Space Telescope Imaging Spectrograph:
Projected Performance through 2013
Paul Goudfrooij
STScI
• Part of procedure to address necessity of ASCS installation
• CCD and MAMA Performance Estimates with/without ASCS
– Science Impact example
TIPS meeting
October 13, 2005
Paul Goudfrooij
1
CCD Performance after SM4
•
•
•
•
•
•
Main contributor of effective throughput loss is increase of Charge
Transfer Inefficiency (CTI = 1–CTE) with time (due to accumulated
radiation damage)
–
Performance degradation continues in current Safe mode
Functional dependence of CTI on source counts, background level,
and elapsed on-orbit time determined in 2003 (Goudfrooij & Bohlin)
–
CCD spectroscopy mode unique to STIS and hence deemed
most relevant for current exercise
Issues related to growth of number of hot pixels similar to those for
ACS CCDs (cf. Marco’s talk), but can be mitigated by dithering
On-orbit increase of dark current typically has less impact than that
of CTI for faint sources
No positive impact of ASCS/STIK expected on CCD performance
Effect of higher temp’s on CTI would need to be calibrated in flight
TIPS meeting
October 13, 2005
Paul Goudfrooij
2
Science Impact of CTI Increase
•
Example of popular use of STIS CCD: Measurement of Black Hole
Mass in center of typical bulge in low-mass galaxy (plenty of mediumand high-mass galaxies already done).
SETUP:
• G750M/8561 Å
• 52x0.2 slit
• CCD binning 2x1
• galaxy with V =
16 mag/arcsec2
TIPS meeting
October 13, 2005
Paul Goudfrooij
3
FUV-MAMA Performance after SM4
• Dark Current Limited: Exp time needed  Dark Counts
–
–
–
Echelle data
FUV-MAMA dark in “glow” region generally increasing with tube
temp, but no clear relation between the two.
 relation between dark in glow region and time since HV turn-on
If tube temp will rise significantly after SM4, might consider
constraining use to only few hours after every HV turn-on
F1 apertures
 NUV-MAMA on

TIPS meeting
October 13, 2005
Paul Goudfrooij
NUV-MAMA off
4
FUV-MAMA Performance after SM4
• Past Long-term Evolution of FUV-MAMA Temperature
– Overall increase with time, but rate of increase declining
– Temp stayed ~ constant with time since 2002
SM3B
Solar Max
SM3A
TIPS meeting
October 13, 2005
Note: Tube Temp is ~
4 ºC lower than
Charge Amp Temp
(latter shown on left)
Paul Goudfrooij
5
NUV-MAMA Performance after SM4
• NUV-MAMA Dark Current
–
–
–
Due to phosphorescence in impurity sites in MgF2 window, where
e– can be in metastable level just below level decaying to emit 
Dark level strongly dependent on thermal environment
Long-term history of dark current (and temp) shows leveling off
Plot by James Davies
SM3A
TIPS meeting
October 13, 2005
SM3B
Solar Max
Paul Goudfrooij
6
NUV-MAMA Performance after SM4
•
•
•
•
Dark can be substantially
reduced with ASCS/STIK
Temp’s (and hence dark
Approx.
current
and exp time for darkdark
limited obs.) down by
level
nearly factor of 2
Dark down extra factor ~2 if
cooling campaigns
established
Would open up whole new
field of applications
–
–
Model of dark
during cooling
campaign
Echelle spectroscopy of
absorption lines in faint,
distant QSOs
NUV imaging of distant,
low surface brightness
galaxies
TIPS meeting
October 13, 2005
Paul Goudfrooij
7
Summary of potential impact of
ASCS on STIS performance
Detector
FUV-MAMA
NUV-MAMA
Impacted by
ASCS?
How?
Science
Impact
What if NO
ASCS?
YES
Positive: Able
to run colder
than lately
Somewhat
lower dark
and better
performance
If temps higher
than now, may
find operational
workaround
YES
Positive: Able
to run colder
than lately
Significantly
lower dark
and better
performance
If temps higher
than now,
somewhat lower
performance
• Performance dependent on post-
CCD
TIPS meeting
SM4 thermal environment
• impact of CTE degradation at
current temp not substantial
• Need recalibration if temp 
NO
October 13, 2005
Paul Goudfrooij
8