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Intro to HST Data Handbooks 8.0 May 2011
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Introduction to the HST Data Handbooks > Chapter 5: HST File Names > 5.4 Associations

5.4
The ACS, NICMOS, STIS, and WFC3 calibration pipelines sometimes produce single calibrated images from associations of many exposures. For example, a NICMOS, ACS, or WFC3 observer might specify a dithering pattern in a Phase II proposal. Those instruments would then take several exposures at offset positions, and the pipeline would combine them into a single mosaic (suffix “_mos” for NICMOS; suffix “_drz” for ACS and WFC3). In this case, the original set of exposures constitutes the association, and the mosaic is the association product.
Similarly, a STIS observer might specify a CR-SPLIT sequence in a Phase II proposal. STIS would gather several exposures at the same pointing, and the STIS pipeline would process this association of exposures into a single image, free of cosmic rays, that would be the association product (suffix “_crj”).
The COS calibration pipeline instead uses associations to process all of COS science data. Please refer to the Chapter 2 (in Part II of the COS Data Handbook) for more details.
When you search the HDA for observations involving associations of exposures, your search will identify the final association product. The rootnames of association products always end in zero (see Table 5.1 above). If you request both Calibrated and Uncalibrated data from the HDA, you will receive both the association product and the exposures that went into making it. The corresponding association table, stored in the file with suffix “asn” and the same rootname as the association product, lists the exposures or datasets belonging to the association. You can read this file using the STSDAS tprint or tread tasks (see Section 2.2.2). The exposure IDs in the association table share the same “ipppss” sequence as the association rootname, followed by a base 36 number “nn” (n = 0-9, A-Z) that uniquely identifies each exposure, and a character t that denotes the data transmission mode for that exposure (see Table 5.1). For association products and sub-products, the last character will be a number between 0-8.
In practice, STIS stores the exposures belonging to associations differently than the other instruments. The exposures belonging to a STIS association all reside in the same file, while the exposures belonging to an ACS, COS, NICMOS, or WFC3 association reside in separate data files. See the relevant Data Structures chapters (in Part II) for more details.

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