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HST Field of View

COS Optical Design
Calibration Lamp Platform Optic Select Mechanism 2 NUV Detector

COS
Cosmic Origins Spectrograph
Far Ultraviolet (FUV) Channel
1-reflection; aberration-corrected along dispersion 2 side-by-side 16k x 1k pixel delay line MCPs TIME-TAG; pulse-height recording 3 gratings; 300-800 spectral range R=3000; 20,000 [1150-2050 е]

FUV Detector

Entrance Aperture

Optic Select Mechanism 1

External light enters at lower left; common path to first element at lower right. FUV path in green; NUV path in blue; PtNe and D2 input calibration beams in tan.

Near Ultraviolet (NUV) Channel
Fully corrected optical design 1k x 1k 25µm/pixel MAMA TIME-TAG recording 4 gratings, 1 imaging mirror and filter 100-800 non-contiguous spectral range R=2000; 20,000 [1700-3200 е]

COS Spectroscopic Mode Summary
The HST focal plane showing the instrument complement after Servicing Mission 4. The COS FUV and NUV channels view the same 2.5 arcsec patch of sky, but not simultaneously. Instrument apertures and separations are shown to scale.

Spectral Element G130M G160M G140L G185M G225M G285M G230L

Nominal Wavelength Range 1150 - 1450 1405 - 1775 1230 - 2050 1700 - 2100 2100 - 2500 2500 - 3200 1700 - 3200

Wavelength Coverage per exp (е) 300 375 820 3 x 35 3 x 35 3 x 41 1 or 2 x 400

Resolving Power () 20000-24000 20000-24000 2400-3500 16000-20000 20000-24000 20000-24000 1500-2800

COS Wavelength Coverage by Mode Mode
G230L G285M G225M G185M G140L G160M G130M 1500 2000 2500 W avelength () 3000

The Cosmic Origins Spectrograph has been built through a collaborative effort between the University of Colorado, Ball Aerospace, and Goddard Space Flight Center. COS will be installed on the Hubble Space Telescope (HST) during HST Servicing Mission 4. The COS principal investigator is James Green (CU). COS is designed to primarily observe point sources. Equipped with state-of-the art detectors and optics and optimized for maximum FUV throughput, COS provides moderate and low resolution spectroscopy throughout the HST-accessible satellite ultraviolet, dramatically increasing faint object discovery power and the spectroscopic efficiency of HST. For further information, please visit: http://www.stsci.edu/hst/cos http://www.stsci.edu/hst/cos

Wavelength coverages available with common COS spectroscopic modes. The ranges for different grating tilts are offset in the vertical direction. Additional settings may be available for some modes. Notice noncontinuous spectral ranges for NUV modes.

April 2007


COS and STIS FUV Throughput Comparison
- 12.5

COS FUV Format
Internal PtNe Wavecal

COS and STIS Detector Characteristic Comparison
COS/FUV Spectral coverage () STIS/FUV COS/NUV
1700-3200 900 (M) 750 (L) (at 2500)

)

- 13.0

ST IS E140M + 0.2x0.2 ap e r t u r e R=45,000

-1

-1

STIS/NUV
1600-3200 350 (M) 900 (L) (at 2500)

- 13.5

1150-1775(M) 1150-1700 1230-2050(L) 2800 (M) 2400 (L) (at 1300) 400 (M) 1700 (L) (at 1300)

log Flux (erg cm

-2

sec

- 14.0 C OS G13 0M R~20,000- 24,000

External Source
C OS G16 0M R~20,000- 24,000

Effective Area (cm2) Resolution ()

- 14.5

- 15.0

- 15.5 1100

1200

1300

1400

1500

1600

1700

1800

W ave le n g t h ( )

The figures above and below compare COS Primary Science Aperture (PSA) and STIS medium resolution (M mode) performance by indicating the limiting flux for which S/N=10 can be achieved in 3600 sec exposures with uniform binning corresponding to R~20,000 (bins of 0.08 е for FUV and 0.12 е for NUV. The superior light gathering power of COS provides >10x the FUV throughput of STIS and up to 70x STIS FUV observing speed. Note: COS Bright Object Aperture (BOA) performance limits are ~100x brighter due to the BOA ND2 filter.


The figure above shows the typical spectrum format for a single COS FUV MCP detector segment. The COS FUV detector consists of two such segments separated by a small gap. Two spectra may be recorded as shown above. The lower spectrum is from an external source illuminating the prime science aperture (PSA); the upper spectrum is from the internal PtNe wavelength calibration lamp which illuminates the wavelength calibration aperture (WCA). The figure below shows the typical spectrum format for the COS NUV MAMA detector. Three separate segments ­ labeled A, B, and C - of the dispersed first-order spectrum are directed by separate pickoff mirrors to illuminate slightly offset positions on the detector. The left-most three stripes are from the internal PtNe wavelength calibration lamp. The three stripes to the right are from an external source illuminating the PSA.
PtNe wavecal C B A External source C B A

H 110000 N/A N/A 110000 M 20000-24000 10000-40000 16000-24000 10000-30000 1000 1500-2800 500 L 2400-3500
32768 1024 (2048) 1024 1024 (2048)

Number of pixels along dispersion Background (cts/sec/resel) Background equivalent F
-2

4.3 x10-5

350 x10

-5

1.9 x10-3

17 x10-3

(erg cm sec )

-1



0.5-8 x10
-1

-18

20 x10-18

1.3-3.8 x10-16

13.8 x10

-16

COS Time-TAG Mode Summary Default mode for FUV and NUV; 32 msec sampling FUV pulse-heights improve background removal Unique TAGFLASH mode acquires wavecal spectra during science exposures; no on-target time lost Fully processed photon list product; can be binned or time-sliced to user requirements

COS flat fields will support S/N=30 and for FP-split observations in certain circumstances S/N~100.

COS and STIS NUV Throughput Comparison
-12.5

COS Imaging Mode Summary Imaging Characteristic COS/NUV
12.5 (full diameter) 4.9 (un-vignetted) 0.07 (2300) 0.02 (3300) 0.024 166 (full diameter) 100 (un-vignetted) ~5 x10-19 (HSTMAG ~ 24.5) 1 (broad-band, 1700-3200 .)

-13.0
log Flux (erg cm -2 sec -1 -1)

STIS E230M + 0.2x0.2 ape r tu r e R=30,000

FOV area (arcsec2)
Y (pix)

-13.5

Broadband throughput Pixel scale (arcsec) Number of pixels

-14.0

-14.5 COS G185M R~16,000- 20,000

-15.0

COS G225M R~20,000-24,000 2200 2400 2600 2800

COS G285M R~20,000-24,000 3000 3200

-15.5 1600

Background equivalent F (erg cm-2 sec-1 -1)
X (pix)

1800

2000

Wave le ngth ()

Number of filters

COS NUV Format