2.2 ACS File Structure

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, it is fetched from the MAST static archive unless newer versions of the pipeline and/or calibration reference files are present. In the latter case, the requested data will be moved to the top of the reprocessing queue.

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.) Single quotes around the extension names are required in Python usage.

  • Global header (PRIMARY): this is always in extension [0], and contains a selection of header keyword values applicable to all information in the FITS file.
  • Science image ('SCI'), error array ('ERR'), and data quality image ('DQ'):

- HRC and SBC are single-detector channels. For these channels, the science image is in extension [1] or ['SCI',1], error array in extension [2] or ['ERR',1], and data quality array in extension [3] 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 [1] or ['SCI',1], error array in extension [2] or ['ERR',1], and data quality array in extension [3] or ['DQ',1]. For WFC1, the science image is in extension [4] or ['SCI',2], error array in extension [5] or ['ERR',2], and data quality array in extension [6] 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 documentation.

- 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 extensions are tabular data required only for WFC, with one extension per chip axis. They are filter-independent corrections for the CCD pixel-grid irregularities resulting from the manufacturing process and contain both X and Y corrections for each WFC CCD chip.

- The ['WCSDVARR'] fits extensions, with one extension per chip axis, hold tabular data which describe small-scale distortions due to filter-dependent non-polynomial distortion corrections which include both X and Y corrections for each WFC CCD chip. Therefore, a WFC flt.fits/flc.fits image has four ['WCSDVARR'] extensions while HRC and SBC only have two ['WCSDVARR'] extensions.

- If the data were reprocessed with a new distortion correction 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 2.1:Data Format for Calibrated and Drizzled ACS Modes


For calibrated science data, WFC1 (chip 1) corresponds to extension ['SCI',2] or [4].

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.

Figure 2.2:WFC apertures Compared with the V2/V3 Reference Frame



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[4]. The WFC1 error and data quality arrays are specified as ['ERR',2] (or [5]) and ['DQ',2] (or [6]), respectively. The readout amplifiers (A,B,C,D) are indicated on the figure. Refer back to Figure 2.1 for the full list of extensions by detector.



 1Simple 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 has become a FITS standard. Representing image distortion corrections using the SIP convention improves the handling of image combination and astrometric information.