Space Telescope Science Institute
DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Chapter 6: Reprocessing with the DrizzlePac Package > 6.1 Beyond the Standard Calibration Pipeline

DrizzlePac is written in Python (with algorithms written in C). Its interface is a departure from conventional IRAF usage that’s familiar to most users. To learn more, please refer to Chapter 5 for an introduction to using the TEAL GUI interface and Python commands to run drizzlepac tasks.
Instrument pipelines provide data calibrated to a level suitable for initial evaluation, but may not be suitable for scientific analysis. When users place a data request at the HST Archive, their data is processed by the On-The-Fly Reprocessing (OTFR) system using the best-available software to calibrate data with the best-available reference files. The same calibration software updates are also propagated in releases of the IRAF/PyRAF STSDAS software package. If it has been a long time since their data was retrieved from the Archive, users are encouraged re-request it to ensure that the data contains the most up-to-date header information and calibrations.
There are presently two major steps in the OTFR process: (1) calibration of individual datasets using instrument-specific calibration software, such as calacs for ACS and calwfc3 for WFC3; (2) combining associated data with AstroDrizzle to produce a combined, distortion-corrected, and largely cosmic ray-free image.
The second step cannot succeed without good results in the first. There may be occasions when pipeline calibration of individual images require custom calibration by the user. Instances when On-The-Fly reprocessing is not ideal may include a user’s preference for self-made calibration reference files, the use of non-default calibration switches, and non-default software parameter values. Reason for these actions could be to improve hot pixels and cosmic ray removal, and deal with image persistence or other additional sources of noise.
For example, NICMOS data may require special attention; images from this camera often contain additional signal in the sky, persistence or pedestal effects (differing bias levels between quadrants in the chip) that require extra processing for removal. For more detailed information on recognizing and removing these effects in NICMOS data, please refer to Chapter 4, Anomalies and Error Sources, in the NICMOS Data Handbook.
Even when individual datasets from the Archive appear well-calibrated, users should consider if reprocessing their images with AstroDrizzle on their home machines is necessary. Drizzled pipeline data was created with conservative values (stored in the MDRIZTAB reference file). Some things to consider, depending on the type of data: drizzling with a finer output scale may produce better cosmic ray rejection. Using a smaller pixfrac will reduce correlated noise. Using both a smaller pixfrac and a smaller scale can produce a sharper PSF. In many cases, one can produce better images with a bit of effort. But if upon examining the pipeline output, if a user believes it is sufficient for his or her purpose, there is no need to re-drizzle the data.

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