A Fresh Start For The COS FUV Detector
C. M. Oliveira (STScI), A. Aloisi, J. Ely, G. Kriss, D. Massa, R. Osten, S. Osterman (U. of Colorado),
C. Proffitt (CSC/STScI), D. Sahnow (STScI), S. Penton (U. of Colorado/STScI)
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ABSTRACT
To forestall possible loss in recorded flux due to charge depletion in the microchannel
plates (or "gain sag”), the location of spectra obtained with the far ultraviolet (FUV) detector of the Cosmic Origins Spectrograph (COS) on HST will be moved to a different position
!"#$%&'$(
on the performance of the COS FUV detector and optics was carried out. Recommendations
Resolution
FUV Detector Gain Sag
Observations of an external source (SK191) were carried out at positions offset in the
The FUV detector of COS is an open face microchannel
plate (MCP) detector with a cross-delay
M5(N+/+.(8*&7;+#(
Q&07(<&;(
cross-dispersion
direction
by +/-3" and +/-6" from the current position (program 12678)
)*1-;*( ;&07( #&;( J0..( I%#$( &==+&%( 07( $*+(
)*+(
'*&77+.(
17( 8:<( Incoming
+9=.1G#( $J1(photons
1=+7F,&'+@(
90'%1'*&77+.(
=.&$+( @+$+'$1%(
'%1##(
@+.&G( .07+(
&71@+D( ( )*0#( $G=+( 1,(
line345(
(XDL)
anode.
strike
the front
surface J0$*(
of &(the
MCP
and2RSN6(
release
in order to characterize
how
the resolution
varies with offset (external data cannot be
"%0;*$+#$( 1,( $*+(
@+$+'$1%(
%+;017#( 0..-907&$+@(
@+$+'$1%(0#(=&%C'-.&%.G(#-"T+'$($1(&7(+>+'$('&..+@(;&07F#&;H(J*0'*(&%0#+#(,%19($*+(9+$*1@(-#+@(,1%(@+$+'07;(=*1$17#D((U&'*(07'1907;(
photoelectrons which are then accelerated and multiplied as they travel down the MCP pores.
"G( &0%;.1J( .07+#H( +/+7$-&..G( .&%;+( =1%C17#( 1,(
=*1$17(+/+7$(=%1@-'+#(&('.1-@(1,('*&%;+(&#(0$(*0$#($*+(@+$+'$1%D((K7(&@@0C17($1($*+(R(&7@(V(.1'&C17H($*+(#0?+(1,($*0#('*&%;+('.1-@H(1%(
obtained beyond
+/-6" due to cross-dispersion constraints on the aperture mechanism).
$*+( '17C7--9( J0..( "+( #*0O+@( @1J7( ,&%(
=-.#+(*+0;*$(&9=.0$-@+(2WM!6H(0#(9+&#-%+@D(
(:/+%(C9+H(&#(91%+('*&%;+(*&#("++7(+B$%&'$+@(,%19(&(;0/+7(%+;017H($*+(@+$+'$+@(WM!(
Each input photon produces an electron
cloud with ~ 10 million electrons (referred to as "gain")
+71-;*(
$1(
*0$(
1-%(
8&.8:<(
.090$#D(
(
)*0#(
"+'19+#(#9&..+%D((P+(#++($*0#(&#(&(#*0O07;(1,($*+($G=0'&.(WM!(@0#$%0"-C17(2WMS6(,1%(&(;0/+7(=0B+.($1(#9&..+%(/&.-+#(J0$*(91%+(-#&;+(
Data obtained at the current position (0") wereJ1-.@(
convolved with several LSF with different
9+&7( $*&$( .&%;+( %+;017#( 1,( $*+( #=+'$%-9(
1,($*+(@+$+'$1%D((!7(+B&9=.+(1,($*0#('&7("+(#++7(07(30;-%+(XH(J*+%+($*+(WMS(,1%(&(=&%C'-.&%(=0B+.(0#(#*1J7(,1%(Y(@0>+%+7$(@&$+#D(()*0#(
which are then collected by the XDL, which uses timing measurements to identify the (x,y)
FWHM until aJ1-.@(
best7++@(
match
was
obtained
between
$1( "+(
+B'.-@+@(
07( $*+(
;&07F#&;(the convolved data and the spectra
#*0O("+'19+#(&(=%1".+9(J*+7($*+(@0#$%0"-C17("+'19+#($11(.1JH(&#($*+(@+$+'$1%(+.+'$%170'#(&%+(71(.17;+%(&".+($1(&''-%&$+.G(%+;0#$+%(
position
and
total
charge
in
the
cloud,
or
pulse
height.
