The COS FUV Lifetime Adjustment PlanAlessandra Aloisi for the COS/STIS team

(with input from D.Massa, S.Penton, C.Proffitt, C.Oliveira, R.Osten, S.Osterman, D.Sahnow)

17 November 2011

SPACETELESCOPESCIENCEINSTITUTE

Operated for NASA by AURA

TIPS Meeting 2

PHA Distribution with Time of COS XDL (FUV) Detector

• COS XDL is a photon-counting micro-channel plate (MCP) detector

• In COS FUV TIME-TAG mode every photon is recorded with:

– position (x,y)– arrival time (t)– total electron charge generated/pulse-height

amplitude (0 ≤ PHA ≤ 31)

• For every detector element, PHA distribution changes with time, as fewer electrons can be extracted from the MCP with usage, the so-called gain-sag effects

• Gain sag leads to a shift of the pulse-height distribution to lower PHA values

• Modal Gain is peak of PHA distribution PHA bin#

of e

vent

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PHA ≥ 2 adopted for filtering as of Dec 21, 2010 PHA ≥ 4 adopted for filtering at

beginning of on-orbit operations

PHA distribution of the counts in the whole region of the extracted spectrum for the same

target observed in Sep 2009 and Dec 2010

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Red: PHA = [4,30] Blue: PHA = [2,30]

G160M/1577 data from program 12424 (sensitivity monitoring) obtained

on Dec 22, 2010

• As the value of the modal gain of a certain pixel slowly decreases, the PHA distribution approaches the minimum threshold imposed by the PHATAB reference file used by CALCOS for filtering of good events, and target photons may be rejected as background events

• E.g., filtering out target photons with a PHA value which has dropped below the original threshold value of 4, produces absorption-like artifacts in the extracted spectra corresponding to those detector regions more damaged by airglow lines.

• Filtering with PHA threshold below 2 not recommended

– Overall background only increases by several %

– Localized background structures appear in the data

Gain-Sag Artifacts in COS FUV Spectra

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Cros

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COS FUV Cumulative Images

• Cumulative images and maps of the PHA distributions as a function of time are built periodically by co-adding all science exposures for both segments A and B of the COS FUV detector

– fully implemented into the pipeline with OPUS build 2010.4

• Majority of counts along the horizontal stripe near middle of detector in Y corresponds to nominal position for science observations

• Other visible stripe around Y=600-650 from spectra of wavelength calibration lamp

• Vertical stripes from bright geocoronal Lyα lines– seen in main spectral region, but also above and

below (the latter from the aperture, PSA or BOA, that is not in use for science observations)

– 4 on segment A (one for each of the 4 FP-POS settings of G140L/1105)

– 20 on segment B (one for each of the 4 FP-POS settings of the 5 G130M settings)

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(Sahnow et al., COS ISR 2011-05)

Dispersion Direction

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Location of Airglow Lines from G130M Setting on COS FUV Segment B

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Modal Gain vs. Time in Segment B

• Modal gain vs. x-pixel position on the detector in a 10-by-6 (in x and y) pixel wide region where spectra fall• Data from cumulative images at different operational HV values and epochs are presented• Figure represent restricted y area on detector, modal gain distribution varies also with y • Modal gain drops everywhere with time where “continuum” of the spectra falls• Modal gain drops even more in regions where geocoronal Lyα falls in G130M/G140L settings, the so-called “holes” • Modal gain = 3 (dotted line) is a benchmark for the onset of severe gain-sag effects • HV increased in Segment B (Mar 2011): modal gain increased by ~ 3 PHA bins (~ 1 year of lifetime gained for the segment)• First “hole” related to geocoronal Lyα observations in G130M/1291 appeared in Jul 2011

Higher HV, SMOV Lower HV, SMOV

End Sep 2011 Feb 2011, before HV increase

Mar 2011, after HV increase

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Modal Gain vs. Time in Segment A

