1.3 COS Quick Reference Guide

Table 1.1: COS Instrument Characteristics

PropertyFUV channelNUV 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



CsI (opaque)

Cs2Te (semi-transparent)



MgF2 (re-entrant)

Wavelength range

< 900–2150 Å

1650–3200 Å

Active area

85 × 10 mm (each for two segments)

25.6 × 25.6 mm

Pixel format (full detector)

16,384 × 1,024 (each for two segments)

1,024 × 1,024

Image size recorded per spectrum

16,384 × 128 (each for two segments, ACCUM)
16,384 × 1,024 (each for two segments, 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

Detector quantum efficiency

~26% at 1335 Å
~12% at 1560 Å

~10% at 2200 Å
~8% at 2800 Å

Typical dark-count rate (away from South Atlantic Anomaly)1

1.38 counts s–1 cm–2
1.99 × 10–6 counts s–1 pix–1
1.19 × 10–4 counts s–1 resel–1

180.80 counts s–1 cm–2
1.13 × 10–3 counts s–1 pix–1
1.02 × 10–2 counts s–1 resel–1

Detector Global Count rate limit2

60,000 counts s1

770,000 counts s1

Local count-rate limit2,3

~1.67 counts s–1 pix–1
~100 counts s–1 resel–1

~200 counts s–1 pix–1
~1800 counts s–1 resel–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 s1 pix1.

Table 1.3: COS Calibration Accuracies


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



Relative photometry (same object at different times)2



Flat field quality (spectral S/N) per resel, using standard techniques



Flat field quality (spectral S/N) per resel, using special techniques



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


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


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

Figure 5.17

NUV flat-field example