1JUL IA ROMAN-DUVAL & COS /ST IS TEAMA U G U S T 1 2 , 2 0 1 4

MOVING COS/FUV TO LIFETIME POSITION 3

See also Poster by Proffitt et al.

2

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

3

GAIN SAG

• COS/FUV is a photon-counting micro-channel plate (MCP) cross-delay 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.

4

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

LP1 HV=175G130M/1309FPPOS=3

Gain-sag holes due to Ly a airglow with 1291

CalCOS PHA threshold

5

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

Ly a airglow holes

G130M/1327 1309 1291

6

PHA

# o

f pix

els

GAIN SAG MANAGEMENT: HIGH VOLTAGE

• Modal gain increases with HV (0.4 PHA bins per HV step)

• COS/STIS team increases HV every 6 months-1year depending on evolution of modal gain to keep modal gain above threshold of 3

• Once maximum operational HV is reached, need to move science spectra to a pristine location on the detector• Move to new lifetime

position

08/11/2012 (right after move to LP2, HV = 163)06/20/2013, HV = 16306/24/2013, HV = 169

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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

LP1: 0”

LP2: +3.5”

LP3: -2.5”

G130M/1291

ehehe

(FUVB)

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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

R/R0 ~ 0.93 at +3.5” (LP2)

R/R0 ~ 0.85 at -2.5” (LP3)(LP1)

Data from 12678 (PI Sahnow)

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)

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G130M CENWAVES: 1055 1096 1222 1291

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

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G130M/1222/FUVB(MODE CLOSEST TO LP1)

XD=-2.06”HV= 1674 FPPOS COMBINED

LP3 spectrum (LOP)of WD0308-565 (flux standard)

Model spectrum

Reference spectrum/2 (FCAL3, 12806)

Spect

rum

Fract

ional D

iffere

nce

+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 LP3SEE 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

Sagged LP1 regions

LP3 spectrum

Extraction AND dq-flagging limits

BOXCAR EXTRACTION

2-ZONE EXTRACTION

Flagged columns

Extraction limits (~99% EE)

dq-flagging limits (80% EE)

NO Flagged columns

See Poster by Proffitt et al.

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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

19

EFFECTS OF GAIN SAG ON FLUX

• When modal gain reaches 3, flux loss occurs

• Below modal gain = 3, flux loss increases steeply with decreasing modal gain

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Modal gain

5% flux loss for modal gain = 3

Fract

ion o

f flux lost

1.0

0.8

0.6

0.4

0.2

0.0

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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

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)

21

LP1 LP2LP3LP1LP3 LP2G140L1280

G130M1222

G130M1291

G130M1327

G160M1577

G160M1623

G140L1280

G130M1222

G130M1291

G130M1327

G160M1577

G160M1623

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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

1055 (99%)1055 (80%)

1291

99%90%80%70%60%

Blue modes overlap with LP1 gain sag holes completely