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In the basic NICMOS data format, shown in Figure 13.4, each readout is represented by 5 arrays, each as a separate extension in the FITS container file. Any of the five arrays may be accessed by specifying its extension number: for example to access the data quality flags, specify the third extension.
The MULTIACCUM mode (Figure 13.5) produces multiple images. The file structure for such a dataset consists of a lattice of the 5 image arrays which are created for each readout, as shown in Figure 13.5.
Figure 13.5: MULTIACCUM Data Format
IRAF Access
Backwards Compatibility
This data structure will be implemented so that the default array is the most meaningful science image array. For example, if you request IRAF to display a dataset, the first science image array would be displayed. Access to the other arrays within a dataset will be via element numbers (1-5; mod 5 for datasets containing multiple readouts) or via a standardized naming convention (see Figure 13.4 and Figure 13.5). It is a critical requirement that existing IRAF tasks be able to access any array in a NICMOS dataset.
FITS File Format
The physical format of NICMOS datasets will be FITS with image extensions. Data will both be delivered to observers and used within IRAF and STSDAS in this format. A modified version of the IRAF kernel has been developed at STScI to support this data format directly. This permits the use of NICMOS data without conversion (i.e., the strfits task is not necessary). This will be distributed with the STSDAS system before Cycle 7.