3.6	When should I recalibrate my data?

The goal of the STScI WFC3 pipeline is to provide data calibrated to a level suitable for initial evaluation and analysis for all users. When data sets are retrieved from the Hubble Data Archive (HDA) they are always reprocessed from scratch using the latest available calibration reference files and software updates. At times, however, observers may require a detailed understanding of the calibrations and processing applied to their data and the ability to repeat, often with improved products or non-default processing options, the calibration process at their home institution. There are several occasions when the data obtained from the HDA may not be ideal and when off-line interactive processing by the user is required:

•	when running calwf3 with personal versions of reference files or when using new reference files downloaded from the WFC3 Web site not yet available in the pipeline,

•	when running calwf3 with non-default calibration parameter values,

•	when images must be cleaned of artifacts such as new hot pixels or cosmic-rays that are not properly cleaned with automatic processing.

While the final drizzled images from MultiDrizzle are produced using parameters that are suitable for a wide range of scientific applications, there are often times when specific datasets would benefit significantly from further customized processing off-line. For example, a different output pixel scale or image orientation may be desired, image offsets may need to be refined, or cosmic-ray rejection parameters might need to be slightly adjusted. The same versions of calwf3 and MultiDrizzle that are used in the STScI pipeline are therefore also available to users in STSDAS and PyRAF. MultiDrizzle provides a single-step interface to the complex suite of tasks in the STSDAS dither package. It is built around the PyDrizzle software and therefore needs to be executed within the PyRAF environment.

As noted above, if a user is interested in recalibrating existing data to simply take advantage of the latest available calibration reference files or calwf3 software, this can be accomplished by retrieving the datasets again from the HDA, where they will be reprocessed using the On-The-Fly-Reprocessing (OTFR) system, thus avoiding the need to reprocess themselves. This is the best way to obtain the advantages of any recent software upgrades, because updates to calwf3 are only released to the public once or twice per year as part of the STSDAS package.

Reprocessing data to use alternate reference files, however, must be done at the user’s home institution. For example, during the early years of WFC3 use on-orbit, the calibrations will naturally evolve, sometimes fairly rapidly. At times the WFC3 team will release to the public “alpha” versions of new calibrations before they are finalized and put into use in the STScI OTFR system. Alternatively, there may be times when a user finds the need to use a custom bad pixel table to handle spurious conditions in their datasets that aren’t covered by the standard bad pixel table used in the STScI pipeline. On these types of occasions users can rerun their datasets through calwf3 or MultiDrizzle using their own customized reference data. Examples of how to do this are discussed below.