Building the Super-CONCAM
Posted: Fri Aug 20, 2004 9:06 am
I have recently been awarded a grant by the Israel Science Foundation to build two CONCAM-like cameras with highly enhanced capabilities relative to the existing devices. In this message, and hopefully in the thread that will follow it, I hope to receive insights from the members of this list.
The purpose of building two cameras is primarily the detection of meteors. As most of you know, meteors are transient phenomena in the upper atmosphere that take place when a meteoroid, which penetrates the atmosphere at high velocity, ablates. The light is produced by the ablation, shock-wave heating, and ionization of the atmosphere next to the meteoroid. The meteor phenomenon takes place at altitudes from ~120 km down to ~70 km and is produced by tiny particles, mostly smaller than a pinhead.
Large meteoroids, from cm to meter-size, produce bolides, fireballs, etc. Even larger meteoroids do not fully ablate when passing through the atmosphere and part of them may reach the ground producing a meteorite. The duration of the luminous phase of a meteor is tenths of a second, though there are longer ones (we measured one that lasted for almost two seconds). In addition, sometimes the meteor will leave a trail that could be visible for a few seconds, and there are rare events in which a train may be visible for tens of minutes; these are "persistent trains" and are produced by chemical reactions in the atmosphere.
The cameras we would like to have should be able to detect as faint a meteor as possible and their images should allow the determination of as many physical parameters of the meteor as possible. As one of the goals of the experiment is the orbit determination for the meteor, we must be able to determine its 3D trajectory while producing light. For this, we need to triangulate the meteor. This implies that the cameras must be located some tens of km apart so that the meteor would show a definite parallax with respect to the stars. We also must estimate its angular velocity, and from this its transversal velocity. This can be done by interrupting the light at a known rate, so that a time exposure of a typical CONCAM would show the stars and the elongated, interrupted, trail of the meteor.
As this Super-CONCAM would produce images typical for a CONCAM, these products could be ingested by the standard CONCAM archives. The added bonus would be in having two rather close cameras with identical performance, except for one parameter. The cameras would be equipped with filters so that, for example, by having a B filter in one camera and a V filter in the other, we could automatically have B and V magnitudes for all the stars above the horizon.
I have a baseline identification of the components for such cameras, and here is where I would appreciate input from the list. The important ingredients are the lens and the CCD. I identified a top-of-the-line fisheye lens that is made by Coastal Optical Systems. It is described at http://www.coastalopt.com/stan_01d.asp. The lens is an f/2.8 with a focal length of 7.45 mm and a back focal distance of 46.5 mm. The image in the focal plane is 22 mm in diameter for a field of view of 185 degrees, with very minor distortions.
The manufacturer can produce this lens with a filter slot; this is important because I believe a filter should not be located above the lens (too big, in this case), or below it (distortions, ghost reflections, etc.) The Coastal modification inserts the filter in between the optical elements of the fisheye lens at a location where the beam is ~collimated, which prevents ghost reflections.
For the CCD, I tentatively setlled on the SBIG Research STL-6303E; this is a 3072 x 2048 pixel CCD with 9 micron pixels and it offers a peak quantum efficiency of 68% at ~6500A. It is important that this is a full-frame CCD, thus the entire surface exposed to light yields detections. The CCD is equipped with a shutter and with a USB connection. A full frame downlinks in 15 seconds. Given the projected size of the field of view from the Coastal lens, the CCD size implies that two opposite edges of the image would not be imaged onto the CCD. We believe this not to be significant because these regions would be very close to the horizon and we would not have to give up much of the science.
These two components provide the basic capability required for a CONCAM. To enhance this to what a Super-CONCAM would require we need to add the filter in the slot provided in the lens, and install a chopper between the back end of the lens and the CCD. Preferrably, the chopping mechanism should be synchronizable, so the choppers in the two cameras operate at the same time.
I would appreciate comments on these ideas and suggestions for alternatives. I would also appreciate ideas about possible choppers, and even preferrably pointers to possible suppliers. Also, comments about the proper choice of filters would be appreciated. For instance, using the SBIG CCD described above, the peak quantum efficiency is at the R band. V is down to ~2/3 of R and B is way down in QE. I would still be OK, though at a QE similar to that of V. To obtain a maximal efficiency, we could design special very broad-band filters; these would then not correspond to the standard Johnson-Cousins set used in astronomy.
