SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
The COS FUV Lifetime Adjustment Plan
Alessandra 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
PHA Distribution with Time
of COS XDL (FUV) Detector
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•
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PHA distribution of the counts in the whole
region of the extracted spectrum for the same
target observed in Sep 2009 and Dec 2010
COS XDL is a photon-counting microchannel 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 pulseheight distribution to lower PHA values
Modal Gain is peak of PHA distribution
# of events
•
PHA ≥ 2 adopted for
filtering as of Dec 21, 2010
11/17/2011
TIPS Meeting
PHA bin
PHA ≥ 4 adopted for filtering at
beginning of on-orbit operations
2
Gain-Sag Artifacts in COS FUV Spectra
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•
•
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
11/17/2011
Gain sag holes
Red: PHA = [4,30] Blue: PHA = [2,30]
data from program
BlueG160M/1577
PHA=[2,30]
Red 12424
PHA=[4,30]
(sensitivity monitoring) obtained
G160M/1577 dataon
fromDec
program
12424, obtained on
22, 2010
Dec 22nd 2010.
TIPS Meeting
3
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
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–
–
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)
Cross-Dispersion Direction
(Sahnow et al., COS ISR 2011-05)
Dispersion Direction
11/17/2011
TIPS Meeting
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Location of Airglow Lines from G130M Setting
on COS FUV Segment B
11/17/2011
TIPS Meeting
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Modal Gain vs. Time in Segment B
Higher HV, SMOV
Lower HV, SMOV
Feb 2011, before
HV increase
End Sep 2011
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•
•
•
•
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•
Mar 2011, after
HV increase
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
11/17/2011
TIPS Meeting
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Modal Gain vs. Time in Segment A
Higher HV, SMOV
Lower HV, SMOV
End Sep 2011
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•
•
•
•
•
•
•
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
11/17/2011
TIPS Meeting
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COS FUV Gain-Sag Effects
•
Once modal gain reaches ~ 3, fraction of counts lost is ~ 5%
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•
•
•
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 !
11/17/2011
Segment B near deepest Lyα hole at ~ 9000 pixels in dispersion direction:
blue original operational HV
green increased HV
8
Timeline for COS FUV Gain-Sag Effects
Segment
B
B
B
B
A
B
B
Cenwave
FP-POS
1291
1309
1327
1318
1105
1300
1327
A
A
B
B
A
A
B
B
B
B
B
A
A
B
B
B
B
B
B
B
A
A
B
B
A
B
B
B
B
1105
Continuum
Continuum
1327
Continuum
1105
1291
1291
1309
Continuum
1318
1105
Continuum
1327
1309
1318
Continuum
1318
1291
1309
Continuum
Continuum
1300
1300
Continuum
Continuum
1300
Continuum
Continuum
3
3
3
3
3
3
1
4
X=7000
X=1100
4
X=4500
1
1
4
1
X=3000
1
2
X=1500
2
4
4
X=4900
2
2
2
X=13000
X=11000
2
4
X=15000
X=8500
1
X=12000
X=14000
Days to PH=3
since 1/21/2011
161
208
292
372
425
428
441
446
495
519
536
540
547
575
577
591
601
618
624
628
636
653
671
690
711
719
748
764
787
800
826
826
830
907
1023
1240
Date
Jul-11 Rest of 2011
Aug-11
Nov-11
Jan-12 First half 2012
Mar-12
Mar-12
Apr-12
Apr-12
May-12
Jun-12
Jul-12 Second half 2012
Jul-12
Jul-12
Aug-12
Aug-12
Sep-12
Sep-12
Sep-12
Oct-12
Oct-12
Oct-12
Nov-12
Nov-12
Dec-12
Jan-13 2013
Jan-13
Feb-13
Feb-13
Mar-13
Apr-13
Apr-13
Apr-13
May-13
Jul-13
Nov-13
Jun-14 2014
First gain sag hole
appears in segment B
First gain sag hole
appears in segment A
Continuum effects start
appearing in both segments
• 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
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Gain-Sag Holes in Segment B of COS/FUV by Apr 2012
G130M settings that will produce these holes
1327
1
2
3
1318
4
1
2
3
1309
4
1
2
3
1300
4
1
2
3
1291
4
1
2
3
4
Detector active area in dispersion direction: ~ 14,000 pixels
Coverage of all G130M settings shown (light blue): ~ 5000 pixels
Effects of Segment B gain sag holes on G160M observations
G160M observations with FP-POS=1,2,3,4 (in wavelength space)
1
2
3
4
What happens to S/N of coadded G160M data on Seg B?
