Building the Super-CONCAM

[nbrosch]

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

[RJN]

Hi Noah,

Congratulations on the funding! I would suggest the term CONCAM4 (or something similar) instead of Super-CONCAM to better give the idea that your instrument would be the next in a continuing series of improving fisheye CCD sky monitors.

Next, I wonder why you are so interested in using filters? There is a tradeoff between the value of obtaining standard (filtered) magnitudes and obtaining more faint meteors. I would think the extra meteors would be more important.

Next, the lens. Yes, the Coastal lens appears really great. Good choice. The only concern is the price. The new Nikon CCD lens discussed in this previous thread appears to have the same throughput and cost a factor of 10 less. The problem is that the Nikon is not obviously waterproofable. Also, as the cost of the lens is less than the CCD, lens cost might not be important.

Next, about the chopper. Here is a fanciful idea: put a rotating spherical mirror in your field of view where half the mirror is dark. The image captured of a bright meteor in the mirror will be chopped, and the speed of the meteor should be able to be determined. Of course the chopper might just be easier.

Last, with the extra pixels, star will track is a shorter amount of time. So you can either rock your CONCAM with the sky or take shorter exposures. Now shorter exposures might be best for discovering more meteors anyway, as discussed here. But exposures less than 3m56s might take you out of synchonization with the 3m56s cadence of the rest of the world's CONCAMs.

- RJN

[lior]

In order to take shorter frames and still maintain the 3:56 cycles, it is optional to perform exposure of around 1:50. This is not possible with our current SBIG CCD model, but a USB interface should allow this for the newer models. In this case, we might need a faster processor in order to make it balanced with the frame rate. An optimized performance might also require more bandwidth.

[Dan Cordell]

Last, with the extra pixels, star will track is a shorter amount of time. So you can either rock your CONCAM with the sky or take shorter exposures. Now shorter exposures might be best for discovering more meteors anyway, as discussed here. But exposures less than 3m56s might take you out of synchonization with the 3m56s cadence of the rest of the world's CONCAMs.

It should be just fine to use the normal 180sec exposures, but c9 and c16 will become more important than c1 and c5.

Of course, we won't know until we've tested it.

Love the entire thing though, that new high-res CCD should be great.

[Zephyr]

I agree with RJN, a super CONCAM would imply a higher resolution and perhaps multiwavelength in detection range.

The other improvement would be to put them in space.

If anyone has seen the Multiwavelength Milky Way site http://adc.gsfc.nasa.gov/mw/mmw_sci.html, I dream of the day you can look at a live version for the whole sky and a 3D Sun.

This would require at least six more SOHOs and overcoming the technical challenge of creating fisheye lenses capable of spectrum wide detection, and radar. I even started emailing the webmaster begging them to include the AMANDA II (http://amanda.uci.edu/) neutron sky map.

I have put this up for discussion in http://www.universetoday.com/ forums.

A smaller target was to put ConCams on some of Earths Co-Orbital companions, im particular Cruithne;

Club 1AU – Asteroids sharing Earth's orbit
There could be "Trojan" asteroids sharing Earth's orbit at the L4 and L5 Lagrange points, like those that share the orbits of Mars and especially Jupiter (see MPC lists). If there are, they will be very hard to spot from the ground. See David Tholen's comments about Trojans in the 24 March 2000 Cambridge Conference Correspondence, and Paul Wiegert's Earth Lagrange/Trojan asteroids page.
Three very small objects, 1991 VG, 1999 CG9, and 2000 SG344, have been spotted with orbits calculated to be so much like the Earth's that they may be artifacts such as J002E3, which appears to be an Apollo rocket stage, or, in CG9's case, may be a chunk of the Moon.
And then there is 3753 Cruithne, which has a bizarre "horseshoe" orbit of 1:1 mean resonance that makes it a very unusual companion of the Earth. Being a companion would otherwise require being a satellite (circling the Earth like the Moon) or a Trojan (following or trailing the Earth in the Earth's own orbit at +/- 60° nodes).
According to those who have studied Cruithne, other candidates for a similar Earth companion status are 1998 UP1 and 54509 2000 PH5. Separately, in an Icarus article, Helena M. Morais and Allesandro M. Morbidelli footnoted that they calculated 1998 UP1 could become such an object, as well as 2000 WN10. They also said that 2002 AA29 appears to be one now, but temporarily.
In their Oct. 2002 Meteoritics & Planetary Sciences article, Martin Connors et al. report that 2002 AA29 is the most truly co-orbital Earth companion asteroid yet found, and that it occasionally transitions into and out of being a natural satellite orbiting the Earth. See A/CC News for more about this story.

http://www.hohmanntransfer.com/topics.htm A/CC

Anyway, goodluck Noah!
Any improvements to Concam is a good thing.