On the other hand, if you are using the POS TARG to move an object of concern “out” of the aperture, failure to specify an ORIENT could result in the target getting pushed further onto the detector (e.g., if your POS TARG assumed an ORIENT of 0, but the observation occurred at an ORIENT of 180, you would move the star further onto the detector). In almost all situations, the preferred method of offset target acquisition is to create an additional target specification for the object of interest (separate RA/DEC or relative RA-OFF/DEC-OFF offset parameters, as described in Positional Offsets). Restricting the ORIENT to a specific value will imply a very tight timing constraint. The POS TARG offsets are defined in detector coordinates and do not correspond to a particular motion on the sky unless a specific ORIENT is defined (e.g ORIENT 85 TO 85). The primary intent of a POS TARG is to move the specified target to a non-standard position in the “aperture.”.If a large number of pointings need to be obtained within one orbital visibility period, it may be more efficient to use a pattern designed for this purpose see Chapter 7: Pointings and Patterns. Changing the pointing in this way can cause overheads to be repeated at each POS TARG pointing.Proposers using this Special Requirement should be aware of the following caveats: Note that a POS TARG is a motion relative to the aperture fiducial point, and so they are not cumulative. Details may be found in the Instrument Handbooks. 9.3.1 FGS POS TARG and Interferometer Coordinate SystemsĪn aperture’s fiducial point is ordinarily close to the geometric center of that aperture.The X-Y coordinate system and the default positioning for each scientific instrument are in these articles: The X and Y positions are in units of arcseconds (i.e., do not enter “arcsec” after each value). Specifies a non-default placement of the target, relative to the aperture fiducial point in the instrument field of view, for the current exposure, which must be on an external target. Patterns used with instruments to create dithers or mosaics fall within “target position,” but the details have been incorporated into a separate chapter because of the scope of the subject. See Exposure Containers: Exposure Sequences, Coordinated Parallels and Patterns. The Pattern Special Requirement has been replaced in the APT User Interface with the use of Pattern containers.
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