NICMOS data are represented by five data arrays for each readout. These arrays contain the:
Each downlinked readout is always represented by these five data arrays. In the basic NICMOS data file format, shown in Figure 12.4, each of these data arrays is stored as a separate image extension in the FITS container file. The MULTIACCUM
mode ( Figure 12.5) produces multiple images. The file structure for such a dataset consists of a lattice of the 5 data arrays that are created for each readout, as shown in Figure 12.5. Compact FITS representations are used to store arrays in which all elements have the same value.
The science image contains the information recorded by the NICMOS detector. The data may be represented as counts (i.e., data numbers) or as count rates (i.e., data numbers per second). Generally the latter is desirable since it is easier to interpret in mosaiced datasets and corresponds closely to flux.
The error array contains an estimate of the statistical uncertainty at each pixel. It is expressed as a real number of standard deviations. This is a calculated quantity based on a noise model of the instrument and its environment.
The data quality flags array provides 16 independent flags for each pixel. Each flag has a true (set) or false (unset) state and is encoded as a bit in a 16 bit (short integer) word. Users are advised that this word should not be interpreted as an integer. NICMOS Data Quality Flag values are summarized in the NICMOS section of the HST Data Handbook.
The samples array is used for one of two purposes:
MULTIAC
C
UM
mode), the samples array denotes the number of samples for each pixel in the corresponding science image.
The integration time array contains the total integration time at each pixel. While initially a simple parameter in some observing modes, combining datasets into composite mosaics and using the information obtained by multiple non-destructive readouts during an integration requires us to keep track of the actual exposure time for each pixel. This array is useful for simple conversions between counts and count rates.
Figure 12.5: Data Format for MULTIACCUM Mode
The physical format of NICMOS datasets is FITS with image and table extensions. Data is delivered to observers and used within IRAF and STSDAS
in this format. The FITS image kernel that became available in IRAF
v2.11 supports the reading and writing of this data format directly. This permits the use of NICMOS data without conversion (i.e., the strfits task is not necessary and not recommended).
Individual FITS image and table extensions are accessed from IRAF tasks by appending to the file name either the index number of the extension, or the combination of the extension name (e.g., SCI, ERR, DQ, SAMP, or TIME) and the extension version number (see Figure 12.4 and Figure 12.5)
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