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| Introduction to Reducing HST Data | |||||
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B.3 Associations
The NICMOS, STIS, and ACS calibration pipelines sometimes produce single calibrated images from associations of many exposures. These associations allow HST pipeline processing to proceed further than it had in the past. For example, a NICMOS or ACS observer might specify a dithering pattern in a Phase II proposal. NICMOS or ACS would then take several exposures at offset positions, and the pipeline would combine them into a single mosaic (suffix
mos). 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 (suffixcrj).When you search the Hubble Data Archive with StarView 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 B.1 above.) If you request both Calibrated and Uncalibrated data from the Archive, you will receive both the association product and the exposures that went into making it. The corresponding association table, located in the file with suffix
asnand 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 2.1.2 in the HST Introduction, Part I). 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 (see Table B.1).In practice, STIS stores the exposures belonging to associations differently than NICMOS or ACS. The exposures belonging to a STIS association all reside in the same file, while the exposures belonging to a NICMOS or ACS association reside in separate datasets. See the relevant Data Structures chapters (in Part II) for more details.
Information on the exposures belonging to an association is also available through StarView (see Chapter 1 of the HST Introduction). You will need to use the standalone version of StarView. On the Welcome Screen, from the pull-down menu button Searches, click on Instruments, then the desired instrument, then click on the Associations button for that instrument. You can then search for the various exposures belonging to an association by entering the rootname of the association in the Association ID field and clicking on Search. An Association Results Screen will display the results of the search, which you can step through using the Scan, Prev, Next buttons. Figure B.1 below gives an example of a NICMOS Association Results Screen. Note the differences between the association rootname and coordinates and those of the individual exposure.
Figure B.1: Association Results Screen from StarView
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |