Users are reminded to consider the large size of WFC exposures when allocating disk space for storing and reprocessing ACS data. Raw images serve as input to the calibration pipeline and have the file sizes (in MB) given in Table 2.4
. A WFC full-frame exposure contains two SCI arrays, while a WFC subarray exposure and exposures from the HRC and SBC detectors each have a single SCI array. The raw image sizes presume that both the SCI and DQ arrays are populated with short integer values, but that the ERR arrays are NULL (all pixels have a value of zero).
processing, the SCI arrays are converted from integer to floating point values. The null ERR array is populated with floating point values, and the null DQ array is populated with integer values. As a result, the size of calibrated images (in MB) is much larger.
The image size in pixels is given in Table 2.5
. Calibrated images taken with the WFC and HRC detectors are smaller in image dimensions than raw images because any prescan and overscan regions have been trimmed during processing.
calibration in the pipeline, AstroDrizzle
combines images in an association to create a product with four extensions: science (SCI), weight (WHT), context (CTX), and header keywords (HDRTAB). Once the distortion is corrected, the size of a drizzled image will be larger (in pixel dimensions and file size) than a calibrated image because the drizzled image's projection on the sky is rhombus-shaped, rather than square. Also, the size of the image offsets, from sub-pixel dithers to image mosaics, will increase the image field of view and hence the image size. The specific dimensions of a drizzled image depends on the image orientation and on which distortion model is in use in the pipeline, and will vary slightly (about 1
pixels) due to the effects of velocity aberration.