$&".+(07(1%@+%($1(%+$&07(@&$&(07$+;%0$GD( (P*+7(
07'1907;(+/+7$#D(((E+'&-#+(1,($*0#H($*+(17"1&%@(+.+'$%170'#(#'%++7(+/+7$#(1,(WM!(Z([H(&7@(8&.8:<(#'%++7#(+/+7$#(J0$*(WM!(Z(\(J*0'*(
observed
at
different
offset
positions.
Figure
3
shows how the resolution, as measured
J+( &==%1&'*( $*0#( .090$H( 0$( J0..( "+'19+(
J0..('&-#+(=&%$(1,($*+(@+$+'$+@(A-B($1("+(.1#$(0,($*+(@0#$%0"-C17(+7$+%#($*+#+(%+;017#D(((
$1( -#+( changes
1$*+%( 9+&7#(
'19"&$( of offset position (POSTARG), with
by the FWHM09=+%&C/+(
of a gaussian,
as $1(
a function
;&07F#&;( J*0'*( +7$&0.( %&0#07;( $*+( 1=+%&C17&.(
The number of electrons, or total charge, in the cloud decreases with usage - the so-called gain
respect to the*0;*(/1.$&;+(2M56(1,($*+(@+$+'$1%D(((
resolution at the current position:
sag effect - and eventually becomes too small to be detected by the electronics, leading to an
!#(#++7(07(30;-%+#(X(&7@(\H($*+(WM!(;&07(1,($*+(@+$+'$1%(@+'%+&#+#(J0$*(C9+(&#(91%+(
effective loss in flux. (For strategies to mitigate gain sag see Poster 136.05).
'*&%;+(0#(+B$%&'$+@(,%19($*+(@+$+'$1%D( (:7+(J&G($1(#*0O($*+(WM!(@0#$%0"-C17("&'b(-=(
0#($1(%&0#+($*+(M5D( (!7(+B&9=.+(1,(#-'*(&(%&0#+('&7("+(#++7(07(30;-%+(YH(J*+%+($*+(M5(
A map of the COS/FUV detector, representing all events that have been collected until March
J&#( %&0#+@( ,%19( Xgi( $1( XidD( ( <-'*( &( %&0#+( *&#( $*+( +>+'$( 1,( #*0O07;( $*+( +7C%+( =-.#+(
2012 is presented
in Figure 1:
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]170$1%07;(&7@(8&.0"%&C17(
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&O+%( WM!( I.$+%07;( "G( 8&.8:<6( &%+( A&;;+@( 07( &( 7+J( Q&07( <&;( )&".+(
2Q<!Q)!E6(%+,+%+7'+(I.+D((U/+7$#(,&..07;(07($*+#+(0@+7CI+@(%+;017#(J0..(
"+(A&;;+@(&7@(+B'.-@+@(,%19(I7&.(BX@#-9(=%1@-'$#("G(8&.8:<D((K,($*0#(
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"+( .1#$H( &7@( $*+G( J1-.@( &==+&%( #090.&%( $1( &"#1%=C17( ,+&$-%+#( 07( $*+(
+B$%&'$+@( #=+'$%-9D( ( 30;-%+( c( #*1J#( $*+( 91@&.( WM!( ;&07( *0#$1%G( 1,(
#-=+%=0B+.( XXcfHXcg( 2&( #-9( 1,( -7"077+@( =0B+.#( hX[YHdYY( $1( hXX\HdYf(
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)*+(;&07(#&;($&".+(J0..("+(09=.+9+7$+@(07($*+(:)3_(W0=.07+(#$&%C7;(07(
<-99+%(\[X\D(
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1=C17D((
Figure 3 shows that moves in the negative cross-dispersion direction lead to a ~20%
higher resolution degradation than similar moves in the positive direction. This behavior
is expected from ray-trace models which predict that the peak of the resolution occurs
between 0" (current position) and +3”.
