1.1 Instrument Capabilities and Design

The Cosmic Origins Spectrograph (COS) is a fourth generation HST spectrometer, designed to enhance the spectroscopic capabilities of HST at ultraviolet (UV) wavelengths. COS was built by Ball Aerospace Corporation to the specifications of Dr. James Green, the Principal Investigator (PI), at the University of Colorado at Boulder in conjunction with the COS Investigation Definition Team (IDT). Designed to primarily observe faint point sources, COS is optimized for maximum throughput, and provides moderate and low resolution spectroscopy in the UV and limited imaging in the NUV.

COS is a slitless spectrograph that employs two circular 2.5 arcsec diameter science apertures, the Primary Science Aperture (PSA) and the Bright Object Aperture (BOA). The PSA is an open aperture and the BOA contains a neutral density filter to attenuate the flux of bright objects. COS also contains two calibration apertures, the Wavelength Calibration Aperture (WCA) and the Flat-Field Calibration Aperture (FCA). Light from external sources does not reach these apertures. Instead they are illuminated by internal calibration lamps. The FCA is not available for observers, but the WCA can be used by observers to obtain wavelength calibration spectra. The WCA can be illuminated by one of two Pt-Ne wavelength calibration lamps. Similarly, the FCA can be illuminated by one of two deuterium flat-field calibration lamps.

The instrument has two channels: a far-ultraviolet (FUV) channel that is sensitive across the 900–2150 Å wavelength range and a near-ultraviolet (NUV) channel that provides wavelength coverage from 1650 to 3200 Å. The COS optical design achieves its high performance, particularly in the FUV, by minimizing the number of reflections in the optical path and the use of large format detectors which maximize the wavelength coverage per exposure. Each channel has its own photon-counting detector and a selection of gratings (Table 1.1). The FUV channel resolution varies with Lifetime Position (LP; see Appendix A) as shown in Figure 1.1. The NUV channel also has a mirror that can be used in two modes for imaging. The FUV channel uses a single reflection system where a high-efficiency, first-order, aspheric holographic grating corrects the beam in the dispersion direction but has low spatial resolution perpendicular to dispersion. Only one channel may be used at a time.

Table 1.1: COS Spectroscopic Modes.

¹ Width of gap between FUV detector segments.
² Note that the resolution R increases approximately linearly with wavelength and is determined from the Line Spread Function (LSF) of COS, which is non-gaussian.
³ R falls with increasing wavelength for the shortest wavelength settings of G130M: R=8,500–11,500 between 940 and 1080 Å.
⁴ Resolution is dependent on wavelength, cenwave, and Lifetime Position. For more detail of the dependence of resolution on Lifetime Position, see Figure 1.1 and COS2025 policies.
⁵ Some shorter wavelengths are recorded in second-order light.

1.1.1 FUV Spectroscopy

The FUV channel employs a large format cross delay line (XDL) detector consisting of two 16384 × 1024 pixel segments, referred to as FUV segments A and B. The segments are separated by a physical gap of 9 mm, which makes it impossible to obtain a continuous spectrum across the two segments with a single setting. The supported central wavelength positions were selected to enable full wavelength coverage of the gap. Table 1.2 shows the wavelength ranges of both segments for all possible FUV grating and central wavelength combinations for FP-POS = 3 (for a discussion on FP-POS and its usage, see the COS Instrument Handbook and "Grating Offset Positions" 1.1.3 below).

Table 1.2: Wavelength Ranges for FUV Gratings for FP-POS = 3.

¹ The central wavelength (cenwave) is (approximately) the shortest wavelength recorded on Segment A.
² All wavelengths recorded here are nominal, due to the uncertainties in the position of the OSM1 mechanism.
³ Cenwaves 800 and 1533 were implemented in Cycle 26.
⁴ The G140L grating in the 800 and 1105 central wavelength settings moves the zero-order image onto segment B. Therefore, only segment A is available for these settings.

