Selection of the New
COS/FUV Lifetime Position
Cristina Oliveira
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
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•
•
COS FUV XDL is a photon-counting
micro-channel plate (MCP) detector
In COS FUV TIME-TAG mode every
photon is recorded with:
–
–
•
# of events
Gain-Sag Artifacts in COS FUV Spectra
position (x,y) and arrival time (t)
total electron charge generated/pulse-height
amplitude (0 ≤ PHA ≤ 31)
For every detector element, PHA
distribution changes with time, shifting
to lower PHA values, as fewer electrons
can be extracted from the MCP with
usage, the so-called gain-sag effect (see
PHA bin
top figure)
•
Events reaching the detector in areas
with low PHA are no longer recorded,
effectively resulting in a flux loss (see
bottom figure)
•
As time progresses the flux loss can
become so severe that the spectrum
needs to be moved to a new lifetime
position
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
(Due to Lya airglow)
2
Timeline of 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
Jan 19 2012
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
TIPS Meeting - COS/FUV Lifetime
Gain sag holes currently
present in COS data
By July 2012 will loose part
of continuum in FUVA
HV
to be raised ~March to avoid this
By September 2012 will
lose part of continuum
in FUVB
Need to move to new
lifetime position before
September 2012
3
Moving to a New COS/FUV Lifetime Position:
What does it Mean?
Constraints on motion of aperture mechanism in Y direction prevents
PSA to be moved beyond +/− 6.0” from current position
Active area of the detector (in pixels)
Geocoronal
Lya emission
Wavecal
location
Current Position at 0”
+6”
+3”
0”
−3”
−6”
COS FUV - Segment B (program 12678,G130M/1291)
17, 2012
JanJan
19 2012
TIPS Meeting - COS/FUV Lifetime
4 4
Parameters Considered in Choosing
New COS FUV Lifetime Position
In order to choose new lifetime position several factors were considered:
•
•
•
•
•
•
•
Resolution at each position
Projected lifetime at each position
Overall flat-field characteristics at each position
Effect of new position on subsequent positions
Use of wavelength calibration lamp spectra at each position
FUV dispersed-light target acquisition at each position
Impact of new lifetime position on aperture mechanism
• Resolution and projected lifetime at each position were the most
critical parameters in choosing the new COS lifetime position
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
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COS FUV Resolution:
Cross-Dispersion (Y) Offsets
• Resolution at Y offsets inferred assuming R = 19,000 at
current position (0.0”)
• Resolution at ±3.0” and ±6.0” measured from program
12678 (other values linearly interpolated from nearest
on-orbit measurements)
• Ray-trace models and on-orbit data predict Y peak
between 0.0” and +3.0”
• Positions above current position have better resolution
than similar positions below by ~ 20% (e.g., similar
resolution between +6.0” and −3.0”)
Assumes R0=19,000 at current pos. (0.0”)
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
X
Y
R
R/R0
0.0”
-6.0”
12000 0.63
0.0”
-5.0”
13000 0.68
0.0”
-4.0”
14000 0.74
0.0”
-3.5”
14500 0.76
0.0”
-3.0”
15000 0.79
0.0”
0.0”
19000 1
0.0”
+3.0”
18500 0.97
0.0”
+3.5”
17750 0.93
0.0”
+4.0”
17000 0.89
0.0”
+5.0”
15500 0.82
0.0”
+6.0”
14000 0.74
-3.0”
-3.0”
12000 0.63
+2.0”
-3.0”
15000 0.79
6
Resolution: Adding Dispersion (X) Offsets
• X offset of 3.0” corresponds ~ to ½ FP-POS
- Needed to interlace gain-sag holes due to Lya airglow at each FP-POS
position
• Mechanism soft stop in +X direction allows max offset of +2.0”
• Data obtained at +2.0” and −3.0” offsets in X direction with a −3.0” offset
in Y (as part of characterization program 12678)
- Resolution at (−3.0”,−3.0”) worse than (0”,−3.0”)
- Resolution at (+2.0”,−3.0”) similar to (0”,−3.0”)
• Ray-trace models support on-orbit results
- In X, resolution peaks between 0.0” and +2.0”
- Resolution with +2.0” offset in X direction similar to resolution with no offset
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
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Resolution: Dispersion + Cross-Dispersion
Offset Examples
Effect of Y offsets in resolution
(0.0”, 0.0”)
(0.0”, 0.0”)
Jan 19 2012
(0.0”,+3.0”)
(0.0”,−6.0”)
Effect of X offsets in resolution
(0.0”, −3.0”)
(−3.0”,−3.0”)
(0.0”, −3.0”)
(+2.0”,−3.0”)
TIPS Meeting - COS/FUV Lifetime
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Resolution: Summary
⇒ Peak of resolution between 0.0” and +3.0” in Y direction
• Moves in Y > 0.0” lead to less resolution degradation than similar moves in
Y < 0.0” (by ~20%)
• Ideally, would like to move to Y = +1.5” (peak as predicted by models).