G(4H6-#\I# #FB-#-Y-M/#'J#4,(=#
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' L - =# ( & & H 0 ( = , / - 3# O V#
,(64&'C#&(=-)#
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0 , 6 E ( = 4 # / B - # M H 6 6 - = /#
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Projected Lifetime at Each Position
G(4H6-# Z# `O-&'CbI# $# 0'3,&# 4,(=# 0,K# `/'Kb# ,=3# M'H=/)# 0,K# `O'@'0b# ,M6'))# >-40-=/# .# M,=# O-# )--=# O-&'C%# # FB-# 0'3,&# 4,(=#
6-;-,&)# B'C# /B-# (&&H0(=,P'=# J6'0# /B-# M'H=/)# 0,K# ,Y-M/)# /B-# 3-/-M/-3# 4,(=# (=# /B,/# 6-4('=%# $6-,)# ,O';-# ,=3# O-&'C# /B-#
MH66-=/#(&&H0(=,/-3#&'M,P'=#CB-6-#3-/-60(=-3#OV#-NK&'6,/'6V#K6'46,0)"#,=3#/B-#)M,&-#J'6#/B-#4,(=#M,=#O-#)--=#(=#/B-#&-4-=3#
/'#/B-#6(4B/%##.6(4B/#^V0,=#$&KB,#,(64&'C#&(=-)#M,=#O-#)--=#(=#;,6('H)#K')(P'=)#(=#/B-#M'H=/)#(0,4-"#,=3#/B-#0'3,&#4,(=#(=#/B-#
M'66-)K'=3(=4# ,6-,)# ,6-# )(4=([M,=/&V# &'C-6# /B,=# (=# /B-# (&&H0(=,/-3# 6-4('=)%# # d;(3-=M-# 'J# ),4# (=# /B-# M'=P=HH0# ()# -;(3-=/# '=#
/B-#&-L#)(3-#'J#/B()#)-40-=/%##
!A#
X#
T#
S#
The lifetime at a new position, i.e. the amount of time when data can be acquired
without suffering from gain sag effects, is a function of the initial gain map at the specific
location, the detector usage, and how close the new position is to the previous position.
Simulations of the evolution of the gain map with time and position in the detector were
carried out for several combinations of dispersion and cross-dispersion offsets. Figure
4 shows a simulation for a cross-dispersion offset of 3.5”, for data obtained with the
G160M grating and B segment:
^V0,=#,&KB,#B'&-)#
9'=P=HH0#),4#
!Z#
$MP;-#,6-,#
!
-6
-5
-4
-3.5
-3
-2
-1
0
1
2
3
3.5
4
5
6
Resolution
Projected
Lifetime
Flat Field
Subsequent
Positions
Wavecal
Target
Acquisitions
Aperture
Mechanism
When all the parameters in Figure 5 above are considered, taking into account that
the resolution and lifetime at each position are the most critical in selecting a new
lifetime position, it becomes clear that the next COS lifetime position should have an
offset of ~ +3.5" in cross-dispersion (+41 pixels in Y). The new lifetime position at +3.5"
should allow us to operate COS/FUV for at least another 3 years, assuming similar usage.
In addition, strategies to increase the lifetime of the new position are being considered these range from starting operations in the new position at a low high voltage level and
incrementally increase it to overcome gain sag effects, to using a weighted extraction
algorithm to extract 1-d spectra.
Move to the New Lifetime Position
Before the planned move to the new lifetime position in early July 2012 several programs
were carried out in order to enable science at this position. These programs include a HV
sweep to select the initial operating voltage at the new position, a focus sweep, and
programs related to verifying the aperture and spectrum placement at the new lifetime
position as well as updates to FUV dispersed light target acquisition parameters (as FUV
target acquisition will be moved to the new lifetime position).
Once the move to the new lifetime position occurs, calibration programs will be executed in
parallel with science observations. These programs will check the wavelength scales and
resolution and will obtain data for flat-fielding and flux calibration.
!
!
!"#$%&$'(%'$
Y Positions (“)
Next Lifetime Position
N0,+C9+(]1/+#(
P*+7( %&0#07;( $*+( M5( 0#( 71( .17;+%( &7( 1=C17H( 0$( J0..( "+'19+( 7+'+##&%G( $1( #*0O( $*+(
+7C%+( #=+'$%-9( $1( &( 7+J( .1'&C17( 17( $*+( @+$+'$1%D( ( E+'&-#+( ;&07( #&;( 17.G( &==+&%#(
J*+%+( .0;*$( *&#( ,&..+7H( &%+&#( 1,( $*+( @+$+'$1%( &"1/+( &7@( "+.1J( $*+( '-%%+7$( #=+'$%&.(
.1'&C17(J0..("+(,%++(1,(&7G(@+;%&@+@(+>+'$#D( ()*+(+>1%$($1(91/+($1(&(7+J(.1'&C17(0#(
-7@+%J&GH(,1%(91%+(07,1%9&C17(=.+&#+(#++(^_+,+%+7'+($1(8%0#C7&`#(=1#$+%a(
When gain sag effects can no longer be overcome by either raising the high voltage of the
detector or by using the FP-split technique (FP-POS), the only solution is to move the spectra to
a pristine region of the detector, i.e, to a new lifetime position - illustrated in Figure 2:
_--E&V#9H0H&,P;-#
9'H=/)#`99ab#
8'3,&#12$#5,(=#
on the detector, i.e., a new lifetime position, in the summer of 2012. In order to maximize science quality over the next five years, a study of the impact of a new lifetime position
)*+(,&%(-.$%&/01.+$(23456('*&77+.(17($*+(81#90'(:%0;07#(<=+'$%1;%&=*(28:<6(#->+%#(.1'&.0?+@(A-B(.1##(@-+($1(;&07(#&;(,%19('17C7-+@(+B=1#-%+($1(.0;*$D((E+'&-#+(1,($*+(717F-70,1%90$G(1,(1"#+%/+@(#=+'$%&(
,&..07;(17($*+(@+$+'$1%H(;&07(#&;(*1.+#(07(+B$%&'$+@(#=+'$%&(I%#$(&==+&%(07($*+(91#$(0..-907&$+@(=1%C17#(1,($*+(@+$+'$1%H($*1#+(&>+'$+@("G("%0;*$(&0%;.1J(.07+#H(&7@(J0..(+/+7$-&..G(09=&'$(.&%;+(%+;017#(1,($*+(
derived from this study for the location of the next lifetime position are presented here.