• Modal gain vs. x-pixel position on the detector in a 10-by-6 (in x and y) pixel wide region where spectra fall• Data from cumulative images at different operational HV values and epochs are presented• Figure represent restricted y area on detector, modal gain distribution varies also with y • Modal gain drops everywhere with time where “continuum” of the spectra falls• Modal gain drops even more in regions where geocoronal Lyα falls in G130M/G140L settings, the so-called “holes” • Modal gain = 3 (dotted line) is a benchmark for the onset of severe gain-sag effects • HV not increased to original SMOV values in Mar 2011 on Segment A• “Holes” related to geocoronal Lyα observations in G140L/1105 suddenly appeared in May/June 2011

Higher HV, SMOV Lower HV, SMOV

End Sep 2011

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COS FUV Gain-Sag Effects• Once modal gain reaches ~ 3, fraction of counts lost is ~ 5%

– comparable to uncertainties in the absolute flux calibration (~ 3-5 %)– comparable to pixel-to-pixel variations due to flat field (~ 5%)

• Fraction of lost counts increases exponentially as modal gain decreases• 100% loss is reached around a modal gain of ~ 1.5• The 100% loss is reached on a timescale that varies from one region to the other depending on

the total counts accumulated as a function of time– in deepest Lyα hole this is reached in 3-4 months– in a typical continuum region this is reached in 6-7 months

Modal gain of 3 is good benchmark for onset of severe gain sag effects and corrective actions must be taken to prevent modal gain from reaching this value !

Segment B near deepest Lyα hole at ~ 9000 pixels in dispersion direction: blue original operational HV green increased HV11/17/2011

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Continuum effects start appearing in both segments

First gain sag hole appears in segment A

First gain sag hole appears in segment B

• Table reports dates when modal gain reaches 3 in certain regions of the spectra

• For Segment B estimates are based on data obtained at lower HV (before Mar 2011) but trends at increased HV are similar

• For Segment A estimates are based on data obtained at current (lower) HV

• Currently revising the predictions of our models based on additional data from Cycles 18 and 19

Timeline for COS FUV Gain-Sag Effects

1327 1318 1309 1300 12911 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

x1dsum with 4 FP-POS (1+2+3+4): S/N = 70% over 200 pix and S/N = 87% over 1600 pix (13% of spectrum)

x1dsum with 1 FP-POS (3): S/N = 0 over 500 pix (3.5% of spectrum)

x1dsum with 3 FP-POS (2+3+4): S/N = 57% over 100 pix and S/N = 81% over 1300 pix (10% of spectrum)

Gain-Sag Holes in Segment B of COS/FUV by Apr 2012

G160M observations with FP-POS=1,2,3,4 (in wavelength space)

Effects of Segment B gain sag holes on G160M observations

1234

What happens to S/N of coadded G160M data on Seg B?

x1dsum with 2 FP-POS (3+4): S/N = 70% over 1000 pix (7% of spectrum)

Detector active area in dispersion direction: ~ 14,000 pixels Coverage of all G130M settings shown (light blue): ~ 5000 pixels

G130M settings that will produce these holes

x1dsum with 4 FP-POS (1+2+3+4): S/N = 0 over 3250 pix (23% spectrum)

x1dsum with 1 FP-POS (1): S/N = 0 over 4000 pix (29% of spectrum)

x1dsum with 3 FP-POS (1+2+3): S/N = 0 over 3500 pix (25% spectrum)

Detector active area in dispersion direction (red + light blue): ~ 14,000 pixelsContinuum area affected by gain sagged (red): ~ 4000 pixels (~ pix 4000 to 8000)

G160M observations with FP-POS=1,2,3,4 (in wavelength space)Effects of Seg A sagged continuum on G160M observations

What happens to S/N of coadded G160M data on Seg A?

x1dsum with 2 FP-POS (1+2): S/N = 0 over 3750 pix (27% of spectrum)

1234

Gain-Sagged Continuum in Segment A of COS/FUV by ~ Jun/Jul 2012

TIPS Meeting 12

Evolution of Gain-Sagged Continuum in Segment A of COS/FUV

• Evolution of Lya-sagged holes not shown • Predictions assume trends at current (lower) HV

HV increase to original SMOV values would allow us to gain ~ 1 year for this segment

May

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Apr 2

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Lighter to dark shade: Increase in time

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Jun

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Lighter to dark shade: Increase in time