Best regards,
Noah Brosch
The purpose of building two cameras is primarily the detection of meteors. As most of you know, meteors are transient phenomena in the upper atmosphere that take place when a meteoroid, which penetrates the atmosphere at high velocity, ablates. The light is produced by the ablation, shock-wave heating, and ionization of the atmosphere next to the meteoroid. The meteor phenomenon takes place at altitudes from ~120 km down to ~70 km and is produced by tiny particles, mostly smaller than a pinhead.
Large meteoroids, from cm to meter-size, produce bolides, fireballs, etc. Even larger meteoroids do not fully ablate when passing through the atmosphere and part of them may reach the ground producing a meteorite. The duration of the luminous phase of a meteor is tenths of a second, though there are longer ones (we measured one that lasted for almost two seconds). In addition, sometimes the meteor will leave a trail that could be visible for a few seconds, and there are rare events in which a train may be visible for tens of minutes; these are "persistent trains" and are produced by chemical reactions in the atmosphere.
The cameras we would like to have should be able to detect as faint a meteor as possible and their images should allow the determination of as many physical parameters of the meteor as possible. As one of the goals of the experiment is the orbit determination for the meteor, we must be able to determine its 3D trajectory while producing light. For this, we need to triangulate the meteor. This implies that the cameras must be located some tens of km apart so that the meteor would show a definite parallax with respect to the stars. We also must estimate its angular velocity, and from this its transversal velocity. This can be done by interrupting the light at a known rate, so that a time exposure of a typical CONCAM would show the stars and the elongated, interrupted, trail of the meteor.
As this Super-CONCAM would produce images typical for a CONCAM, these products could be ingested by the standard CONCAM archives. The added bonus would be in having two rather close cameras with identical performance, except for one parameter. The cameras would be equipped with filters so that, for example, by having a B filter in one camera and a V filter in the other, we could automatically have B and V magnitudes for all the stars above the horizon.
I have a baseline identification of the components for such cameras, and here is where I would appreciate input from the list. The important ingredients are the lens and the CCD. I identified a top-of-the-line fisheye lens that is made by Coastal Optical Systems. It is described at http://www.coastalopt.com/stan_01d.asp. The lens is an f/2.8 with a focal length of 7.45 mm and a back focal distance of 46.5 mm. The image in the focal plane is 22 mm in diameter for a field of view of 185 degrees, with very minor distortions.
The manufacturer can produce this lens with a filter slot; this is important because I believe a filter should not be located above the lens (too big, in this case), or below it (distortions, ghost reflections, etc.) The Coastal modification inserts the filter in between the optical elements of the fisheye lens at a location where the beam is ~collimated, which prevents ghost reflections.
For the CCD, I tentatively setlled on the SBIG Research STL-6303E; this is a 3072 x 2048 pixel CCD with 9 micron pixels and it offers a peak quantum efficiency of 68% at ~6500A. It is important that this is a full-frame CCD, thus the entire surface exposed to light yields detections. The CCD is equipped with a shutter and with a USB connection. A full frame downlinks in 15 seconds. Given the projected size of the field of view from the Coastal lens, the CCD size implies that two opposite edges of the image would not be imaged onto the CCD. We believe this not to be significant because these regions would be very close to the horizon and we would not have to give up much of the science.
These two components provide the basic capability required for a CONCAM. To enhance this to what a Super-CONCAM would require we need to add the filter in the slot provided in the lens, and install a chopper between the back end of the lens and the CCD. Preferrably, the chopping mechanism should be synchronizable, so the choppers in the two cameras operate at the same time.
I would appreciate comments on these ideas and suggestions for alternatives. I would also appreciate ideas about possible choppers, and even preferrably pointers to possible suppliers. Also, comments about the proper choice of filters would be appreciated. For instance, using the SBIG CCD described above, the peak quantum efficiency is at the R band. V is down to ~2/3 of R and B is way down in QE. I would still be OK, though at a QE similar to that of V. To obtain a maximal efficiency, we could design special very broad-band filters; these would then not correspond to the standard Johnson-Cousins set used in astronomy.
Best regards,
Noah Brosch