'17C7--9(&#(J+..D((K7(1%@+%($1(=%+#+%/+($*+(@&$&(L-&.0$G(&7@(+B$+7@($*+(1=+%&C17&.(.0,+C9+(1,(8:<H(#$%&$+;0+#(*&/+("++7(09=.+9+7$+@(J*0'*(09=&'$(7+&%.G(+/+%G(&#=+'$(1,(8:<(1=+%&C17#D(()*+#+(07'.-@+(
'*&7;+#($1(@+,&-.$(1"#+%/07;(#+L-+7'+#H(09=%1/+@(9170$1%07;H(7+J('&.0"%&C17(=%1'+@-%+#H('*&7;+#($1(@+$+'$1%(+.+'$%170'#H(&7@(&('19=.+$+(%+.1'&C17(1,($*+(0..-907&$+@(=1%C17(1,($*+(@+$+'$1%D((
)*+,$-../#0$1$23,4567$89:./;.$
The resolution of the new lifetime position is expected to be within ~10% of the resolution at
the original position. Small changes to the wings of the line spread function are predicted;
new LSF models have been computed and will be made available to users once they are
validated by calibration observations at the new lifetime position. The flux and wavelength
calibrations of the new lifetime position are expected to be very similar to those at the
original position. All the calib
ration observations will be executed shortly after moving to the new lifetime position and
updates to calibration reference files, if needed, will be promptly made.
%$
Modeling Gain Sag Effects
Using data acquired during the 2+ years of on-orbit operations models were constructed to
predict the evolution of gain sag at the current position as a function of time and position on the
detector. While the information lost due to gain sag holes (gain sag in regions illuminated by Lyα
airglow) can in principle be recovered by the use of multiple FP-POS positions, the same is not
true when broad pieces of the continuum start being affected by gain sag. The models predict
that by September 2012 a non-negligible portion of the detector will suffer from gain sag
effects, forcing us to move to a new lifetime position by this time. In order to add a safety
margin to this time estimate the move to a new lifetime position was planned to occur in
early July 2012.
Maximizing Science Quality
To maximize science quality over the next five years, a study of the impact of a new lifetime
position on the performance of the detector and optics was carried out. The parameters
considered are: resolution, projected lifetime and overall flat-field characteristics at each
position, the effect of the next lifetime position on subsequent positions, the ability to use the
wavelength calibration lamp, the ability to perform dispersed light target acquisitions, and the
impact of a new lifetime position on the aperture mechanism.
For each simulation, the % of pixels with modal gain less than or equal to 3
(corresponding to a flux loss of ~5%) was computed.
These simulations indicate that positions with positive cross-dispersion offsets allow us
to mitigate gain sag effects for a longer period of time than similar positions with
negative offsets, while offsets in the dispersion direction (to interlace Lyα holes) do not
prolong the overall lifetime of the new position.
Other Parameters Affecting Choice
of Lifetime Position
Although not as critical as the resolution and lifetime at each position, other parameters
were also considered in the selection of the new lifetime position. The constraints from all
the parameters are represented in Figure 5:
The move to the new lifetime position will be transparent to users preparing their Phase II
with APT. No changes will be made to the location of the COS NUV science or target
acquisitions.
The move to the new lifetime position has been delayed due to COS suspending operations
on Apr 30 2012, because of a count rate violation observed in the FUV detector. The NUV
detector was returned to science operations on May 14. The ongoing process to recover the
FUV detector takes a very cautious approach and is similar to the process used to turn on
the FUV detector after COS was installed in HST. This process will be completed by June
12. Return to science operations is expected to occur around June 15. The move to the new
lifetime position is expected to occur around middle to late July.