The Asteroid Finder Focal Plane

Transcription

1 The Asteroid Finder Focal Plane H. Michaelis (1), S. Mottola (1), E. Kührt (1), T. Behnke (1), G. Messina (1), M. Solbrig (1), M. Tschentscher (1), N. Schmitz (1), K. Scheibe (2), J. Schubert (3), M. Hartl (3), K. Lenfert (3) (1) DLR e.v. Institute of Planetary Research Rutherfordstraße Berlin/Germany harald michaelis@dlr.de (2) DLR e.v. Optical Information Systems Rutherfordstraße 2, Berlin/Germany karsten.scheibe@dlr.de (3) Kayser-Threde GmbH. wolfratshauser Straße 48, München/Germany js@kayser-threde.de ABSTRACT The DLR Institute of Planetary Exploration has proposed a novel design for a space instrument accommodated on a small satellite bus (SSB) that is dedicated to the detection of inner earth objects (IEOs) from a low earth orbit (LEO). The low pointing stability of the satellite bus, the stray light and thermal environment in LEO represent the major design drivers for achieving the required limiting magnitude of 18.5 (V-band). In order to cope with the design drivers, DLR has proposed a novel focal plane consisting of four Electron Multiplying CCDs (EMCCD) and their associated electronics. 1. INTRODUCTION The DLR Institute of Planetary Exploration has proposed a novel design for a space instrument accommodated on a small satellite bus (SSB) that is dedicated to the detection of inner earth objects (IEOs) from a low earth orbit (LEO). The low pointing stability rate of the satellite bus, the stray light and thermal environment in LEO represent the major design drivers for achieving the required limiting magnitude of 18.5 (V-band). The instrument design is based on a focal plane consisting of four electron-multiplying CCDs (EMCCD). These detectors operate at a high frame rate and very low effective readout noise, in order to compensate the spacecraft s Sensors, Systems, and Next-Generation Satellites XIII, edited by Roland Meynart, Steven P. Neeck, Haruhisa Shimoda, Proc. of SPIE Vol. 7474, 74741F 2009 SPIE CCC code: X/09/$18 doi: / Proc. of SPIE Vol F-1

2 pointing jitter. The telescope optics is based on an off-axis anastigmatic design (TMA). A reflective Schmidt-type corrector plate enables a corrected 2 2 field of view to be achieved by the fast F/3.4 telescope with near diffractionlimited performance. The absence of center obscurations or spiders in combination with an accessible intermediate field plane and exit pupil allow for efficient stray light mitigation. To accommodate the passive thermal stabilization scheme and the necessary structural stability, HB-Cesic was selected as material for the telescope structure and mirrors. This new composite ceramic material is highly promising for space telescope applications. The electronics design comprises high speed signal and data processing chains for on-board acquisition, filtering, accumulation and compression of the CCD data. One of the most important tasks of the on-board processing software is to implement the image stabilization function. For this purpose the images are oversampled, guide stars are automatically identified and tracked, and the individual short-exposure images are shifted and co-added with sub-pixel accuracy. During this process, spurious events as cosmic ray hits or dark spikes are identified and eliminated. Some more detailed instrument design features, as shown in the poster, are described below. 2. RATIONALE Current dynamical models predict a sizable population of asteroids (IEOs) whose orbits lie entirely within the Earth s orbit ( ~103 with D>100m) Those objects are very difficult to be detected from Earth because they always appear close to the Sun. However, an imaging instrument on a small satellite can observe closer to the Sun and for a longer period of time Therefore, DLR has proposed a novel imaging instrument accommodated on a small satellite bus (SSB) that is dedicated to the detection of IEOs from a low earth orbit (LEO) Fig.1: AsteroidFinder small satellite 3. INSTRUMENT KEYS REQUIREMENTS The instrument requires high sensitivity to detect asteroids with a limiting magnitude larger than 18.5m (V-Band) at 30 solar elongation and astrometric accuracy of 1 arcsec (1 σ). This requires a telescope aperture greater than 400cm2, a detector with high quantum efficiency (peak>90%) and low noise, which is only limited by Zodiacal background. The instrument requires a field of view of minimum 2 x2 for sufficient sky coverage (detection efficiency) The instrument has to fit in a small satellite. Therefore the instrument mass is limited to 25kg and the power consumption has to be below 100W. Proc. of SPIE Vol F-2

