Popular Astronomy

Share to Facebook Share to Twitter Stumble It More...

Meteor Imaging with DSLR Cameras

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: 

Introduction:

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.

Technology Differences

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.

The Challenge

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.

ISO "speed"

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.

Pixel sizes

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.

 
Practical issues
 
DSLR cameras need more support than film cameras for successful imaging. Figure 3 shows some of the items needed for DSLR imaging.
Figure 3: Accessories needed for DSLR meteor imaging.
 
The individual items will be described as we go along.
 
Low temperatures and high humidity do not mix well with most electronic equipment. This is exactly what we are subjecting our DSLR cameras to! When setting up use a decent torch (F). Make sure you are on solid ground and that the tripod is locked properly. Even in temperate climates like the UK, the temperatures can drop quickly and the relative humidity shoots up resulting in the lens dewing and then ultimately the camera body itself. Remember that the camera may well be pointing at the sky for hours on end.
 
The first measure is to use a deep lens hood (L), but make sure the hood doesn't impinge on the field of view.
 
Use a dust blower (B) before each session, this will help keep them clean longer.
  
Figure 4: Deep Lens Hood with heating ribbon.
Unlike older film cameras which used simple mechanical cable releases, DSLR's invariably need some form of electronic controller (J). Whilst offering considerable flexibility in terms of exposure control, number of exposures and interval setting, they are not so robust. A large number of third party controllers are available at considerably less cost than branded ones, but be aware that the quality can be variable. Make sure that you do your homework before buying one.
 
For best dew control, some sort of heater (C) (see Figure 4) is necessary. Commercial ones are available or they can be home made with a little care. More details and instructions are given in the recommended further reading list at the end. 
 
 
Batteries, Data Cards & Notes
 
As the night progresses, the battery will fail so you will NEED a couple of fully charged extras (K). Keep the spares warm, that is keep them inside the house or in your pocket if further afield. Also since DSLR's will be using much shorter exposures than film you'll be taking hundreds of images so have an extra data card or two (I). Finally a cover over the whole camera is a good idea to protect your investment and ensure  the best lifespan of your camera. This can be made from bubble wrap or other suitable packing material (M, underneath the desicant bags). Just tape (G) it in place - but keep it loose fitting. You will need to change batteries and data cards. If this can be done without major dis-assembly all the better.
 
Figure 5: The camera set up for a night's observing.
It is good practice to keep a note of what's going on (E + H). Note the Time of sequence starts, Field of View details, Target details (regular monitoring or shower) etc along with all the usual visual observing details if conducting a watch. Use a low level red light (D) when writing notes as the night goes on or for changing batteries and data cards to preserve your night vision.
 
Focussing
 
Once the camera is set up, the focus needs to be checked. Depending on the lens, the real infinity focus tends to be just short of the infinity mark on the lens. (this is also an issue when using older lenses on DSLRs, the difference can be significant). Using a camera with a "live view" mode one only needs to centre a bright star and focus. For cameras without live view small increments should be made (starting from the endstop of the focus range) until best focus is achieved. Note or mark this position. 
If using a modern lens, marking options are limited due to the chassis design but older lenses (used with an adapter) can have tape attached and marked. 
 
Exposure times
 
The last thing is to determine the optimum exposure. Whereas with film exposures were minutes long with CMOS sensors the exposure are much shorter. The length of exposure will be dictated by the ambient light levels and your choice of ISO setting. Something around 30 seconds at ISO 800 is a good starting point. However from areas of severe light pollution this might be too long an exposure. But all is not lost and this is one of the key benefits of DSLR meteor imaging, We have immediate feedback. No waiting to find out if it's all a disaster! The exposure could be shortened but this will result in many more frames being taken or the ISO reduced with a resultant loss of sensitivity. It's all a compromise. With the image digital data we have the final option of image processing. Even with very bright looking frames, provided the image is not fully saturated, the brightness and contrast can be adjusted on a PC very easily. This may reveal meteors not initially seen. 
 
