Observing Meteors with a Video Camera
Figure 1: Northern Taurid fireball recorded by the video camera of William Stewart from Ravensmoor on 2012 Nov 08.
Visual meteor observing has traditionally been seen as a low cost and easy way to get involved in astronomy, particularly when visual observation are involved. Imaging could become expensive, both in the initial outlay for camera(s) and in terms of wasted film exposures that contained no meteor images. Photographic film is also only likely to record the brightest meteors.
Over the past decade, there have been big advances in imaging technology.DSLR cameras have eliminated the expense of wasted film and their improved sensitivity means that they are able to record fainter (and hence a greater number of) meteors.
The results of video observing can be particularly impressive. A “still” image of a fireball can look good, but is easily surpassed by a video showing a fireball moving across the sky and exploding. There is also no need retrospectively review a recording that lasts the whole duration of the observing session as software now exists that can automatically recognise the genuine meteors and exclude aircraft and satellites.
Figure 2: The same Northern Taurid fireball recorded by Alex Pratt from Leeds
Some observers will be satisfied to merely record videos of meteors. However, by collaborating with other observers it is possible to analyse images of the same meteor obtained from different locations. Using triangulation, it is then possible to determine the meteor’s atmospheric speed and trajectory and from this to work out the original solar system orbit of the meteoroid.
In addition to improving the understanding of major meteor showers, such work is also proving invaluable when it comes to the identification of minor meteor showers.
Visual observers have reported the existence of hundreds of minor showers over the decades, but video imaging, and the precision analysis that it enables, allows observers to determine which are genuine and which were spurious.
One should bear in mind that meteor showers are dynamic with activity waxing and waning as the orbit of the meteoroid stream changes due to the effects of gravity and radiation pressure. Hence there is considerable value in continuing to monitor meteor showers year after year.
Figure 3: The ground path of the Taurid fireball, determined via triangulation.
For example, nowadays the Geminids are probably the most reliable and spectacular meteor shower. However, they were not active 200 years ago.
Similarly, how will the current showers evolve in the coming decades? What new showers will become visible?
One network of collaborating video observers is the NEMETODE network (http://www.nemetode.org ) which operates in the UK and Ireland.
William Stewart, is part of this group, and has provided most of the information contained in the following guide to the equipment required to carry out the video observation of meteors.
Figure 4: The original solar system orbit of the meteoroid that gave rise to the fireball.
Its a lot simpler than it used to be ... and one type of video camera is in widespread use.
Video observations obviously require a video camera and for a number of years models have been available that can capture excellent live views of the night sky without the need for image intensifier technology (thus reducing cost and complexity). Live video (as opposed to a camera that integrates an image over a period of a few seconds or minutes) facilitates not only a precise measure of the meteor velocity but also an accurate time for when the event occurred, key requirements when determining the original orbit of the meteoroid.
For many years the camera of choice has been the Watec 902H and enhancements to the model have ensured this continues to be the case. It is sensitive, robust, is readily available with 1/2” and 1/3” format sensors and provides 25 frames per second at a resolution of 752 x 582. As of 2013, new units retail for around £300 while second hand units can be purchased online for under £125.
A compromise between sky coverage and number of meteors captured
'Trial and Error' is not a good approach because you don't get to see many meteor showers each year. Here is how to get it right first time ...
It is not possible to know in advance where a meteor will occur and so the natural instinct is to use a wide angle lens to maximise the chance of a capture. However, this will only detect bright meteors, whereas a lens with a longer focal length, while having a smaller field of view (FOV), will detect fainter meteors - and for every bright meteor there are many fainter ones. Typically lenses with a focal length in the range 3.8mm to 12mm are employed, yielding a respective field of view (FOV) of 80° to 20°. A longer focal length lens is better suited to a suburban location that suffers from light pollution and has, due to local obstructions, limited options for where the camera can be pointed. Bear in mind that lenses are designed for cameras with a particular sensor size and so using, for example, a lens for a 1/3” sensor on a 1/2” sensor camera will result in vignetting and significant distortion/degradation of the image at the edge of the FOV.
Another consideration is the focal ratio or “speed” of the lens: f/1.2 or better will give good results. The sensitivity of camera sensors has now increased to such an extent that manufacturers no longer have demand for ultra fast lenses. Lenses of f/1.2, 1.4 and 1.6 are readily available and fast lenses are still produced by manufacturers such as Computar (TG3Z2910FCS-IR), Tokina (TVR0398DCIR) and Fujinon (YV2.8×2.8LA-SA2) - expect to pay around £50.
Figure 5: The change in the Limiting Magnitude when changing Focal Ratio for a given focal length lens.
Figure 5 shows how the limiting magnitude changes with focal ratio for a given focal length. As can be seen, a lens of a particular focal length operating at f/1.0 will detect meteors a full magnitude fainter than one of the same focal length operating at f/1.6. A faster lens can make the difference between detecting 20 meteors per night and detecting 50 - 60. Whatever is chosen, to allow accurate astrometry to be performed on the meteors captured, the camera and lens combination must be capable of detecting stars down to magnitude +3. Due to the transient nature of meteors and the detection / triggering capabilities of the software, the faintest detected meteors are typically 1-2 magnitudes brighter than the stellar limiting magnitude.
The good news is that you don't need an expensive high spec PC. You do, of course, need to make sure that you have enough storage capacity to hold the recorded meteor videos.
The capture software is not processor intensive; hence typical desktop PCs from the early 2000s running Windows XP will suffice The NEMETODE Ravensmoor Node utilises IBM ThinkCentre M51 3.2GHz P4 HT units which can be picked up for around £40. Typical CPU load is around 20% and the only modification from the standard specification has been to add additional RAM and a 300GB Hard Disk Drive (HDD) .
