SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
The COS/STIS Team @ STScI
5/16/2013
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Our Diverse COS/STIS Team
Alessandra
Aloisi
Julia Duval
Justin Ely
Phil Hodge
5/16/2013
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TEAM LEAD
Alessandra Aloisi
TEAM DEPUTIES
Cristina Oliveira & Charles Proffitt
CALIBRATION
Julia Duval
Justin Ely
B
L
O
C
K
S
PIPELINE
Azalee Bostroem
Svea Hernandez
USER SUPPORT
Paule Sonentrucker
Nolan Walborn
Svea Hernandez
OPERATIONS
Steve Penton
Sean Lockwood
Calibration planning (Phase 1 & Phase 2), monitoring, data
analysis, inputs for reference files & ETCs, science data quality
verification
Back end of the ground system, DPAS, CALCOS/CALSTIS,
formatting and delivery of reference files to Pysynphot
Phase I & II support, including CS reviews, APT, ETC, IIHBs,
DHBs, ISRs/TIR publication, Help Desk
Target acquisitions, front end of the ground system (flight software,
TRANS, instructions), anomaly investigations
COS FUV XDL
Cristina Oliveira
5/16/2013
COS & STIS MAMAs
Charles Proffitt
CHANNELS
STIS CCD
John Debes
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Cosmic Origins Spectrograph (COS)
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Designed for maximum spectroscopic
sensitivity at  ~ 20,000 in FUV
10-30x STIS performance
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Installed on Hubble to replace
COSTAR during SM4 (May 2009)
4 years of operations in total on Side 1 !
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Single 2.5” aperture (PSA or BOA)
One-reflection FUV channel 1150-2050
Å using FUSE detector (2 side-by-side
16k x 1k pixel delay-line MCPs)
NUV channel 1700-3200 Å using STIS
MAMA spare detector (1k x 1k)
Medium (~ 20,000) and low (~3,000)
resolution gratings
Some sensitivity in 900-1150 Å range
Some limited imaging capabilities in
NUV with ~0.025“ per pixel
5/16/2013
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COS Design at a Glance
5/16/2013
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Space Telescope Imaging
Spectrograph (STIS)
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Highly versatile spectrograph
Replaced FOS (and GHRS) with 2D spectroscopy
during SM2 (Feb 1997)
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Operated for ~ 4 years on Side 1 and ~ 3 years on
Side 2
– Side 2 repaired during SM4 in May 2009
11 years of operations in total and still counting !
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3 detectors (can use only one at a time)
FUV MAMA: 1150-1700 Å, 1k x 1k, ~0.025”
per pixel
NUV MAMA: 1600-3200 Å , 1k x 1k, ~0.025”
per pixel
CCD: 1650-11,000 Å , 1k x 1k, ~ 0.05” per pixel
Large set of gratings, slits, and modes !
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Long slit 1st order spectra
High-dispersion UV echelle spectra with resolution
 up to ~ 100,000
Imaging and spectroscopic coronagraphy
Imaging and slitless spectroscopy
5/16/201
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STIS Optical Design
5/16/2013
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Highlights of current COS/STIS work
• COS
– Gain sag and lifetime positions
– New G130M modes
– FUV TDS
– FUV Dark Rates
• STIS
– CCD CTE effects
5/16/2013
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PHA Distribution with Time
of COS FUV Detector
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COS XDL is a photon-counting microchannel plate (MCP) detector
In COS FUV TIME-TAG mode every
photon is recorded with:
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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 gainsag effects
Gain sag leads to a shift of the pulseheight distribution to lower PHA values
Modal Gain is peak of PHA distribution
PHA distribution of the counts in the whole
region of the extracted spectrum for the same
target observed in Sep 2009 and Dec 2010
# of events
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PHA ≥ 2 adopted for
filtering as of Dec 21, 2010
3/19/2013
PHA bin
PHA ≥ 4 adopted for filtering at
beginning of on-orbit operations 9
COS FUV Modal Gain vs. Time
Higher HV (175), SMOV
Lower HV (167), SMOV
Segment B
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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
(Segment B)
Modal gain drops everywhere with time
where “continuum” of the spectra falls
Modal Gain
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Modal gain drops even more in regions
where geocoronal Lyα falls in G130M
settings, the so-called “holes”
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X pixel
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Higher HV, test Dec 21 2010
Lower HV, Dec 16 2010
Data from cumulative images at different
epochs and operational HV values are
presented up to Dec 2010
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faintest targets contribute to holes
CALCOS filtering
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PHA = 2
brightest targets contribute to continuum
Currently [2,30] (see dotted line)
Still in the process of implementing
position-dependent filtering
Modal gain = 3 is a benchmark for the
onset of severe gain-sag effects (flux
losses of ~ 5%)
Modal gain = 5 is a benchmark for the
onset of severe x-walk effects (up to ~