The aim is to get an exposure that results in the best signal to noise ratio for a given lens/ISO combination. Experimentation is the key. With a few test exposures you should now be ready to set the controller. Each brand/model have different operational settings but in general the camera itself should be set to "B". The camera will then respond to the signal from the controller. Set the number of exposures per run, set download interval, set exposure length and now you're ready to do.  
 
Before pushing the start button
 
STOP!. At this point do a once over of the whole system. This final check can save much frustration when you realise you've left the lens cap on or forgotten to connect the heater to the battery, the lens dews up and you've just shot an hours worth of nothing.
 
Once underway, it's good practise to monitor the battery level at intervals throughout the imaging session. If you stop hearing the camera shutter operating then it's probably safe to assume the battery has expired! However it's not good for the battery to be completely depleted each time. The ambient temperature, how the camera is insulated and the exposure length will determine how long any given battery will last. When to change the battery before it discharges is matter of judgement. Each camera usually has a power indicator of some sort and as experience is gained it will become apparent when the best time to switch is. 
 
At the end of the session the camera might well be thoroughly chilled even with a cover and dew zapper. Care must be taken not to put the camera straight into a warm environment. If this is done it will surely be covered with condensation and possibly inside as well as outside. This is best avoided! Place the camera in it's bag but don't seal it shut. Leave the bag in a cooler area, perhaps a hall to slowly warm up. A vital element in the camera bag should be several bags of silicon desicant (A). This helps considerably with keeping both camera and lenses in good shape. It helps absorb water vapour and prevent fungus growth on optical elements. Convenient sizes are 50g and 100g. Get a pack of ten, put them in with all your optics. It's money well spent! 
 
Despite the improvement in performance a lot of shots will still need to be taken to catch a reasonable number of meteors. Meteors in the range around ~ +1 will look like short streaks (see Figure 6). The coloured lines can be ignored as they were caused by a grating attached in front of the lens in an attempt to capture the spectrum of bright meteors. 
 
Meteor or not ?
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.
 
DSLRs are excellent at recording things in the sky that look like meteors - but aren't!
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.
 
In particular, satellites are very problematic. These can look just like faint meteors.
 
There are a few clues that can help.
 
Meteors tend to have a particular "shape", possibly with a terminal flare. Most satellites are uniform tracks BUT of course it's not that simple. As some satellites orbit they maintain a particular orientation with respect to the earth resulting in a flare from sunlight glinting off of reflective surfaces, solar panels etc.
 
When imaging in colour, bright meteors often have a green component in the trail. This is from an emission of light caused by oxygen molecules being excited by the passage of the meteor through the atmosphere.
 
 
In Figure 7, the meteor at the bottom left of the image flares just outside the frame towards the bottom left.
 
The greenish tint mentioned can be seen in the tail. To the centre right of the train THREE satellites can be seen. 
 
Comparing the satellite trails carefully to the meteors in Figure 6, subtle differences can be seen.
 
Another clue, of course, is that when imaging shower meteors, the trails can be traced back to the shower's radiant.
 
Care must always be taken when examining any image for potential meteors.
 
Figure 8: All-sky lens used for imaging fireballs.
Special lenses like fisheye lenses (see Figure 8) can be used to produce dramatic results as those in Figure 9 (below).
 
However due to the small physical aperture and small image scale these lenses are only really good for fireball patrols.
 
Modern DSLR cameras offer much more potential than their film based predecessors.
 
With a little effort one can produce excellent and scientifically valuable images. 
 
 
Figure 9: Mag -10 Perseid Fireball imaged by Bill Ward using Canon 350D, 8.5mm f3.5 lens, 30 second exposure ISO 800.
Recommended further reading:
 
1: Meteors (Philips Observers Handbook) Neil Bone, 1993
 
2: Meteors and Meteorites (Crowood Press) Martin Beech, 2006