THE OBSERVATIONS OF THE NEAR–EARTH OBJECTS WITH THE ZA–320M PULKOVO TELESCOPEDevyatkin A.V., Aleshkina E.Yu., Barshevich K.V., Baturina G.D., Bekhteva A.S., Gorshanov D.L., Krakosevich O.V., Kouprianov V.V., L’vov V.N., Smekhacheva R.I., Sochilina A.S., Tsekmejster S.D.
The Russian Academy of Sciences Central Astronomical Observatory at Pulkovo
The Cassegrain ZA-320M Pulkovo telescope (D=32cm, F=320cm) is mainly used for observations of the Near-Earth Objects (NEOs are asteroids and comets), but also some other Solar system bodies, including geostationary objects (GEOs), constitute a considerable part of our observation program.
The scientific activities include the observations, their ephemeris support and processing (L’vov et al., 2002). Great attention is paid to the real nature of the objects and the methodology of their observations.
Now the ZA-320M is a fully automated telescope (Kanaev et al., 2002, Devyatkin et al., 2004). It operates in accordance with the pre-defined schedule by an optimal choice of the observation conditions for each object. There is also the possibility of the detailed remote control of, and the manual intervention into, the sequence of operations. All facilities including the imaging CCD camera, the pointing engine, the guidance (tracking), the filter wheel, and the dome are mechanized and can be remotely controlled. The precise time is maintained by the local Pulkovo time service.
Since April 2005 the ZA–320M telescope is equipped with the FLI IMG1001E CCD camera (1024x1024 of 24? pixels). As compared with the former SBIG ST–6 camera, the field of view (28' x 28') is increased by an order of magnitude. The camera has a lower readout noise, higher sensitivity, and faster readout (of less than 3 seconds). All these features help to do more accurate measurements with an increased number of reference stars, to observe fainter objects (now the limiting magnitude is equal to about 20m.5 with a 5 minute exposure), and rapidly changing events.
The CCD frames are processed by the APEX and APEX-A software packages (Devyatkin et al., 2000, Devyatkin et al., 2004) that can determine the coordinates and magnitudes of objects. APEX-A can use various modern star catalogs (USNO–A2.0, USNO– B1.0, UCAC–2) with a choice of the objects’ approximation model and a method of reduction. The output data are presented in the standard MPC format.
The Table 1 illustrates some results of NEO observations. The column headers are as follows: Object, an object’s designation; N, a number of observations; (O - C)αcos δ, the total mean value of (Î–Ñ) right ascension differences; σαcos δ, the mean error of a single RA. Observation; (O - C)δ, the total mean value of (Î–Ñ) declination differences; σδ, the mean error of a single Decl. Observation; Δm, the object’s magnitude range.
Table 1. Some results of NEO observations at Pulkovo
Some objects with variable brightness were also observed. For example, the asteroid 1999 HF1 appeared to be a quasi-variable object with a period near 20 minutes. The amplitude varies from date to date in the range of 0.1m – 0.4m. The total instrumental frequency band (~350–1000 nm) was used. The mean photometric accuracy of these observations is 0.06m. Fig. 1 illustrates the brightness variation for this object within one of the observation series.
Fig. 1. The brightness variation of 1999 HF1 during 40 minutes of observations
The ephemeris support is based on the EPOS software package (L’vov et al., 2005). The EPOS package contains the database for about 300 thousand asteroids and 500 comets. Currently there are 3.5 thousand near–Earth asteroids (NEAs) in the database. The data are permanently updated using several well- known sources including the MPCs and Dr E. Bowell’s catalogs. It is possible to distribute the objects of various classes to different catalogs, to work with an user’s orbit elements for real and model objects, and to get selection in accordance with various conditions. The EPOS package provides an effective ephemeris support for observers. For example, it is possible to get a list of objects visible within the specified sky area at a pre-defined time-moment, or a list of objects observable in a specified place and night, or a current list of potentially hazardous objects, and of close approaches of asteroids to the major planets within the specified time-span. A high-precision ephemeris with many useful parameters may be computed for any object. The program also helps to estimate the observation accuracies, to reject observations with large errors, and to identify an object. Finally, the EPOS package can generate the impressive pictures of orbital motions of several objects in space and their apparent motions among the stars on the sky. Besides the incorporated data, the EPOS package can use various modern numerical ephemerides and star catalogs distributed on CDs.
Now the ephemeris support covers all NEOs accessible for observations at Pulkovo, including the objects discovered most recently. The observations at Pulkovo are not always feasible due to the weather conditions or rapidly varying brightness of NEOs. Nevertheless, the ZA–320M telescope shows a high level of readiness and productivity. This instrument is the leading one in NEA observations, being made in the CIS, and takes the 27th place in the world rating as for May 1, 2005. This fact is illustrated by Table 2.
Table 2. NEODys Database Statistics of NEA Observations for May 1, 2005
During the years 2001-2005 more than 8000 observations of the Solar system bodies were made including more than 4000 observations of 450 NEAs and about 1000 observations of 21 comets. The observations were processed, the results being sent to the Minor Planet Center (see the MPC Circulars in the reference list). The mean accuracy of one observation varies in the range of 0.2'' through 0.4'' and depends mainly on the brightness and angular motion of an object.
Much attention is paid to investigation and improvement of the ZA–320 telescope automatic control system and of the observation methods for obtaining the high quality astrometric and photometric results.
In addition to the above mentioned follow–up activity, experimental observations of other types are in progress now. The first one is the astrometric monitoring of the close apparent approaches of asteroids to the stars, or to each other. It is high-accuracy astrometry with only one reference star or with no stars. The apparent angular distances between two or more objects are measured; this value being a function of orbital elements. The second one is related to the photometry by occultations of stars by asteroids. These two methods are tested. The first one is using the window with a decreased number of pixels of the CCD-matrix. This helps to reduce the readout-time to reasonable values. The second one is related to measurements of a broken star trace obtained by observing with the switched-off clock drive.
In spite of the most northern location and unstable weather, the Central (Pulkovo) Astronomical Observatory now proves to be one of the efficient world NEO follow–up centers. This activity will be more intensive (including the regular GEO observations) after reconstructing and putting into operation the MTM-500 telescope.
Acknowledgements. This work is partially supported by the INTAS-01-0669 grant.
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