New Observation Results from A Rotating-drift-scan CCD System

Transcription

1 New Observation Results from A Rotating-drift-scan CCD System TANG Zhenghong, MAO Yindun, LI Yan, YU Yong Shanghai Astronomical Observatory

2 TANG Zhenghong

3 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?

4 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-driftscan CCD was presented as a possible solution for observing faint space debris in low orbit.

5 Principle of CCD: (1) Photoelectric effect: photons charges

6 Principle of CCD: 2) Transfer charges from one trap (pixel) to another by change voltage of electric pole. 2V 10V 2V 2V 2V 10V 10V 2V 2V 10V 10V 2V (a) charges (b) (c) 2V 2V 10V 2V 2V 2V 10V 2V (d) (e)

7 Exposure in Stare mode (normal) (1) Telescope tracks the object. (2) The image of one star covers some fixed pixels.

8 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.

9 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.

10 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.

11 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: mm CCD array: 3056 * 3056 pixel CCD Fov: 8.4 * 8.4 degree

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13 Limited magnitude for 1s exposure

14 5 Automatic observation control The automatic observation program are developed. Main function includes: prediction observation task arrangement automatic observation.

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16 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.

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18 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.

19 8 Observation results (1) Radar calibration satellite (01520, D=35cm) Obs.time Range (km) Azimuth (Deg.) Altitude (Deg.) Exp. Time (s) SNR :05: :58:

20 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 Cosmos 1275 DEB/ :54: :05:

21 8 Observation results (3) laser-ranging satellite. (13~14mag) Name/ Number Diameter (cm) Obs.time Range (km) Exposure Time(s) SNR Lageos01 /08820 Lageos02 /22195 Stella / :07: :56: :39:

22 8 Observation results Precision estimation Compare observation result with standard orbit of AJISAI satellite Obs.date Arc length (s) sigma_a* ( ) Sigma_H ( )

23 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.

24 Thanks!