8.10 Special Cases
8.10.1 Early Acquisitions and Preliminary Images
In some situations an observer may need to obtain an independent ultraviolet image of a region in order to ensure that no objects violate safety limits and that the target to be observed can be acquired by COS successfully. Such an early acquisition should be included in the Phase
I proposal, and the observation should not use a photon-counting detector. The UVIS channel on WFC3 is recommended, but observers are encouraged to consult with an STScI instrument scientist.
8.10.2 Extended or Multiple Targets
Because most COS target-acquisition schemes were developed with the implicit assumption that the target is a point source, acquisitions of extended or multiple sources may require more careful planning.
If the target is sufficiently uniform and its coordinates are well known, then a target acquisition may not be required. To ensure that the brightest region of an extended source falls into the aperture, an
CENTER=BRIGHTEST may be sufficient (Section 8.3).
ACQ/PEAKXD acquisitions with the NUV detector should be avoided for extended sources, because light from the three spectral stripes will overlap on the detector (Section 8.5). The pattern of dwell points for
ACQ/PEAKD and FUV
ACQ/PEAKXD should be tuned to the extent of the target.
Imaging acquisitions of extended sources are subject to special bright-object screening procedures. If an ETC calculation shows that the source is below the NUV local count rate limit (50 cts/s/pixel) under the assumption that it is a point source (the worst-case scenario), then it passes. If it does not pass as a point source, but the maximum local count rate as an extended source is <5.9 cts/s/pixel, then the visit is safe. Above 25 cts/s/pixel, the visit is unsafe. Between 5.9 and 25 cts/s/pixel, the visit will be checked for safety by multiple reviewers. For more information, consult with your contact scientist.
Complex targets—such as two stars with very small angular separation, multiple bright knots, etc.—may confuse the acquisition algorithms. In such cases, consider an offset target acquisition, discussed in. Take care when doing target acquisition for a target in a binary system since it is possible for the pointing to end up between the two stars. The coordinates of the target must be corrected for the orbital phase of the target in the binary system.
8.10.3 Offset Target Acquisitions
When targets are faint or lie in crowded fields, direct acquisition of the primary science target may be difficult or uncertain. In such cases, an offset acquisition, in which acquisition of a nearby field target is followed by a short slew to the science target, may be appropriate.
The size of the offset is limited by the requirement that the guide stars remain within the fields of view of their respective FGSs. Offset acquisition slews routinely involve displacements up to 1 arcmin and can be larger. Offset slews have a typical accuracy of ±0.003 arcsec. The centering of the initial offset target should be refined (via either
ACQ/PEAKXD+ACQ/PEAKD) before the offset maneuver. For offset acquisitions, bright-object considerations apply to the acquisition target, the science target, and their fields. The region between the two targets does not need to be checked, because the shutter is closed during the slew. Refer to Chapter 9 for a discussion of the modest overheads associated with the offset-acquisition spacecraft movement.
In unusual cases, including highly uncertain target coordinates or knotty, extended sources for which high wavelength accuracy is required, an offset target acquisition, followed by an additional
ACQ/PEAKXD+ACQ/PEAKD on the primary target, may be employed.
8.10.4 Acquisition Failure Actions and Diagnostics
Should any stage of the TA fail or a Local Rate Check (LRC) violation occur during a TA exposure, then the subsequent acquisition procedures in that visit (such as
ACQ/PEAKD) will not be executed, but the science exposures will still occur. Note that HST will be left pointing at the last commanded position, which may differ substantially from the initial pointing.
Many quantities useful for evaluating the success of COS TAs are recorded in the COS TA data products (the
_spt files). Table 5 of COS ISR 2010-14 lists these keywords and their meanings.
COS Instrument Handbook
- Chapter 1: An Introduction to COS
Chapter 2: Special Considerations for Cycle 28
- • 2.1 COS FUV Detector Lifetime Positions
- • 2.2 Central Wavelength Settings Added in Cycle 26
- • 2.3 Use of the G285M Grating is Discouraged
- • 2.4 COS Observations Below 1150 Angstroms: Resolution and Wavelength Calibration Issues
- • 2.5 Time-Dependent Sensitivity Changes
- • 2.6 Spectroscopic Use of the Bright Object Aperture
- • 2.7 Non-Optimal Observing Scenarios
- • 2.8 NUV Spectroscopic Acquisitions
- • 2.9 SNAP, TOO, and Unpredictable Source Programs with COS
- • 2.10 Choosing between COS and STIS
- 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
- 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
- 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
Chapter 13: Spectroscopic 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