HST Primer for Cycle 25
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Hubble Space Telescope Primer for Cycle 25 > Chapter 4: Cycle 25 Scientific Instruments > 4.4 Cosmic Origins Spectrograph (COS)

4.4
The Cosmic Origins Spectrograph (COS) is an ultraviolet spectrograph designed to optimize observations of point sources. It was installed during Servicing Mission 4 in the instrument bay previously occupied by COSTAR. COS is designed to be a very high throughput instrument, providing medium to low resolution ultraviolet spectroscopy. The instrument has two channels for ultraviolet spectroscopy: Far-ultraviolet (FUV) and Near-ultraviolet (NUV).
4.4.1 Far-Ultraviolet Channel (COS/FUV)
COS/FUV uses a single optical element to disperse and focus light onto a crossed-delay-line (XDL) detector; the result is high ultraviolet sensitivity from about 900 to 2050 with resolving power for different modes ranging from R ~ 1300 to ~ 17,000 (for detailed information on the resolving power of the FUV channel, please see the COS Instrument Handbook). This detector has heritage from the FUSE spacecraft. The active front surface of the detector is curved; to achieve the length required to capture the entire projected COS spectrum, two detector segments are placed end-to-end with a small gap between them. Each detector segment has an active area of 85 10 mm, digitized to 16384 1024 pixels, and a resolution element of 6 10 pixels.
The FUV channel has three gratings: G140L provides nearly complete coverage of the FUV wavelength range in a single exposure with a resolving power R ~ 1300 to 3500. G130M spans wavelengths between 900 and 1450 , and G160M covers the wavelength range between 1400 to 1775 ; both these gratings provide resolving power R between 13,000 and 17,000 for wavelengths longer than 1150 (see the next paragraph for resolving power at shorter wavelengths). For all three FUV gratings, a small segment of the spectrum is lost to the gap between the two detector segments. This gap can be filled by obtaining two exposures offset in wavelength.
COS Observations Below 1150
COS observations can be obtained below 1150 using either the G140L grating with the 1280 setting, or the G130M gratings with the 1055 , 1096 , or 1222 settings.
G130M Grating with the 1055 , 1096 , and 1222 Settings:
The G130M spectral resolution at the 1055 and 1096 central wavelength settings has been substantially increased above values offered during previous cycles; they now allow resolutions between 8000 and 10,000 at many wavelengths below 1150 . These settings cover the 900 to 1236 wavelength range with an effective area of approximately 20 cm
2 between 900 and 1050 . The effective area increases steeply towards longer wavelengths, exceeding 1000 cm2 by 1150 . Users should note, however, that for these modes, the focus values have been set to optimize the resolution over a limited part of their wavelength range (see Figure 4.1).
The resolution values quoted here are based on ray-trace models, so actual resolution may be slightly different. However, preliminary comparison with on-orbit test data appears consistent with the predictions of these models. At the 1055 setting, these ray-trace models predict that a resolution between 7000 and 8500 can be obtained on segment B of the COS FUV detector between 900 and 970 , while for the 1096 setting, resolutions between 7000 and 10,000 can be obtained between 940 and 1080 . These settings complement the G130M 1222 central wavelength setting that was first offered in Cycle 20, which provides resolution of 10,000 to 13,000 between 1067 and 1172 . At longer wavelengths, the resolution offered by any of these settings will be inferior to that available with the original complement of G130M central wavelength settings (1291 , 1300 , 1309 , 1318 , and 1327 ). Some targets that are too bright to observe at longer wavelengths with the COS G130M grating may be observable on the B segment with the 1055 and 1096 settings by turning off the A segment which covers the longer wavelengths. However, in this case, there is no usable wavelength calibration lamp spectrum recorded, and the spectrum observed on the B segment cannot be automatically corrected for mechanism drift or zeropoint offsets. For such configurations, GO wavecals have to be inserted following specific rules described in the COS Instrument Handbook.
Figure 4.1: Spectral Resolution at LP2 as a Function of Wavelength for the B segment of the COS/FUV Detector with the 1055, 1096, 1222, and 1291 Central Wavelength Settings.
The predicted resolution at LP2 as a function of wavelength for each segment of the COS FUV detector for 1055 , 1096 , 1222 , and 1291 central wavelength settings. These predictions are based on ray-trace models. Comparisons with on-orbit results less than 1140 are preliminary. The spectral resolution at LP4 is expected to be within 15% of the values shown here. For more detailed information about the spectral resolution at LP4 please see the COS Instrument Handbook.
4.4.2 Near-Ultraviolet Channel (NUV)
COS/NUV uses a 1024 1024 pixel Cs2Te MAMA detector that is essentially identical to the STIS/NUV MAMA except that it has a substantially lower dark count. Four gratings may be used for spectroscopy. Portions of the first-order spectra from the gratings are directed onto the detector by three separate flat mirrors. Each mirror produces a single stripe of spectrum on the detector. For the low-dispersion grating, G230L, one or two first-order spectrum stripes are available, each covering ~400 of the entire 1700 to 3200 range at resolving power R ~2100 to 3900, depending upon wavelength. For high-dispersion gratings G185M, G225M, and G285M, three non-contiguous stripes of 35 are available in each exposure at resolving powers of 16,000 to 24,000. Panchromatic (1650 to 3200 ) images of small fields (less than two arcseconds) may also be obtained at ~0.06 arcsec or greater resolution in imaging mode.
4.4.3 COS FUV XDL Gain Sag
