gently warming the lens and the air very near it will help prevent dew forming. Some observers use low-voltage, battery-powered dew shields, usually of home manufacture. Placing the camera inside a heated box is an another option. Even a humble hot-water bottle close to the lens can help!
- Multiple cameras with a rotating shutter Although more expensive to establish and run, a "nest" of 4 to 6 cameras set as an inward-facing ring covering the strip of sky at 50° elevation should produce more trails, and a useful addition to this group is a rotating shutter. This looks rather like a broad helicopter rotor blade and is set up so that the blades will pass periodically in front of all the camera lenses and break the exposure about 30 times per second. (It is important the exact rotation speed is known and maintained.) On a photograph, this will make any meteors appear as chopped-up straight lines, rather than the solid arcs of the star trails, which not only makes the meteors easier to spot, but also provides valuable information on each meteor's speed. Great care needs to be exercised when using this kind of set-up however, as for optimum operation the motor may need to use mains electricity, and the shutter can cause serious injury as it rotates.
- All-sky cameras With wide-field camera lenses, there is a trade-off between the advantage that a larger area of sky can be imaged than with an ordinary lens, and the disadvantage that the thicker lens means only brighter meteors can still be imaged. These factors roughly cancel one another for typical wide-angle lenses. However, if we move out further to the "fish-eye" lenses, capable of imaging an entire hemisphere – so, the whole visible sky appears on a single image – the thickness of the lens means in general only negative magnitude meteors will be captured, and only those of fireball class (magnitude –3 and brighter) are liable to be recorded at all well. This kind of system works best if it is automated to cover the whole sky for as much of the night as remains clear, since fireballs are uncommon, and typical capture rates away from the major shower maxima trend firmly towards the "nights per meteor" class, rather than "meteors per night". The data can be very valuable, because rarely, some fireballs may drop meteorites, and a good fireball image may be essential in helping to recover those. The nature of this kind of imaging means film can have significant advantages – perhaps needing only two or three frames per night – but if linked to a computer, a large number of much shorter-duration digital frames can be stored, and with the right software, automatically checked for potential meteor trails. Not all such cameras use fish-eye lenses, as some have the camera suspended above a high-optical-quality, hemispherical mirror instead. The potential results, and the system operation, are otherwise identical.
The IMO's "Handbook for Photographic Meteor Observing", available online in PDF format, provides a great deal more detail about meteor photography, though almost exclusively using film. Pages 55–59 of this book contain more on constructing and using a rotating shutter, for instance, while Part 2 (pages 25–36) discusses fireball patrol work in-depth, using all-sky cameras. There is also a topic on the SPA's Observing Forum where discussions by observers about digital cameras suitable for meteor work can be found.