Evolution of Gain-Sagged Continuum in Segment B of COS/FUV

• Evolution of Lya-sagged holes not shown • Predictions assume trends at lower HV but trends at increased HV similar

HV increase to original SMOV values already performed in March 2011

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Gain-Sag Effects on COS FUV Spectra: 1

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• Gain sag in FUV detector leads to:- localized holes in spectra

• produced by Lyα airglow (~ 100 pixels wide each)

• mostly on Segment B, but some starting to appear on Segment A

• mitigated operationally by use of multiple FP-POS positions

Use of all 4 FP-POS positions enforced starting from Cycle 20- depression of continuum in some areas of each segment compared to other

nearby areas (due to shift of PHA distribution to lower values with usage)• effects on continuum hard to overcome operationally with FP-POS because involving

a large fraction of pixels

TIPS Meeting 15

Gain-Sag Effects on COS FUV Spectra: 2

• If trends from earlier data (higher HV for Segment B) continue and no further action is taken, effects of gain sag difficult to overcome within ~ 1 year (Sep 2012) even when using several FP-POS positions

- first hole on Segment B appeared in Jul 2011

- multiple holes on both Segments by early Apr 2012

- 25% of continuum on Segment A severely affected by end of Jun 2012 (but currently considering increasing the HV to initial on-orbit values)

- 15% of continuum on Segment B severely affected by Sep 2012

• With no further action taken, whole science spectrum severely compromised - on Segment A within 2 years (HV increase would give ~ 1 additional year)

- on Segment B within 3 years

An action of some kind is required to mitigate severe gain-sag effects within the next 10 months (Sep 2012)!

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• Gain-sag effects at current lifetime position can be mitigated by raising operational HV of 2 COS FUV segments, similarly to what done for Segment B in March 2011:

– Currently considering raising HV in Segment A to value initially used in SMOV (target date Feb/Mar 2012)

– Investigated raising HV of Segment B (and eventually A) to values greater than those already used on-orbit

Only ~ 2 hours of TV testing at higher HV, not currently planning to go this way !

• Effects of gain sag can also be mitigated by moving to new lifetime position– Extensive preparatory work in progress; needs to be completed within the next ~

6 months or so to be able to move to a new lifetime position by Summer 2012– Up to ~ 100 external (E) orbits and ~ 25 internal (I) orbits might be required.

These orbits have been or will be used for:• preparatory work to characterize all new FUV lifetime positions• Work to enable/calibrate next FUV lifetime position to use for science• work to enable/calibrate science at NUV off-nominal positions (if needed) not yet

included (additional ~ 60 E and ~ 10 I orbits)

Currently working towards moving to new FUV lifetime position by Summer 2012 !

Mitigation of COS FUV Gain-Sag Effects

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Constraints on Number of Available FUV Lifetime Positions

OP-01 assumes 0.875 mm (~3 arcsec) steps between FUV lifetime positions, for a total of 5 lifetime positions, but might be necessary to consider positions more or

less closely spaced Number of available lifetime positions will depend on several factors, including:

• Detector effects (e.g., gain map, bad pixels, etc.)• Optical effects (e.g., resolution degradation, asymmetry of LSF, etc.)• Physical limitations of the aperture mechanism (how far it can be moved)• Mechanical limitations of aperture mechanism for NUV target acquisitions

(i.e., number of certified aperture moves)• Paths that allow lamp and external light to reach detector in improper ways

(e.g., avoidance of safing detector events similarly to what happened in program 12096)• E.g., capability to simultaneously flash the lamp in TAGFLASH mode

• Keeping wavelength calibration spectra on the detector for use of TAGFLASH• Keeping target spectra on the detector• How tightly will be able to pack positions in cross dispersion

Our driving principle will be optimizing COS FUV science over next 5 years !

