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 Instrument 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) 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.

GratingWavelength range (Å)Bandpass per exposure and FUV Gap1 (Å)Resolving Power R = λ/FWHM2Dispersion (mÅ pixel-1

FUV Channel

G130M

900–1236

295/16

up to 11,5003

 9.97

1065–1365

296/15.7

10,000–15,0004

 9.97

1150–1450

292/14.3

12,000–24,0004

 9.97

G160M

1405–1775

360/18.1

13,000–24,0004

12.23

G140L

~900–21503

>1150/112

1,500–4,0004

80.3 

NUV Channel

G185M

1700–2100

3 × 35

16,000–20,000

37  

G225M

2100–2500

3 × 35

20,000–24,000

33  

G285M

2500–3200

3 × 41

20,000–24,000

40  

G230L

1700–32005

(1 or 2) × 400

2,100–3,200

390   

1 Width of gap between FUV detector segments.
2 Empirically determined FWHM of the LSF, which is not Gaussian. R increases approximately linearly with wavelength.
3 R falls with increasing wavelength. R=8,50011,500 between 940 and 1080 Å.
4 Resolution is dependent on wavelength, cenwave, and Lifetime Position. For more detail of the resolution at LP4, see Figure 1.1 and http://www.stsci.edu/hst/instrumentation/cos/proposing/cos2025-policies.
5 Some shorter wavelengths are recorded in second-order light.

Figure 1.1: COS FUV Spectral Resolution Depends on Lifetime Position.

This figure shows the spectral resolution R = λ/Δλ versus wavelength for 5 central wavelengths (cenwaves), four from the G130M grating (1055, 1096, 1222, 1291), and one from the G160M grating (1577). The two blue mode settings (1055 and 1096) remain at Lifetime Position 2 (LP2). All other settings shown are now at LP4, but were previously at various Lifetime Positions. The resolution curves shown are generated by the Code V optical models, which has been validated through on-orbit observations. The resolution increases with wavelength for the 1222, 1291, and 1577 settings, and vary with Lifetime Position.

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.

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

Grating

Central wavelength setting (Å)1

Recorded wavelengths2

Segment B

Segment A

G130M

1055

899–1040

1055–1196

1096

940–1080

1096–1236

1222

1067–1207

1223–1363

1291

1134–1274

1291–1431

1300

1144–1283

1300–1441

1309

1154–1294

1309–1450

1318

1163–1303

1319–1460

1327

1172–1313

1328–1469

G160M

15333

1342–1515

1533–1707

1577

1386–1559

1577–1751

1589

1397–1571

1589–1762

1600

1409–1581

1601–1774

1611

1420–1594

1612–1786

1623

1432–1606

1625–1798

G140L

8003

N/A4

815–1948


1105

N/A4

1118–2251


1280

<900–1165

1280–2391

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


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.

Grating

Central wavelength setting (Å)1

Recorded wavelengths

Stripe A

Stripe B

Stripe C

G185M

1786

1670–1705

1769–1804

1868–1903

1817

1701–1736

1800–1835

1899–1934

1835

1719–1754

1818–1853

1916–1951

1850

1734–1769

1833–1868

1931–1966

1864

1748–1783

1847–1882

1945–1980

1882

1766–1801

1865–1900

1964–1999

1890

1774–1809

1872–1907

1971–2006

1900

1783–1818

1882–1917

1981–2016

1913

1796–1831

1895–1930

1993–2028

1921

1804–1839

1903–1938

2002–2037

1941

1825–1860

1924–1959

2023–2058

1953

1837–1872

1936–1971

2034–2069

1971

1854–1889

1953–1988

2052–2087

1986

1870–1905

1969–2004

2068–2103

2010

1894–1929

1993–2028

2092–2127

G225M

2186

2070–2105

2169–2204

2268–2303

2217

2101–2136

2200–2235

2299–2334

2233

2117–2152

2215–2250

2314–2349

2250

2134–2169

2233–2268

2332–2367

2268

2152–2187

2251–2286

2350–2385

2283

2167–2202

2266–2301

2364–2399

2306

2190–2225

2288–2323

2387–2422

2325

2208–2243

2307–2342

2406–2441

2339

2223–2258

2322–2357

2421–2456

2357

2241–2276

2340–2375

2439–2474

2373

2256–2291

2355–2390

2454–2489

2390

2274–2309

2373–2408

2472–2507

2410

2294–2329

2393–2428

2492–2527

G285M

2617

2480–2521

2596–2637

2711–2752

2637

2500–2541

2616–2657

2731–2772

2657

2520–2561

2636–2677

2751–2792

2676

2539–2580

2655–2696

2770–2811

2695

2558–2599

2674–2715

2789–2830

2709

2572–2613

2688–2729

2803–2844

2719

2582–2623

2698–2739

2813–2854

2739

2602–2643

2718–2763

2837–2878

2850

2714–2755

2829–2870

2945–2986

2952

2815–2856

2931–2972

3046–3087

2979

2842–2883

2958–2999

3073–3114

2996

2859–2900

2975–3016

3090–3131

3018

2881–2922

2997–3038

3112–3153

3035

2898–2939

3014–3055

3129–3170

3057

2920–2961

3036–3077

3151–3192

3074

2937–2978

3053–3094

3168–3209

3094

2957–2998

3073–3114

3188–3229

G230L

2635

1334–17332

2435–2834

1768–19673

2950

1650–2050

2750–3150

1900–21003

3000

1700–2100

2800–3200

1950–21503

3360

2059–24584

3161–35605

2164–23613

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.


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.

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).

Figure 1.2: Grating Offset Positions (FP-POS).

This figure shows spectra of an emission-line source obtained at all four FP-POS positions using the G185M grating with a central wavelength setting of 1850. The individual plots show the collapsed counts from the stripe B spectra versus the uncalibrated x-pixel coordinates. Note that the three features marked 1, 2, and 3, shift slightly for each FP-POS position.
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.


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. 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.