6.3.3 Target Position Exposure-level Special Requirements

Target Position Special Requirements (e.g., POSition TARGetSAME POSition) are used to restrict the pointing of HST observations. The Astronomer's Proposal Tool (APT) is used to enter the requirements into the proposal.

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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.


Most general special requirements directly or indirectly restrict the times when observations (and/or their component visits) can be scheduled. These should be used to provide the schedulers at STScI with enough constraints to ensure that the observations are properly scheduled. Special requirements should not be used unless necessary to accomplish the scientific objectives of the proposal.


(red star) The Pattern Special Requirement has been replaced in the APT User Interface with the use of Pattern containers. See Exposure Containers: Exposure Sequences, Coordinated Parallels and Patterns.

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 Chapter 7: Pointings and Patterns.

POSition TARGet <X-value>,<Y-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. The X and Y positions are in units of arcseconds (i.e., do not enter “arcsec” after each value). The X-Y coordinate system and the default positioning for each scientific instrument are in these articles:

 An aperture’s fiducial point is ordinarily close to the geometric center of that aperture. Details may be found in the Instrument Handbooks.

(red star) Note that a POS TARG is a motion relative to the aperture fiducial point, and so they are not cumulative.

Proposers  using  this  Special  Requirement  should  be  aware  of  the  following caveats:

  • 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 Chapter 7: Pointings and Patterns.
  • The primary intent of a POS TARG is to move the specified target to a non-standard position in the “aperture.”

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). Restricting the ORIENT to a specific value will imply a very tight timing constraint. 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).

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).

SAME POSition AS <exposure>

SAME POS AS <exposure> requests that the current exposure be done at the same position on the sky and with the same telescope roll as <exposure>. Note that <exposure> must be in the same visit as the current exposure. This requirement is implicit for exposures within a visit with the same aperture, target, and POS TARG combination.

This requirement is used in many astrometric observations, so that the telescope doesn’t try to center successive targets in the astrometer pickle before observing it.

For other instruments, SAME POS AS should be used sparingly and with caution. For example, SAME POS AS 1 will cause the spacecraft to return to the pointing of exposure 1. Thus if the current exposure has a different (non concentric) aperture from 1 and specifies SAME POS AS 1, the target will be placed in the aperture used by exposure 1, not the aperture currently requested. Further, specifying SAME POS AS an onboard acquisition exposure will undo the offsets determined in the acquisition process.

(red star) Do not use SAME POS AS with dithering patterns (Chapter 7: Pointings and Patterns) because it will negate them. SAME POS AS means exactly the same position as another exposure.

For engineering parameters (SAME POS)

SAME POS AS <exposure> can be used if the pointing of an internal exposure is required to be the same as that of an external exposure in the same visit, or if the same orient needs to be maintained across different obsets in a single visit.

Care must be employed when using the special requirement "SAME POS AS <exposure>" during an exposure where the spectral element is a ramp filter (any filter starting with "FR" or "LRF"), since this special requirement will cause the ramp observations to be executed with the aperture used in the earlier <exposure>. If the latter aperture is not centered on the ramp aperture and/or does not encompass the full ramp aperture, the ramp observations will not have the desired central wavelength and transmission efficiency. To use a ramp filter with this special requirement you must contact your Contact Scientist or an Instrument Scientist (via the helpdesk (https://stsci.service-now.com/hst).

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Change Log

Version Cycle 31 July 2023

      Fixed the link to the Instrument Handbooks in the POS TARG section