Prolonged exposure to light causes the COS FUV XDL detectors to become less efficient at photon-to-electron conversion, a phenomenon called “gain sag.” When a particular region of the detector is increasingly used, there is a corresponding decrease in the “pulse height” of the charge cloud generated by an individual photon. As long as the pulse heights are above a minimum threshold needed to distinguish real photons from background events, there is no loss in sensitivity. But as the average pulse height in a particular region approaches and then drops below this threshold, real photon pulses are increasingly misidentified as background, causing a decreasing effective throughput. Since the amount of gain sag increases with the total amount of previous illumination, the effects first appear in regions of the detector that are illuminated by the bright Lyman Alpha airglow line, but eventually, the entire spectrum is affected.
STScI is undertaking a number of actions to mitigate the effects of gain sag and extend the lifetime of the COS FUV XDL detector. On July 23, 2012, the COS FUV spectrum was relocated by 3.5" (about 1 mm) to a part of the detector where significant gain sag had not yet occurred. This move, in conjunction with careful management of the high voltage of the COS FUV detector, has allowed us to obtain data that is free of gain sag artifacts during Cycle 20, Cycle 21 and a fraction of Cycle 22. On February 9, 2015, the COS FUV spectrum was relocated by -6" to another part of the detector at lifetime position 3 (LP3). This LP move along with continued management of the high voltage of the COS FUV detector, has allowed us to obtain data that is free of gain sag artifacts during part of Cycle 22, Cycle 23 and part of Cycle 24. FUV spectra will be moved yet again to a fourth lifetime position (LP4) during Cycle 24 in July 2017 to keep mitigating gain sag effects. The spectral resolution of the COS G130M and G160M gratings at this new position may be 15% lower than at LP3, with a possibly larger resolution decline for the G140L grating.
A new spectral extraction algorithm is used by the COS pipeline to calibrate data obtained at LP3 and will also be used to calibrate data obtained at LP4. This new extraction method is optimized for observations of point sources using the PSA aperture only. FUV spectra of extended sources and FUV spectra obtained with the BOA will be poorly calibrated. As a result, proposers planning to use COS/FUV for science observations with the BOA aperture (whether point source or extended) should carefully consider the impact of the poor calibration on their science goals. See the COS website for updated information on the calibration of the LP4 position.
4.4.4 Optimizing the Science Return of COS
Fixed-pattern noise in the COS detectors limits the signal-to-noise that can be achieved in a single exposure (see Section 5.8 of the COS Instrument Handbook). A simple way to remove these detector features is to obtain exposures at multiple FP-POS or CENWAVE settings, both of which shift the spectrum on the detector, and combine them in wavelength space. This is especially important for the COS FUV detector as the fixed pattern noise is larger and more poorly characterized than that of the NUV detector. In addition, the consistent use of multiple FP-POS positions in the G130M and G140L 1105 settings will spread the bright geocoronal Lyman Alpha illumination and significantly delay the appearance of gain sag effects.
Since this simple shift-and-add technique significantly improves the signal-to-noise ratio of the resulting spectrum, and will extend the lifetime of the COS FUV detector, the use of multiple FP-POS for each CENWAVE setting is required unless a strong scientific justification to do otherwise is provided in Phase I. This can be achieved by using the FP-POS=ALL parameter in APT for each CENWAVE or by spreading out the four FP-POS positions over multiple orbits within a visit for each CENWAVE or over multiple visits to the same target. Proposers who do not intend to use all four FP-POS for each CENWAVE setting must justify their observing strategy in the Phase I proposals. A modest reduction in observational overheads will not normally be considered sufficient justification for not using all four FP-POS settings. In addition, in order to continue mitigating the effects of gain sag caused by airglow emission in the FUV B detector, when observing with the G130M grating, STScI reserves the right to switch the G130M central wavelength setting of any program to another G130M central wavelength setting (cenwave). This is to insure that the G130M usage in Cycle 25 is such that a single cenwave is not unreasonably causing the detector to sag. Users that require a specific G130M cenwave must justify it in the phase I.
COS exposures may be obtained in either a time-tagged photon address (TIME-TAG) mode, in which the position, arrival time, and pulse height (for FUV observations) of each detected photon are saved in an event stream, or in accumulation (ACCUM) mode, in which only the positions of the photon events are recorded. In TIME-TAG mode, which is the default observing mode, the time resolution is 32 milliseconds. ACCUM mode is designed for bright targets with high count rates that would otherwise overwhelm the detector electronics. Because the lower information content of ACCUM data reduces their utility for archival researchers, its use must be justified for each target. For details, see Section 5.2 of the COS Instrument Handbook.
In both TIME-TAG and ACCUM mode, the Astronomer’s Proposal Tool (APT) automatically schedules wavelength calibration exposures, either during science exposures or between them (see Section 5.7 of the COS Instrument Handbook). The COS data reduction pipeline (CALCOS) uses these data to adjust the zeropoint of the wavelength solution for the extracted spectra. It is possible to suppress the taking of wavelength calibration spectra, but since it significantly lessens the archival quality of COS data, it must be justified.
Observers who wish to employ non-optimal observing techniques must strongly justify their observing strategy in the “Description of Observations” section of their Phase I proposal. Non-optimal observing techniques should not normally be adopted solely for the purpose of producing a modest reduction of the observational overheads; in such cases the observer should normally just request adequate time to use the recommended optimal strategy (see Section 9.2 of the Call for Proposals).

Hubble Space Telescope Primer for Cycle 25 > Chapter 4: Cycle 25 Scientific Instruments > 4.4 Cosmic Origins Spectrograph (COS)

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