1 resel = 6 pixels) and resolution
degradation by ~ 20%
Moving to a New COS FUV Lifetime Position
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New position located at -0.05 and +3.5” in dispersion- and cross-dispersion
directions
Science operations at new position started on Jul 23 2012
Active area of the detector (in pixels)
Current Position at 0”
Geocoronal
Lya emission
Wavecal
location
New Position at 3.5”
+6”
+3”
0”
−3”
−6”
COS FUV - Segment B
Jan 17, Lifetime
2012
(COS FUV
Position Exploratory Program 12678, G130M@1291)
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Data Quality at
New COS FUV Lifetime Position
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Data quality meets expectations
– Gain sag ameliorated and “holes” from Lyα
exposure are avoided
– Resolution ~ 90% of that at original position
– Throughput at all wavelengths within ~ 2%
of that at original position
– Detailed re-calibration observations at new
position have being analyzed; no changes
were found
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New set of model LSFs computed with
Code V at new FUV position
– Excellent agreement with observations
– Theoretical LSFs soon to be released to the
community
http://www.stsci.edu/hst/cos/performance/spectral resolution/
Update on New COS FUV Lifetime Position
Calibration
Enabling
Exploratory
Program
ID/Activity
Title
# Orbits
Executed
Document
Status of Documentation
1
12676
COS/FUV Characterization of Detector
Effects
12 Internal
Massa et al. (2013a)
Under review (ISR)
2
12677
COS/FUV Mapping of Stray PtNe Light
Through FCA
16 Internal
Oliveira et al. (2013a)
Published (ISR 2013-02)
3
12678
COS/FUV Characterization of Optical Effects
6 External
Sahnow et al. (2013)
To be completed (ISR)
4
Aperture Mechanism Moves
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Proffitt et al. (2013a)
Under review (ISR)
5
Summary:
Choosing the Next FUV Lifetime Position
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Oliveira et al. (2013b)
To be completed (ISR)
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Requirements for FUV Enabling and Lifetime
Calibration Programs
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Osten et al. (2013a)
Published (TIR 2013-02)
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12793/FENA1
FUV Detector High Voltage Sweep:
Choosing HV
16 Internal
Kriss et al. (2013)
To be completed (ISR)
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12795/FENA2
FUV Spectrum and Aperture Placement
3 External
Proffitt et al. (2013b)
Published (ISR 2013-03)
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12796/FENA3
FUV Focus Sweep
9 External
Oliveira et al. (2013c)
Published (ISR 2013-01)
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12797/FENA4
FUV Target Acquisition Verification
13 External
Penton et al. (2013)
To be completed (ISR)
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FCAL1
FUV Wavelength Scales
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Sonnentrucker et al. (2013)
Under review (ISR)
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12805/FCAL2
FUV Spatial & Spectral Resolution
7 External
Roman-Duval et al. (2013)
Under review (ISR)
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12806/FCAL3
FUV Flux Calibration, Flat Fields,
Time Dependent Sensitivity Transfer
25 External
Massa et al. (2013b)
To be completed (ISR)
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12807/FCAL4
FUV BOA Operations
1 External
Debes et al. (2013)
Under review (ISR)
Summary:
FUV Enabling and Calibration Programs
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Osten et al. (2013b)
To be completed (ISR)
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3/19/2013
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COS FUV Modal Gain vs. Time:
Original Lifetime Position
Data up to end of July 2012 before Lifetime move
Segment A
Segment A:
• At time of lifetime move,
large part of continuum
below PHA = 5, thus
affected by X-walk effects
• No large holes due to Lyα
in G140L
Segment B
Segment B:
• At time of lifetime move,
continuum above PHA = 5
• Lots of deep holes due to
Lyα in G130M with deepest
holes in FP-POS = 3
3/19/2013
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COS FUV Modal Gain vs. Time:
New Lifetime Position
Data since Lifetime move at end of July 2012
Segment A
Segment A:
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Well above PHA = 5 (including
low gain region) so not yet
affected by X-walk effects
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Additional 12-18 months left
before continuum reaches
PHA = 5
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No holes developed yet
Segment B
3/19/2013
Segment B:
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Continuum getting close to
PHA = 5 where X-walk effects
start to be substantial
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Several holes due to Lyα in
G130M are starting to appear
and getting close to PHA = 5
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Mandatory use of all FP-POS
is helping in spreading the
damage around
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Resurrecting FUSE:
New COS G130M Modes
t = 2918 sec FUSE
t = 330 sec COS
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5/16/2013
