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|Figure 1: Bright mag -6 Geminid fireball imaged by Bill Ward using Canon 350D, 30mm f1.4 lens,30 second exposure, ISO 800.|
Do you own a DSLR camera? If so, you may have already given thought to capturing images of meteors. You may be unsure however as to how to get the best results from meteor imaging. The good news is that Bill Ward has already worked out most of the answers for you and has provided this useful guide:
With advances in imaging technology, film based meteor photography is almost extinct. An image that contains no meteors is still disappointing, but at least it hasn't wasted a frame on a roll of film that you paid for.
Whilst the basic operation of DSLR imaging is the same as film imaging, there are a few practical differences between the two that need to be considered.
Imaging meteors with film cameras has always been a standard supplement to visual observing. However film emulsions suffered from two significant limitations when used for meteor imaging.
The first is a very low "detector efficiency" This limits the magnitude of the faintest object than can be detected. Due to the chemistry involved the very best emulsions were only approximately 4% efficient. To look at it another way this means that only 4% of the light available from the object actually goes into producing the image.
The second issue is called "reciprocity failure". This non linear sensitivity decrease over long exposures results in a further reduction in sensitivity. This was paradoxically a slight advantage. It allowed the exposure to be many minutes long without complete "fogging" of the film by the light of the sky background. The faintest meteor magnitude recordable was however still limited by the increase in fog level as the exposure length increased. Together, these limited the ultimate performance of films in meteor imaging. Even during shower maxima a single camera equipped with a good lens and fast film might only record a few of the brightest meteors.
Modern digital SLR cameras equipped with what are called CMOS detectors are now the norm. Electronic detectors have vastly improved detector efficiency, so much more of the light is used to produce an image. In effect more meteors are captured. Perhaps more significantly electronic sensors are essentially linear devices. This in effect removes reciprocity failure. As long as the shutter is open the image will be building up from the start to the end of the exposure with no tailing off of sensitivity, the effective exposure limited only by the capacity of the detector pixel before saturation occurs.
Standard film cameras of the 80's and 90's were robust, metal framed and fully mechanical. Examples included Zenits and Practikas from Eastern Europe. Some observers also used medium format cameras such as the Russian Lubitel. These were triggered using simple mechanical cable releases. Also at this time were more up market manual Nikons and Canons from Japan. These had electronic controller options which led them (the Canons in particular) to be used in multiple camera set ups.
Modern DSLR's are somewhat different. They tend to be lighter weight with plastic bodies and of course now fully electronic. The sensors used are generally specified by a total pixel count, but most are physically smaller than the area of a 35mm film frame. These smaller sensors are known as APS devices and are in the region of 22mm x 15mm. The smaller sensor will affect the choice of lens to be used and the all electronic nature of the cameras means they need a little more care and attention than did the older mechanical ones.
Meteors represent an unusual imaging challenge. Even during major showers they are essentially random in occurrence on the sky, they are brief events and fast moving. Whether film of electronic the sensor is exposed to a very small amount of light for a short time.
To record the most meteors the maximum coverage of the sky and the maximum amount of light needs to be delivered to the sensor. In terms of the lenses these parameters are essentially mutually exclusive.
To image a large area of sky a wide field lens is requirement. However with the smaller APS sensors the focal length of the lens needs to be shorter than the equivalent coverage lens in 35mm film format. This is often referred to as the magnification factor. Given the ratio of the APS to 35mm format, it is around 1.6.
|Figure 2: Lens adapters can be used to mount older lenses on DSLR cameras. This picture shows one that allows Pentax M42 threaded lenses to be used on Canon DSLR cameras.|
For example, a 50mm lens on an APS format camera has the same coverage as an 80mm on a 35mm film format (50 x 1.6 = 80mm). So to get the same sky area coverage as a standard 50mm lens on 35mm film format would require a lens af approx 30mm on APS format, that is 50 / 1.6 = 31.25mm. Thus to get a wide field on APS requires very short focal length lenses. However if you happen to have a 28mm or 35mm lens from an older camera these can be used very effectively on APS format DSLRs. Adapters can be purchased from many photographic suppliers that allow older lenses to be used on modern cameras.
To get the maximum amount of light onto the sensor a "fast" lens is needed. A 50mm f1.8 lens was the standard on film cameras and was a good general performer. However, f1.8 28mm or 35mm lenses tend to be expensive, but regular f2.8 types are much less so. Combined with all the other digital advantages they can produce great results. In practical terms, use the fastest lenses you have.
Observers can also adjust the ISO "speed". The ISO setting on DSLR cameras is only included to give an analogue of film so that manual settings can be adjusted to suit the creative need of conventional photographers. From a meteor observers point of view a more technological perspective is needed. As we are dealing with an electronic sensor the ISO setting should be thought of as an amplifier gain setting. The higher the ISO setting the more electronic gain is applied to the photo-electron signal level of the pixels. There is a downside to more gain (a higher ISO setting) on a DSLR camera. In film a higher ISO speed meant grainier images. In electronic terms the higher the ISO setting (more amplifier gain) leads more electronic noise being added to the image. In ordinary imaging the exposure is so short it's not really a problem. With the relatively long exposures used in meteor observing it can become noticeable if too high an ISO setting is used. Each camera/sensor has it's own optimum that can be determined by detailed testing but on average ISO 800 or 1600 is suitable. Under dark skies higher ISO settings can be used.
The final difference between film and digital is the way the light is actually collected. In film the light photons strike grains in the emulsion that are subsequently chemically processed to produce the final image. In digital CMOS sensors the collecting areas, the pixels, are fixed. The have a definite dimension in a fixed array. The modern trend for more and more pixels which gives a higher resolution in ordinary imaging actually plays against the meteor imager. The pixels in a CMOS detector might have pixels as small as 4 microns across (0.000004m) In theory this is good news but as the image of a meteor moves across the detector it spends proportionally less time over a given pixel the smaller it is. This limits the available electronic signal and in practical terms gives us a fainter image. A larger pixel is better, up to a point! As every meteor image will be subject to an almost infinite variety of circumstances it's impossible to define an optimum pixel size. If the pixel is too large, noise and sampling issues become a problem so we don't want an overly large pixel. Cameras with pixel sizes of 6 to 9 microns will generally be the best for meteor observing.
|Figure 3: Accessories needed for DSLR meteor imaging.|
|Figure 4: Deep Lens Hood with heating ribbon.|
|Figure 5: The camera set up for a night's observing.|
|Figure 6: Two first magnitude Geminids imaged by Bill Ward using Canon 1000, 30mm f1.4, 30 second exposure, ISO 800 plus 500 lpm grating.|
|Figure 7: Bright Perseid meteor with three satellite trails imaged by Bill Ward using Canon 1000D 28mm f2.8 lens, 30 second exposure, ISO 1600.|
|Figure 8: All-sky lens used for imaging fireballs.|
|Figure 9: Mag -10 Perseid Fireball imaged by Bill Ward using Canon 350D, 8.5mm f3.5 lens, 30 second exposure ISO 800.|