Monitoring Faint Space Debris with Rotating Drift-Scan CCD

Transcription

1 Monitoring Faint Space Debris with Rotating Drift-Scan CCD Zhenghong TANG, Yindun MAO, Yan LI, Xudong ZHANG, Yong YU Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai200030,China Abstract: Normal drift-scan CCD is often used to survey the sky to get images of stars in the time-delay and integrate (TDI) mode at the apparent sidereal rate. With drift-scan CCD, the track of stars can be realized even when the telescope keeps idle state. The orbits of middle and low orbit space debris are in different directions. To observe these objects for long exposures, drift-scan CCD camera needs to be rotated to make the direction of the pixel line parallel to the orbit of one object. Since the drift-scan mode can track objects for some time, the small telescopes with rotating drift-scan CCD can catch small and faint space debris. According to the preliminary estimation, one 'D=30cm, F=25cm' telescope can observe space debris with size of 20cm*20cm, if using a 3K*3K CCD with a pixel size of 12 micrometer square. 1. Introduction Charge Coupled Devices (CCDs) were invented in the 1970s and originally applied as memory devices. Their light sensitive properties were quickly exploited for imaging applications and they produced a major revolution in Astronomy. They improved the light gathering power of telescopes by almost two orders of magnitude. CCD caused a revolution in observational astronomy, comparable to the invention of the telescope. They provide very high sensitivity, linear response to light, mechanical stability, and mainly astronomical images in the digital form, which can be immediately processed by computers. Drift-scan CCD, also called time delay integration (TDI), is an imaging technique permitting the image of the sky to drift across a CCD at the focal plane of a telescope, while clocking the parallel registers of the CCD at a rate that transports the accumulating charge at the same rate that the star generating that charge is moving across the CCD. In its simplest form, involving a stationary telescope and drift motion due to the Earth's rotation, this requires that the CCD parallel registers are precisely oriented in the east-west direction and that the clocking rate is dependent upon the declination as the inverse cosine of the distance from the celestial equator. More complicated schemes involve precisely driven telescopes and drive-dependent clocking rates.

2 First used in astronomical CCD imaging in the early 1980's at the University of Arizona by Dr. Tom Gehrels of the Spacewatch asteroid search group and Dr. John McGraw of the Zenith Telescope project, scan-mode imaging has several limitations and benefits. Using undriven telescopes, scanning is limited to the equatorial regions of the sky. As one progresses to more northerly or southerly declinations, star images no longer drift across the CCD in straight lines, but begin to move along arcs, producing star images elongated in the north-south direction. Since the correct CCD clocking rate varies as declination, the condition will also arise that the clock rate will be correct for the center of the CCD but not the north or south edge, producing Gaussian star images at the center but not the edges. These limitations may be avoided by using precisely-driven telescopes and drive-coordinated cameras that scan the sky along great circles. There are, however, some wonderful benefits to the technique. Because a resulting image pixel is the sum of charge collected by all of the photosites in the CCD imager column producing that pixel, pixel-to-pixel variations in sensitivity are averaged out, resulting in very beautiful raw images. These raw images may be calibrated using synthetic column-averaged scan-mode flat-fields, producing final images with essentially no noise contributed by the flat-fielding process. These synthetic calibration flats are produced by averaging all of the pixels in the columns of a scan-mode flat image. The result is a single line of column-averaged pixels as wide as the starting image. A synthetic flat image is then created by repeating this same line for the full length of the image. If one starts with a 1024x1024 flat image, the final synthetic flat is effectively the result of averaging 1024 flat images. The noise is reduced by square root of 1024 or 32. Through special control, the lines that charges scan over can be limited to a certain number, that s called part-time drift-scan. With this technique, time will be save when there is no necesity to let charges scan all lines of the CCD. 2. Basic idea of rotating drift-scan The apparent magnitude of space debris is quite faint because of the small size of them. Table 1, 2 and 3 show the apparent magnitude of a 30cm*30cm, 10cm*10cm and 2cm*2cm object on different orbit and different phase angle (the angle of Sun-object-telescope). Here the background of the Sky was assumed as 20mag, and seeing as 2, albedo as 0.5, SNR>3. Table 1. Apparent magnitude of a 30cm*30cm object on different orbit and different phase angle (the angle of Sun-object-telescope). Phase angle Orbit

3 altitude 200km km km km km km km km km Table 2. Apparent magnitude of a 10cm*10cm object on different orbit and different phase angle (the angle of Sun-object-telescope). Phase angle Orbit altitude km km km km km km km km km Table 3. Apparent magnitude of a 2cm*2cm object on different orbit and different phase angle (the angle of Sun-object-telescope).

