11.2 COS Target Acquisition Modes

GO's preparing Phase 2 proposals which are critically dependent upon precise astrometry (mainly spectroscopic observations) should be aware that a subset of existing ACS and WFC3 datasets in the MAST archive have astrometric errors due to imperfect fits to GAIA stars during post-processing. This may result in incorrect scale factors in the image headers leading to errors on the order of ~1 arc second. Observers should consult the Phase 2 Proposal Instructions and the MAST www page. Please see item 3 in the WFC3 STAN Issue 31, June 2020 for more information. A description of the WCS naming conventions used in ACS and WFC3 data is at Improvements in HST Astrometry. Instructions on resetting the WCS to the pipeline defaults is given in the Jupyter notebook Using updated astrometry solutions.

Observations with HST's COS instrument generally require a target acquisition before obtaining science data, and the Astronomer’s Proposal Tool (APT) has parameters for specifying these observations.

COS FUV target acquisitions are not currently offered with the new FUV cenwaves G140L/800 and G160M/1533.

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Format definitions

Boldface type indicates the name of an APT parameter or a value for a parameter.

(red star) Black text indicates an important note.

Magenta text indicates available but unsupported parameters (requires prior approval from STScI).

Red text indicates restricted parameters (for STScI use only).

Brown text indicates text file parameters.

Items in brackets - <value> - are required values.

Items in square brackets - [<value>] - are optional.


Mode = ACQ/SEARCH Config = COS/FUV 
Mode = ACQ/PEAKXD Config = COS/FUV 
Mode = ACQ/PEAKD Config = COS/FUV 
Mode = ACQ/SEARCH Config = COS/NUV
Mode = ACQ/IMAGE Config = COS/NUV 
Mode = ACQ/PEAKXD Config = COS/NUV
Mode = ACQ/PEAKD Config = COS/NUV 

For COS science observations of targets whose coordinates are not known to at least 0.4", you need to use the target search (ACQ/SEARCH Mode) and either the image acquisition (ACQ/IMAGE Mode) or peakup stages (ACQ/PEAKXD Mode followed by ACQ/PEAKD Mode) to center your target. For targets whose coordinates are accurate to 0.4" or better, the target search step (ACQ/SEARCH) can be skipped at the GOs own risk. For more details on these modes, see the COS Instrument Handbook.

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