SPACE
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SCIENCE
INSTITUTE
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TIPS / COS Update
The NUV Gratings
18 October 2007
Last COS TIPS presentation: 21 December 2006
NUV grating characteristics
 COS NUV grating blanks coated with Au and Cr
 Original COS NUV gratings (G185M, G225M, G285M, and
G230L) also were MgF2 coated over Al to protect reflective
surface
 Routine design process shifts Wood’s anomaly out of spectral
region of interest to shorter wavelengths
– Anomalous distribution of energy in diffracted light; highly polarization
dependent; sensitive to groove spacing and depth comparable to
wavelength diffracted
– this is a resonance effect – it will not show up in bulk reflectivities, such
as from flat-mirror witness samples, but as a complicated wavelength and
polarization-dependent grating efficiency modulation.
Keyes – 18 October 2007
Slide 2 of 19
NUV Gratings
 For COS NUV gratings the addition of the last layer of coating
(MgF2) apparently shifts Wood’s anomaly directly into
bandpass of interest decreasing throughput substantially
– Solution 1: use MgF2-coated “longer-wavelength” grating in shorter
wavelength region where throughput is nominal
> Consequence: resolution degraded as Dl does not change
> Shifting G225M for use in G185M region works well, G285M to G225M does
not
– Solution 2: do not apply MgF2 coating to Al surface; anomaly stays at
design location
> This is employed for G225M and G285M gratings
> Consequence: Al oxidizes within days of deposition; forms thin coating;
literature indicates oxide layer reaches maximum depth of ~5 nm.
Keyes – 18 October 2007
Slide 3 of 19
Variances in measured COS sensitivities
in vacuum 2006/2003
Bare Al
Bare Al
Keyes – 18 January 2007
Slide 4 of 19
Grating Efficiency Issues:
 2003 T/V data compared to 2006 T/V data
- this data shows significant variations in efficiency between 2003 and
2006, including indications that some channels at some wavelengths
improved in efficiency by as much as 40% and others diminished by ~25%
- the majority, but not all, of these discrepancies can be explained by the
polarization sensitivity of COS, and the difference in polarization
generated by the 2003 Calibration delivery system and the 2006 calibration
delivery system
- this polarization sensitivity has been demonstrated on the flight spares,
and in the flight optics
COS polarization data
Polarization and COS
1.800
1.700
Vertical/Horizontal
1.600
1.500
1.400
1.300
1.200
1.100
1.000
0.900
0.800
0.700
1200
1700
2200
2700
Wavelength (Anstroms)
Scrambled, CDS+RAS/Cal
Unscrambled, CDS Only
3200
Grating Efficiency Monitoring
 Grating efficiency monitoring tests of the NUV gratings
(no monitoring is done of the FUV gratings)
–
a sequence of exposures are performed on the NUV gratings at
various wavelength settings using the internal lamps, and the count
rates are recorded – ratios of count rates in different channels at the
same wavelength (when possible) are also calculated – this process
should remove the possible effects of lamp variability
 These tests indicate that the lamp is slowly losing
brightness (not unexpected and at an acceptable rate) – and
that G225M and G285M are experiencing steady and
continuing degradation
COS Spectral Layout
Internal Wavecals and Science Spectra
NUV
MAMA
–
–
PtNe
Wavecal
External
Science
C
C
B A
B A
Obtain (continuous or flashed) internal PtNe spectra
at same time as science exposure
Track internal PtNe lines and apply shifts to science
spectrum (all events time-tagged) in COS data
pipeline
Keyes – 18 October 2007
Slide 9 of 14
G185M / G225M
G225M / G285M
Left: G185M / G230L Both MgF2
Right: G225M / G230L
G285M / G230L
Keyes – 18 October 2007
Slide 10 of 19
COS NUV Performance
Changes
TV06/TV03 Performance ratios with coupon
ratios (dashed)
1.1
1.05
TA1 Raw Ratio
1
TV06/TV03
G225 Ratio
G285 Ratio
0.95
G185 Ratio
0.9
G230 Ratio
'COS Aft' Al/MgF2
0.85
CW-9 Al/MgF2
CW-14 Al/MgF2
0.8
225F1 Bare Al
0.75
285D1 Bare Al
225F2 Bare Al
0.7
170
190
210
230
250
270
290
310
Wavelength
Compare G285M and G225M to G230L, TA1, and
to witness coupons (dashed)
NUV Performance Summary
 The degradation of the G225M and G285M gratings are
real – at the rate of 1.4%/year and 4.4%/year respectively.
