MOVING COS/FUV
TO LIFETIME POSITION 3
JULIA ROMAN-DUVAL & COS/STIS TEAM
AUGUST 12, 2014
See also Poster by Proffitt et al.
1
OUTLINE
• Gain sag management and timeline for LP3 move
• Choosing the next lifetime position
• Spectral resolution
• Gain-sag mitigation and spectral quality
• Strategy for the LP3 move
• Improved extraction at LP3
• Conclusion
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GAIN SAG
• COS/FUV is a photon-counting micro-channel plate (MCP) crossdelay line (XDL) detector
• Pulse-height amplitude, PHA ≈ 20.5 log10 ne – 127.5, corresponds to
total electron cloud generated
• Modal gain = peak of PHA distribution (narrow for given location and HV)
• Usage of the detector and subsequent charge extraction causes
localized declines of the modal gain, the so-called gain sag effect.
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EFFECTS OF GAIN-SAG ON FLUX
• When modal gain reaches 3, >5% flux loss occurs
• Fraction of flux lost increases exponentially with decreasing modal gain below 3
CalCOS PHA
threshold
LP1 HV=175
G130M/1309
FPPOS=3
Gain-sag holes due to Ly a
airglow with 1291
4
IMPRINT OF USAGE ON GAIN MAP
Map of modal gain on FUVB with HV = 163 (lowest setting used in operations)
Obtained in January 2014, 18 months after start of LP2 operations
LP2
LP1
G130M/1327
1309
1291
Ly a airglow holes
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GAIN SAG MANAGEMENT:
HIGH VOLTAGE
• Modal gain increases with HV
(0.4 PHA bins per HV step)
• Once maximum operational
HV is reached, need to move
science spectra to a pristine
location on the detector
• Move to new lifetime
position
# of pixels
• COS/STIS team increases HV
every 6 months-1year
depending on evolution of
modal gain to keep modal
gain above threshold of 3
08/11/2012 (right after move to LP2, HV = 163)
06/20/2013, HV = 163
06/24/2013, HV = 169
PHA
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LIFETIME POSITIONS
Constraints on the motion of the aperture mechanism in the Y direction prevent the
PSA aperture to be moved beyond +/- 6.0” from LP1
(FUVB)
G130M/1291
LP2: +3.5”
LP1: 0”
LP3: -2.5”
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LP3 TIMELINE AND LOCATION
• Move to LP2: July 23, 2012
• Move from LP2 to LP3 scheduled for February 2015
• LP2 lifetime is shorter than LP1 due to increased COS/FUV
usage (UV initiative)
• LP3 location is a compromise between the following
constraints:
• Maximizing spectral resolution:
• At LP3 and future lifetime positions
• Maximizing lifetime of best area of COS/FUV detector
• Impact of previously sagged regions on flux accuracy
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IMPACT OF LIFETIME MOVE ON
SPECTRAL RESOLUTION
• Resolution is a function of position on the detector
• Maximizing spectral resolution of COS/FUV implies squeezing
lifetime positions as close to LP1 as possible
Data from
12678 (PI Sahnow)
R/R0 ~ 0.93 at
+3.5” (LP2)
R/R0 ~ 0.85
at -2.5” (LP3)
(LP1)
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IMPACT OF LP1 GAIN SAG HOLES ON
SPECTRAL QUALITY AT LP3
• Different gratings/cenwaves occupy different positions on the detector
• Location of LP3 constrained effects of LP1 gain sag on spectral quality of
highest modes (closest to LP1)
G130M CENWAVES: 1055 1096 1222 1291
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BLUE MODES FUTURE LOCATION
 Blue modes (1055, 1096) overlap with LP1 sagged
regions completely for LP3 ~ -2.5”
 Blue modes cannot be properly calibrated at -2.5”
 Blue modes could fit at -3.5” at the sacrifice of resolution
 1222 just fits under LP1 for LP3 = -2.5”
- Far wings of the cross-dispersion profiles overlap with LP1
without significant flux loss
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IMPACT OF LP1 GAIN SAG HOLES ON
SPECTRAL QUALITY AT LP3
• We tested spectral quality of highest modes (1222, 1291, 1280) at -2.06”
and -2.33” below LP1 as part of lifetime optimization program (LOP)
• Analysis showed that effect of LP1 gain sag holes is <5% at positions
lower than -2.06” for all settings except blue modes (G130M/1055, 1096)
• 1222 must be operated at HV = 167 to reduce effects of LP1 gain-sag
XD=-2.06”
HV= 167
4 FPPOS COMBINED
Reference spectrum/2 (FCAL3, 12806)
Fractional Difference
G130M/1222/FUVB
(MODE CLOSEST TO LP1)
Model
spectrum
Spectrum
LP3 spectrum (LOP)
of WD0308-565 (flux standard)
+5%
-5%
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STRATEGY
• Move all modes to -2.5” below LP1, except blue
modes (1055, 1096)
• Blue modes will stay at LP2 with minimal impact on spectral
quality with spectral dithering
• G130M/1222 will be operated at slightly higher HV (171/167 on
A/B) to reduce impact of LP1 gain sag holes on LP3 data
• Projected lifetime of LP3 is similar to LP2 and LP1
• Impact of factor 2 increase in usage (UV initiative)
mitigated by optimized management of HV
• Ability to produce calibrated spectra at LP3 that
are not affected by gain-sag relies on improved
spectral extraction techniques (2-zone extraction)
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TWO-ZONE EXTRACTION AT LP3
SEE POSTER BY PROFFITT ET AL.
