1.3 COS Quick Reference Guide

Table 1.1: COS Instrument Characteristics

Property	FUV channel	NUV channel
Entrance apertures	2.5 arcsec diameter circle: clear (PSA) or attenuated (BOA)
Detector plate scale	~0.11 arcsec per pixel¹ 1.1 arcsec per resel (G130M cross-dispersion)	23.5 mas per pixel 70.5 mas per resel (imaging mode)
	0.0285 arcsec per pixel 0.171 arcsec per resel (G130M along-dispersion²)	23.8 mas per pixel 71.4 mas per resel (G185M along—and cross-dispersion directions)

¹ Exact value of plate scale depends on grating and lifetime position.
² Raw plate scale.

Table 1.2: COS Detector Characteristics

	FUV XDL (A/B)	NUV MAMA
Photocathode	CsI (opaque)	Cs₂Te (semi-transparent)
Window	None	MgF₂ (re-entrant)
Wavelength range	< 900–2150 Å	1650–3200 Å
Active area	85 × 10 mm (two)	25.6 × 25.6 mm
Pixel format (full detector)	16,384 × 1,024 (two)	1,024 × 1,024
Image size recorded per spectrum	16,384 × 128 (two, `ACCUM`) 16,384 × 1,024 (two, `TIME-TAG)`	1,024 × 1,024
Pixel size	6 × 24 μm	25 × 25 μm
Spectral resolution element size (= resel)	6 × 10 pix	3 × 3 pix
Quantum efficiency	~26% at 1335 Å ~12% at 1560 Å	~10% at 2200 Å ~8% at 2800 Å
Typical dark-count rate (away from SAA)¹	1.38 counts s^–1cm^–2 1.99 × 10^–6counts s^–1 pix^–1 1.19 × 10^–4counts s^–1resel^–1	180.80 counts s^–1cm^–2 1.13 × 10^–3counts s^–1pix^–1 1.02 × 10^–2 counts s^–1resel^–1
Detector Global Count rate limit²	60,000 counts s^–1	770,000 counts s^–1
Local count-rate limit^2,3	~1.67 counts s^–1pix^–1 ~100 counts s^–1resel^–1	~200 counts s^–1pix^–1 ~1800 counts s^–1resel^–1
Screening limits for bright objects	see Table 10.1
Dead-time constant	7.4 μs	280 ns

¹ The dark rate varies with time. The reported values are measured as of September 2020. FUVA and FUVB values differ slightly (see Table 7.1). See also COS Dark Rate Monitoring for updated values.
² Count rates higher than these limits will trigger a detector shut-down. Bright-object screening limits are lower. See Chapter 10.
³ The FUV values are not applicable to the COS FUV G130M blue cenwaves, 1055 and 1096. The local count rate limit for these two modes is 0.2 counts s⁻¹pix⁻¹.

Table 1.3: COS Calibration Accuracies

Property	FUV channel	NUV channel
Wavelength zero point: M gratings	15 km s^–1	15 km s^–1
Wavelength zero point: L gratings	150 km s^–1	175 km s^–1
Wavelength scale¹	15 km s^–1	15 km s^–1
Absolute photometry²	5%	5%
Relative photometry (same object at a different times)²	2%	2%
Flat field quality (spectral S/N) per resel, using standard techniques	35:1	100:1
Flat field quality (spectral S/N) per resel, using special techniques	100:1³	100:1

¹ With updated dispersion solutions, the wavelength scales for the FUV M gratings are now accurate to 7 km s^–1. See the COS ISR 2018-22 for more details.
² Value for the NUV channel applies to spectroscopy, not imaging.

³ See Section 5.8.1.

Table 1.4: Useful Figures and Tables

Topic	Source	Content
Usage planning	Table 5.1	Grating parameters
	Table 5.3	FUV grating wavelength ranges
	Table 5.4	NUV grating wavelength ranges
	Table 10.1	Count-rate screening limits
Aperture parameters and PSFs	Figure 3.2	BOA transmission
	Figure 3.6	Model LSFs for the COS FUV channel
	Figure 4.10	Model LSFs for the COS NUV channel
	Figure 3.7	Resolving Power of FUV gratings
	Figure 6.4	Cross section of the COS imaging PSF
	Figure 8.5	Relative transmission of the COS PSA in the NUV
	Figure 13.29	HST focal plane and COS aperture
Effective Area	Figure 5.1	FUV spectroscopy
	Figure 5.2	NUV spectroscopy
	Figure 6.3	NUV imaging
Acquisitions	Figure 8.1	Examples of spiral search patterns
	Figure 8.2	Point-source images with all apertures and mirrors
	Figure 8.3	ACQ/IMAGE exposure times
	Figure 8.4	Dispersed-light exposure times
Detector characteristics	Figure 4.1	FUV XDL detector schematic layout
	Figure 4.9	NUV MAMA detector layout
	Table 7.1	Detector background count rates
Overheads and observing parameters	Table 5.2	TAGFLASH exposure durations
	Table 9.1	Overhead estimates for Phase `I` proposals
	Table 9.2	Generic observatory overhead times
	Table 9.3	Overhead times for OSM1 movements
	Table 9.4	Overhead times for OSM2 movements
	Table 9.5	Science exposure overhead times
Celestial backgrounds	Figure 7.1	Sky background versus wavelength
	Figure 7.2	Moon, Earth and zodiacal-light background levels
	Figure 7.3	Galactic extinction model
	Table 7.3	Earthshine and zodiacal light fluxes
	Table 7.4	Strengths of airglow lines
Data quality	Figure 5.13	FUV flat-field example
Data quality	Figure 5.17	NUV flat-field example

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COS Instrument Handbook

1.3 COS Quick Reference Guide

Table 1.3: COS Calibration Accuracies

Table 1.4: Useful Figures and Tables

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