STIS Instrument Status
Charles Proffitt & the STIS team
Paul Goudfrooij, Tom Brown, James Davies, Rosa
Diaz-Miller, Linda Dressel, Jessica Kim Quijano,
Bahram Mobasher, Mike Potter, Kailash Sahu, David
Stys, Jeff Valenti, Nolan Walborn, Ivo Busko, Phil
Hodge, Paul Barrett, & Ralph Bohlin.
Thanks are also due to many former members of the
STIS/spectrographs group and to the STIS IDT for their
numerous contributions to STIS calibration.
STIS Calibration Status
• STIS Side 1 Failure and Side 2 Recovery
• Detector Health and Performance
• Recent and Future Calibration Enhancements
Side 1 Failure and Recovery
• Main STIS bus fuse blew on May 16, 2001.
• Side 1 electronics completely unusable, and onorbit repair not possible.
• STIS reactivated using redundant Side-2
electronics on 3 – 11 July, 2001.
• MAMA detector performance unaffected.
• Some effects on CCD performance.
– Higher CCD read noise.
– No active CCD detector temperature sensor or active
CCD temperature control on side 2.
Read-noise increased from ~4.4 to ~5.4 e-/read on side 2 (for
AMP D with gain = 1 and 1x1 binning – other amps/gain/
binning show similar increases).
STIS Amp D, Gain = 1, unbinned CCD read-noise vs. time.
The extra noise is in the form of a coherent, narrow band
pattern noise that varies from 16 – 18 kHz. For some images,
Fourier filtering can reduce this noise (see poster by Tom
Brown and STIS ISR 2001-05 for more details).
UGC 2847 observed with the STIS CCD in September 2001 by program
8951 (PI Doug Richstone).
Variable CCD detector temperature and dark current.
• Side 1 control loop kept the STIS CCD at –83 ºC.
• Side 2 CCD temperature sensor doesn’t work, so TEC is
run at constant current, and temperature varies.
• Results in variable dark current (+/- 15%).
• CCD Housing temperature (OCCDHTAV ~ +18 ºC)
correlates with dark current (~7%/°C) and is used to scale
dark (see poster by T. Brown and STIS ISR 2001-003).
• Even with temperature scaling, the dark subtraction doesn’t
work quite as well as it did for side 1 (background residuals
in the dark subtracted images are about 30% larger than on
side 1 data from April 2001).
• Other CCD monitors continue previous trends apart from
offsets due to lower mean detector temperature.
• CTE will be discussed by Paul Goudfrooij in next talk.
MAMA Detector Performance – Dark Current
• NUV MAMA dark current dominated by window
phosphorescence.
• Short term dark current varies exponentially with T.
• Long term behavior depends strongly on radiation
environment with weak dependence on temperature.
Comparison of NUV MAMA charge amplifier temperature (OM2CAT) with
mean dark rate for NUV dark monitor exposures.
• Current NUV dark current is about 12% lower than
during cycle 8, despite higher detector temperatures.
• Current distribution slightly flatter as function of T.
• Dark rate now always > 0.0008 cnts/sec/lo-res-pixel.
• Pipeline dark scaling has been updated for change in behavior.
• New drk reference files follow changes in dark distribution.
FUV MAMA has
an intermittent
glow centered in
the upper left
quadrant of the
detector.
This glow has
become patchier
and more frequent
over time.
The lower right
corner remains
free of the glow,
with mean rate of
6.6e-6 cnts/s/pixel
In 1998, the enhanced FUV glow was rare; now its absence is the rarity.
Count rate in glow region [200:400, 600:800] of FUV dark monitor exposures.
FUV MAMA dark rate (glow) tends to increase with
- Observation date
- Detector temperature
- Length of time detector high-voltage has been on (strongest
correlation)
•
•
•
•
•
First orbit after SAA passages has lowest dark current.
Recommend that faint targets be placed close to the bottom
of the detector (for spectra as much as POS TARG 0 –6.8).
External cooling of detector would help, but unlikely to
completely eliminate enhanced FUV dark current.
