Part II: ACS Data Handbook

2.3 Data Storage Requirements

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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. The WFC image includes two SCI arrays, while the HRC and SBC detectors have a single 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 than raw images because the overscan regions have been trimmed during processing.

After calibration, MultiDrizzle combines the science, data quality, and error arrays for each detector into a single science, weight, and context extension. Once the distortion is corrected, the size of the drizzled images will be larger (in pixels and also in MB) than the calibrated images due to the fact that their projection on the sky is rhombus-shaped, rather than square. The specific dimensions of the drizzled image depend on the image orientation and on which distortion model is currently in use in the pipeline and will vary slightly (~1-2 pixels) due to the effects of velocity aberration.

Table 2.4: Size (in MB) of raw, calibrated, and drizzled ACS images for each detector. The drizzled image sizes are distortion-corrected only and assume no additional rotation, dither offset or scale change.

Detector	Size of FITS file (Sraw)	Size of FITS file (Scal)	Size of FITS file (Sdrz)
WFC (2 Chips)	68.7 MB	168 MB	215 MB
HRC (1 Chip)	4.4 MB	10.5 MB	16 MB
SBC (1 Chip)	4.2 MB	10.5 MB	16 MB

 
Table 2.5: Size (in pixels) for raw, calibrated, and drizzled ACS images for each detector. The drizzled image sizes are distortion-corrected only and assume no additional rotation, dither offset or scale change. The size of drizzled images will vary slightly due to velocity aberration effects.

Detector	Xraw	Yraw	Xcal	Ycal	Xdrz	Ydrz
WFC (2 Chips)	4144	2068	4096	2048	4213	4240
HRC (1 Chip)	1062	1044	1024	1024	1164	1138
SBC (1 Chip)	1024	1024	1024	1024	1397	1341

 

While the size of calibrated, drizzled HRC and SBC images is comparable to that of STIS or WFPC2, the ACS WFC images are over 16 times as large. The following equation can be used to estimate the minimum amount of free storage required for processing associated ACS data:

where:

• D_min is the minimum free disk space required for processing,
• n is the number of exposures in each CR-SPLIT set or REPEAT-OBS set,
• S_raw is the size of the raw exposure (from Table 2.4),
• S_cal is the size of the calibrated exposure (from Table 2.4),
• S_drz is the size of the distortion-corrected exposure (from Table 2.4),
• p is the percentage shift (in pixels) across all dither positions

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