2.8 Prime and Parallel Observing: MAMA Bright-Object Constraints

The STIS MAMA detectors are subject to damage at high-illumination rates, as explained in greater detail in Section 7.7. To protect the instrument, we have established limits on the maximum count rate at which the detectors may be illuminated. These count-rate limits translate into a set of configuration-dependent, bright-object screening magnitudes. The spectroscopic bright-object screening magnitudes are summarized in Table 13.45, and the imaging counterparts in Table 14.40.

STScI will perform screening of all MAMA exposures prior to scheduling. Targets not established as safe for the configuration in which they are being observed will not be scheduled. Observations that pass screening but are lost on orbit due to a bright-object violation will not be rescheduled. Observers are responsible for assuring that their observations do not violate the MAMA count-rate limits. A detailed description of the MAMA bright-object limits and the observers' responsibility is presented in Section 7.7 and in STIS ISR 2000-01.

Observers should note that the ETC global count-rate calculations for STIS echelle modes include a correction factor that takes into account the substantial amount of scattered light that falls outside the spectral extraction region but which still falls on the detector. In Cycle 25, this factor was updated in the ETC based on new measurements, and for many central wavelength settings this resulted in a substantial increase in the predicted global count rate. Observers are advised to redo all calculations with the most recent version of the ETC to verify that the proposed observation is feasible. Additional details may be made available on the STIS website or via Space Telescope Analysis Newsletters (STANs).

To assure that STScI can adequately screen observations, special constraints are imposed on parallel observing with the MAMAs:

  • No pure parallels are allowed using STIS.
  • Coordinated parallels are not allowed with MAMA imaging modes or with the NUV/MAMA-PRISM.

  • Coordinated parallels are allowed for other (non-PRISM) spectroscopic modes used with the MAMA detectors only if an exact spacecraft orientation (ORIENT) is requested and the RA and Dec of the parallel field determined. Note that the specification of an exact orient usually limits the scheduling of observations to a ~4–8 week period each year. The observer is responsible for assuring that observations do not violate the MAMA count-rate limits both for coordinated parallel MAMA observations and for primes.

  • Coordinated parallels with STIS CCD modes may be performed without any required restrictions on the allowed range of orientations.

Targets that are one magnitude or more fainter than the magnitude limits in the screening tables generally pass screening. For a target that may exceed a screening limit after correction for extinction and slit losses, observers must provide UV or optical data adequate to establish safety according to the guidelines detailed in Section 7.7. If such data are not available, they must be obtained in a safe HST configuration (see Section 7.7 for more details) or from another source; if obtained with HST, the required orbits must be requested in Phase I.

Normal MAMA bright object limits apply to all objects that may be within 5 arcseconds of the entrance aperture. In addition, there are special limits that apply to any extremely bright object that may be within 13.5 arcseconds of the aperture. Full details are given in Section 7.7.4.

Please also note that those proposing MAMA target-of-opportunity observations should provide an explanation in the Phase I proposal of how the safety of the proposed observations of the target will be ensured.