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1. Preparatory work to determine number of available lifetime positions, characterize them, assess scientific and operational restrictions at each position, and select next lifetime position – To be completed ASAP in order to select next position

These activities will be performed once

2. Work to enable science at new lifetime position– To be completed before start of science operations at new position

These activities will have to be performed at each new FUV lifetime position

3. Work to calibrate science at new lifetime position– To be performed in parallel with science operations at new position

These activities will have to be performed at each new FUV lifetime position

COS FUV Lifetime Adjustment Work

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Part 1: Activities to be performed to Characterize New Lifetime Positions

• Characterization of FUV detector effects– Mapping of the COS/FUV detector with Deuterium lamp through FCA to map flat field

• total # of internal orbits less than total # of orbits required with external target for similar S/N– Mapping across the FUV detector of stray light through FCA when using wavelength calibration PtNe

lamp through WCA (unexpected effect observed when program 12096 executed in Mar 2010, led to global count rate violation and shut-down at + 6” but not at + 3”)

• Characterization of FUV optical effects– Mapping of FUV optical effects across detector using external source to look at resolution, cross-

dispersion profiles, etc.

FUV observations executed over past 4 months (Aug–Nov 2011) and completed as of today !

• Study of impact of moving aperture position for NUV target acquisitions and NUV observations– Obtain spectra and images of external target and wavelength calibration lamp in the NUV as the PSA

aperture is moved in cross dispersion • Operations at new lifetime positions originally conceived to be executed with NUV TA at current position• Exploring possibility of using the same aperture position for NUV TA and FUV spectroscopic observations• May be forced to also move NUV aperture position for routine NUV spectroscopic observations due to limitations in

the number of movements that aperture mechanism has been certified for– Mapping across the NUV detector of stray light through FCA when using wavelength calibration PtNe

lamp through WCANUV observations on hold until we sort out if the COS aperture mechanism is indeed a limiting factor

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Part 2: Activities to be performed to enable Science at New Lifetime Position

• Aperture (PSA) Location and Alignment• FUV High Voltage Sweep

– Obtain data at different HV values to determine what is the minimum HV value that can be used without compromising quality of the data

– Will allow us to keep increasing HV to counteract the effects of gain sag and will extend lifetime of each new detector position

• FUV Focus Sweep– Perform a small focus sweep for each grating to determine the best focus for

operations at the new lifetime position– Will allow us to use the lifetime position at the highest resolution possible

• FUV Target Acquisition Update– Update parameters used by FUV target acquisition algorithms

Currently planning to execute this enabling part in Mar-Apr 2012 for start of science operations at new lifetime position in Jul 2012

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Part 3: Activities to be Performed to Calibrate Science at New Lifetime Position

A “spot check” approach will be considered before committing all the orbits as executed during SMOV and initial on-orbit calibration.

Only PSA calibrations will be considered (BOA not used for COS FUV observations).

• FUV wavelength calibration & lamp template spectra– zero-point of the wavelength dispersion solution for all grating/cenwave settings through

external sources (based on assumption that dispersion solutions do not change)– lamp template spectra for all grating/cenwave/fp-pos settings– New spectral extraction regions for target, background regions, and wavecal lamp spectra

• FUV flux calibration, flat-fielding, and bad pixel regions– Data obtained for the flux calibration will be used to produce a grid-wire flat and to update

the bad pixel regions (data from previous detector characterization will also be used)

• FUV spectral and spatial resolution– Determination of changes of flux and spectral/spatial resolution as the position of the

source is stepped across the aperture

Currently planning to execute these calibrations in Jul-Sep 2012

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Part 1: Characterizing COS/FUV Lifetime Positions

• 3 FUV programs approved by HMO and already executed – Mapping of COS/FUV Detector Effects with D Lamp (12676, PI Massa, 12 orbits)

completed (Jul 27 – Aug 1)– COS/FUV Mapping of PtNe Stray Light through FCA (12677, PI Oliveira, 16 orbits)

completed (Aug 29 – Sep 30)– Mapping of COS/FUV Optical Effects (12678, PI Sahnow, 6 orbits)

completed (Oct 14 – Nov 17); last data acquired today still to be analyzed• Analysis of the data from these FUV exploratory programs under completion • Need for programs to assess NUV target acquisition and science at off-nominal

positions (due to limitations in the number of certified moves of the aperture mechanism) still under consideration by COS/STIS teamMeeting with HMO and IDT to finalize choice of next FUV lifetime position

currently scheduled for Dec 9, 2011

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Part 1: Results so far

• 12677 mapped stray light from the PtNe lamp through the FCA – Results as expected, with stray light absent at +5”, but present at +6” (similarly to

what seen in 12096 back in Mar 2010)• 12676 mapped flat field and gain over most of active FUV detector region using