To date no process has been identified that can explain the
observed behavior
 Over the past 3 years measurements of the flight spare
gratings have shown very similar degradation rates (1%
and 4.5%)
 Measurements of the witness sample mirror coupons have
shown no degradation over the past 5 years
Summary of observations



Little loss of efficiency in FUV channels
Anomalous efficiency loss in G225M and G285M
(high density, bare aluminum) – External and internal
calibration data give similar results
GSFC analysis shows gratings and coupons are very
clean
Possible explanations for loss of
efficiency considered to date
 Is efficiency loss due to simple hydrocarbon contamination?
– This is not consistent with FUV, coupon or TA1 data – we would
expect more loss in the FUV channels and in short wavelength
coupon data than we see
– wash/analysis of witness samples inside instrument revealed no
significant contamination
 Is loss due to migration of Au substrate into Al?
– No gold was observed in GSFC analysis, not consistent with
Al/MgF2 optic stability, and we would expect to see a similar
reflectivity loss in the bare aluminum witness coupon data
Possible explanations for loss of
efficiency considered to date (cont)
 Is the ‘loss’ due to change in test setups – change in polarization of
calibration signal?
– Polarization testing of COS indicates that this cannot explain all of
the loss in apparent efficiency
– NUV efficiency monitoring indicates continuing, steady
degradation
 Could in-air NUV testing be polymerizing hydrocarbons onto mirrors
and gratings, with stronger effect on high line density optics than on
low density or purely reflective optics?
– GSFC testing rules this out
Possible explanations for loss of
efficiency considered to date (cont)
 Could pin-holes be forming in Al coating or could mixing
of Au through Cr layer into Al be degrading reflectivity?
– pin-holing and layer mixing cannot be ruled out on flight gratings without
examination of flight units ; however no indications of either in detailed
special tests of spare gratings at GSFC
 JY modeling of gratings with a thick (9nm) Al2O3 layer is
consistent with our observed performance.
– This thickness is twice what is suggested likely by literature
– Continuing effort at GSFC to determine actual thickness of oxide
layer – possible update at Oct COS MSR next week
Current Status Summary – Oct 2007
 Degradation is real for G225M and G285M; no effect seen
for MgF2 coated NUV or FUV optics
 COS gratings show definite polarization sensitivity which
will be evaluated on orbit; most polarized astronomical
sources <5% polarized
 Current degradation rate projects to COS ≥ 4x STIS
efficiencies per exposure at launch (for likely COS target
flux-levels)
 Contamination has been ruled out
 Observational uncertainty and systematics have been ruled
out
Current Status Summary – Oct 2007
(cont)
 JY modeling with a thick (9nm) Al2O3 layer is consistent
with the observed G225M/G285M performance; literature
suggests that this is thicker than we should expect to see by
~2x; effort continues at GSFC to determine oxide layer
thickness
 Will it continue on orbit? – unknown at present
 Spare gratings have been fabricated and coated; are in
testing at GSFC at present
 No grating swap is likely at this point
Keyes – 18 October 2007
Slide 18 of 19
Background Information
Keyes – 18 October 2007
Slide 19 of 19
G285M flight and spare grating
performance changes
Estimated G285M-D (flight) vs G258M-E (spare)
0.7
Absolute Efficiency (Unpolarized)
G285M-E 2/1/02 (G285M spare) Bare Al
0.6
G285M-E 2/9/07 (G285M spare) Bare Al
0.5
G285M-D 1/16/02 (G285M Flight) Bare Al
G285M-D 12/06 est. (G285M flight) Bare Al
0.4
G285M-D 2006 modeled by
multiplying 2002 grating only values by
the 2003-2006 change in COS
G285M efficiency, and removoing
estimated TA1 contribution.
