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THE CURRENT STATUS OF THE VSOP MISSION AND SCIENTIFIC OBSERVATIONS

The In-orbit Checkout (IOC) of HALCA is continuing and already several milestones have been achieved. Images of several bright radio galaxies and quasars at 1.6 GHz and 5 GHz have been made with HALCA with the participation of many ground stations, particularly the VLBA. All three correlators are producing good quality visibility data. The pointing of HALCA is better than 5' and is now sufficiently accurate for 5 GHz observations. All tracking station types have recorded data which has been correlated at one or more of the correlators.

There are still problems which must be fixed before HALCA data can be routinely recorded and correlated and full implementation of general observing time (GOT) observations can begin. The estimated date is about November 1, 1997, however limited GOT observations have already begun. The major problems the mission is still trying to resolve are: (1) Some tracking stations are just beginning to come on-line; (2) Observations are limited to small sun- source angle ranges for a variety of reasons; (3) Pointing must be verified with a tracking pass over Japan before an observation can start; (4) The tape copiers are somewhat behind schedule, so the use of general arrays is somewhat limited; (5) The calibration data is of limited quality; (6) Some of the operational elements still need software completion or modification.

THE 22 GHz HALCA SENSITIVITY

One important change in the capabilities of HALCA is the high effective system temperature of 4000K or more for the 22 GHz system. This is more than 20 times worse than expected. This low sensitivity is thought to be caused by >90% absorption of the signal between the feed and the low noise amplifier. It is believed that the loss was caused by a misalignment of the 22 GHz wave-guide during launch. Although more tests and calibrations at 22 GHz will be made, it is likely that only the very strongest sources in the sky can be detected at 22 GHz, and that any successful 22 GHz observations will require considerable effort on the part of mission personnel.

It is unlikely that any 22 GHz proposals will be observed as `normal' GOT observations; hence, most of the highly rated and unique 22 GHz experiments submitted by the scientific community cannot be observed. Since these represent about 50% of the total proposal pool, the mission is considering a possibility of modification of the original AO proposal policy.

EARLY GOT/IOC OBSERVATIONS

The policies for VSOP-PI interaction, until about November 1, 1997, are:

1. The main goal of the observations during this initial period is to improve the reliability of the entire VSOP system as quickly as possible. Hence, any scheduled observation can be cancelled or modified by the mission to meet any immediate needs of calibration. If there is a failure in the space part of an experiment, the ground part will not be correlated unless needed for testing.

2. Although some early HALCA observations overlap VSOP proposals, and are so designated, they are IOC observations and the choosing of resources, scheduling of time and correlation of the data will be handled by the appropriate mission groups. Many engineering tests are being made, tracking station support is not fully developed, and many experiments need multiple correlation passes. Although the mission may image the observations in order to debug the system, those images associated with observations which overlap a GOT proposal will not be used for scientific purposes.

3. For these IOC observations that overlap GOT proposals, should the data be of sufficient quality for scientific analysis, the data will be sent to the PI. If a PI wishes to come to a correlator center (Mitaka, Penticton or Socorro), or to other mission facilities (Calgary, Dwingeloo, ISAS or JPL), please contact the center and the VSOG. In any case, after the data are sent out, interaction with the correlator and mission will be of utmost importance to both the imaging goals and the debugging of the system, including the new AIPS software. If the early IOC observations of any source are not of sufficient quality to fulfill the PIs scientific goals, the source will be reobserved later in the mission.

We thank you for your patience as the In-Orbit Checkout of HALCA continues, and we hope that this clarifies the present and future status of the mission. We look forward to producing exciting images in the very near future. Please send any comments or suggestions to vsog@vsop.isas.ac.jp

Prof. H. Hirabayashi

(Project Scientist)