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ACS Data Handbook > Chapter 2: ACS Data Structure > 2.3 Data Storage Requirements

2.3
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 image typically contains two SCI arrays, while 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).
During calacs 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 the overscan regions have been trimmed during processing.
After calacs calibration in the pipeline, AstroDrizzle combines images in an association to create a product with three extensions: science (SCI), weigh (WHT), and context (CTX). 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 to 2 pixels) due to the effects of velocity aberration.
Table 2.4: Size (in MB) of Raw, Calibrated, and Drizzled ACS Images for Each Detector
Size of FITS File1 (Sdrz)
1
The drizzled image sizes are only distortion-corrected and assume no additional rotation, dither offset, or scale change.
 

Table 2.5: Size (in Pixels) for Raw, Calibrated, and Drizzled ACS Images for Each Detector
Xraw
Yraw
Xcal
Ycal
Xdrz1
Ydrz1
1
The drizzled image sizes are only distortion-corrected and assume no additional rotation, dither offset, or scale change. The size of drizzled images will vary slightly due to velocity aberration effects.

While the size of calibrated, drizzled HRC and SBC images is comparable to that of STIS, WFPC2, or WFC3 IR data, the ACS WFC images are over 16 times larger and comparable to that of WFC3 UVIS. The following equation can be used to estimate the minimum amount of free storage required for processing associated ACS data:
 
 
where,
Dmin is the minimum free disk space required for processing,
n is the number of exposures in each “CR-SPLIT” set or repeated exposures for an observation,
m is 2 when both regular and CTE-corrected WFC products are being produced. Otherwise m is 1,
Sraw is the size of the raw exposure (from Table 2.4),
Scal is the size of the calibrated exposure (from Table 2.4),
Sdrz is the size of the distortion-corrected exposure (from Table 2.4),
p is the percentage shift (in pixels) across all dither positions

ACS Data Handbook > Chapter 2: ACS Data Structure > 2.3 Data Storage Requirements

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