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Hubble Space Telescope (HST) is an orbiting astronomical observatory operating from the near-infrared into the ultraviolet. Launched in 1990 and scheduled to operate at least through 2020, HST carries and has carried a wide variety of instruments producing imaging, spectrographic, astrometric, and photometric data through both pointed and parallel observing programs. MAST is the primary archive and distribution center for HST data, distributing science, calibration, and engineering data to HST users and the astronomical community at large. Over 500 000 observations of more than 30 000 targets are available for retrieval from the Archive.

May 14, 2018

This is to notify HST users that all observations obtained or reprocessed in the approximate timeframe from June 2017 through May 2018 will be reprocessed to ensure that the Universal Time (UT) timestamps on all associated data products are accurate.

The HST Science Operations Ground System uses a set of time correlation calibration parameters to translate scheduled command activity times between Universal Time (UT) and spacecraft clock counts for execution by the spacecraft and instrument computers. Clock correlation is required to maintain the proper UT time assignment because of the intrinsic on-board clock drift rate. HST engineers recently discovered that the science ground system has been using an erroneous set of time correlation parameters for the past 11 months. This has introduced small, but increasing errors in the assignment of execution activity UT times to associated data products ranging from ~0.5 seconds on day 2017.179 (June 28, 2017) up to ~2.75 seconds when the error was corrected on day 2018.124 (May 4, 2018).

The data reprocessing has begun and should be completed within about 2 months for all affected datasets. STScI expects that this will have little impact on most data, but we encourage users to examine their data products. We apologize for this oversight and any inconvenience this may cause in your research efforts.

Track reprocessing progress here:

Please contact the archive helpdesk ( if you have any questions regarding this issue.

Bill Workman and Faith Abney

Real Time Satellite Tracking for HST.

Map of HST observations