x1dsum with 1 FP-POS (3): S/N = 0 over 500 pix (3.5% of spectrum)
x1dsum with 2 FP-POS (3+4): S/N = 70% over 1000 pix (7% 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)
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)
Gain-Sagged Continuum in Segment A
of COS/FUV by ~ Jun/Jul 2012
Detector active area in dispersion direction (red + light blue): ~ 14,000 pixels
Continuum area affected by gain sagged (red): ~ 4000 pixels (~ pix 4000 to 8000)
Effects of Seg A sagged continuum on G160M observations
G160M observations with FP-POS=1,2,3,4 (in wavelength space)
1
2
3
4
What happens to S/N of coadded G160M data on Seg A?
x1dsum with 1 FP-POS (1): S/N = 0 over 4000 pix (29% of spectrum)
x1dsum with 2 FP-POS (1+2): S/N = 0 over 3750 pix (27% of spectrum)
x1dsum with 3 FP-POS (1+2+3): S/N = 0 over 3500 pix (25% spectrum)
x1dsum with 4 FP-POS (1+2+3+4): S/N = 0 over 3250 pix (23% spectrum)
Evolution of Gain-Sagged Continuum
in Segment A of COS/FUV
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•
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
11/17/2011
TIPS Meeting
Apr 2013
Feb 2013
Mar 2013
May 2012
Jul 2012
Oct 2012
Lighter to dark shade: Increase in time
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Evolution of Gain-Sagged Continuum
in Segment B of COS/FUV
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•
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
11/17/2011
TIPS Meeting
Jun 2014
Nov 2013
May 2013
Dec 2012
Sep 2012
Jun 2012
Lighter to dark shade: Increase in time
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Gain-Sag Effects on COS FUV Spectra: 1
• 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
11/17/2011
TIPS Meeting
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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
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on Segment A within 2 years (HV increase would give ~ 1 additional year)
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on Segment B within 3 years
An action of some kind is required to mitigate
gain-sag effects within the next 10 months (Sep 2012)!
11/17/2011
TIPS Meeting
severe
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Mitigation of COS FUV Gain-Sag Effects
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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 !
11/17/2011
TIPS Meeting
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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:
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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)
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•
•
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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 !
11/17/2011
TIPS Meeting
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COS FUV Lifetime Adjustment Work
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
11/17/2011
TIPS Meeting
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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, crossdispersion 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 WCA
NUV observations on hold until we sort out if the COS aperture mechanism is indeed a limiting factor
11/17/2011
TIPS Meeting
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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
11/17/2011
TIPS Meeting
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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
11/17/2011
TIPS Meeting
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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 team
Meeting with HMO and IDT to finalize choice of next FUV lifetime position
currently scheduled for Dec 9, 2011
11/17/2011
TIPS Meeting
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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
-
11/17/2011
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
TIPS Meeting
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Part 1: Results so far
•
Current gain map with modal gain of PHA
distribution (segment A top, segment B at
former lower HV bottom)
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–
•
•
•
Cross-Dispersion Direction
•
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?)
11/17/2011
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Part 1: Results so far
•
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
–
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
–
•
•
Cross-Dispersion Direction
•
Current plan of uniform FP-POS usage should
give better results than shown
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
11/17/2011
25
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
11/17/2011
TIPS Meeting
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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
Resolution Peak may be between 0” and +2”, so in terms of resolution + direction preferred
11/17/2011
TIPS Meeting
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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
11/17/2011
TIPS Meeting
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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
11/17/2011
TIPS Meeting
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