12.9 Parallel Observing with STIS

The second Servicing Mission installed solid-state data recorders on HST. The volume capacity of these recorders is roughly ten times that of the mechanical tape recorders in use for Cycles 1 through 6. Coupled with changes to the ground system and the flight software of the second-generation instruments designed to fully exploit this capability, this translates into a greatly increased capability for parallel observing.

STIS can be used to observe simultaneously with ACS, COS, WFC3, or FGS. Figure 3.2 shows the HST field of view following the installation of COS and WFC3 during the HST Servicing Mission 4. STIS, ACS, COS, and WFC3 are shown, with their fields of view drawn to scale, in their relative focal plane positions. The three STIS cameras share a common field of view; only one can be used at a time.

The policy for proposing for parallel observations and technical advice on parallel observing are provided in the Call for Proposals/Phase II Proposal Instructions. We remind you that there are two types of parallel observations:

  • Coordinated parallels, in which you explicitly link the taking of exposures in parallel to your own prime scientific exposures.
  • Pure parallels, in which exposures are taken in parallel with other observers' prime exposures. For Cycle 31 only pure parallel proposals using WFC3 or ACS CCDs will be considered by the TAC. See the HST Call for Proposals for more details.

Both coordinated and pure parallels must be explicitly proposed in Phase I. Implementing parallels requires significant resources; only those recommended by the TAC process will be implemented. If you are considering coordinated parallels, you may wish to consider constraining the orientation of HST, to place an object of interest in the parallel instrument's field of view. ORIENT constraints do affect observation scheduling, however, and should not be entered lightly (see Chapter 11).

12.9.1 Coordinated Parallels: Using STIS in Parallel with Other Instruments

Observations for which STIS is the parallel instrument are likely to be most useful when the full STIS field of view is used. If you wish to use a small slit, then STIS should be used as prime, and the other imaging instruments used in parallel with it.

If STIS is used as the secondary instrument in coordinated parallel observations, the STIS exposures cannot contain both external and internal exposures. Assuming the first exposure is external, all STIS exposures will be declared external. Therefore no internal exposures are allowed. This includes any user-specified internals, such as fringe flat fields, as well as automatic internals, such as auto-wavecals. If STIS is used as the prime instrument, this restriction does not apply.

For coordinated parallels where STIS is prime, automatic wavecals occur during the visibility period—not during occultation. Since many CCD observations request CR-SPLITs, in general, this is not an impact for STIS CCD coordinated parallels. However for long MAMA exposures, auto-wavecals that would have occurred during occultation in the absence of coordinated parallels are now shifted into the visibility period. This reduces the time available for science exposures. In addition, the buffer management overhead associated with the last MAMA science exposure now occurs prior to the auto-wavecal, further reducing the time available on target.

CCDFLATs for fringe correction of long wavelength CCD exposures can be taken in the same orbit as the coordinated parallel and forced into the occultation period when the prime STIS CCD exposures fill the visibility period. The CCDFLAT exposures are required to be specified outside the Prime + Parallel Group. However, they should be specified immediately after the last parallel exposure and they can be grouped as a non-interruptible sequence with the Prime + Parallel Group by highlighting both the Prime + Parallel Group and the CCDFLAT exposures then selecting Edit-Group-New Sequence in the APT menu toolbar and specifying that the CCDFLAT sub-exposures occur during the same orbit as the STIS observation they will correct.

The MAMA detectors cannot be used for pure parallel observing. For Cycle 31, coordinated parallels will not be allowed with STIS MAMA imaging modes and the STIS NUV-MAMA PRISM mode. However, coordinated parallels will be allowed with other STIS MAMA spectroscopic modes, provided that an explicit ORIENT is specified and precise RA and Dec coordinates for the parallel field are given.

The MAMA detectors are subject to bright object protection limits; see Section 2.8.

Three types of STIS exposures which have particular scientific utility with STIS as the parallel instrument are:

  • Optical imaging taking advantage of the ability to go deep very fast with the 50CCD wide-open mode.
  • Optical slitless spectroscopy.
  • UV slitless spectroscopy (available only for coordinated parallels with exact ORIENT specification).