Allsky cameras for meteors
Posted: Wed Jan 31, 2007 11:51 pm
I originally sent this to Bob Nemiroff and a few others as a private communication, but Bob thought it would be of general interest and asked me to repost it here. I'm discussing allsky cameras that are specifically intended for recording meteors.
As you may know, I operate a network of allsky cameras in Colorado, intended specifically for collecting meteor data. We have had cameras running for over five years now. The project is underfunded, and we are interested in making it as wide as possible, so we've been careful to seek the most cost effective approach. Currently, that means using PC164C cameras with fisheye lenses. We are able to set up each station for well under $1000, and I'm making some changes now that will reduce that to about $500 (less the computer, which does not need to be new or fast). The only problem with the PC164C is that it uses a 1/3" format sensor, and the fisheye doesn't quite cover it. With this setup we lose about 15° of sky in two directions. That isn't a serious problem since most of the installations are in locations that have a few natural obstructions, and in any case the most interesting events are usually higher than that. I also have one camera that uses a Watec 902HS. With its 1/2" sensor the entire sky is imaged, although it adds about $170 to the cost of a station.
For meteor detection you do not want an integrating camera like a StellaCam. You must collect true video (25/30 fps) or you lose all velocity information. With multistation integrated images you can get a path, from which you can make reasonable estimates for possible fall sites, but you can't get the initial state vector you need to calculate the orbit of the parent body, which is one of the most important things you should want to do when studying meteors. The velocity profile is also required for estimating the mass and composition of the meteoroid. If your only goal is meteor monitoring, save the money and use a simple video camera. If you use an integrating camera for its other features, be sure to set its frame rate to 25/30 fps when you are monitoring meteors.
Integrating cameras tend to be somewhat less sensitive than video cameras to meteors, since the effective exposure time is usually set by the time the meteor image is actually on a pixel. Both cameras will therefore have about the same signal, but the integrating camera will have somewhat higher noise from dark current and sky background. Our video cameras are sensitive to just under magnitude 2 in individual frames, and to better than magnitude 3 in externally stacked frames. This provides an ample number of reference stars for astrometric calibration.
Generally, high resolution is not required. We normally operate at only 320x240, but using sub-pixel centroid calculations and considering the best fit between multiple frames, our position accuracy is around 3 arcmin, which at typical meteor distances is only around 100 meters.
The cameras run day and night. A few have fixed iris lenses, but they haven't shown any sign of damage from the Sun. Nearly all of the most sensitive cameras these days use Sony SuperHAD exView sensors, and these slowly develop hot pixels due to cosmic ray damage. We notice this since many of our cameras are at high altitude, but it doesn't seem to impact the performance in any significant way.
From dark sites, we average 10 meteors per night, although the count varies considerably with the time of year and moon interference (there is a frequency chart at http://www.cloudbait.com/science/showers.html). Fireballs (mag -4 or brighter) are much more common than we expected, averaging about 0.7 per day (very large fireballs are documented at http://www.cloudbait.com/science/fireballs.html).
I'm in the process of getting all of our data online, which involves converting a lot of old data to a new format. About 15,000 events are available at http://meteor.cloudbait.com, with another 10,000 still waiting to be added. The site currently provides the ability to view still images of each event, and to map them. Soon to be added will be the ability to make orbital calculations for multistation events.
If you have any questions about these cameras, please let me know and I'll be happy to answer them. Given my experience over the last five years, I'm quite convinced that simple video is the best approach for monitoring meteors. If you're also interested in work that benefits from longer exposures, I'd consider setting up separate cameras for each application.
As you may know, I operate a network of allsky cameras in Colorado, intended specifically for collecting meteor data. We have had cameras running for over five years now. The project is underfunded, and we are interested in making it as wide as possible, so we've been careful to seek the most cost effective approach. Currently, that means using PC164C cameras with fisheye lenses. We are able to set up each station for well under $1000, and I'm making some changes now that will reduce that to about $500 (less the computer, which does not need to be new or fast). The only problem with the PC164C is that it uses a 1/3" format sensor, and the fisheye doesn't quite cover it. With this setup we lose about 15° of sky in two directions. That isn't a serious problem since most of the installations are in locations that have a few natural obstructions, and in any case the most interesting events are usually higher than that. I also have one camera that uses a Watec 902HS. With its 1/2" sensor the entire sky is imaged, although it adds about $170 to the cost of a station.
For meteor detection you do not want an integrating camera like a StellaCam. You must collect true video (25/30 fps) or you lose all velocity information. With multistation integrated images you can get a path, from which you can make reasonable estimates for possible fall sites, but you can't get the initial state vector you need to calculate the orbit of the parent body, which is one of the most important things you should want to do when studying meteors. The velocity profile is also required for estimating the mass and composition of the meteoroid. If your only goal is meteor monitoring, save the money and use a simple video camera. If you use an integrating camera for its other features, be sure to set its frame rate to 25/30 fps when you are monitoring meteors.
Integrating cameras tend to be somewhat less sensitive than video cameras to meteors, since the effective exposure time is usually set by the time the meteor image is actually on a pixel. Both cameras will therefore have about the same signal, but the integrating camera will have somewhat higher noise from dark current and sky background. Our video cameras are sensitive to just under magnitude 2 in individual frames, and to better than magnitude 3 in externally stacked frames. This provides an ample number of reference stars for astrometric calibration.
Generally, high resolution is not required. We normally operate at only 320x240, but using sub-pixel centroid calculations and considering the best fit between multiple frames, our position accuracy is around 3 arcmin, which at typical meteor distances is only around 100 meters.
The cameras run day and night. A few have fixed iris lenses, but they haven't shown any sign of damage from the Sun. Nearly all of the most sensitive cameras these days use Sony SuperHAD exView sensors, and these slowly develop hot pixels due to cosmic ray damage. We notice this since many of our cameras are at high altitude, but it doesn't seem to impact the performance in any significant way.
From dark sites, we average 10 meteors per night, although the count varies considerably with the time of year and moon interference (there is a frequency chart at http://www.cloudbait.com/science/showers.html). Fireballs (mag -4 or brighter) are much more common than we expected, averaging about 0.7 per day (very large fireballs are documented at http://www.cloudbait.com/science/fireballs.html).
I'm in the process of getting all of our data online, which involves converting a lot of old data to a new format. About 15,000 events are available at http://meteor.cloudbait.com, with another 10,000 still waiting to be added. The site currently provides the ability to view still images of each event, and to map them. Soon to be added will be the ability to make orbital calculations for multistation events.
If you have any questions about these cameras, please let me know and I'll be happy to answer them. Given my experience over the last five years, I'm quite convinced that simple video is the best approach for monitoring meteors. If you're also interested in work that benefits from longer exposures, I'd consider setting up separate cameras for each application.