|Space Telescope Science Institute|
|COS Instrument Handbook|
As the result of our analysis of the target acquisition (TA) performance of COS during SMOV and Cycle 17, 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 changes below. Table 8.3 provides our recommended values for optional parameters for each COS TA type.
• 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, ACQ/PEAKXD+ACQ/PEAKD, or a second 2 × 2 ACQ/SEARCH which uses FLUX-WT weighting.
• 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.
• If at all possible, use STRIPE=MEDIUM (stripe B) for NUV spectroscopic ACQ/PEAKXDs. (This is the default.)
• Use SCAN-SIZE=5, STEP-SIZE=0.9, and CENTER=FLUX-WT-FLR for most ACQ/PEAKD centerings. For the most accurate AD centering possible, use SCANSIZE=9, STEP-SIZE=0.6, and CENTER=FLUX-WT-FLR. When a fast TA is required and lower centering accuracy can be tolerated, use SCAN-SIZE=3, STEP-SIZE=1.3, and CENTER=FLUX-WT.
A, B, BOTH 31 For NUV: STRIPE=MEDIUM DEFAULT3A, B, BOTH 51
For FUV exposures, data from detector segment B are ignored by the ACQ/PEAKXD centroiding algorithm, regardless of the value of the SEGMENT parameter.
For ACQ/PEAKD, use of the special parameter CENTER=DEFAULT is recommended. This parameter sets CENTER=FLUX-WT if NUM-POS=3 and CENTER=FLUX-WT-FLR if NUMPOS=5, 7, or 9.