Instruments Operation Science Information Miscellaneous
| SOT | Solar Optical Telescope - Functions |
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Sequence control of the Focal Plane Package (FPP) observations is managed by observation tables in Mission Data Processor (MDP). Sparate observation tables are prepared for FG observation and for SP observation. The table contains several lists of commands for acquiring observables on a time interval schedule. Commands for taking observables (macro commands) are issued according to the tables, and the FPP takes action in response to them. The contents of the tables are composed from pre-arranged science observing plans and are uploaded from the ground station. Science data (CCD images) are acquired by the FG and SP CCD cameras. Multiple images may be exposed to derive observables such as Dopplergrams and magnetograms. In those cases, exposed data are processed in the FPP in real time to reduce the data amount. The processed science data is then transferred to the MDP via a high-speed parallel interface. Because of limited telemetry downlink bandwidth, data can be compressed in pixel depth (16 to 12 bit compression) as well as in 2-dimension (image compression). MDP reforms compressed data to CCSDS packets and sends them to Data Handling Unit (DHU) for recording in Data Recorder (DR).
The MDP has eight kinds of look-up tables to perform the 16-to-12 bit compression with different compression curves. For image compression of FPP data, two algorithms are available for different compression parameter tables; One is 12bit JPEG DCT lossy compression and the other is 12bit DPCM lossless compression. According to studies with simulated FPP data, filtergram data may be compressed to 3 bits/pixel by the JPEG algorithm and Stokes vector data to 1.5 bits/pixel, when noise due to lossy compression is comparable to photon noise level in the data, although compression ratio is highly dependent upon the nature of the images.
When you plan SOT observations, you should take account of the usage of the data recorder and the schedule of telemetry downlink stations. The capacity of the Solar-B data recorder is 8 Gbits in total, of which 5.6 Gbits may be available for FPP data with an assumption of 70% allocation. The 4M bps high telemetry downloads 1.7 Gbits of FPP data in one ground station contact (10 min duration). If 15 stations are scheduled per day, the FPP can acquire 25.5 Gbits data per day. For example, when we have an observation with the rate averaged for a long period, the output rate of the observation needs to be 300 Kbps after compression. Another extreme example is to perform an observation with bursts of data output, which provides high cadence observation with a large CCD frame size. The possible maximum data rate in post compression is 1.3 Mbps in nominal observations. When we stop XRT and EIS observations, the possible maximum rate increases to 1.8 Mbps. In the burst observation mode, it takes about 1 hour to completely fill in the 5.6 Gbits area of the data recorder. The data will require about three ground station contacs to complete its downlink.