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WFC3 Data Handbook V. 4.0 |
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All WFC3 science data products are two-dimensional images that are stored in Multi-Extension FITS files, which can be manipulated directly in the STSDAS environment in either PyRAF or IRAF. The structure of these data products is based on the ACS, NICMOS, and STIS file format. All images taken during an exposure are bundled in a single FITS file, with each image stored in a separate FITS image extension (see Section 2.2 of the Introduction to the HST Data Handbooks). The WFC3 file structure differs for UVIS and IR data, as explained in the following sections.The WFC3 UVIS detector is similar in structure to the ACS WFC detector, with two chips butted together to form a complete detector array. As shown in Figure 2.1, each chip has 4096 x 2051 imaging pixels, with 19 rows and 30 columns of virtual overscan at the long and short inside edges respectively, and 25 columns of physical overscan on each side. As a result, full-frame raw images have a total of 4206 x 4140 pixels, and after overscan subtraction in the calibration process, calibrated images have a total of 4096 x 4102 pixels.Figure 2.1: Schematic of a raw, full-frame WFC3 UVIS image.The UVIS detector operates only in ACCUM mode to produce time-integrated images. As with the ACS WFC, the data read from the two chips are stored in separate image sets, or "imsets" (see Section 2.2 of the Introduction to the HST Data Handbooks) within a single FITS file. Each imset contains three data arrays that are stored in three separate image extensions:For a single full-frame UVIS exposure, this results in a FITS file containing the following: the global or primary header unit, and 6 image extensions, 3 for each imset corresponding to each of the chips of the detector. As seen in Figure 2.1 CHIP1 (UVIS1) is above CHIP2 (UVIS2) in y-pixel coordinates, but it is stored in imset 2 in the FITS file, shown graphically in Figure 2.2. Thus, the chip-extension notation is counterintuitive. To display the science image for CHIP1, the user must specify the second science extension "file.fits[sci,2]". Similarly, the data quality and error arrays for CHIP1 are specified as "[dq,2]" and "[err,2]", respectively. Note that subarray UVIS readouts contain only 3 data arrays, because the data come from only one chip.Figure 2.2: Format for WFC3 UVIS data. Note that for UVIS data, UVIS1 (CHIP1) corresponds to extension [sci,2].The WFC3 IR channel uses a 1024 x 1024 pixel detector. Reference (bias) pixels occupy the 5 rows and columns on each side of the detector, thus yielding bias-trimmed images with dimensions of 1014 x 1014 pixels, as shown in Figure 2.3.Figure 2.3: Format of a raw full detector WFC3 IR image.Figure 2.4: Format for WFC3 IR data. Note that for IR data, readouts are stored in reverse chronological order.This array contains 16 independent flags indicating various status and problem conditions associated with each corresponding pixel in the science image. Each flag has a true (set) or false (unset) state and is encoded as a bit in a 16-bit integer word. Users are advised that this word should not be interpreted as a simple integer, but must be converted to base-2 and each bit interpreted as a flag. Table 2.5 lists the WFC3 data quality flags.
Table 2.5: WFC3 Data Quality flags. This array is present only for IR data. It is a 16-bit integer array and contains the number of samples used to derive the corresponding pixel values in the science image. For raw and intermediate data files, the sample values are set to the number of readouts that contributed to the science image. For calibrated files, the SAMP array contains the total number of valid samples used to compute the final science image pixel value, obtained by combining the data from all the readouts and rejecting cosmic ray hits and saturated pixels. Similarly, when multiple exposures (i.e., REPEAT-OBS) are combined to produce a single image, the SAMP array contains the total number of samples retained at each pixel for all the exposures.