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Observation of Lunar Impact Flashes with the SPOSH
Camera: System Parameters and Expected Performance
R. Luther (1,2), A. Margonis (2), J. Oberst (2,3), F. Sohl (3) and J. Flohrer (3)
(1) Humboldt Universität zu Berlin, Germany, (2) Technische Universität Berlin, Germany, (3) Institute of Planetary Research, German Aerospace Center (DLR), Berlin, Germany. 1. Introduction
surface temperature) and 5778 K (solar surface temperature): Observations of meteors in the atmosphere of the Earth have a long historic tradition and brought up knowledge of meteoroid population and streams in near Earth space (amongst others). Only recently resulting in 19,300 nm infrared radiation and 500 nm observations of meteoroid impacts on the dark side of visible light peaks, respectively. Thus, as the SPOSH the Moon became technically possible. Since the first camera system is most sensitive to visible light, confirmed Earth based observation of a lunar impact Earthshine is the dominant background signal for our flash in 1999 [e.g. 2] more than 50 impact flashes have been registered [1]. Meteoroids bombarding the Moon are not slowed down by an atmosphere and The amount of background radiation recorded by one impact with high velocities of up to 70 km/s, causing pixel depends on the background emitting area. The a light flash of about 10 to 100 ms duration. area covered by one pixel depends on the Continuous observations of the dark hemisphere of instantaneous field of view (IFOV) that can be the Moon enable the possibility to improve data of the meteoroid population as well as to determine impact time and location which can be used for seismic analysis and interior structure determination. Therefore, it is important to study the various system with dPixel being the size of a pixel (13,6 μm for the parameters that determine the possibility of a SPOSH camera) and f is the focal length of the successful lunar impact flash detection, which we telescope. Greater focal length and smaller pixel size have implemented by numeric simulations. In result in a smaller IFOV, hence, in a smaller surface particular, we want to evaluate the performance of area covered by one detection unit and less the camera head of the SPOSH camera system [3] background radiation. In this way, instrumental set- up influences the limiting magnitude for lunar impact flash detection. However, smaller IFOV results in a 2. Influence of System Parameters
smaller coverage on the lunar surface and therefore in a lower probability of detecting lunar impacts. In order to determine the limiting magnitude of an observational system we simulate the background Another important characteristic of the observational signal seen during the observation. It consists mainly system is the integration time of the camera of two types of radiation: lunar thermal radiation and electronics and CCD. Fastest observations are done the sunlight reflected from Earth to the Moon and by systems with frame rates of up to 60 halfframes back to Earth, called Earthshine. Each type of per second [e.g. 4]. Slower systems would collect radiation is emitted with a characteristic intensity in a more background signal instead of impact flashes certain wavelength. As CCD chips differ in their and, thus, suffer from reduced signal-to-noise ratios. Binning of CCD’s can increase frame rates on cost of wavelength, we evaluate the amount of background resolution, but increases also the lunar surface per electrons by simulating the two main signal spectra detection unit by the binning factor and thus as black body radiation. The wavelengths of compensates the lower integration times (see maximum intensity can be calculated by Wien’s displacement law for temperatures of 150 K (lunar In contrast, changes in telescope aperture have an 4. Summary
effect on the signal-to-noise ratio. A larger aperture directly increases the amount of collected photons Evaluating all observational system parameters is per detection unit. The Poisson-distributed noise important for understanding measurements of lunar increases by the square root of the increased signal. impact flashes and background noise. Impact flash Thus, doubling the diameter will approximately measurements will shed light on the properties of the impacting objects for studies of the origin of the meteoroid population and for assessments of collision hazards. Successful observations rely on improving the signal-to-noise ratio and balancing of lunar impact probability which depends on the 3. Observational System
system limiting magnitude as well as lunar surface coverage. We are planning further observations with the SPOSH camera head at the Liebenhof observatory At the conference, we will present further parameters near Berlin. Two telescopes from the observatory and of our observational system and will discuss its their specifications are shown in Fig. 1. Both allow complete coverage of the dark hemisphere of the Moon. Although the TEC Refractor has a smaller Acknowledgements
aperture, the system could be less sensitive to light reflections due to its conception as refractor and thus This research has been supported by the Helmholtz might suffer less from losses in signal-to-noise ratio Association through the research alliance "Robotic due to reflections from the bright lunar hemisphere. Exploration of Extreme Environments". In contrast, the Baker Ritchey Chrétien has a larger aperture and due to the larger focal length covers less lunar surface per pixel, resulting in reduced References
Earthshine background and better signal-to-noise [1] Bouley, S. et al. “Power and duration of impact flashes on the Moon: Implication for the cause of radiation.” Icarus 218 (2012): 115-124. [2] Cudnik, B.M. et al. “Ground-based Observations of high velocity Impacts on the Moon’s surface – The Lunar Leonid Phenomena of 1999 and 2001.” Lunar and Planetary Science XXXIII (2002): 1329. [3] Oberst, J. Et al. “The Smart Panoramic Optical Sensor Head (SPOSH) – A camera for observations of transient luminous events on planetary night sides.” Planetary and Space Science 59 (2011): 1-9. [4] Yanagisawa M., N. Kisaichi. “Lightcurves of 1999 Leonid Impact Flashes on the Moon.” Icarus 159 (2002): 31-38. Observatory: TEC 140 Apo Refractor (left, 140 mm aperture, 980 mm focal length, 0,81° field of view) and Baker Ritchey Chrétien (right, 250 mm aperture, 1268 mm focal length, 0,62° field of view).

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