3 4. DESIGN DRIVER Detection sensitivity requires high suppression of stray light (10 orders of magnitude) originating externally from Sun and Earth illumination in the LEO orbit but also internal stray light coming from bright stars have to be suppressed below the limiting magnitude. The instrument has to cope with the moderate pointing stability rate of the small satellite which is currently specified with 7.5 arcsec/s (3σ). 5. TELESCOPE Fig.2: optical layout of the telescope The telescope is based on an all-reflective three-mirror anastigmatic (Cook) optical design with a Schmidt corrector. It was chosen because it provides: Effective straylight baffling No obscuration High image quality (PSF, distortion) Compact dimensions It is based on all-hb-cesic mechanical design with Si coated HB-Cesic mirrors because of thermal reasons. It permits a purely passive thermal design. The key telescope parameters are summarized in Table 1. Table1: telescope key parameters F.L. 760 mm Effective Aperture 474 cm2 f ratio 1:3.4 FOV 2 IFOV 3.5 /pix Proc. of SPIE Vol F-3

4 6. CCDS, FOCAL PLANE AND READOUT ELECTRONICS Fig.3: configuration of the focal plane and readout electronics The focal plane with the CCD detectors is the key part of the instrument. The CCD detector and the detector electronics has to provide high quantum efficiency and low effective noise at high readout frequency. Therefore, the focal plane consists of four backside illuminated 1kx1k Electron Multiplying CCDs (EMCCD) operating nominally at 5fps to freeze image motion caused by S/C pointing instability rate. The EM gain is adjustable up to 50 resulting in an effective readout noise of below 2 electrons rms at 5.6 MHz readout frequency. Output Fig.4: block diagram of the CCD electronics The resulting signal to noise (see Figure 5) ratio was estimated with the following assumptions: Proc. of SPIE Vol F-4

5 readout noise: MHz F 2 : 2 Dark current: 205 Dark current spike: 5 time average dark current Dark current noise: sqrt of dark current spikes Background: 15 electrons/pix/sec Signal: 34 electrons/pix/sec EMCCD gain: 50 Frame time: 200ms Number of accumulated: (parameter) Temperature: -10 C to -80 C (parameter) Fig.5: Estimated signal to noise ratio It can be concluded that a SNR of greater than 5 can be reached after accumulation of more than 50 images at a temperatures below -60 C. The CCD control- and analog signal processing is provided by four Front-End Electronics (FEE) channels (one for each CCD) and an interconnection module (IM) that acts as interface between the four FEE channels, the DPU and the power supply unit (PSU); see Figure 4 and 6. Fig.6: functional blocks of the CCD electronics Proc. of SPIE Vol F-5

6 7. DIGITAL PROCESSING UNIT (DPU) The DPU interfaces the instrument and the satellite IO. It performs: Preprocessing of the CCD- data: - masking, DSNU,PRNU, - identifying and calculation of guide stars, - centroid / PSF, cosmic hit reduction, - alignment correction, - image accumulation, - data compression) A schematics of the DPU processing chain is shown below in Figure 7. DPU Input Guide stars Output Series of images Cosmic hits Alignment Accumulated, aligned and compressed image Fig.7: DPU processing chain Proc. of SPIE Vol F-6

7 8. ESTIMATED INSTRUMENT RESOURCES The estimated AF- instrument resources are summarized below. Mass: Power: Total: 25 kg Total (nom): 70W Telescope: 21 kg Detector electronics: 31W Electronics: 4 kg DPU+Mem 24W PSU 15W Volume: Data Volume: Telescope: 40x57x54 cm³ Generation rate ~ 1 Mbit/s Electronics: 20x10x10 cn³ REFERENCES 1. Janesick, J. R., Scientific Charge Coupled Devices SPIE Press, 2001 Proc. of SPIE Vol F-7