D lamp flats- Gain map shows wings of current sagged region have more impact on next position

than expected- Maximizing lifetime may require an aperture offset in along-dispersion (AD) direction

as well as cross dispersion (XD) to minimize overlap of Lyα sagged regions- Need to study trade-offs between offsets, spectral resolution and purity, overlap of gain

sag regions, and the amount of available detector area for future lifetime positions. Best short term solution might limit future positions

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• Current gain map with modal gain of PHA distribution (segment A top, segment B at former lower HV bottom)

– Lower HV (or even a lower value) will be used at new lifetime position

– At this voltage we would already have several holes at the current position

• Red & Black regions (modal gain ≤ 3) show significant loss from gain sag at the lower HV used for segment B

• XD region occupied by combined continua of different gratings spans about 27 pixels (2.7”)

• Wings of full aperture Lyα spots extend even further

• One XD direction (the − direction) less affected by gain sag than the other, so currently preferred for next position

Small vertical bars on right show standard extraction boxes for (left to right) G130M, G160M, & G140L (may currently be larger than needed?)

Part 1: Results so far

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Cross-Dispersion Direction

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• Predicted gain map if previous usage pattern assumed until Jun 2012 at current position and for another 1.5 years at new position shifted in XD

– Current plan of uniform FP-POS usage should give better results than shown

• At −3” XD offset, without an AD offset (top), deep holes extend into new continuum region much sooner than with an AD offset (below), limiting time at new position before problems arise

– Damage could be spread around more systematically among FP-POS positions to buy back some time, but solar cycle is expected to make Lyα 2-3 times stronger so it may not help

• Adding an AD offset of ~3” would reduce overlap in wings while keeping XD spacing small enough to allow future lifetime positions at ± 6” (tested in last visit of 12678)

• However, according to ray-trace models AD move affects LSF (and resolution) more than XD move

Part 1: Results so far

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Cross-Dispersion Direction

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Effect of Aperture Offsets on Spectral Resolution (Ray-Trace Models)

Dispersion LSFs (in pixels) as a function of aperture position (Detector blur and MFWFE not included)• Resolution declines more

rapidly with AD offset than with XD offset

• Note marked asymmetry when offsetting in AD direction

• We may not be at either the optimal XD or AD position with current on-orbit alignment– Little or no existing data to

verify or refute this suggestion

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Resolution vs XD Position in Program 12678: Part 2

• Data at 0” position convolved with Gaussians of various σ until best fit achieved with data at POSTARG position (±3” and ±6”)

• R=16,000R assumed at 0”• Results are for segment B, but Segment

A follows same trends• Observed degradation better than

model predictions by 2x or so at central wavelength used (1291 Å)

• 30% degradation at most (observed at extreme −6” position)

• Data at −3” with AD offset still to be analyzed

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Resolution Peak may be between 0” and +2”, so in terms of resolution + direction preferred

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Parts 2 & 3: Enabling and Calibrating Science at New COS/FUV Lifetime Position

• Mini-SMOV working group formed at this time and started to meet weekly (Lead: Rachel Osten, co-Lead: Jerry Kriss)

• Completed definition of mini-SMOV requirements– Original SMOV4 requirements already triaged for this purpose in Spring 2011

• Already identified and removed requirements that do not apply anymore• Identified requirements that still hold

– Identified new FUV requirements that apply to new lifetime position – NUV requirements on hold at this point

• In the process of completing identification of on-orbit activities based on final list of mini-SMOV requirements

• Phase II programs will be prepared based on these activities and will be approved through a PIT-like process

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To be continued….

• A lot of critical work will be performed by the COS/STIS team over the next several months

• Stay tuned for additional TIPS presentations on this or related matters:– COS FUV Gain-Sag Effects– Choosing the New COS FUV Lifetime Position– The mini-SMOV Plan to Enable/Calibrate Science at the New COS FUV

Lifetime Position

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