Haven’t been able to find a practical strategy to predictably
lower FUV MAMA dark rates for the observations that
most need them (see STIS TIR 2002-02 by Tom Brown).
New reference files delivered to track the increasing
number of FUV MAMA hot pixels over time.
MAMA Sensitivity Changes
See also Bohlin’s talk, poster by Stys, and STIS ISRs 2001-01R & 1999-07
Time dependent sensitivity for other STIS modes.
• MAMA M modes for a given detector have same
time/wavelength dependence as L modes.
• MAMA imaging and echelle modes time dependent
sensitivity changes are consistent with MAMA L modes.
• For CCDs CTE effects must be corrected first, but residual
G230LB changes similar to G230L; G430 and G750
changes are smaller (see R. Bohlin’s talk).
• Pipeline now corrects 1st order MAMA spectra for time
dependent sensitivity. CCD and echelle will follow.
• Time dependent throughput in SYNPHOT soon, including
MAMA imaging.
Echelle Calibration Improvements
• Echelle Scattered Light Algorithm
– Algorithm by Lindler and Bowers now in pipeline.
– See Valenti et al poster and STIS ISR 2002-01.
• Echelle Blaze Correction for Spectral Offsetting
– Monthly spectral offsetting designed to spread wear on
detectors, but causes differential flux errors with
wavelength in each order (see Chuck Bowers’ talk).
– A version of the software fix by Lindler and Bowers now
in pipeline for primary echelle wavelengths.
– Monthly echelle spectral offsetting turned off starting
Aug 2002 (no excessive usage at individual detector
locations).
• Improved Echelle Dispersion Solutions (Lindler)
– Better accounts for monthly spectral offsets
Coming soon:
• Time dependent sensitivity for echelles.
• Better flux calibration at secondary wavelengths.
E1 Aperture Positions
Special CCD aperture positions defined at
row 900 to minimize CTE losses during
readout of 1st order spectra.
(4X fewer charge transfers => _ CTE loss).
Advantages:
• More counts in given time.
• Differential CTE losses distort spectra.
• CTE “tails” of cosmic rays and hot pixels
are substantially reduced near the top of
the CCD.
E1 Aperture Positions
Some disadvantages:
• Only ~12” between aperture bar top of
chip, so less room for extended targets.
• For point sources contemporaneous IR
fringe flats don’t work as well at E1
positions. If S/N > about 50:1 needed for l
> 7500 Å, then do observations near row
512. (But see Eliot Malumuth’s poster).
• E1 Position vignetting and aperture
throughput corrections not finalized, but
will be well before Cycle 12 Phase 2.
For most CCD spectral observations we
recommend using the E1 aperture positions.
Other Planned Calibration
Enhancements
• CTE correction formula (see Paul Goudfrooij’s talk).
• Better NUV-PRISM flux and wavelength calibrations
for off-axis data.
• Software tools to analyze slitless spectra.
• Better smoothing of 1st order spectroscopic backgrounds
• New low order flats for CCD spectral modes to improve
the flux calibration vs. position along slit.
• Improved pixel-to-pixel flats for all detectors.
• Better on-line selection of imaging and spectroscopic
PSFs for post-pipeline analysis.
STIS Splinter Session
• If you have comments for the splinter
session, see Jeff Valenti during break.
• Specview demo by Ivo Busko.
• Discussion of future calibration of STIS.
•
Average dark current and hot pixels slightly reduced because of
lower mean operating temperature, but otherwise consistent with
previous trends.
• CTE effects continue to increase (see Paul Goudfrooij’s talk).
Median dark current
Mean dark current
MAMA temperature variations driven by cycling of
low voltage power supply during SAA passages
NUV MAMA charge amplifier (solid) and tube (dotted) temperatures.
NUV MAMA dark current driven by window phosphorescence.
Model originally developed by Randy Kimble and Ed Jenkins.
•High energy particle impacts populate meta-stable states.
•Thermal collisions depopulate meta-stable states => UV photon