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| Part II: ACS Data Handbook | |||||
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2.2 ACS File Structure
The ACS calibration pipeline assembles data received from HST into datasets, applies the standard calibrations, and stores the uncalibrated datasets in the HST Data Archive. The structure of these data products is based on the STIS and NICMOS file format and consists of multi-extension FITS files which store science (SCI), data quality (DQ) and error (ERR) arrays, as shown in Figure 2.1.
ACS WFC data come from two CCD chips and are treated as separate observations, with a SCI, DQ and ERR array for each chip, and with both chips being stored in the same FITS file. The result is a FITS file with 6 data extensions plus a global header for a single WFC exposure. The WFC apertures are plotted with respect to the V2/V3 reference frame in Figure 2.2 and are oriented such that the x-axis is approximately to the right and the y-axis is approximately straight up. For pipeline data products, chip 2 is below chip 1 in y-pixel coordinates and was therefore defined as extension 1. Thus, the chip/extension notation is counterintuitive. To display the science image for chip 1, the user must specify the extension 'file.fits[sci,2]'. Similarly, the data quality and error array for chip 1 are specified as [dq,2] and [err,2].
A single HRC or SBC exposure comes from a single chip and has only 3 data extensions plus a global header. While the raw and calibrated WFC images contain 6 data extensions, the drizzled product will always contain 3 data extensions, no matter which detector was used. When WFC data is drizzled, both chips are included in a single FITS extension.
Figure 2.1 illustrates the basic format for storing ACS images. An uncalibrated (RAW) exposure contains a primary header plus, for each chip, a SCI extension (in 16-bit integer format), an empty ERR extension, and an empty DQ array. The 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).
MultiDrizzle removes the effects of the geometric distortion and produces a drizzled (DRZ) product which has the multi-extension FITS file shown in Figure 2.1. The SCI extension contains the distortion-corrected data (32-bit float), the WHT extension contains the weight mask (32-bit float), and the CTX extension contains the context image (32-bit integer). For more information on these DRZ product extensions, refer to "Data Products" in Section 4.4.
Figure 2.1: Data format for calibrated and drizzled ACS modes. Note that for calibrated science data, WFC1 (chip 1) corresponds to extension [sci,2].
Figure 2.2: WFC apertures compared with the V2/V3 reference frame. The readout amplifiers (A,B,C,D) are indicated on the figure. In detector pixel coordinates, WFC1 (chip 1, which uses amps A,B) is on top. The science image for chip 1 is therefore specified as [sci,2].
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |