DARTS of the Month

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Figure 1: 35 images among the 107 objects from the VSOP survey program by Dodson et al. (2008).

Data Archive for HALCA

Science data from HALCA is different from those from other satellites for astrophysics. HALCA, which was the radio astronomy satellite, co-observed the astronomical objects with ground radio telescopes. We combined all of the telescopes with the VLBI (Very Long Baseline Interferometry). We call the Japanese space VLBI mission "VSOP" (VLBI Space Observatory Programme). We can get useful science data after we got the correlation between 2 antennas, not after the acquisition of the satellite data with the data rate of 128 Mbps.

The date from HALCA was received at Usuda 10 m link station in Japan, and other 4 link stations in the world. The length of a pass at the link station is 5 hours, and the data with a single pass was about 300 Gbytes. This was a huge data in 1997 when HALCA was launched. The only solution to record and to send the data to the correlation center, was magnetic tapes in 1997, though we have lager capacity disks, wider bandwidth network, and computer capability now.

The data with the same rate are also produced at each ground radio telescopes joined the observation. The correlation center was located at NAOJ (National Astronomical Observatory in Japan), at Soccoro, US for VLBA, and at the Dominion Observatory in Penticton, Canada. All the correlation process was carried out in each correlation centers. Users of the VSOP received their data from each correlation center.

Typical VSOP observation was about 12 hours, which corresponded to the 2 orbit of HALCA. About 10 ground radio telescopes are joined the observation. Each telescope produced the data with 128 Mbps data rate, and the total output of the data was about 7.5 Tbytes. We can reduce this huge data to about 2-20 Gbytes with the correlation process. These data recorded as FITS-IDI (Interferometry Data Interchange) format, and archived at each correlation center. We can read FIT-IDI format by a interferometry data reduction package called AIPS (see: http://www.nrao.edu/aips/). Some of the person tends to use difmap(see: ftp://ftp.astro.caltech.edu/pub/difmap/difmap.html) for imaging instead of AIPS.

VSOP was basically a telescope open to all astronomers. The proprietary period of the data right for PI (Principle investigator) was 18 months after PI received the data from the correlation center. The typical data size for PI was about 10 Gbytes, and it was not good for the network capacity to send the huge data for the users. Archive of the data was done at the correlation center, and the user who want to have the data directly contacted to the correlators and asked them to send the data from them.

We started a project to make on-line data archive for VSOP in ISAS, after we know that the correlation center in Canada closed and stopped their user support at the end of VSOP operation. Now the user can get the data with contacting the space VLBI group in ISAS. We have completed preparation of the on-line data about 70 % of all data and will move on-line data archive system soon, with the collaboration of DARTS system.

Recently the last results of the VSOP survey observation was published. (Dodson et al. 2008, Ap. J. Suppl., 175, 314: Figure 1). We have not only the raw data, but also the calibrated data and the image data. We hope to open those data archives, too.

Yas Murata (ISAS)

July 2008