2.3 Data Storage Requirements

Users are reminded to consider the 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 image files (Table 2.4) 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: Minimum Size (in MB) of Raw, Calibrated, and Drizzled ACS Images for Each Detector

Detector	Size of FITS File (S_raw)	Size of FITS File (S_cal)	Size of FITS File^a(S_drz)
WFC (2 Chips)	34.4 MB	168.0 MB	220.0 MB
HRC (1 Chip)	2.3 MB	10.6 MB	16.0 MB
SBC (1 Chip)	2.1 MB	10.6 MB	28.0 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	X_raw	Y_raw	X_cal	Y_cal	X_drz^a	Y_drz^a
WFC (2 Chips)	4144	2068	4096	2048	4221	4334
WFC1-1K^b	1046	1024	1024	1024	1063	1029
WFC1-2K^b	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
WFC1-512	534	512	512	512	531	512
WFC1-1K	1046	1024	1024	1024	1063	1030
WFC1-2K	2070	2046	2048	2046	2127	2076
WFC2-2K	2070	2046	2048	2046	2090	2109

^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, so those shown are approximate and representative.

^bThese are available but unsupported modes. Subarrays at the bottom of the table below SBC are the old WFC subarrays.

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 raw ACS data, since they do NOT have Hubble Advanced Product (HAP) headerlet extensions:

$\begin{array}{l}\displaystyle D_{\mathrm{min}} = nS_{\mathrm{raw}} + m(1+n)S_{\mathrm{cal}} + m(2.1 + p^2)S_{\mathrm{drz}}\end{array}$

where,

D_min 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,
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.

«Previous Next»

ACS Data Handbook
- Preface
  - • Handbook Structure
  - • Typographic Conventions
- • Acknowledgments
- • Change Log
- Chapter 1: ACS Overview
  - • 1.1 Instrument Design and Capabilities
  - • 1.2 Basic Instrument Operations
- Chapter 2: ACS Data Structure
  - • 2.1 Types of ACS Files
  - • 2.2 ACS File Structure
  - • 2.3 Data Storage Requirements
  - • 2.4 Headers and Keywords
- Chapter 3: ACS Calibration Pipeline
  - • 3.1 Mikulski Archive for Space Telescopes - MAST
  - • 3.2 Pipeline Overview
  - • 3.3 Structure of calacs
  - • 3.4 calacs Processing Steps
  - • 3.5 Manual Recalibration of ACS Data
- Chapter 4: ACS Data Processing Considerations
- Chapter 5: ACS Data Analysis
  - • 5.1 Photometry
  - • 5.2 Astrometry
  - • 5.3 Polarimetry
  - • 5.4 Coronagraphy
  - • 5.5 Ramp Filters
  - • 5.6 Spectroscopy with the ACS Grisms and Prisms

2.3 Data Storage Requirements

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