Astronomical Experiments for the Chang E-2 Project Maohai Huang 1, Xiaojun Jiang 1, and Yihua Yan 1 1 National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road,Chaoyang District, Beijing 100012, China tel: +86-10-82727382; mhuang@bao.ac.cn Abstract. Two astronomical experiments have been proposed and accepted for further study for the second phase of China s lunar exploration programme (the Chang E Programme), which is envisaged to operate a lander and a rover on the surface of the moon. The first experiment is an interferometer experiment in the very low frequency (VLF, f < 15MHz) regime of radio frequencies with at least degree-level spatial resolution. The goals include observing solar storm activities, Coronal Mass Ejections, Auroral Kilometric Radiation, and planetary radiation in the solar system, studying the origin of Cosmic Rays, spectral properties of pulsars, surveying ionized hydrogen in the Galaxy, and exploring coherent radio emissions. The second experiment is to operate a small optical telescope and conduct continuous precision photometry to study properties of stellar interiors through observations of oscillations in stellar luminosity and surface temperature, and search for extrasolar planets, particularly the telluric planets. INTRODUCTION China s space exploration programme to the moon, the Chang'E Programme, is envisaged to have three phases: the lunar orbiter (Chang'E-1) phase, the lander-rover phase (Chang E-2), and the sample return phase. The lander-rover phase will deploy a lander and an exploring vehicle (the rover) to conduct scientific experiments on lunar surface with a number of objectives. The lander will land on the moon with the rover and provide a stable platform for experiments. The rover is expected to travel with its onboard instruments and could be separated from the lander by a distance of several kilometers. Two astronomical experiments proposed for the Chang E-2 phase have been accepted for further study. They are an interferometer experiment in the very low frequency (VLF, f < 15MHz) regime of radio frequencies with at least degree-level spatial resolution and a small optical telescope and conduct continuous precision stellar photometry. VERY LOW FREQUENCY INTERFEROMETRY The Earth s ionosphere makes it practically impossible to conduct ground-based study of VLF radiation coming from space. Past experiments, such as the Radio Astronomy Explorer-2 mission, have observed in VLF from space at very low spatial resolution of the level of one radian (Alexander et al. 1975). Ground observations in the past either yielded no spectral information or only had space resolution better than a few degrees (see summary in Dwarakanath and Udaya Shankar 1990; Roger et al 1999; and review in Wielebinski 2003). Because of the great brightness of the sky at VLF (10 5 ~ 10 7 K) in order to map the sky at these frequencies the system does not need very long integration time or low noise receiver design to attain adequate signal to noise ratios (Woan 1996). Limited spatial resolution is the observational bottle neck in VLF study. Our experiment will employ at least two sets of dipole antennae, one on the lander and one on the rover of Chang E- 2. VLF radiation are measured off these Hertzian antennae and sampled at Nyquist rate of the highest frequency in the observed bandwidth. The digitized signal undergoes minimum processing before downlinked to the ground and processed. Data processing on the ground will map the sky at different frequencies at angular resolution of at least International Lunar Conference 2005 1
degree-level using interferometry or phased array techniques. The angular resolution is limited by the intrinsic resolution of the array, λ/d, where λ is the observing frequency and D the distance between the two antennae. For 10MHz observation using two antennae 10km apart, the intrinsic angular resolution of the array is better than 0.2 degrees. The following is a simulation of a simplified observing scenario to assess major interferometric performance, assuming that a few minutes of observing is performed every 24 hours during the lunar day (i.e. only when there is sunlight on the landing site) for half a year, with one antenna being moved away on the rover form the one fixed on the lander at a constant speed. Because the dipole antennae are shorter than the working wavelengths, they have have an intrinsic beam that cover most of the visible sky at any moment. Figure 1 shows the u-v coverage of the observation after half a year. It can be seen that the u-v coverage is well distributed in radial and azimuth coverage. FIGURE 1. u-v coverage of simulated observation. Figure 2 shows the synthesized beam shape plotted at different scales. Most power of the beam is concentrated in the main beam, which has a FWHM of 11arcminutes. International Lunar Conference 2005 2
