2.3 Data Storage Requirements

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).

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 any prescan and overscan regions have been trimmed during processing.

After calacs 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 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

Detector

Size of FITS File (Sraw)

Size of FITS File (Scal)

Size of FITS Filea(Sdrz)

WFC (2 Chips)

34.4MB

168.0 MB

220.0 MB

HRC (1 Chip)

2.3 MB

10.6 MB

16.0 MB

SBC (1 Chip)

12.2 MB

10.6 MB

128.2 MB

aThe 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

Detector

Xraw

Yraw

Xcal

Ycal

Xdrza

Ydrza

WFC (2 Chips)

4144

2068

4096

2048

4221

4334

WFC1-1K

1046

1024

1024

1024

1063

1029

WFC1-2K

2070

2046

2048

2046

2124

2073

WFC1A-1K

2072

1024

2048

1024

2026

1082

WFC1B-1K

2072

1024

2048

1024

2079

1070

WFC2C-1K

2072

1024

2048

1024

2047

1091

WFC2D-1K

2072

1024

2048

1024

2111

1091

WFC1A-2K

2072

2068

2048

2048

2082

2075

WFC1B-2K

2072

2068

2048

2048

2124

2075

WFC2C-2K

2072

2068

2048

2048

2089

2112

WFC2D-2K

2072

2068

2048

2048

2142

2123

WFC1A-512

2072

512

2048

512

1994

594

WFC1B-512

2072

512

2048

512

2055

576

WFC2C-512

2072

512

2048

512

2027

574

WFC2D-512

2072

512

2048

512

2096

569

HRC (1 Chip)

1062

1044

1024

1024

1163

1134

SBC (1 Chip)

1024

1024

1024

1024

1423

1640

aThe 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