0.3
0.2
0.1
0
250
260
270
280
290
Wavelength
300
310
320
The Degradation Projection Scenario
 Linear extrapolation of observed degradation to 9/11/2008
launch date at ~ 0.5% per month yields additional ~11% loss of
sensitivity before COS gets to orbit; compared to TV I values:
– G285M will have lost ~25% throughput waiting to fly
– G225M will have lost ~20% throughput waiting to fly
 In comparisons to follow, we have assumed 25% degradation
(i.e., launch throughput = 0.75 x TV I throughput)
Keyes – 18 October 2007
Slide 21 of 19
Observing Efficiency Comparison:
COS vs STIS – darks included
HST orbits required to reach S/N=10 at 2500 Ǻ (R~20,000 (0.12 Ǻ) binning)
-13.0
-13.5
log Flux (erg cm
-2
sec -1 Ǻ-1 )
-14.0
STIS E230M + 0.2x0.2 aperture
3x7 binning
COS G225M; 3x7 binning
25% degradation;
worst-case dark
-14.5
-15.0
-15.5
COS G225M; 3x7 binning
no degradation;
ground dark
-16.0
COS G225M; 3x7 binning
no degradation;
worst-case dark
-16.5
0
10
20
30
40
50
HST Orbits
60
70
80
COS G225M; 3x7 binning
25% degradation;
ground dark
90 – 18 January
100 2007
Keyes
Slide 22 of 19
Impacts: Ratios of exposure to achieve same S/N
with degraded sensitivity
COS Exposure ratio
to reach S/N = 10
COS/STIS Exposure ratio
to reach S/N = 10
25%-degraded Sλ
vs no-loss COS Sλ
with COS ground dark
(with worst-case on-orbit dark)
25%-degraded COS Sλ
vs STIS Sλ
with COS ground dark
(with worst-case on-orbit dark)
1.e-13
1.33 (1.34)
0.43 (0.44)
1.e-14
1.35 (1.41)
0.29 (0.36)
1.e-15
1.45 (1.64)
0.09 (0.24)
5.e-16
1.50 (1.69)
0.07 (0.23)
2.e-16
1.62 (1.74) [>40 orbits]
0.05 (0.22) [>40 orbits]
1.e-16
1.68 (1.76) [>40 orbits]
0.04 (0.21) [>40 orbits]
Object
Flux
Keyes – 18 October 2007
Slide 23 of 19
Impacts – Single COS grating setting used
 Single COS grating setting used:
– Bright limit (ignore background): simply increase science exposures to
compensate for sensitivity loss: 1.25-1.3x to achieve same S/N
> In bright limit: STIS/COS(no-loss) exposure ratio ~3x; so COS Sλ must
degrade to 0.33 TV I level for COS=STIS efficiency
– Faint “limit”:
> For a 40-orbit COS observation to achieve S/N=10 at 2500 Ǻ.:
• No-loss case with ground dark: Fλ=2.0 e-16
• No-loss case with worst-case dark: Fλ=4.0 e-16
• With 25% degradation and ground dark: Fλ=2.5 e-16 ; however, this
flux requires 26 orbits in no-loss case (1.5x longer with degradation)
• With 25% degradation and worst-case dark: Fλ=5.5 e-16 ; however, this
flux requires 24 orbits in no-loss case (1.7x longer with degradation)
– the limiting flux for a STIS 40-orbit observation is 1.2e-15
Keyes – 18 October 2007
Slide 24 of 19
Impacts: Summary and Questions
 For observing the fainter targets with degraded Sλ, two considerations
important:
– Brighter limiting flux for observation at a particular S/N
– Increase of exposure time to reach a target at a specific S/N
 For most STIS targets the modest difference in COS limiting flux due to the
degradation does not appear to be an important consideration
 Targets with the faintest fluxes attempted by STIS (~1.e-15) in 40 orbits are
important, but with 25% degradation and worst-case background would
require ~12-13 orbits for a single grating setting with COS (or ~7-8 orbits
if no degradation).
– Question: is 8 versus 12 orbits significant for science at this flux level?
> Depends on number of targets and/or COS grating settings required
> In most cases where multiple COS grating settings are needed; COS
probably would not be chosen as the SI of choice
 The difference in limiting flux between 25% degradation and no
degradation (for S/N=10 at 2500 Å and worst-case background) is 6. e-16
versus 4. e-16 (or for best-case dark, 2. e-16)
Keyes – 18 October 2007
Slide 25 of 19