FIGURE 2. The shape of the synthesized beam of the interferometry observation scenario. The FWHM of the main beam is 11arminutes. When this beam is convolved with a simulated double Gaussian source shown in the left panel of Figure 3, the resulting dirty map is shown in the right panel of the same figure. The FWHM size of the source Gaussians is 1 degrees. The two Gaussians are 4.6 degrees apart. The size of the map is 20 degrees on each side. International Lunar Conference 2005 3
FIGURE 3. The left panel shows a pair of simulated sources with Gaussian distribution in brightness. The right panel is the dirty map when the source shown left is convolved with the beam shown in Figure 2. The selection of source configuration in Figure 3 shows large structure inspite of an imperfect beam. It is expected that after CLEAN techniques is applied the map quality will improve significantly. The simulation shows that the interferometry observation is capable of producing reasonable maps of the sky even before cleaning algorithms is applied. The effect of systemic errors in clock synchronization, baseline and antennae orientation, and uncertainties in electric properties of lunar surface environment tend to reduce the final spatial resolution and position accuracy of sources in the sky. These factors will be analyzed in later works. Degree-level resolution in the VLF regime allows study of a broad range of phenomena. Promising areas include solar and solar system observations and Galactic phenomena studies. For observing within the solar system, the objectives include solar storm activities, Coronal Mass Ejections, Auroral Kilometric Radiation from the Earth, and planetary radiation in the solar system. For Galactic sources, studying objectives include the origin of Cosmic Rays, spectral properties of pulsars, surveying ionized hydrogen in the Galaxy, and exploring coherent radio emissions. LUNAR-BASED OPTICAL TELESCOPE The second experiment, the Lunar-based Optical Telescope (LOT), is to operate a small (~300mm diameter) optical telescope to conduct continuous precision photometry study. it aims at two major scientific objectives: 1. studying properties of stellar interiors through observations of oscillations in stellar luminosity and surface temperature. 2. Search for extrasolar planets, particularly the telluric planets. Because of stable moon surface, lack of atmosphere interference, and long observable window (typically around 14 days), brightness variation of sub-10ppm can be observed. LOT is also capable for making ultra-high precision monitoring of other variable objects from the surface of the moon. International Lunar Conference 2005 4
FIGURE 4. Lunar-based Optical Telescope (LOT) assembly The scientific objectives have three main requirements to the LOT: 1. Photometric precision must reach 10ppm for stars brighter than V=4; 2. Temporal coverage should be as long as possible, in the case of a lunar-based telescope, should be around 10-14 days; 3. Sky coverage of the LOT must provide adequate sources for the observations during the life time of the telescope. TABLE 1. Preliminary Specifications of the telescope Parameters Specifications Diameter 300mm f-ratio of the primary mirror f/1.5 Optical design Ritchey-Chretien reflector system f-ratio f/8 scale at Cassegrain focus 85.9arcsec/mm mount modified Az-Alt mount, automatic pointing and tracking, tracking accuracy is better than 10 arcsec/sec Detector Marconi 47-20 frame transfer CCD, 1024x1024, back illuminated, pixel size 13x13micron field of view 19.1 x 19.1 arcmin (with Marconi 47-20) working wavelength 380 700 nm Mass and dimensions International Lunar Conference 2005 5
telescope OTA w/baffle and shield telescope mount telescope OTA telescope baffle and shield telescope mount 25kg 15kg 380mm X 710mm TBD 360mm X 580mm CONCLUSIONS Two astronomical experiments are proposed and accepted for further study for the second phase of China s lunar exploration programme, one experiment is an interferometer experiment in the very low frequency (VLF, f < 15MHz) radio frequencies, the other experiment is to operate a small optical telescope and conduct continuous precision photometry. We discuss and describe the design and objectives of the experiments. ACKNOWLEDGMENTS Many helpful suggestions have been contributed by colleagues of the community, including Profs. Rendong Nan, Jian Zhang, and Huli Shi, who we are indebted to. The studies have been supported by NAOC and LEPC. REFERENCES Alexander J.K. et al., Scientific instrumentation of the Radio-Astronomy-Explorer-2 satellite, Astro. & Astrophys, 40, 365-371 (1975) Dwarakanath, K.S., and Udaya Shankar, N., A Synthesis Map of the Sky at 34.5 MHz, JApA, 11, 323-410 (1990) Roger, R.S., et al, The Radio Emission from the Galaxy at 22 MHz, Astro. and Astrophys. Supp. 137, 7-19 (1999) Wielebinski, R., History of Radio Continuum Survey in proceedings of The Magnetized Interstellar Medium, edited by B. Uyaniker, W. Rich & R. Wielebinski Woan, G., Design Considerations for a Moon-Based Radio Telescope Operating at Frequencies below 16 MHz in Proceedings of Large Antennas in Radio Astronomy, Edited by C. G. M. Van't Klooster and A. Van Ardenne, 1996, p.101 International Lunar Conference 2005 6