COS Instrument Handbook for Cycle 24

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8.9	Recommended Parameters for all COS TA Modes

As the result of our analysis of the target acquisition (TA) performance of COS during SMOV and during Cycles 17 through 21, 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, based on ETC simulations.

•	Signal-to-noise (S/N) is important. Use S/N > 40 for PSA and BOA TAs, except for BOA ACQ/IMAGE, which should have S/N > 60. A S/N of 25 on detector segment A is sufficient for FUV ACQ/PEAKXD exposures.

•	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 в 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. It is the user’s responsibility to verify that target coordinates are correct.

•

If at all possible, use STRIPE=MEDIUM (stripe B) for NUV spectroscopic ACQ/PEAKXDs. (This 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.

•

Use NUM-POS=5, STEP-SIZE=0.9, and CENTER=FLUX-WT-FLR for most ACQ/PEAKD centerings. For the most accurate AD 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

Acquisition Type	Description	SCAN-SIZE or NUM-POS	STEP-SIZE (arcsec)	Optional Parameters	Recommended Values	Recom- mended S/N
ACQ/ SEARCH	Spiral pattern; multiple exposures	2	1.767	CENTER= FLUX-WT, FLUX-WT-FLR, BRIGHTEST   For FUV: SEGMENT= A, B, BOTH	FLUX-WT	40 (PSA and BOA)
		31			FLUX-WT-FLR
		4			FLUX-WT-FLR
		5			FLUX-WT-FLR
ACQ/ IMAGE	Initial and confirmation images (NUV only)					40 (PSA) 60 (BOA)
ACQ/ PEAKXD	One exposure			For NUV: STRIPE= SHORT, MEDIUM, LONG   For FUV: SEGMENT= A, BOTH2	For NUV: STRIPE=MEDIUM   For FUV: SEGMENT=BOTH   (These are the default values.)	For NUV: 40   For FUV: 25 on segment A
ACQ/ PEAKD	Linear pattern; multiple exposures	3	1.3	CENTER= FLUX-WT, FLUX-WT-FLR, BRIGHTEST, DEFAULT3   For FUV: SEGMENT= A, B, BOTH	FLUX-WT	40
		51	0.9		FLUX-WT-FLR
		7	0.6		FLUX-WT-FLR
		9	0.6		FLUX-WT-FLR

1

Recommended number of acquisition steps.

2

For FUV exposures, data from detector Segment B are ignored by the ACQ/PEAKXD centroiding algorithm, regardless of the value of the SEGMENT parameter.

3

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.