4 Phase angle Orbit altitude 200km km km km km km km km km To track the space debris in middle and low orbits around the Earth, when the prediction of the orbit is bad, it is necessary to use drift-scan technique. But normal drift-scan CCD only can be used to track the stars at the apparent sidereal rate. To observe objects in middle and low orbits, the CCD cameras should be equipped with a rotating system which can make the direction of pixel line parallel to the orbit. Here is the basic procedure of the rotating-drift-scan (RDS) to observe space debris in middle and low orbit: (1) Point telescope to the first position of the object and rotate the CCD to make its line parallel to the orbit plane; (2) Expose a short time under stare mode (to get images of stars); (3) Drift-scan in part-frame mode continuously (until the images of objects appear and disappear on the CCD frames); (4) Fast reduction on CCD frames of part-time drift-scan mode. (to detect the circular/elliptical images); (5) After the object left the CCD, expose a short time again; (6) Calculate the correction of orbit inclination and speed from >2 images of objects which appear on part-time drift-scan frames; (7) Point telescope to next position, rotate the CCD to the new inclination angle and drift-scan with new speed; (8) Repeat steps (2)~(7) again, till the object moves over the horizon (drift-scan with new speed).

5 The advantages of RDS are the following: (1) No need for good orbit prediction. Which means sigma of inclination prediction is enough, if it is smaller than 1/3 width of the CCD field of view, and when the object appears in CCD is not important. (2) Long exposure time can be realized when observing faint objects. Since time of drift-scan of object can be prolonged when needed, faint objects can be caught when the field of view is big. Objects in higher orbit will pass the FOV for longer time, while low orbit objects with fast speed can be tracked easily. (3) Precise position and magnitude can be obtained with stars from stare mode observation before and after drift-scan mode as the reference, CCD imaging model parameters and their variation can be calculated separately, and the parameters of CCD frames, that the object appears in, can be calculated from time interpolation. (4) Many positions in one observable arc can be obtained to fit the orbit. 3. Preliminary results of RDS system During , a test RDS system was finished under the collaboration between Shanghai Astronomical Observatory (China) and Nikolaev Astronomical Observatory (Ukraine) and was used to observe some middle and low orbit objects. The basic parameters of the system are: (1) Telescope: D=10cm F=50cm Mount: 1.56m telescope (2) CCD: Apogee U9000 pixels number: 3K*3K pixel size: 12um*12um (3) FOV: 4 deg* 4 deg Figure 1 shows the photo of the test system.

6 Figure 1. The 10cm objective with rotating-drift-scan CCD camera mounted on the 1.56m telescope Table 4 gives the statistic of the preliminary observation of middle and low orbit objects with RDS code type Exposure time of Rotating angle Orbit altitude drift-scan (s) (deg) (km) GPS , GPS , MEO , LEO , LEO , LEO , LEO , LEO , LEO , LEO , LEO LEO LEO

7 Figure 2-6 show the CCD frames obtained from RDS system for one GPS satellites and one LEO satellites. Figure 2. CCD frame of 1s exposure with stare mode Figure 3. Drift-scan image of GPS at time T1 (30s exposure)

8 Figure 4. Drift-scan image of GPS at time T2 (30s exposure) Figure 5. Drift-scan image of LEO at time T3 (2s exposure)

9 Figure 6. Drift-scan image of LEO at time T4 (2s exposure) 4. Discussion Table 5 lists limited magnitude of different telescopes with different exposure time (equipped with a U9000) D (mm) F (mm) FOV (deg) L-mag (exp=60s) L-mag (exp=10s) L-mag (exp=5s) L-mag (exp=1s) mag 14.1mag 13.4mag 11.6mag mag 15.1mag 14.4mag 12.6mag mag 15.7mag 14.9mag 13.2mag mag 17.1mag 16.3mag 14.6mag Table 6 gives the apparent magnitude of different size objects of different orbit size(cm) orbit(km) 2*2 5*5 7.5*7.5 10*10 20*20 30*30 50*

10 Table 7 gives the time that different orbit objects pass the FOV of different telescopes (unit is second) Orbit(km) D=250mm D=400mmD=600mm D=1000mm s From tables 5-7, it can be concluded that a telescope of 'D=30cm, F=25cm' can observe space debris with size of 20cm*20cm, if using a 3K*3K CCD with pixel size of 12 micrometer square. A D=400mm, F=600mm telescope equipped with a U9000 CCD can catch space debris with the size of ~10cm*10cm. Reference: Gehrels, T., 1986, Instrumentation and Research Programmes for Small Telescopes. Proceedings of the 118th. Symposium of the International Astronomical Union, held in rch, New Zealand, December 2-6, Editors, J.B. Hearnshaw, P.L. Cottrell; Publishers, D. Reidel Publishing Company, Dordrecht, Holland. Sold and distributed in the U.S.A. and Canada by Kluwer Academic Publishers, Norwell, Massachusetts, ISBN # LC # QB88.I , P. 285, 1986 Wetterer, Charles J.; McGraw, John T.; Hess, Thomas R.; Grashuis, Randy, 1996, Astronomical Journal v.112, p.742 McGraw, J. T.; Angel, J. R. P.; Sargent, T. A., In: Conference on Applications of Digital Image Processing to Astronomy, Pasadena, Calif., August 20-22, 1980, Proceedings. (A ) Bellingham, Wash., Society of Photo-Optical Instrumentation Engineers, 1980, p