1.1	Instrument Capabilities and Design

FUV Spectroscopy

NUV Spectroscopy

Grating Offset Positions (FPPOS)

NUV Imaging

Data Collection Modes

The Cosmic Origins Spectrograph (COS) is an HST fourth generation 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; however, this is restricted to observatory calibration programs.

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 1650-3200 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 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 completely 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

Grating	Normal wavelength range (Å)1	Bandpass per exposure (Å)	Resolving Power R = λ/Δλ	Dispersion (mÅ pixel–1)
FUV Channel
G130M	9002 – 1450	300	16,000 – 21,000	9.97
G160M	1405 – 1775	370	16,000 – 21,000	12.23
G140L	900 – 21503	>1100	1,500 – 4,000	80.3
NUV Channel
G185M	1700 – 2100	3 × 35	16,000 – 20,000	34
G225M	2100 – 2500	3 × 35	20,000 – 24,000	34
G285M	2500 – 3200	3 × 41	20,000 – 24,000	40
G230L	1700 – 3200	(1 or 2) × 400	2,100 – 3,200	390

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1

Normal wavelength ranges are for the primary (default) central wavelength setting.

2. Modes covering wavelengths below 1130 Å are expected to have a much reduced resolving power of about 2000.

3. Second order contamination possible longward of 2150 Å.

FUV Spectroscopy

The FUV channel employs a large format cross delay line (XDL) detector consisting of two 16384 x 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.

Table 1.2: Wavelength Ranges for FUV Gratings for FPPOS = 3

Grating	Central wavelength setting (Å)	Recorded wavelengths
		Segment B	Segment A
G130M	10551	900 - 1041	1055 - 1197
	10961	940 - 1081	1096 - 1237
	12221	1068 - 1208	1223 - 1364
	1291	1132 – 1274	1291 – 1433
	1300	1141 – 1283	1300 – 1442
	1309	1150 – 1292	1309 – 1451
	1318	1159 – 1301	1318 – 1460
	1327	1168 – 1310	1327 – 1469
G160M	1577	1382 – 1556	1577 – 1752
	1589	1394 – 1568	1589 – 1764
	1600	1405 – 1579	1600 – 1775
	1611	1416 – 1590	1611 – 1786
	1623	1428 – 1602	1623 – 1798
G140L2	1105	N/A3	1105 – 2253
	12304	<300 – 1095	1230 – 2378
	1280	<500 – 1165	1280 – 2405

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1

These are new modes that are currently being characterized, so these numbers are preliminary. 1055 and 1096 central wavelength settings have been offered starting from Cycle 19 and 1222 central wavelength mode starting from Cycle 20.

2

It is not yet clear how much of the G140L segment B short wavelength (< 900 Å) ranges will be available due to uncertainties in the HST OTA throughput. This is currently being investigated.

3

The G140L grating and 1105 central wavelength setting moves the zero-order image onto segment B. Therefore, only segment A is available for this setting.

4

Beginning in Cycle 18, the G140L 1230 setting was replaced by the 1280 setting.

NUV Spectroscopy

To retain efficiency utilizing the square format of the NUV detector, three mirrors simultaneously image three, fully aberration-corrected, spectra onto a single 1024 x 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

Grating	Central wavelength setting (Å)	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 – 17331	2435 – 2834	1768 – 19672
	2950	1650 – 2050	2750 – 3150	1900 – 21002
	3000	1700 – 2100	2800 – 3200	1950 – 21502
	3360	2059 – 2458	3161 – 35603	2164 – 23612

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1

The wavelengths listed for central wavelength 2635 Å in stripe A are for completeness only and also in case a bright emission line falls onto the detector. Note that the NUV detector’s sensitivity at these wavelengths is extremely low. To obtain a low-resolution spectrum at wavelengths below about 1700 Å we recommend G140L and the FUV channel.

2

The values in shaded cells are wavelength ranges as seen in second-order light. In these cases the achieved dispersion is twice that for first-order mode. Note, however, that some first order light may contaminate the spectrum depending on the SED of the target.

3

The spectrum longward of 3400 Å may be contaminated by second order spectrum, depending on the SED of the target.

Grating Offset Positions (FPPOS)

For each NUV and FUV central wavelength setting there are four grating offset positions (FPPOS=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 1.1 shows the shifts in uncalibrated x pixel coordinates of the stripe B spectra for all four FPPOS positions.

Figure 1.1: Grating Offset Positions (FPPOS)

This figure shows spectra obtained at all four FPPOS 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 FPPOS position.

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 quite good, the field of view is very small. Furthermore, 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 observation. 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. 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 use sub-arrays since the 18MB 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 x 128, are shown in Figure 2.2.