8.5 ACQ PEAKXD Acquisition Mode
ACQ/PEAKXD sequence is used to improve centering in the cross-dispersion (XD) direction. We recommend a minimum S/N of 40 for all dispersed-light target acquisition exposures.
The steps executed for NUV
ACQ/PEAKXD sequences are:
- A short exposure of the Pt-Ne wavelength calibration lamp through the WCA aperture is obtained. The spectrum is collapsed along the dispersion direction, its centroid is calculated, and the center of the target aperture is computed.
- A target spectrum is recorded for the user-specified time using a sub-array tailored to each grating and central wavelength (excluding edge effects and airglow lines). The spectrum is collapsed along the dispersion direction.
- The target XD location is assumed to be the median position of the collapsed spectrum.
- The slew required to move the target spectrum in the XD direction to the center of the aperture is computed.
- The telescope is slewed by the calculated offset to center the target in the XD direction.
The user must specify the aperture (PSA or BOA, typically the same as for the science exposure), the grating and central wavelength, and the exposure time. The use of
MIRRORB is not allowed. The stripe (
LONG, corresponding to stripes A, B, or C) to be used in the computation may be specified; however, the default stripe B (
MEDIUM) is recommended for most settings, as it achieves the best centering.1
Note: For extended sources observed with the NUV detector, light from the three spectral stripes may overlap. In this case acquisitions will likely fail and should be avoided.
As described in Section 4.1.7, the COS FUV detector exhibits gain sag. Gain sag effects are alleviated by periodic lifetime moves, which place the FUV spectrum on unsagged regions of the detector (see Section 5.12).
One consequence of the gain sag effect is the mis-registration of photon events in the cross-dispersion (XD) or Y direction, commonly referred to as Y walk. While Y walk does not adversely affect science data, it can reduce the accuracy of target acquisitions obtained in dispersed light. If the target is centered in the aperture, but the Y walk shifts its spectrum in the XD direction, then the FUV
ACQ/PEAKXD algorithm at previous lifetime positions (LP1–LP3) would miscalculate its centroid and move the target away from the aperture center.
The FUV Segment B is more affected by Y walk and gain sag than Segment A, mainly due to the intense geocoronal Lyman-α emission that hits the detector during G130M observations. To combat the effects of Y walk, the LP1–LP3 FUV
ACQ/PEAKXD algorithm was modified to use only Segment A data.
From Cycle 25 onward (October 2, 2017), the FUV
ACQ/PEAKXD sequence differs from the algorithm used in previous cycles. It uses only the total number of counts and is not affected by Y walk. For FUV
ACQ/PEAKXD acquisitions, either segment A or B may be used when available, but use of
SEGMENT=DEFAULT is recommended. All G140L
ACQ/PEAKXD acquisitions and those with G130M cenwaves restricted by the COS 2025 rules use only segment A. In the case of cenwave G140L/1280, detector segment B is left on by default but counts from segment B are not used for acquisition calculations. The ETC correctly calculates the needed exposure time for cenwave 1280 acquisitions, and also lists the count rate for segment B. Any count rate violation warnings for segment B are real and should not be ignored.
ACQ/PEAKXD algorithm works much like
ACQ/SEARCH except that, instead of a spiral, the spacecraft is moved linearly along the XD axis between exposures. An array containing the total counts at each dwell point is constructed. Its centroid is computed, and the telescope is moved to center the target in the aperture in the XD direction. The user must specify the aperture, grating, central wavelength, and the exposure time at each dwell point. The use of
MIRRORB is not allowed. The number of steps, called
NUM-POS, may be 3, 5, 7, or 9. The
STEP-SIZE is given in arcseconds. There are three options for the centering algorithm,
BRIGHTEST, and they work just as described in Section 8.3. For most applications, we recommend the use of
CENTER=FLUX-WT, as this combination (which is the default) is the fastest pattern that centers targets to within the requirements. Observers who wish to use
NUM-POS=5 are advised to use
CENTER=FLUX-WT-FLR. The special parameter
9. If NUM-POS is 5, 7, or 9, then the value of STEP-SIZE should be manually entered in APT. While any combination of NUM-POS and STEP-SIZE is allowed, it should be noted that for PEAKXD NUM-POS=3 with STEP-SIZE=1.3 and NUM-POS=5 with STEP-SIZE=0.9 are the only two combinations that are routinely tested.
