Optical Observations at the Zimmerwald Observatory

2008

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12 pages

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The Zimmerwald Observatory, located 10 km south of Bern, belongs to the Astronomical Institute of the University of Bern (AIUB). The main instrument is the 1 m Zimmerwald Laser and Astrometry Telescope (ZIMLAT) that is used for satellite laser ranging observations (SLR) and for optical observations of Earth-orbiting satellites, in particular of space debris in high altitude orbits. Astrometric and photometric measurements, as well as light-curves are acquired while performing optical observations. Additionally, minor planets and Near-Earth objects (NEOs) are in the observation schedule. ZIMLAT is operated every night if weather permits. With the observations acquired at Zimmerwald AIUB ensures the necessary follow-up observations of space debris objects discovered during the ESA surveys performed at the ESA Space Debris Telescope (ESASDT) on Tenerife. Beside this, the AIUB provides observations from the Zimmerwald Observatory to the Pulkovo cooperation of optical observers (PULCOO)....

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conditions. A filter wheel allows observations in different wavelengths reaching from blue to infrared. Switching of the filter can be done within a few seconds controlled by the telescope software. A special shuttering mode (“smear mode”) was developed to register the exposure epoch with an accuracy of about 1ms. An error of about 7ms corresponds to an along-track error of 0.1 arcsec for objects in geostationary orbits.

Figure 2: 1 m ZIMLAT telescope

Figure 3: 0.2 m ZimSMART telescope in completely opened dome Due to the quite large pixel scale of ZimSMART of about 5 arcsec the obtained astrometric positions cannot be compared with the ones from ZIMLAT. After processing, positions of reference stars are usually better than 1 arcsec, but we still observe systematic errors at the periphery of the image. Future improvements of the mapping model should result in a astrometric position accuracy of about 1/10 of the pixel scale. However, it must be kept in mind that the main tasks of ZimSMART are the search for unknown bright objects, and the acquisition of follow up observations for building up and maintaining a catalogue of space objects. For these tasks it is not necessary to reach an astrometric performance level better than a few arcsec.

Figure 4: Process flow for the routine optical observations at Zimmerwald observatory were not found. Planning is usually made before the nightly observations start, but may be run during night as well (or again), if changes in the observations schedule are needed. The planning has to ensure that all data for the nightly work are in place at Zimmerwald observatory and that the observer is instructed if special actions are required. The updated orbital elements from the catalogues, provided from internal work or from external sources, are used to plan the observations of the next observation night (green box). In the planning we used knowledge from past observations to improve the observation schedule. In particular we pay attention to ephemeris quality, determined brightness, or whether objects

Figure 5: ZIMLAT time used for optical observations (hours per night in Zimmerwald) Figure 6 gives the number of position measurements per month, which is the second important performance figure. It is possible, but not shown here, to classify this parameter per observation campaign or per object class, or even for different time intervals than one month. A detailed analysis of Figure 6 must, however, always take into account the actual observation time. We may conclude from the plot that the very high degree of automation allows an average of more than 1300 position measurements per months. This estimated average number of position measurements has to be interpreted carefully, as it is a function of object selection, summed telescope time, and weather conditions. Faint or difficult to search for objects result in less position measurements compared to bright or easy to find re- acquisition objects. In summer the available telescope time is shorter than in winter. Weather conditions (thin clouds, etc.) or the Moon (phase and = appearance) may significantly raise the detection threshold. In March 2005 the prominent peak is partly due to the mentioned SLR outage.

Figure 6: Successful position measurements per month, ZIMLAT only

indicated with blue boxes, those performed at AIUB in Bern with green boxes, while the processes performed at the Zimmerwald observatory are in red boxes. ‘Semi- automated’ means that a few manual interactions are necessary, e.g., to start a script, while ‘automated jobs’ are operated fully autonomously. 7. Examples

Ephemerides have to be determined using the TLEs for the Zimmerwald observations. ZIMLAT or ZimSMART can either use these ephemerides. The observations from both instruments are automatically processed at the observatory and sent to AIUB in Bern. Merging these observations with all available observations of a specific object closes the loop. The main loop for the Zimmerwald observations is indicated in Figure 8 by bold arrows. First, an orbit determination and/or improvement is performed for all objects with new observations. Then, all available orbits in the AIUB catalogue are propagated to 0:00 UT of the following night and TLEs generated

In order to acquire more information on the sizes, shapes and possibly the material of the debris objects with high area-to-mass ratios, light curves are acquired with ZIMLAT. The light curves show a wide variety of signatures, ranging from periodic or random variations of several magnitudes over time spans of a few minutes to constant brightness over 10 minutes. Moreover, the behavior may change completely for one and the same object from one observation to the next. All this is indicative of randomly tumbling objects with complicated shapes. Figure 9 shows two light curves of the object ‘EGEO21’. Both light curves show significant periodic variations. However, the amplitudes and the periods of these variations are very different. The peak-to-peak variations range from 1 to 2 magnitudes and the periods from 50 to 250 seconds. The apparent magnitude of this object is highly variable — although showing distinct periodic signatures over short time spans of a few minutes - indicating an object in a random tumbling motion with a rather complex shape and probably including some highly reflective surfaces.