The ACS calibration pipeline assembles data received from HST into datasets, applies standard calibrations (so that calibrated image header keyword values can be stored in the Archive database), and stores the uncalibrated datasets in the HST Data Archive. The structure of these data products is based on the STIS file formats (and also similar to WFC3 and COS data structures).
Image data and associated data quality information are stored in multi-extension FITS files: the science image (SCI), error array (ERR), and data quality image (DQ), as shown in
Figure 2.1. The global header always has extension [0]. For more information about primary and extension FITS headers, please refer to the
Introduction to the HST Data Handbooks.
ACS WFC data come from two CCD chips; each chip’s data has three components: the SCI group contains the science image, the ERR group contains the error array, and the DQ group has the image data quality flags. Data from both chips are stored in one FITS file—the result, for each WFC exposure is a FITS file with 6 data extensions plus a global header.
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 to 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. To display the science image for WFC1, the user must specify the extension
fits[sci,2] or
fits[4]. Similarly, the error and data quality arrays for WFC1 are specified as
[err,2] (or
[5]) and
[dq,2] (or
[6]), respectively.
Figure 2.1 illustrates the basic format for storing ACS images. An uncalibrated (
raw.fits) image contains a primary header, and, for each chip, a SCI extension (in 16-bit integer format), an empty ERR extension, and an empty DQ array. Calibrated product from
calacs contains a primary header, plus, for each chip, a SCI extension (in 32-bit float format), an ERR extension (32-bit float), and a DQ extension (16-bit integer).
While raw and calibrated WFC images contain six data extensions, the drizzled product will always contain three data extensions.
MultiDrizzle removes the effects of the geometric distortion and produces a drizzled (
drz.fits) product which has the multi-extension FITS file structure shown in
Figure 2.1. The first (SCI) extension contains the distortion-corrected data (32-bit float), the second (WHT) extension contains the weight mask (32-bit float), and the third (CTX) extension contains the context image (32-bit integer). For more information on these drizzled image product extensions, please refer to the
MultiDrizzle Handbook.
