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Most of the time delay between the taking of the data and the conversion into images is taken up by the time it takes to deliver the telemetry to the instrument teams.
The process starts inside the spacecraft when the telescope images are converted into telemetry packets by the instrument electronics. These packets are delivered across the spacecraft bus and stored in the solid state recorder until a telemetry connection is available. The recorder can store about 1.5 days worth of data, although this depends on the SECCHI daily scheduling. About once or twice a day a connection is made with one of the NASA Deep Space Network (DSN) antenna stations around the world to downlink the data. The recorder isn't always emptied on every pass, so sometimes the data being downlinked is from today, and sometimes from yesterday or even the day before. Now that the spacecraft are so far away, it takes some effort to get down all the data, and not overfill the recorder.
The telemetry collected from the various DSN stations is then collated together into a single daily telemetry product. This is done in a central facility which handles all the NASA missions supported by the DSN. Duplicate packets are filtered out, and the packets are arranged in time order. This product is then delivered as a file transfer to the STEREO Mission Operations Center, where it is then made available to the instrument teams on the web. The instrument teams then download these files and process them.
All the telescopes onboard STEREO belong to the SECCHI instrument suite, run out of the Naval Research Lab (NRL). Up until the point the data reach NRL, they are still in packet format. It takes specialized instrument-specific software to convert the packets into images, and this is done at NRL. This is typically done after a delay of two days, because it takes that long to get a complete packet file which covers the entire day. Once the telemetry is processed into FITS image files at NRL, they are automatically copied over to the STEREO Science Center and processed into JPEG images. Copying over the data and processing them into JPEG format takes a few hours.
NASA Goddard Space Flight Center
Greenbelt, MD 20771
Source: Private email