cosmic origins spectrograph hubble space telescope james c. green university of colorado cosmic...

Cosmic Origins SpectrographHubble Space Telescope

James C. Green University of Colorado

Cosmic Origins Spectrograph

Instrument Design

and Capabilities



• Hubble 4th-generation instrument for Servicing Mission 4 (Spring 2004)

• Replaces COSTAR in Bay 4• New ultraviolet spectrograph with more

than 10 times greater sensitivity than previous or existing UV instruments

• Will probe the formation of structure and evolution of matter in the Universe by studying the furthest objects and the intervening material

• PI Institution: University of Colorado• Industrial Partner: Ball Aerospace,

Boulder, Colorado• Additional partners: UC-Berkeley, STScI,

GSFC, SwRI, U. Wisconsin• Web site: http://cos.colorado.edu



COS Science Requirements• The COS Science Goals require:

– point-source spectroscopy at UV wavelengths– medium spectral resolution (R > 20,000)– highest possible throughput– broad wavelength coverage in one exposure

• These goals are met using a combination of:– Quasi-Rowland circle spectrograph design (FUV) with only 1 reflection– high-efficiency 1st-order holographic gratings– large-format, solar-blind detectors– HST’s capabilities

• large collecting area• UV coatings• excellent pointing stability• superb image quality (after aberration correction)

• NUV channel to backup STIS and provide complementary observing capability



• COS has 2 channels to provide low and mediumresolution UV spectroscopy– FUV: 1150-1775Å, NUV: 1700-3200Å

• FUV gratings: G130M, G160M, G140L

• NUV gratings: G185M, G225M, G285M, G230L

– All M gratings have a spectral resolution requirement of R 20,000

NUV MAMADetector

(STIS spare)

CalibrationPlatform

FUV XDLDetector

OSM2: G185M, G225M,G285M, G230L, TA1

OSM1: G130M,G160M, G140L,NCM1

Aperture Mechanism:Primary Science Aperture,Bright Object Aperture

Optical bench(not shown):

re-use of GHRSbench



COS FUV Spectroscopic Modes

Nominal Wavelength Resolving Power

Grating Wavelength Range (R =

b

Coverage a per Exposure

G130M 1150 - 1450 Å 300 Å 20,000 - 24,000

G160M 1405 - 1775 Å 375 Å 20,000 - 24,000

G140L 1230 - 2050 Å > 820 Å 2500 - 3500

a Nominal Wavelength Coverage is the expected usable spectral range delivered by each grating mode. The G140L grating disperses the 100 - 1100 Å region onto one FUV detector segment and 1230 - 2400 Å onto the other. The sensitivity to wavelengths longer than 2050 Å or shorter than 1150 Å will be very low.

b The lower values of the Resolving Power shown are delivered at the shortest wavelengths covered, and the higher values at longer wavelengths. The resolution increases roughly linearly between the short and long wavelengths covered by each grating mode.



• COS FUV and NUV flight detectors meet or exceed performance requirements (QE, resolution, flat-fields, etc.)

• FUV gratings:– FUV holographic gratings fabricated by Jobin-Yvon (France) on

SVG/Tinsley substrates and coated at GSFC

– Flight FUV gratings (G130M, G160M, G140L) have been delivered which meet or exceed requirements

– G140L-blazed R&D effort on-going at J-Y could, if successful, deliver additional 30-50% throughput improvement for that mode

FUV Grating& Test

Grating Efficiency(Measured/Spec)

Detector QE(Meas/Spec)

Channel Throughput(Meas/Spec = %)

G130M @ 1304Å .45/.36 .33/.25 .15/.09 = 165%

G160M @ 1560Å .55/.36 .23/.17 .13/.06 = 207%G140L @ 1407Å .33/.24 .25/.19 .082/.046 = 179%



FUV Effective Area

3000 cm2

1000 cm2

Sensitivity ~ S/N = 10 in 10,000s for F = 1 x 10-15 ergs/cm2/s/Å



COS NUV Spectroscopic Modes Nominal Wavelength Resolving Power

Grating Wavelength Range (R = b

Coverage a per Exposure

G185M 1700 - 2100 Å 3 x 35 Å 18,000 - 27,000

G225M 2100 - 2500 Å 3 x 35 Å 20,000 - 27,000

G285M 2500 - 3200 Å 3 x 35/41 Å 20,000 - 27,000

G230L 1700 - 3200 Å 1-2 x 500 Å 850 - 1600

a Nominal Wavelength Coverage is the expected usable spectral range delivered by each grating mode, in

three non-contiguous strips for the medium-resolution modes. The G230L grating disperses the 1st-order spectrum between 1700 - 3200 Å along the A&B stripes on the NUV detector. G230L also disperses the 400 - 1400 Å region onto one of the outer spectral stripes and the 3400 - 4400 Å region onto the other. The shorter wavelengths will be blocked by an order separation filter and the longer will not register because the detector is solar blind. The G230L 2nd-order spectrum between 1700 - 2200 Å may be detectable along the long wavelength stripe.

b The lower values of the Resolving Power shown are delivered at the shortest wavelengths covered, and the higher values at longer wavelengths. The resolution increases roughly linearly between the short and long wavelengths covered by each grating mode.



NUV Effective Area

800 cm2

200 cm2 Sensitivity ~ S/N = 10 in

10,000s for F = 1 x 10-15 ergs/cm2/s/Å



Calibration Sub-system

cosmic origins spectrograph hubble space telescope james c. green university of colorado cosmic...

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