HST Primer for Cycle 21

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).

Far-Ultraviolet Channel (COS/FUV)

COS Observations Below 1150 Å

COS/FUV uses a single optical element to disperse and focus the light onto a crossed-delay-line (XDL) detector; the result is high ultraviolet sensitivity from about 900 Å to 2050 Å with four resolving power modes: R ~ 1500, R ~ 2500, R ~ 10,000, and R ~ 20,000 (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, and 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 x 10 mm, digitized to 16384 x 1024 pixels, and a resolution element of 6 x 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 ~ 1500 to 4000. G130M spans wavelengths between 900 Å and 1450 Å, and G160M covers the wavelength range between 1400 Å to 1775 Å. Both these gratings provide resolving power R between 16,000 and 21,000 for wavelengths longer than 1150 Å (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 offsets in wavelength.

COS Observations Below 1150 Å

COS observations can be obtained below 1150 Å using either the G140L grating with the 1280 Å setting, or the G130M gratings with the 1055 Å, 1096 Å, or 1222 Å settings.

G140L Grating with the 1280 Å Setting

The G140L/1280 Å setting has R ~ 1500 to 4000 (increasing approximately linearly with wavelength), with a wavelength range from less than 900 Å to 1165 Å (FUVB), with an effective area of 10 cm2 between 900 Å and 1050 Å, and increasing rapidly to greater than 1000 cm2 at 1150 Å.

G130M Grating with the 1055 Å, 1096 Å, and 1222 Å Settings

The G130M spectral resolution at the 1055 Å and 1096 Å 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 Å. These settings cover the 900 Å to 1236 Å wavelength range with an effective area of approximately 20 cm2 between 900 Å and 1050 Å. The effective area increases steeply towards longer wavelengths, exceeding 1000 cm2 by 1150 Å. 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 Å setting, these ray-trace models predict that a resolution between 8000 to 10,000 can be obtained on segment B of the COS FUV detector between 900 Å and 970 Å, while for the 1096 Å setting, resolutions between 8000 and 12,000 can be obtained between 940 Å and 1080 Å. These settings will complement the G130M 1222 Å central wavelength setting that was first offered in Cycle 20, and which provides resolution of 12,000 to 15,000 between 1067 Å and 1172 Å. 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 Å, 1300 Å, 1309 Å, 1318 Å, and 1327 Å). 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 Å and 1096 Å 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.

More information about the G130M modes can be found in the COS Instrument Handbook.

Figure 4.1: Predicted Resolution as a Function of Wavelength for 1055 Å, 1096 Å, 1222 Å, and 1291 Å Central Wavelength Settings in Each COS FUV Segment

The predicted resolution as a function of wavelength for each segment of the COS FUV detector for 1055 Å, 1096 Å, 1222 Å, and 1291 Å central wavelength settings. These predictions are based on ray-trace models. Comparisons with on-orbit results less than 1140 Å are preliminary.

Near-ultraviolet Channel (NUV)

COS/NUV uses a 1024 x 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 Å of the entire 1700 Å to 3200 Å 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 Å are available in each exposure at resolving powers of 16,000 to 24,000. Panchromatic (1650 Å to 3200 Å) images of small fields (less than two arcseconds) may also be obtained at ~0.05 arcsec resolution in imaging mode.

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) and now falls on a part of the detector where significant gain sag has not yet occurred. This should eliminate the “Lyman Alpha holes” and other gain sag artifacts for data obtained during Cycle 21. Throughput at this new location is very similar to that of the original position before the effects of gain sag, However, spectral resolution may be as much as 10% lower than the original resolution. See the COS Web site for additional information about calibration of this new lifetime position.

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 6.8 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 1280 Å settings will spread the bright geocoronal Lyman Alpha illumination and significantly delay the appearance of gain sag effects. Because 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, proposers using the FUV channel who do not intend to use all four FP-POS settings for each CENWAVE setting must provide strong scientific justification for their observing strategy. A modest reduction in observational overheads will not normally be considered sufficient justification for not using all four FP-POS settings.

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 6.2 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 6.7 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).