• Current COS 1D spectral extraction collapses the
spectrum over a large box
• Extraction and flagging limits are identical
• A bad or sagged pixel in the far wings of the profiles causes
a whole column to be discarded
• Since wings of LP3 profiles overlap with LP1 gain-sagged
regions, chunks of LP3 spectra would be flagged with
current box extraction
• Two-zone extraction relies on the knowledge of the
2D flux distribution
• Only flags bad or sagged pixel if its flux contribution is
negligible
• LP3 location was chosen so that 2-zone extraction provides
properly calibrated spectra
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IMPROVED EXTRACTION AT LP3
See Poster by Proffitt et al.
BOXCAR EXTRACTION
Sagged LP1
regions
Flagged
columns
LP3 spectrum
Extraction AND
dq-flagging
limits
dq-flagging
limits (80% EE)
2-ZONE EXTRACTION
NO
Flagged
columns
Extraction limits
(~99% EE)
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PREPARATION OF THE LP3 MOVE
• Optimization phase (LOP) complete (January-May, 2014)
• Determined position of LP3 to be -2.5”
• Enabling phase (LENA) under way
• Obtain enabling parameters at -2.5”, such as aperture placement,
pointing, plate scales, focus, and target acquisition parameters
• Calibration phase (LCAL) under way
• 4 LCAL programs have been drafted and will be finalized by the end
of August
• LCAL programs will allow us to obtain:
•
•
•
•
•
Line spread functions
Wavelength scale
Flux calibration
2D cross-dispersion profiles (for extraction)
Flat-fields (L-flats and P-flats)
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CONCLUSION
• Move to LP3 scheduled for February 2015
• Move all modes to -2.5” below LP1, except blue modes (1055,
1096)
• Blue modes will stay at LP2 with minimal impact on spectral quality with
spectral dithering
• 1222 will be operated at slightly (4 steps) higher HV than other modes
• Improved spectral extraction technique (2-zone extraction) being implemented
for LP3 operations
• Spectral resolution loss between LP1 and LP3 is expected be
15% (5-8% between LP2 and LP3)
• Spectral performance will otherwise be identical to LP1 and LP2
• Projected lifetime of LP3 is similar to LP2, but shorter than LP1
due to UV initiative (increase usage by factor ~2)
• Strategic planning and optimization of lifetime positions will
carry us out to 2020 (for overlap with JWST)
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BACKUP
SLIDES
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EFFECTS OF GAIN SAG ON FLUX
1.0
• When modal gain
reaches 3, flux loss
occurs
Fraction of flux lost
0.8
0.6
0.4
• Below modal gain
= 3, flux loss
increases steeply
with decreasing
modal gain
5% flux loss for
modal gain = 3
0.2
0.0
1.0
1.5
2.0
2.5
Modal gain
3.0
3.5
4.0
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G130M/1055/1096 AT -2.5”
• Blue modes (1055 and 1096) are too wide to be placed
closer to LP1 than -3.5”
• Completely overlap with LP1 sagged regions
• Cannot be moved to -2.5” with the other FUV modes
20
IMPACT OF LP1 GAIN SAG HOLES ON
SPECTRAL QUALITY AT LP3
• Different grating/cenwave combinations occupy different positions on
the detector
• Location of LP3 constrained effects of LP1 gain sag on spectral quality
of highest modes (closest to LP1)
G140L
1280
LP3
G130M
1222
G130M
1291
G130M
1327
G160M
1577
G160M
1623
LP1
LP2
LP3
LP1
LP2
G140L
1280
G130M
1222
G130M
1291
G130M
1327
G160M
1577
G160M
1623
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LOCATION OF LP3 WITH RESPECT TO
MODAL GAIN
G130M CENWAVES: 1055 1096 1222 1291
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RELATIVE POSITIONS OF GRATINGS
• G140L is always positioned the highest on the
detector
• G140L/1280 on FUVB only covers a small range in X
on the right side of the detector (low S/N)
• Relative positions of the 3 gratings is the same at
different lifetime positions (though profiles change
with XD position)
G140L profile
low S/N
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LOCATION OF LP3 WITH RESPECT TO
MODAL GAIN
Blue modes overlap with LP1 gain sag holes completely
1055 (99%)
1055 (80%)
1291
99%
90%
80%
70%
60%
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