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New Observation Results from A Rotating-drift-scan CCD System
TANG Zhenghong, MAO Yindun, LI Yan, YU Yong
Shanghai Astronomical Observatory
2011-10
1 Background
Up to now, the number of space debris larger than 10cm is over 18,000, larger than 1cm is over 100,000. plenty of objects.
Space debris have potential collision risk to
operational spacecraft/satellite. They must be observed precisely. space safety.
Observing faint space debris needs large
telescopes when using normal CCD. too expensive to afford.
Any Solutions?
2 Principle of Rotating-drift-scan (RDS) CCD technique
Drift-scan CCD, also called TDI CCD, was used to observe stars since 1980s, later widely used in survey purpose.
In 2006, the idea of Rotating-drift-scan CCD was presented as a possible solution for observing faint space debris in low orbit.
2V 10V 2V 2V
(a) charges
(b)
2V 10V 10V 2V 2V 10V 10V 2V
2V 2V 10V 2V 2V 2V 10V 2V
(c)
(d) (e)
Principle of CCD: 2) Transfer charges from one trap (pixel) to another
by change voltage of electric pole.
Exposure in Stare mode (normal)
(1) Telescope tracks the object.
(2) The image of one star covers some fixed pixels.
Drift-scan mode
(1) Telescope does not track the object,
(2) The image of one star moves on the CCD,
charges are drifted with the same speed.
Principle of RDS CCD:
Rotate the CCD camera to let direction of charge movement parallel to that of object movement. Stop engines of telescope. Then drift CCD charges with the same speed of the object, i.e. charge-tracking, so the image of the object will be circular.
3 Observational procedure of RDS
(1)Point telescope to first direction that object will appear. Rotate CCD to make the charge movement direction parallel to movement direction of the object. .
(2)Before the object enters FOV, expose short time in stare mode (1s, for stars).
(3)Perform drift-scan mode continuously till the object left FOV.
(4)Expose short time in stare mode again.
(5)Point the telescope to next predicted direction, and rotate CCD to predicted angle.
(6)Repeat step (1) – (5), till the object disappears.
4 Parameters of the new system
A Rotating-drift-scan (RDS) CCD system has been developed in the past two years.
Diameter: 300 mm
Focal length: 250 mm
CCD camera: Apogee U9000
CCD pixel size: 0.012 mm
CCD array: 3056 * 3056 pixel
CCD Fov: 8.4 * 8.4 degree
5 Automatic observation control
The automatic observation program are developed.
Main function includes:
prediction
observation task arrangement
automatic observation.
6 Procedure of data reduction
Detect stars from stare mode frames
Match stars with reference catalog
Detect object from drift-scan mode frames
Calculate positions and magnitude of objects
Since the telescope keeps stable during one round of observation, each pixel of CCD corresponds to the fixed azimuth and altitude. The positions of objects can be calculated with the help of reference of stars.
7 Advantages of RDS technique
(1) No need of precise orbit prediction
The exact time when the object will appear in FOV is not necessary, since drift-scan can be operated earlier than prediction.
(2) Faint object can be got by small telescope
Exposure time of the faint object can be extended through charge-tracking.
(3)Precise position and magnitude can be obtained
Both images of object and reference stars are circular.
8 Observation results
(1) Radar calibration satellite (01520, D=35cm)
Obs.time Range
(km)
Azimuth
(Deg.)
Altitude
(Deg.)
Exp.
Time (s)
SNR
20100427
04:05:38
1280 -98.6 54.7 5.0 79.2
20100503
03:58:34
1562 -58.2 31.1 5.0 42.7
8 Observation results
(2) space debris (13~14magnitude)
Name/
Number
RCS
(m*m)
Obs.time Range
(km)
Exposure
Time(s)
SNR
FENGYUN
1C
DEB/29746
0.028 20100312
01:54:17
3785 15 5.2
Cosmos
1275
DEB/12730
0.15 20100312
04:05:52
1168 8 20
8 Observation results
(3) laser-ranging satellite. (13~14mag)
Name/
Number
Diameter
(cm)
Obs.time Range
(km)
Exposure
Time(s)
SNR
Lageos01
/08820
60 20100808
22:07:15
6200 15 10
Lageos02
/22195
60 20100808
20:56:10
6500 15 13
Stella
/22824
24 20100809
03:39:20
1900 12 25
8 Observation results
Precision estimation
Compare observation result with standard orbit of AJISAI satellite
Obs.date Arc length
(s) sigma_A* (″)
Sigma_H (″)
2011-03-07 520 6.1 6.5
2011-03-28 388 5.8 4.3
2011-03-30 553 6.9 7.3
2011-03-31 456 5.9 3.2
9 To be done later
When the low orbit object pass through the big FOV (~8 degree), their speed and direction are not stable, it is necessary to change them to get better image of objects based on prediction.
When faint images appear in the CCD frames(SNR~3), more powerful detection program is needed.