• Need to move further away from current position (at least 3.0”) to
avoid existing regions where gain has already sagged
• Resolution at +3.0” similar to resolution at current location
• Resolution at +6.0” similar to resolution at -3.0”
⇒ Peak of resolution between 0.0” and +2.0” in X direction
• Offset of +3.0” slightly better to minimize gain sag effects in the first 1-2 yrs
• Not feasible due to aperture mechanism constraints
• Would likely lead to worse resolution
⇒ To optimize resolution best to move as close as possible to +3.0” in Y
(with or without X = +2.0” offset)
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
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Projected Lifetime at Different Positions
The projected lifetime at each position is a function of the initial gain map
at the specific position, the detector usage, and how close the new
position is to the previous position
To determine the projected lifetime at each position:
– Data obtained in 12676 were used to model the evolution of gain sag as a
function of time and position in the detector
– Images were generated for pure Y moves and moves with a -3.0” and +2.0”
X offset
•
•
•
Histograms of the modal gain distribution were determined, for the regions where
the different gratings project the spectra, for each lifetime position
The % of pixels with modal gain ≤ 3 was determined for each case.
A modal gain of 3 corresponds to a 5% loss in flux
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
10
G160M/FUVB (0.0”,+3.5”)
G160M/FUVB (0.0”,+3.5”)
Jan 19
Dec
09,2012
2011
TIPS
COS
Meeting
Lifetime
- COS/FUV
Brainstorming
Lifetime
MODAL
GAIN
vs.
TIME
time = 1 yr (July 2013)
time = 3 yr (July 2015)
11
Summary: Analysis Modal Gain vs Time and Offset
•
Positions with Y > 0.0” allow us to mitigate gain sag effects for a longer
period of time than similar positions with Y < 0.0”
– For FUVB, after ~3 years, the % of pixels with modal gain ≤ 3 is approx. the same
regardless of position, indicating that newly formed gain sag regions at that position
dominate other effects (except for G140L at -3.0”)
– For FUVA, after ~3 years, the % of pixels with modal gain ≤ 3 is much larger for Y < 0.0”
than for similar Y > 0.0” positions
• G160M spectra are narrower and fall lower in the detector than G130M, leading to asymmetric
gain sag structure from the currently used position
– The evolution of the modal gain with time for positions at +3.5” or beyond is similar,
indicating that they are in a detector location not affected by the gain sag of the current
position
• The position at +3.0” is not as pristine as that at +3.5”, and after ~3 years it has the double
amount of pixels with modal gain ≤ 3
•
Positions with X = -3.0” or X = +2.0” offsets are not significantly better at
mitigating gain sag than positions without X offsets
• Slightly delay gain sag of the Lya regions at the new position in the first 1-2 yrs
• So not prolong the overall lifetime of the new position
–
In ~3 yrs the % of pix with PHA<3 is similar because gain sag at that location dominates
⇒ To optimize lifetime at the new position (due to gain sag) best to move to
+3.5” or beyond in the Y direction
(no huge benefit in moving in X direction)
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
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Summary: Analysis of COS Lifetime Positions I
• To optimize resolution best to move as close as possible to +3.0” in Y (with or
without +2.0” X offset)
• To optimize lifetime at the new position (due to gain sag) best to move to +3.5” or
beyond in the Y direction (with or without X offset)
• X direction offset may be slightly advantageous only in the first 1-2 years because it
delays appearance of new gain sag holes
• Overall flat-field characteristics don’t change much as a function of Y position, but
positive moves are slightly preferred
• In terms of bad pixel map, moves with Y > 0.0” are slightly preferred to moves with Y <
0.0”. However, no region is so badly affected as to have a significant weight in the
choice of the next lifetime position(s)
• Positions in either the positive or negative Y directions lead to flat fields not
significantly different from that at current location
• Geometric correction accuracy in X and Y comparable for lifetime positions between 6.0” and +6.0” for both segments. No impact in choice of next lifetime position.
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
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Summary: Analysis of COS Lifetime Positions II
• To optimize the number of lifetime positions, best separation in Y between
positions is 3.0”-3.5”.
• “Weighted” extraction can be used to reduce separation
•
Wavecal operations affected only for most extreme positions
• Tagflash operations will have to be changed for lifetime positions beyond Y = +5.0”. All
other positions can use tagflash without modifications
• Only lifetime positions at Y = -4.0” or lower will have to deal with WCA spectrum falling
on region of detector potentially affected by gain sag
• Target acquisition algorithm will have to be modified for lifetime position at Y = 6.0”.
• Other lifetime positions require no modifications
• The lifetime of the COS aperture mechanism does not constrain the lifetime
positions
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
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Constraining the Next Lifetime Position
Y Positions (“)
-6
-5
-4
-3.5
-3
-2
-1
0
+1
+2
+3
+3.5
+4
+5
+6
Resolution
Projected
Lifetime
Flat Field
Subsequent
positions
Wavecal
Target Acquisition
Aperture
Mechanism
Best location for next
lifetime position at Y=+3.5”
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
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New COS FUV Lifetime Position and Beyond
• The following scenario could be considered for future positions (only next
position selected; future positions to be evaluated at later time):
Lifetime position 2 at (0”,+3.5”)
Lifetime position 3 at (0”,−3.0”)
Lifetime position 4 at (0”,+6.0”) – TAGFLASH changes needed to avoid FCA leak issue –
• To extend lifetime of each position
-
Start at low HV and incrementally increase HV to overcome gain sag (modal gain
increases by ~ 3 PHA bins per 8 HV steps)
-
Decrease size of extraction boxes currently used
-
Use some sort of “weighted extraction” to avoid CALCOS throwing out entire columns
when only a few pixels at the edges of extraction box have sagged below the
threshold (due to overlap with previously sagged position)
• These 3 additional positions will enable us to operate for additional ~ 9
years while mitigating gain sag effects
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
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END
Jan 19 2012
TIPS Meeting - COS/FUV Lifetime
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