11.5 Target Positioning

Target position special requirements are used to restrict the pointing of HST observations. Details on the different target position options and implementing these special requirements in Phase IIs can be found in Section 6.3.3 of the HST Phase II Proposal Instructions (P2PI). Below we summarize some STIS use cases for target position options. 

11.5.1 POSition TARGet (POS TARG)

POS TARG 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. Changing the pointing in this way can cause overheads to be repeated at each POS TARG pointing. 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 Section 11.3).

Below are some of the STIS use cases for POS TARG:

  • Custom dithering: POS TARG is most commonly used in some special science cases for custom dithering with the STIS-ALONG-SLIT pattern or to implement a user-specified pattern (see Section 11.3.2 for more details).
  • Crowded fields: In overcrowded regions, POS TARG can be used to set a position relative to an acquisition/peakup of an isolated reference source. Though in many cases this can be accomplished without a POS TARG by using the preferred method of specifying an OFFSET reference star for the acquisition (see Table 3.1 in the P2PI).

  • Spatial scanning: For scanned observations with G750L or G750M, POS TARG can be used to better align the stellar scans (often taken with the 52x2 aperture) with the fringe flats (generally taken with 52x0.1) – to account for the slight difference in the X positions of the two apertures (as tabulated in the Science Instrument Aperture File). See Section 12.12.3 for more details.
  • Coronagraphy: Using POS TARG to adjust the positioning of a target behind coronagraphic bars is an Available-but-Unsupported mode (see Sections 12.10 & 12.11 on coronagraphy).

11.5.2 SAME POSition AS (SAME POS AS)

SAME POS AS specifies that the current exposure be done at exactly the same position on the sky and with the same telescope roll as a reference exposure. Do not use SAME POS AS with dithering patterns (see Section 11.3), as it will negate them.