1.1.2 NUV Spectroscopy

To provide maximum wavelength coverage on the square format of the NUV detector, three mirrors simultaneously image three, fully aberration-corrected, spectra onto a single 1024 × 1024 Multi-Anode Micro-channel Array (MAMA) detector. Consequently, three separate regions of the spectrum are imaged onto the detector. These spectral regions, referred to as stripes A, B, and C, each span the physical length of the detector in the dispersion direction—but are not contiguous in wavelength space. The allowable grating positions were defined with two objectives: the capability of obtaining full spectral coverage over the NUV bandpass and maximizing scientific return with a minimum number of grating positions. As a result, several of the supported central wavelength positions were selected to maximize the number of diagnostic lines on the detector in a single exposure. Table 1.3 shows the wavelength ranges of the three stripes for all possible NUV grating and central wavelength combinations.

Table 1.3: Wavelength Ranges for NUV Gratings for FP-POS = 3.

¹ The central wavelength setting (cenwave) corresponds to the approximate midpoint of stripe B.
² For central wavelength 2635 Å, the stripe A wavelengths are listed for completeness only (and in case a bright emission line falls onto the detector). The NUV detector's sensitivity at these wavelengths is extremely low. To obtain a low-resolution spectrum at wavelengths below ~1700 Å we recommend the FUV grating G140L.
³ The values in shaded cells are wavelength ranges observed in second-order light. Their dispersion is twice that of the first-order spectrum. First-order flux, from wavelengths twice those of the listed range, will be present at the ~5% level.
⁴ Lyman α may be present in second-order light.
⁵ Longward of 3200 Å, second-order light may be present. At these wavelengths, the flux calibration applied by calcos is unreliable.

1.1.3 Grating Offset Positions (FP-POS)

For each NUV and FUV central wavelength setting there are four grating offset positions (FP-POS=1–4) available to move the spectrum slightly in the dispersion direction. This allows the spectrum to fall on different areas of the detector to minimize the effects of small scale fixed pattern noise in the detector. Figure 1.2 shows an example of the shifts in uncalibrated x-pixel coordinates of the NUV stripe B spectra for all four FP-POS positions.

1.1.4 NUV Imaging

COS imaging may only be done with the NUV channel and the spectral coverage includes the entire NUV bandpass from ~1650–3200 Å. This mode utilizes a flat mirror with two available mirror settings, MIRRORA and MIRRORB. The first setting uses a primary reflection off the mirror surface, and the second setting provides an attenuated reflection. MIRRORB and/or the BOA may be used to obtain images of brighter objects, but MIRRORB produces a secondary image and the BOA produces an image with coma that degrades the spatial resolution (Figure 5.2 and Figure 5.3).

While the spatial resolution of COS NUV MIRRORA (Section 1.2) images can be good, the field of view is very small. Furthermore, because COS uses the aberrated PSF from the optical telescope assembly (OTA), and because the optics image the sky onto the detector, not the aperture, the image includes some light from sources out to a radius of about 2 arcsec. However, only point sources within about 0.5 arcsec of the aperture center have essentially all their light imaged, and so the photometric interpretation of a COS image can be inherently complex.

1.1.5 Data Collection Modes

COS has two modes of data collection, TIME-TAG and ACCUM, and only one mode can be used for a given exposure. In TIME-TAG mode the position, time, and for FUV, pulse height of each detected photon are tabulated into an events list, while in ACCUM mode the photon events are integrated onboard into an image. TIME-TAG data have a time resolution of 32 ms, and can be screened as a function of time during the post-observation pipeline processing to modify temporal sampling and exclude poor quality data. COS is optimized to perform in TIME-TAG mode, although ACCUM mode is fully supported in the pipeline processing. Note that in ACCUM mode no "walk" correction is made (see Section 3.4). ACCUM mode should be used primarily for UV bright targets that can not be observed in TIME-TAG mode due to high count rates. Users should note that FUV data taken in ACCUM mode store only a portion of the full detector since the 18 MB of onboard memory cannot hold a complete FUV image (containing both detector segments). ACCUM mode omits only the wavecal region and unused detector space, therefore the FUV ACCUM subarrays contain all of any external spectrum. The FUV ACCUM subarrays, whose sizes are 16384 × 128, are shown in Figure 2.2 for LP3.