Calibration of the Cosmic Origins Spectrograph

James C. GreenUniversity of Colorado

The CU COS Instrument TeamProject Scientist: Cynthia FroningInstrument Scientist: Steve OstermanSteve PentonStéphane BélandDetector Scientist: Jason McPhate (Berkeley)

Gone – but not forgottenErik WilkinsonKen BrownsbergerJohn AndrewsJon MorseDennis EbbetsDave Sahnow

• 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

– M gratings have spectral resolution 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 2400 - 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.

* N2 purge data through FUV detector door window.

* Portion of FUV detector flat-field obtained during component-level testing.

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 16,000 - 20,000 G225M 2100 - 2500 Å 3 x 35 Å 20,000 - 24,000 G285M 2500 - 3200 Å 3 x 41 Å 20,000 - 24,000 G230L 1700 - 3200 Å (1 or 2) x 400 Å 1500 - 2800 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 middle strip on the NUV detector. G230L also disperses the 400 - 1400 Å region onto one of the outer spectral strips and the 3400 - 4400 Å region onto the other. The shorter wavelengths will be blocked by an order separation filter and the longer will have low thruput on the solar blind detector. The G230L 2nd-order spectrum between 1700 - 2200 Å will be detected along the long wavelength strip.

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.

Increasing Wavelength

Point source Spectra Calibration Stripes

25.60 square

2.80 2.80

NCM3a

NCM3a

NCM3b

NCM3b

NCM3c

NCM3c

1.45 2.255.75 4.95

All dimensions in mm

• NUV spectra projected onto the NUV MAMA detector

Wavelength Accuracy

• Extragalactic moderate resolution programs generally require absolute wavelength accuracy of ~ +/- 1 resel ( = +/- 15 km/s), with relative accuracy of 1/3 resel rms across the spectrum.

• Some programs that require higher accuracy can use “tricks” to obtain needed calibration e.g., using known wavelengths of ISM lines along sight-line.

Target Acquisition• COS is a “slitless” spectrograph, so the precision of target acquisition (placement

of target relative to calibration aperture) is the largest uncertainty for determining the absolute wavelength scale.

• Goal is to center targets routinely in science apertures to a precision of+/- 0.1 arcsec (= +/- 10 km/s).

• Throughput is relatively insensitive to centering due to large size of science apertures; centering of +/- 0.3 arcsec necessary for >98% slit throughput.

• The aberrated HST PSF centered in the COS Primary Science Aperture.

Fundamental Design Priorities

1: Sensitivity

2: Bandpass

3: Spectral Resolution

High Sensitivity

High throughput achieved through a minimum of reflections - ion etched blazed holographic gratings - windowless, opaque photocathode (NUV)

COS throughput measured by direct comparison to RASCAL reference detector – effective areas calculated by multiplying by HST area and reflectivity

For Comparison - COS Throughputs:G130M @ 1216 Å (peak) = 19.7%, @1430 Å = 9.0%G160M @ 1430 Å = 11.7%, @1669 Å = 5.8%G140L @ 1248 Å = 10.7%, @1723 Å = 0.65%G185M @1777 Å = 3.9%, @2085 Å = 2.3%G225 M @ 2262 Å = 2.7%, @ 2497 Å = 2.8%G285M @ 2659 Å = 2.2%, @ 2998 = 1.0%G230L @ 1846 Å = 3.0%, @ 2998 Å = 0.8%

Spectral Resolution

Spectral Resolution determined by measuring the FWHM of best Gaussian fit to emission lines:

FWHM (m) * dispersion (Å/m) = / = Resolution

This technique will underestimate the true resolution if the emission line is not monochromatic

COS has an imaging mode

• TA1 : implemented for target acquisition and correction of initial installation errors

• Broad Band Imaging in the NUV – no filters – response dictated by the MAMA QE curve

COS Imaging

Plate scale = 0.024″ / pixelFWHM resolution (radial) 2.7 pixels =

0.064″ (FWHM is ~ 78% encircled energy)Field of View = 2.5″ - 3″ (fuzzy edge)(40 resol diameter - 105 pixel diameter

image)

For comparison, ACS WFC/HRC has 80/82% encircled energy in 0.25″

COS Flat Fielding

• NUV MAMA – classic flat filed techniques yield high signal/noise capabilities

COS Flat Fielding

• FUV detector show hex pattern consistent with MCP manufacturing process –

• Stable geometric distortion map can be developed and applied

• Classic flat field techniques do not fully remove the structure - redistribution

COS Bonus Feature

Potential capability from 912 - 1150Å

G140L places these wavelengths on the detector with spectral resolution of ~ 1000

Grating efficiency at 1048Å measured – implying 20-50 cm2 effective area at 1048Å – assuming that HST mirror coatings behave as grating coating