1.3	COS Quick Reference Guide

Table 1.1: COS Instrument Characteristics

Property	FUV channel	NUV channel
Entrance apertures	2.5 arcsec round: clear (PSA) or attenuated (BOA)
Detector plate scale	0.114 arcsec per pixel 1.14 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-dispersion)	23.8 mas per pixel 71.4 mas per resel (G185M along- and cross-dispersion directions)

Table 1.2: COS Detector Characteristics

		FUV XDL	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)		16384 × 1024 (two)	1024 × 1024
Image size recorded per spectrum		16384 × 128 (two, ACCUM) 16384 × 1024 (two, TIME-TAG)	1024 × 1024
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		3.03 cnt s–1 cm–2 4.37 × 10–6 cnt s–1 pix–1 2.67 × 10–4 cnt s–1 resel–1	159 cnt s–1 cm–2 9.92 × 10–4 cnt s–1 pix–1 8.93 × 10–3 cnt s–1 resel–1
Detector global count rate limit2	TIME-TAG mode	30,000 cnt s–1 (but counts lost at rates above ~21,000 cnt s–1)
	ACCUM mode	~15,000 cnt s–1 per segment	~170,000 cnt s–1
Local count-rate limit2		~1.67 cnt s–1 pix–1 ~100 cnt s–1 resel–1 3	~200 cnt s–1 pix–1 ~1800 cnt s–1 resel–1
Screening limits for bright objects		see Table 10.1
Dead-time constant		7.4 μsec	280 nsec

---

1

The dark-count rate values are projections to the middle of Cycle 21 (April 2014).

2

Count rates higher than these limits will trigger a detector shut-down. Bright-object screening limits are lower. See Chapter 10.

3

These values are not applicable to the COS FUV G130M blue cenwaves, 1055 and 1096. We expect the local count rate limits for those modes to be lower by a factor 2-3. However, the calibration data allowing the derivation of the local count rate limits for the 1055 and 1096 cenwaves are still being worked on. Users should check the COS Web site for updates on those values.

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 scale	15 km s–1	15 km s–1
Absolute photometry	5%	5%
Relative photometry (same object at a different time)	2%	2%
Flat field quality (spectral S/N) per resel, using standard techniques	35:11	100:1
Flat field quality (spectral S/N) per resel, goal	100:1	100:1

---

1

S/N of 100:1 is possible; see Section 5.8.1.

Table 1.4: Useful Figures and Tables

Topic	Source	Content
Usage planning	Table 5.1	Grating parameters
	Table 5.4	FUV grating wavelength ranges
	Table 5.5	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 4.9	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 13.28	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	Example of 3 × 3 spiral search pattern
	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 4.8	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 5.12	FUV flat-field example
	Figure 5.14	NUV flat-field example