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 25, 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, except for ACQ/IMAGE, which requires S/N > 20 through the PSA and S/N > 30 through the BOA.
  • FUV ACQ/PEAKXD exposures in Cycle 25 and beyond take place at LP4 and use ACQ/PEAKXD and the optional parameters NUM-POS, STEP-SIZE, and CENTER. The default pattern of NUM-POS=3, STEP-SIZE=1.3, and CENTER=FLUX-WT should be used for most targets.
  • 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. 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 2.8. 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. (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 (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.

Acquisition TypeDescriptionSCAN-SIZE or NUM-POSSTEP-SIZE (arcsec)Optional ParametersRecommended ValuesRecommended 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)


 

 

 

 

20 (PSA)
30 (BOA)

ACQ/
PEAKXD
(NUV)

One exposure


 

 

STRIPE=SHORT,
MEDIUM,
LONG

STRIPE=MEDIUM2
 
(This is the default value.)

40

ACQ/
PEAKXD
(FUV)

Linear pattern; multiple exposures

31

1.3

CENTER=FLUX-WT, FLUX-WT-FLR,
BRIGHTEST,
DEFAULT3
 
SEGMENT=
A, B, BOTH

FLUX-WT

40

5

1.0

FLUX-WT-FLR

7

0.8

FLUX-WT-FLR

9

0.65

FLUX-WT-FLR

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 The G230L grating has special recommendations, see Section 8.5.1.
3 For ACQ/PEAKD and FUV ACQ/PEAKXD, 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 NUM-POS=5, 7, or 9.