ACQ/PEAKXDs on extended sources are possible, but they require a
STEP-SIZE combination tuned to the extent of the source.
1 There are special restrictions with the G230L grating, which positions first-order light on the detector only for certain stripes. The
MEDIUM stripe is required for the 2635 central wavelength, the
SHORT stripe is required for the 3360 wavelength, and either
MEDIUM may be used for the 2950 and 3000 wavelengths. The
LONG stripe may not be used with G230L at all.
COS Instrument Handbook
- Chapter 1: An Introduction to COS
- Chapter 2: Proposal and Program Considerations
- Chapter 3: Description and Performance of the COS Optics
- Chapter 4: Description and Performance of the COS Detectors
Chapter 5: Spectroscopy with COS
- 5.1 The Capabilities of COS
- • 5.2 TIME-TAG vs. ACCUM Mode
- • 5.3 Valid Exposure Times
- • 5.4 Estimating the BUFFER-TIME in TIME-TAG Mode
- • 5.5 Spanning the Gap with Multiple CENWAVE Settings
- • 5.6 FUV Single-Segment Observations
- • 5.7 Internal Wavelength Calibration Exposures
- • 5.8 Fixed-Pattern Noise
- • 5.9 COS Spectroscopy of Extended Sources
- • 5.10 Wavelength Settings and Ranges
- • 5.11 Spectroscopy with Available-but-Unsupported Settings
- • 5.12 FUV Detector Lifetime Positions
- • 5.13 Spectroscopic Use of the Bright Object Aperture
- Chapter 6: Imaging with COS
- Chapter 7: Exposure-Time Calculator - ETC
Chapter 8: Target Acquisitions
- • 8.1 Introduction
- • 8.2 Target Acquisition Overview
- • 8.3 ACQ SEARCH Acquisition Mode
- • 8.4 ACQ IMAGE Acquisition Mode
- • 8.5 ACQ PEAKXD Acquisition Mode
- • 8.6 ACQ PEAKD Acquisition Mode
- • 8.7 Exposure Times
- • 8.8 Centering Accuracy and Data Quality
- • 8.9 Recommended Parameters for all COS TA Modes
- • 8.10 Special Cases
- Chapter 9: Scheduling Observations
Chapter 10: Bright-Object Protection
- • 10.1 Introduction
- • 10.2 Screening Limits
- • 10.3 Source V Magnitude Limits
- • 10.4 Tools for Bright-Object Screening
- • 10.5 Policies and Procedures
- • 10.6 On-Orbit Protection Procedures
- • 10.7 Bright Object Protection for Solar System Observations
- • 10.8 SNAP, TOO, and Unpredictable Sources Observations with COS
- Chapter 11: Data Products and Data Reduction
Chapter 12: The COS Calibration Program
- • 12.1 Introduction
- • 12.2 Ground Testing and Calibration
- • 12.3 SMOV4 Testing and Calibration
- • 12.4 COS Monitoring Programs
- • 12.5 Cycle 17 Calibration Program
- • 12.6 Cycle 18 Calibration Program
- • 12.7 Cycle 19 Calibration Program
- • 12.8 Cycle 20 Calibration Program
- • 12.9 Cycle 21 Calibration Program
- • 12.10 Cycle 22 Calibration Program
- • 12.11 Cycle 23 Calibration Program
- • 12.12 Cycle 24 Calibration Program
- • 12.13 Cycle 25 Calibration Program
- • 12.14 Cycle 26 Calibration Program
- • 12.15 Cycle 27 Calibration Program
- • 12.16 Cycle 28 Calibration Program
- • 12.17 Cycle 29 Calibration Program
- • 12.18 Cycle 30 Calibration Program
Chapter 13: COS Reference Material
- • 13.1 Introduction
- • 13.2 Using the Information in this Chapter
- 13.3 Gratings
- • 13.4 Spectrograph Design Parameters
- • 13.5 The Location of COS in the HST Focal Plane
- • 13.6 The COS User Coordinate System
- • Glossary