A comparative study of current and planned NEO surveys
Alain Maury - June 1999
There are currently several operating Near Earth Objects (NEOs) surveys. The goal is to perform a complete, or as complete as possible, inventory of Earth Crossing objects, asteroids and comets, in order to detect the potentially hazardous objects a long time in advance in order to make sure that Earth path is safe for the future, or in the contrary case, to be able to take actions, with ample time to do so.
Some history
The first Near Earth Object was discovered visually more
than 100 years ago. Visual detection of Near Earth Objects is not very
efficient, and very soon after this time, most astronomers were using photographic
plates and astrographs, performing a visual detection of asteroid images
on the developed photographic plate. Detection was either made because
the asteroid's motion left a trail on the photographic plate, or by using
2 plates taken after a given time interval, and using either blinking or
stereo detection. Almost nothing changed before 1989, when the Spacewatch
group, led by Tom Gehrels at the University of Arizona, convincingly showed
that CCDs were much more efficient than photographic plates. I was a matter
of year before other groups converted to electronic detection.
Many groups are currently engaged in asteroids surveys.
It is however known that the current effort is many times smaller than
would be necessary in order to finish this inventory in a relatively short
time. Therefore, it is expected than more groups will join this effort,
and that the groups currently involved in NEO searches will tend to to
upgrade their detection system in the future.
Survey efficiency :
It is interesting to study the efficiency of each system being used or planned in order to see where improvments can be made.
It seems relatively obvious that a survey system will be efficient if it uses a wide field of view, a very efficient detector, is able to observe many nights per month, while loosing as little time as possible not collecting photon (reading out the CCD detector for example). A commonly accepted figure of merit for a survey quoted many times in the litterature, is :
W = Solid angle seen by the detector on the sky (in square degrees)
D2 = Square of the diameter of the telescope (in meters)
DQE = Detective Quantum Efficiency of the detector
Eff. = Observationnal efficiency of the detection (ratio of the time spent collecting photons versus total observing time)
Short preliminary discussion :
- For a given telescope, W is a measure of the number of square millimeters of detector in the focal plane. If a is the area of the detector, one can write
It is therefore possible to rewrite formula 1 as :
This clearly shows the importance of the focal ratio F/D of the telescope being used.
- D is usually fixed, and increasing its value usually is very costly. font size=+1>DQE depends of the detector. Since detection is usually performed using unfiltered detector, it can be taken from 2 to 4% for photographic plates, to around 35% for thick front illuminated CCDs, to 85% for thinned back illuminated CCDs. While photographic surveys are basically obsolete, the choice is with either relatively inexpensive but less efficient CCDs to a very efficient but terribly expensive device.
- Eff. could be taken in a wide variety of ways. It could be taken as the number of seconds spent effectively collecting photons in a year, divided by 31536000 (365x24x60x60). It would include the fact that we have to close our domes during the day, during cloudy nights, during nights where the full moon is up in the sky, that there are some nights which are or not allocated to NEO survey on some telescopes, and finally that using some observing modes, one must readout the CCD, and during this time, one clearly is not collecting photons. In practice, some of these parameters are very difficult to avoid. One can optimize by placing the telescope in a very good night. Very often one is left with an already existing telescope, or whatever telescope time is given to him/her by a telescope allocation committee. In order to compare existing and future system, we took Eff. as the ration of the time collecting photon versus total exposure time. I.e. a measure of the CCD controller efficiency, ignoring the other parameters.
- Stare mode, where an exposure is taken, the shutter closed, and the CCD readout during a given time. On "normal" telescopes, great care is taken to have a very low readout noise. In survey telescopes, which are usually very fast telescopes, used without filters, the sky background is the dominant source of noise. It is therefore much more efficient to have a fast readout CCD controller.
- Scan mode : In this mode, the shutter is left open, and the CCD readout is performed while the integration is being done. In the sidereal tracking mode, the telescope is simply idle (no tracking), and the stars are drifting across the CCD. If the CCD is rotated so that its line are parallel to the motion of the stars, and if the electric charges are moved synchronously with the star image motion across the CCD, a band of the sky is recorded, at a rate of 15 degrees per hour times the height of the field of view.
- Frame Transfert mode : In order to avoid loosing time reading out the CCD, some CCDs, called transfert frame CCDs can be used. They contain a storage zone to which the image is very quickly transferred (in a few milliseconds). The next exposure can be started, while the storage area is readout. No time reading out is lost.
I have not pushed further yet the adequacy of the survey system to the current distribution of unknown asteroids, nor have I taken care to remove trailing losses effects while detecting fast moving objects.
Technical characteristics of currently existing NEO survey telescopes, with corresponding factor of merit :
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detector size (mm) |
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62x62=3775 |
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31x62=1887 |
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31x31=944 |
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31x31=944 |
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49x49=2416 |
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62x62=3775 |
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47x61=2890 |
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Notes :
- I have not taken central obstruction into account, not having the correct data for all the telescopes.
- NEAT has access problems to the telescope, but may resume to 6 nights per month.
- SCAP does have access to the Xinglong Schmidt telescope only when conditions are not good enough for the main cosmology project undertaken at this telescope (non photometric nights, twilight, full moon, etc...).
- Spacewatch will improve its CCD array, and will also start using a larger 1.8meter telescope.
- LINEAR is actually a 1960x2560 pixels of 24 microns device, used in binned 2x2 mode.
- LINEAR exposure times varies from 4 to 11 seconds, with an average of 6 seconds. The goal is to achieve 600 fields (1200 square degrees) x 5 per night, short or long night ( moon or no moon).
- ODAS has stopped observations in April 99.
What lays ahead in the future :
Included here are the configurations, either new or updated, which should become operationnal in the next few years:
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Notes : the 90% rule of engineering (90% of the project takes 90% of the time, and the remaining 10% take also 90% of the time) prevent to include completion times. Below are some notes concerning the different projects.
- A 4K, 15 microns CCD (Loockeed CCD485A) is in reality 4080 active pixels, hence is 61.2mm wide.
- Catalina will update to a larger Schmidt corrector (and mirror).
- Catalina south is going to use the so called "Upsalla Schmidt" at AAO as is, then change the optical configuration.
- LONEOS will double the current CCD size, replacing their current thick 2x4K by 2 2x4K thinned, as well as increase the readout rate.
- Spacewatch should get a major update, with a focal reducer and a much larger CCD array (in fact a new optical tube).
- Spacewatch II should get into operation. However it is likely that it will use a faster than sidereal readout rate, giving a much larger hourly coverage.
- Bisei ( Spaceguard Japan) has clearly the potential to become a leader in this field. The 50cm telescope does not use the full size of the CCD but is limited to optical field of view of the telescope.
- A second LINEAR telescope should very quickly get into operations
- In March 2000, NEAT should start using a 1.2meter USAFRL telescope, with the same field of view as their current telescope, but during 18 nights per month.
- NEAT should start observing at Palomar with a single or dual 4K camera. In the future, a much larger camera, with ~20 degrees field of view should become available.
- DMT, the dark matter telescope would use an 8.4 primary mirror, for a 6.9 meter effective aperture. The CCD camera has a 550mm circular diameter
About this document :
This is still a very temporary version. Last update, June
23rd 1999.
For any comments or modifications, please send me an
email.