The ACS, NICMOS, STIS, and WFC3 calibration pipelines sometimes produce single calibrated images from associations of many exposures. For example, an ACS, NICMOS, 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” or "drc" 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 COS science data. Please refer to Chapter 2 of the COS Data Handbook for more details.
When you search MAST 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 2.1). If you request both raw and calibrated data from MAST, 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. The list of files is stored as a binary table in the first extension (
EXTNAME = ASN) of the association file (see Chapter 3 for more information about data formats). You can read this file using the astropy.io.fits Python package (see Section 4.4). The exposure IDs in the association table share the same “
ipppss” sequence as the association rootname, followed by a base 36 number “
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 2.1). 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 chapters concerning data structure in the instrument-specific data handbooks for more details.
Introduction to the Hubble Space Telescope Data Handbooks
- 1. Obtaining HST Data
- 2. HST File Names
- 3. HST File Formats
- 4. HST Data Analysis
- 5. Observation Logs