|Space Telescope Science Institute|
|ACS Data Handbook V7.1|
The ACS calibration pipeline assembles data received from HST into datasets, applies standard calibrations (so that calibrated image header keyword values can be entered in the Archive database), and stores uncalibrated datasets in the HST Data Archive. When a user requests data, the OTFR (On-The-Fly-Reprocessing) system delivers freshly-calibrated data, processed by calacs and AstroDrizzle, as multi-extension FITS files.Data from the Archive arrives as multi-extension FITS files that have three main categories, as listed below. (Please see Figure 2.1 for a graphical representation of this information.)
• Global header: this is always in extension , and contains a selection of header keyword values applicable to all information in the FITS file.
- HRC and SBC are single-detector channels. For these channels, the science image is in extension  or [sci,1], error array in extension  or [err,1], and data quality array in extension  or [dq,1].
- WFC data comes from two chips, each with its own science, error, and data quality arrays. For WFC2, the science image is in extension  or [sci,1], error array in extension  or [err,1], and data quality array in extension  or [dq,1]. For WFC1, the science image is in extension  or [sci,2], error array in extension  or [err,2], and data quality array in extension  or [dq,2]. See Figure 2.2 for a visual representation of the WFC chips projected onto the V2,V3 axis.
• AstroDrizzle extensions:
Drizzled data from the pipeline have the suffix drz.fits, and additionally drc.fits for WFC CTE-corrected data. These products are delivered as multi-extension FITS files with three image data extensions: science image [SCI], weight image [WHT], and context image [CTX]. However, AstroDrizzle also adds new FITS extensions to the flt.fits/flc.fits and drz.fits/drc.fits files, as summarized below. For more information on drizzled data, please refer to the The DrizzlePac Handbook.
- For flt.fits/flc.fits images: during OTFR processing, AstroDrizzle updates the WCS of these images with distortion corrections. Linear distortion corrections (scale, rotation, and time-dependent skew) are incorporated into the CD-Matrix. Coefficients for higher order polynomial functions that describe the distortion corrections are stored as SIP1 header keyword values. Some distortion corrections, however, cannot be expressed as equations and have to be stored in array form as FITS extensions.
- The [D2IMARR] FITS extension, only required for ACS/WFC, contains detector column width corrections.
- The [WCSDVARR] FITS extensions hold tabular data describing small-scale distortion effects due to optics, one extension per chip axis. Therefore, a WFC flt.fits/flc.fits image has four [WCSDVARR] extensions while HRC has two [WCSDVARR] extensions. SBC has no residual distortion extensions.
- If the data were reprocessed with a new distortion corrections reference file, a FITS extension called [WCSCORR] is added that contains a history of WCS changes.
- For drz.fit/drc.fits images: a fourth extension has been added, called [HDRTAB], which is a compilation of important header keywords that have unique values for each input image.Figure 2.2 shows the WFC apertures plotted with respect to the V2,V3 reference frame, and oriented such that the x-axis runs approximately towards the right and the y-axis runs approximately straight up. In pipeline data products, WFC2 is displayed below WFC1 (along the y-axis) and is therefore designated as extension 1.In pipeline data products, WFC2 is displayed below WFC1 (along the y-axis) and is therefore designated as extension 1. To display the science image for WFC1, the user must specify the extension fits[sci,2] or fits. The WFC1 error and data quality arrays are specified as [err,2] (or ) and [dq,2] (or ), respectively. The readout amplifiers (A,B,C,D) are indicated on the figure.Simple Image Polynomial (SIP) convention (Shupe, et. al, 2005). This convention has been in use for describing the geometry of Spitzer Space Telescope images, and it is expected to become a FITS standard in the near future. Representing image distortion corrections using the SIP convention will improve the handling of image combination and astrometric information.