11.4 Fixing Orientation on the Sky

STIS users, particularly those using the long slits to observe extended sources, commonly wish to specify the orientation of the slit on the sky. Specific orientations may also be used to place multiple point sources in a long slit (to be observed simultaneously) or to make sure that overly bright nearby stars are excluded. Observers planning coordinated parallel observations may also wish to specify the orientation of the HST focal plane, so as to place the appropriate instrument to cover a given patch of sky. Setting the orientation of the telescope means to effectively constrain the times when the observations can be scheduled, since HST must maintain a spacecraft orientation (sometimes called roll angle) which keeps its solar panels roughly perpendicular to the incoming sunlight. A roll ranges report, available in the APT Visit Planner, shows the range(s) of roll angle possible for a given observation as a function of observation date.

The orientation of the spacecraft (and therefore of the STIS long slits which are fixed in relation to the HST focal plane) is controlled by the ORIENT special requirement, which is entered during Phase II. The Phase II Proposal Instructions contain a detailed description of orientations and how to specify them. A specific orientation can be set, or a range of allowed orientations (e.g., 90–110 degrees) can be given. The tighter the constraints, the more difficult it will be to schedule the observation.

The ORIENT parameter gives the orientation of the HST focal plane projected onto the sky and is defined by the U2 and U3 axes. Figure 3.2 shows the HST focal plane containing all the HST instruments, with the U2 and U3 axes defined. Figure 11.1 shows the relationship between these axes and the position angle (PA) of the STIS long slits on the sky. Note that the long slits are approximately aligned with the detector's AXIS2, i.e., they are directly perpendicular to the dispersion axis (AXIS1). The important point to note is that if you fix the orientation of a long slit on the sky to be PA = Χ, where Χ is measured in degrees east of north, then the ORIENT parameter (which determines where the other HST instruments lie for parallel observations) is given as Χ+45 or Χ+225 degrees. Likewise, for PRISM mode observations, if you wish to fix the orientation of the spatial direction (i.e., perpendicular to the dispersion) to be X, then the ORIENT parameter should be set to X+45 or X+225 degrees. It is possible during Phase II to specify more than one permissible ORIENT range. The allowed and forbidden orientation ranges may be displayed in the Aladin image of the field in APT.

Users who wish to determine their orientation requirements using existing HST images should consult the HST Observatory website on determining an observation's orientation.  It is especially important to be careful when using HST data taken before 15 Sept. 1997, where there may be errors of ~0.5 degrees in the ORIENT and PA_V3 header keywords.

We show two examples below. Figure 11.10 illustrates how to set the ORIENT parameter to place the long slit along the M87 jet. Figure 11.11 illustrates how to set the ORIENT parameter to fix the dispersion axis for PRISM observations to be perpendicular to a double star system.

This relation between ORIENT and position angle will satisfy most needs. Observers with extremely stringent orientation requirements, however, should be aware that each STIS aperture has a specific U3 offset angle which is close to, but not equal to, 45°. In Table 11.2 below we list the offset angles for all supported spectroscopic slits. If ultimate precision is required, observers may wish to use the offset angles given in this table instead of the standard 45°. Observers should also note that the position angles for the 52" long slits have been revised by up to 0.33°.
 

Table 11.2: Offset Angles Between Supported STIS Slits and the Spacecraft Orientation Reference Vector U3.