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

TANG Zhenghong, MAO Yindun, LI Yan, YU Yong

Shanghai Astronomical Observatory

2011-10

TANG Zhenghong

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.

Principle of CCD:

(1) Photoelectric effect: photons charges

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

Limited magnitude for 1s exposure

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.

Thanks!