Resolution of G130M/1055/1096 greatly
improved by focus adjustment
• Data quality at these wavelengths
comparable to FUSE
• New focus implemented in FSW on
Dec 10, 2012
GO observations with G130M/1055/1096 at
new focus started in Jan 2013
GO observations with G130M/1222 started
immediately after move to LP2 (Jul 2012)
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COS FUV TDS & Solar Activity
LP1
LP2
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TDS Breaks
HV change FUVB
Switch to LF2
HV change FUVA
3/19/2013
COS FUV sensitivity showed
steep drop in late 2011 (up to
20% per year), coincident with
period of very high solar
activity (curves at the bottom:
green is smoothed version of
the blue curve )
Subsequent decline much
more modest (4% to 6% per
year)
HV increase on segment A in
March 2012, caused a small
(~2%) increase because data
at lower HV were affected by
gain sag
Latest TDS break appeared
shortly after moving to new
position (LP2)
Currently TDS trends flattened
out again (2-3% per year) and
are at this point almost
wavelength independent
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Average COS FUV Dark Rate & Solar Activity
Segment A
Segment A: Increase in
scatter and baseline
rate
Solar cycle analogue;
10.7 cm radio flux
Segment B
Segment B: Increase in
scatter only with no
corresponding
baseline increase
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Overall dark rate, 2D structure, and distribution of pulse heights (PHD) continue to evolve with time for
both segments
Correlation with solar cycle, though behavior is different for Segment A and B
Dark rate on Segment A mysteriously reset to 2009 values, 2D structure, and PHD after Dec 2012 HST Safe
(no similar change seen after either the COS suspend in Apr 2012 or the Feb 2013 HST Safe)
3/19/2013
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2D Structure of the COS FUV Darks
Segment A
Segment B
11/23/2009
12/20/2012
12/27/2012
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Recent changes to PHA filtering (from 2-30 to 2-23) has decreased observed dark rate by ~20%
An even stricter pixel-by-pixel PHA filtering is under implementation
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Sorting out implications for spectral extraction and background subtraction
Increased frequency (2x) of dark observations in Cycle 20 allows to:
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achieve higher S/N data for 2D dark images and improvements to background subtraction method
better track trends, identify anomalous changes, and help in the creation of 2D dark corrections
5/16/2013
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STIS CCD CTE Effects
Van Dixon 2011, STIS ISR 2011-02
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11 years in the harsh onorbit environment have
significantly affected STIS
CCD in terms of CTE
A STIS algorithms exists to
partially correct:
 Flux of extracted spectra
 Flux and astrometry of
point sources in images
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However, no way to
correct for added noise
due to CTE trails of hot
pixels and cosmic ray
Red spectrum of a V ~ 18 mag SNa at the default position (S/N ~ 3)
Black spectrum of another V ~ 18 mag SNa of similar brightness at the E1 position (S/N ~ 17)
5/16/2013
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Towards a Pixel-Based CTE
correction for STIS CCD
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Investigated nature of STIS CCD CTE
trails to evaluate feasibility of a pixel-based
CTE correction
Used ACS script to perform preliminary
evaluation of STIS CCD data
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Amplifier
STIS hot pixel CTE trails qualitatively similar
to ACS/WFC3
Trail length only weakly T dependent
Preliminary tests on individual dark frames
and science data yield good correction of
CTE trails
Have not yet fully optimized CTE
parameters for STIS
Need to finalize characterization of time
dependence of CTE
Updated ACS script to run on STIS CCD images as a stand-alone tool soon to be
released to the community
Pixel-based CTE correction for STIS appears to be achievable
Sep 25, 2012
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Additional STIS CCD CTE Efforts
• Comparing subarray versus full-frame images to evaluate effect of
CCD readout speed on CTE
– Understanding this will allow improved pixel-based CTE corrections for
subarray data, and might inform impact of timing changes on CTE
• Evaluating science benefits of possible CCD readout changes
– Since STIS CCD can be readout from either end of detector, doing a
parallel split readout (amp D + amp A) would reduce maximum
transfers by 50% for the lower ½, although at cost of increased read
noise (from 5.6 e- to 8.5 e-) due to noisier amplifier (A) on bottom of
detector
– Doubling readout speed might improve CTE from current ~0.999 to
~0.9999, but it is unclear if this can be done without a noise penalty
• Charge injection strategies may exist, but a compelling science
need has not yet been identified
It is doubtful at this point that we can scientifically justify
expensive hardware testing, so pixel-based CTE correction still
remains the most promising way to mitigate STIS CTE effects
3/19/2013
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