1.2 Observing with COS
1.2.1 Target Acquisitions
The two COS entrance apertures are 2.5 arcsec in diameter. To ensure that the target is centered in the aperture, a target-acquisition procedure must be performed at the beginning of each visit (but not at the beginning of subsequent orbits within the visit).
The COS flight software provides two methods for acquiring and centering a target in the aperture. The first method obtains a direct image of the aperture with the NUV channel and moves the telescope to the center of light. The second method centers the target using its dispersed spectrum and can be performed with either the NUV or FUV channel. For both methods, the target's center of light can be computed from either a single exposure or from a series of exposures that map out a grid on the sky. Target acquisitions are described in Chapter 8.
1.2.2 Observing Modes: Spectroscopic and Imaging
While COS was designed as a spectrograph, the NUV channel can also be used for imaging observations. The COS/NUV plate scale of 23.5 mas per pixel provides the highest spatial sampling of any instrument aboard HST. The image is corrected for the telescope's spherical aberration, but is degraded by zonal (polishing) errors on HST's primary and secondary mirrors (see Chapter 3). Because COS' detectors are photon-counting, there are limits to the brightness of sources for a given configuration. For the PSA, the NUV imaging count-rate screening limit of 50 counts per second per pixel (Table 10.1) corresponds to a GALEX NUV magnitude of 17.6.
1.2.3 Observing Modes: TIME-TAG
and ACCUM
COS provides two observing modes, TIME-TAG
and ACCUM
. In TIME-TAG
mode the position, arrival time, and (for the FUV channel) pulse height of each detected photon are recorded in the memory buffer. With regard to the accuracy of the arrival time, the HST spacecraft computer (HST486) clock has a precision of 125 milliseconds (1/8th second per tick). Our goal is to maintain the clock's accuracy compared to UTC to within 10 milliseconds. In ACCUM
mode, only the locations of arriving photons are recorded.
TIME-TAG
mode is preferred because it allows for more sophisticated data reduction. For example, an observer may compare data from the night and day sides of the orbit or compute the count rate of an object whose intensity varies on short time scales. TIME-TAG
observations through the primary science aperture (PSA) allow the taking of occasional wavelength-calibration spectra during an exposure. These spectra are used by the COS data-reduction pipeline, CalCOS, to correct drifts in the spectrum due to small motions of COS' Optics Select Mechanism (OSM). ACCUM
mode is designed for observations of targets that are too bright for TIME-TAG
mode. Because the lower information content of ACCUM
data reduces their utility for archival researchers, its use must be justified for each target.
Both TIME-TAG
and ACCUM
modes may be used with either the FUV or NUV channel. For more information comparing TIME-TAG
and ACCUM
see Section 5.2.
1.2.4 Typical Observing Sequences
In the majority of cases the following sequence of events will produce high-quality data:
- Acquire the object using COS/NUV
ACQ/IMAGE
. This typically takes about three minutes total time. See examples in Chapter 9. Obtain spectra in
TIME-TAG
mode using theFP-POS=ALL
setting andFLASH=YES
so the spectra can be corrected for flat-field anomalies and OSM drifts. (For dual-segment spectroscopy at LP5 with cenwave 1291,FP-POS=ALL
is not available) The COS Exposure Time Calculator webpage (ETC) provides a means of calculating essential parameters, such as theBUFFER-TIME
.Obtain additional spectra during subsequent orbits to achieve the desired signal-to-noise ratio or wavelength coverage.
1.2.5 Observing Bright Objects with COS
Observing bright objects with exposure times longer than necessary for a maximum achievable signal-to-noise ratio (SNR) have recently been found to significantly impact the lifetime of COS. This SNR value for an observation is limited by the fixed pattern noise on the detector, which is a function of grating and the number of FP-POS obtained. ETC SNR values above this limit derived from longer exposure times do not generally yield improvements in usable signal. This extra use of the detector also unnecessarily reduces its lifetime, which should be avoided.
Exposure times of bright objects should therefore be limited within an orbit to not exceed the maximum achievable SNR for a given combination of grating and number of FP-POS used. Users should consult the 50th percentile measurements listed in Table 1 of COS ISR 2023-11. Exceptions to this guideline should be justified explicitly in the Phase I proposal. See also Section 5.8 for more details.
-
COS Instrument Handbook
- Acknowledgments
- 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
- • 10.9 Bright Object Protection for M Dwarfs
- 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
- • 12.19 Cycle 31 Calibration Program
- Chapter 13: COS Reference Material
- • Glossary