8.9 Recommended Parameters for all COS TA Modes

As the result of our analysis of the target acquisition (TA) performance of COS, we have adjusted many COS TA parameters and have refined our recommendations and guidelines for routine COS TAs. We present these recommendations and guidelines in the previous sections and summarize the most significant instructions below. Table 8.3 provides our recommended values for optional parameters for each COS TA type.

• All TA modes provide good centering. For optimal wavelength accuracy, use NUV imaging mode; otherwise, use the mode that is fastest, as determined by the Orbit Planner in APT.
• Signal-to-noise (S/N) is important. Use S/N > 40, except for ACQ/IMAGE, which requires S/N > 20 through the PSA and S/N > 30 through the BOA.
• By default, FUV acquisitions for gratings G130M and G160M take place in the same Lifetime Position used for the grating/cenwave combination. FUV acquisitions for grating G140L take place at LP4 even though science exposures are taken at LP3. In standard cases users do not need to specify a choice of LIFETIME-POS in APT. Users observing with grating G160M at LP4 (Section 9.5.1) should select LIFETIME-POS=LP4 under optional parameters on APT.
• A single ACQ/SEARCH is not sufficient to CENTER a COS point-source target in the aperture. Always follow the first ACQ/SEARCH with an ACQ/IMAGE or with ACQ/PEAKXD+ACQ/PEAKD.
• ACQ/SEARCH exposures are not required for targets with coordinate accuracies of ±0.4 arcsec or better in the ICRS/GSC2 frame. Spending extra time to validate target coordinates is the best way to save TA time. On-line catalogs often have coordinate errors that exceed 0.4 arcsec. It is the userʹs responsibility to verify that target coordinates are correct.
• For NUV spectroscopic ACQ/PEAKXDs, only a few cenwaves have been proven to be reliable. Users should choose one of these cenwaves, which are listed in Section 8.4. The G185M, G225M, and G230L gratings each have three or four recommended cenwaves, while the G285M grating has none. For a full discussion of this issue, see COS ISR 2017-05.
• If at all possible, use STRIPE=MEDIUM (stripe B) for NUV spectroscopic ACQ/PEAKXDs, which is the default. Primarily, this is because the WCA-to-PSA offsets in the flight software are optimized for stripe B in most modes. Choosing a different stripe will result in systematic offsets of the initial spectrum of order of a few pixels, which generally are still within typical target coordinate uncertainties.
• FUV ACQ/PEAKXD uses NUM-POS=3, STEP-SIZE=1.3, and CENTER=FLUX-WT as defaults. In most cases there is no need to change, or enumerate, these parameters.
• There are no defaults for FUV ACQ/PEAKD.  Use NUM-POS=5, STEP-SIZE=0.9, and CENTER=FLUX-WT-FLR for most ACQ/PEAKD centerings. For the most accurate AD (ACQ/PEAKD) centering possible, use NUM-POS=9, STEP-SIZE=0.6, and CENTER=FLUX-WT-FLR. When a fast TA is required and lower centering accuracy can be tolerated, use NUM-POS=3, STEP-SIZE=1.3, and CENTER=FLUX-WT.

Table 8.3: COS Acquisition Modes, Options, and Recommended Values.

¹ Recommended value. For ACQ/PEAKD, FUV ACQ/PEAKXD, and ACQ/SEARCH, use of special parameter CENTER=DEFAULT is recommended.
² The G230L grating has special recommendations, see Section 8.5.1.
³ For ACQ/PEAKD, FUV ACQ/PEAKXD, and ACQ/SEARCH, use of special parameter CENTER=DEFAULT is recommended. In the case of ACQ/PEAKD and FUV ACQ/PEAKXD this parameter sets CENTER=FLUX-WT if NUM-POS=3, and CENTER=FLUX-WT-FLR if NUM-POS=5, 7, or 9. For ACQ/IMAGE this parameter sets CENTER=FLUX-WT if NUM-POS=2 and CENTER=FLUX-WT-FLR if NUM-POS > 2.