Gone are the days when you had to spend hours the next day watching a playback of the overnight video
There are a number of choices available for the acquisition software and these determine the choice of interface card to allow the video signal to be streamed into the PC. The days of recording an entire video stream of the night sky, then spending an equal (or greater) amount of time visually examining the video for meteors, then manually estimating magnitude and radiant are, thankfully, over. Modern systems use tuneable motion detection algorithms, whereby the video stream is continuously read into a memory buffer and only those sequences that contain movement are saved as discreet clips to the HDD for later analysis. Sequences in the buffer containing no movement are discarded. Analysis of the clips is automated with minimal user interaction required.
Your choice of software will depend on whether you are planning to become part of a video observing network and the software already in use by the network.
The first software option is MetRec (Meteor Recogniser). This is a well established and widely used program that not only has excellent motion detection capabilities but also performs real time monitoring of local seeing conditions. In addition the software determines, on the fly, meteor brightness, velocity and equatorial coordinates and assigns probable meteor shower association based on the working list of meteor showers maintained by the International Meteor Organisation. MetRec is a good choice for observers who operate as “single stations” as opposed to part of a coordinated network. The software is freely available from the MetRec website but requires the use of a particular type of frame grabber card (Matrox Meteor) which is no longer produced. Second hand units are available but these typically cost around £125.
Another option is SonotaCo’s UFO Suite. DO NOT be put off by the name - it is nothing to do with 'flying saucers' - it is a set of highly capable programs designed for capturing and analysing any objects moving through the atmosphere. Indeed, while primarily designed for meteors, UFO Capture software has also been used to detect other atmospheric phenomenon such as sprites above lightning storms. Like MetRec, this software is well established and widely used. Note however that it does not support real-time monitoring of local seeing conditions nor does it perform the analysis (magnitude, speed, position, radiant and shower association) on the fly - these are performed later using a separate programme called UFO Analyser. This suite's greatest strength is the capability of a third programme, called UFO Orbit, which takes the analysed data from individual observers and automatically determines parameters for the radiant, velocity, trajectory and prior solar system orbit.
Depending on exchange rates, the software licence for UFO Capture costs around £125 (following an initial, free, 30 day trial period). The other two programmes (UFO Analyser and UFO Orbit) are free. The software does interface with readily available TV cards: the NEMETODE network make use of Osprey 210 Video Capture Cards, the cost of which have fallen significantly in recent years - new ones can be purchased online for under £30. Cheaper TV cards are available though in the authors’ experience, quality is variable with some contributing to lost frames. USB based devices such as the ClimaxDigital unit (£20) have also been successfully used.
The UFO Suite is a good choice for observers operating as part of a network who take steps to ensure common volumes of the meteoric layer are simultaneously observed.
You may be able to use something that you already have
In the NEMETODE set up, a laptop style power supply was originally used to convert 230V AC to 12V DC (to power the camera and the iris on the lens), though the more permanent installation at Ravensmoor has now switched over to a dedicated, higher current, enclosed power supply that supports three camera systems. The Leeds node uses 12V DC transformer plugs.
The bottom line ...
A budget of around £450 will purchase a highly capable system that can generate good scientific results. This is not a trivial amount of money nor is it an outrageous one. As ever one has to weigh up the pros and cons of cost (initial outlay) versus benefit (how much will it get used?).
Good news, you don't have to stay awake all night or monitor the cloud cover ... and you can do other observing while the video camera is running
There are several issues here:
Starting the system running at the start of the night and stopping it at the end
Making use of short lived clear spells
Protection from the elements (rain, wind, ice, etc)
Freeing up your time for other activities (including sleep)
A significant attraction of a video-based meteor
Figure 6: Three of the video cameras operated by William Stewart.
detection system is the ease with which much of the data collection can be automated, thus avoiding the need to set up and pack away the equipment in every observing session. A suitably protected system can be left to run every night irrespective of the sky conditions. With minimal additional financial outlay, a meteor system can be made permanent and configured to run for multiple consecutive nights (irrespective of forecast conditions) with no user intervention whatsoever. During cloudy spells your system might record bats or aircraft moving across the cloudy background, but if the sky clears while you are asleep then the system will capture those meteors that appear during such a cloud break.
The first step is a weather-proof enclosure for the camera and lens. You may choose to make your own - competent individuals have made their own using lengths of drain-pipe and perspex covers but with commercial units that include mounting brackets and incorporate a heater to de-mist the front window available for around £50 users may decide that the time to manufacture / risk of failure is not worth the financial saving. Bear in mind that commercial units are typically designed to point down instead of up and hence what may provide good protection from the elements in one orientation may not work in another. NEMETODE cameras use units from MAP Security sourced via eBay – search on “ebay map security cctv enclosure” – and have found them to be excellent.
It is essential that you don't operate the camera when the Sun is in the sky. Hence you need a mechanism to automatically switch the camera and lens iris on and off. This can be as simple as powering the 230VAC to 12VDC power supply via a mains time-switch of the type that is often used to automatically switch lights on and off. These are readily available for under £5 from electrical or DIY stores and presetting them to activate / deactivate at the time of sunset and sunrise is a straightforward exercise. These times will need to be adjusted during the course of the year in line with changes in local sunset / sunrise times … this can be inconvenient and so other potential solutions include operating the power supply in line with a light sensor
Thus a modest increase to the budget can significantly enhance utilisation and help justify the initial outlay. Over the first year, assuming less than 15% of the hours of darkness are clear, the cost works out at under £1 per hour.
William Stewart and Tony Markham