Status of the STIS Instrument after SM4
C. R. Proffitt1, A. Aloisi2, R. C. Bohlin3, W. V. Dixon4, P. Goudfrooij3, T. Gull5, D. Lennon2,
D. Lindler6, R. A. Osten3, M. A. Wolfe3, B. E. Woodgate5, & W. Zheng4
1STScI/CSC, 2STScI/ESA, 3STScI, 4JHU, 5NASA/GSFC, 6Sigma
Space Corporation.
ABSTRACT
The Space Telescope Imaging Spectrograph (STIS) was installed in the Hubble Space Telescope (HST) in February of 1997. It operated until August of 2004, when an electrical malfunction in a power supply
forced cessation of operations. On May 17, 2009, during the fourth EVA of SM4, astronauts Michael Good and Mike Massimino undertook an eight-hour spacewalk, during which they successfully repaired
STIS by replacing the STIS LVPS-2 circuit board containing the failed component. We review the scientific capabilities and operational status of STIS after this repair. In most respects, STIS operates in much
the same way as it did prior to the 2004 failure. The degradation of the STIS CCD due to radiation damage is consistent with extrapolation of previously-observed trends. Changes in optical throughput are
modest, and for most modes also continue previous trends. Internal and external alignments of the instrument are also similar to those of 2004. The biggest surprise is that the dark current for the NUV MAMA
detector is several times larger than had been expected and is only slowly decreasing towards its expected range. We discuss how these changes will affect science with STIS now and in the future.
Initial Tests during SM4
Immediately after the replacement of the new LVPS-2R circuit board
and the installation of the new cover for the electronics box containing it,
a brief aliveness test was conducted to verify the functioning of the new
circuit board. This verified that the new board could supply the voltages
necessary to operate the STIS mechanisms and heaters. Following the
return of the astronauts to Atlantis, a more extensive functional test of
the STIS instrument was conducted that exercised as much as possible of
the STIS electronics and mechanisms. This test showed that the STIS
instrument was still functional.
SMOV Plan and Execution
Following the release of HST after SM4, a period of Science Mission
Observatory Verification (SMOV) began. The primary goal for STIS
during SMOV was to verify that the instrument was ready to undertake
the planned GO science program. A number of the more time
consuming calibrations, such as detailed pixel-to-pixel flats, were
deferred until after SMOV period. The STIS SMOV plan is outlined in
Figure 1. Information on each of the individual SMOV programs can
be found at http://www.stsci.edu/hst/scheduling/program_information.
Spectroscopic Throughputs
The optical throughputs for most STIS modes have steadily declined
since launch. This is believed to be primarily due to the build up and
darkening of contaminants on the optical surfaces within the
instrument. Trends for the sensitivity of the low order MAMA G140L
and G230L modes over the history of STIS are shown in Figures 2 and
3. At most wavelengths, the G230L throughput in 2009 is close to what
would have been expected from a simple extrapolation of 2004 values,
while for most wavelengths of the G140L, the change from 2004 values
are very small.
The trends for CCD 1st order modes are shown in the
accompanying poster by Wolfe et al (poster 463.20). The G230LB
trends are very similar to those of G230L, while G430L and G750L
continue to decline linearly by about 0.28%/yr and 0.23%/yr
respectively. Trends for the medium resolution first order gratings are
similar to those seen for the low order modes.
Post-SM4 throughput changes for medium-resolution echelle
modes are consistent with the trends seen in the MAMA low-order
modes. However, the high dispersion echelle gratings appear to show
larger throughput declines than other STIS gratings (Figure 4). This is
complicated by small changes in the echelle blaze angles, and a larger
than usual focus excursion during the first E140H sensitivity monitor
observation (squares in Fig. 4). Analysis of additional data is ongoing.
NUV MAMA Dark Rate
The dark count rate of the STIS NUV MAMA is dominated by a
phosphorescent glow from the detector window (Ferguson & Baum
1999). Meta-stable states are populated by cosmic ray impacts, and
then, days or weeks later, are thermally excited to a state from which
they can decay, producing an NUV photon. On short time scales, the
dark count rate is controlled by the thermal transition rate, and varies
like e–(ΔE/kT), but when the detector has been cold for an extended
period of time, a larger number of meta-stable states become populated,
and when the detector is then warmed back up again there is a period
of enhanced dark rate until a new equilibrium is reached.
Figure 6: A simple model of the STIS NUV MAMA phosphorescent window
glow, shows the process by which meta-stable states in window impurities
are populated by cosmic rays and then de-excited by a thermally induced
transition to an unstable state.
Figure 4: For one spectral order of each echelle grating, we show the relative
throughput changes for sensitivity monitor observations of BD 28°+4211. Also
shown are piecewise linear fits to these echelle measurements (solid lines),
and similar fits to the same wavelength region of the corresponding low
dispersion modes (dashed lines). The square symbol in each panel highlights
the first of the two post-SM4 sensitivity monitor measurements.
Thermal Environment
Figure 1. The planned sequence of STIS SMOV programs are shown. Each
box gives the HST program ID, a brief description of the program, and the
dates on which it executed. The boxes with double-lined borders denote those
activities that included observations of external astronomical targets. Other
programs were entirely internal to the STIS Instrument. Activities primarily
devoted to calibration of the CCD detector are shown in green, those for the
FUV MAMA in blue, and the NUV MAMA in red. Orange boxes show those
activities that checked the external focus and alignment.
The activities shown in Figure 1 included a number of activities
designed to check the functioning of the STIS electronics and
mechanisms, as well as the alignment of the optics. Most of these tests
showed that performance was not significantly different from that in
2004. All of the gratings project spectra onto the same locations on the
detector, mechanisms work as before, and the stability of the optical
bench is similar to that seen previously. The absolute alignment of STIS
with the Fine guidance sensors (FGS) was found to be within 0.3” of the
nominal value. While this is more than sufficient for standard
acquisitions, it is comparable to the expected GSC2 guide star accuracy,
and in the coming months this alignment will be adjusted to further
improve the initial pointing accuracy.
The CCD detector continued to accumulate radiation damage
during the time STIS was inoperable. Because of the complexity of the
effects of radiation degradation on the STIS CCD, full details of the current
post-SM4 CCD detector performance are described in the accompanying poster
(463.20) by Wolfe et al.
One anomaly was seen in the STIS electronics. One of the four
redundant amplifiers (Amp B) of the STIS CCD developed a flaw in the
readout chain that causes a bit-slip in all images read out with this
amplifier - the least significant bit is lost. Similar behavior in this
amplifier had been seen twice in STIS ground testing in 1996, but the
fault has apparently now become permanent. Since all STIS CCD
science is done using the D amplifier, which has always had the lowest
readout noise, this flaw will have no effect on GO observations.
In the optical, most of the STIS calibration lamps have fluxes
very close to what was seen in 2004, however, in the far UV the
wavelength calibration lamps continued to show declining fluxes.
In the remainder of this poster, we will discuss those changes
that will have the most significant effect on observers.
Figure 2: The change in the throughput of the STIS G140L averaged over 50
Å bins is plotted as a function of time over the history of STIS. Data are from
sensitivity monitor observations of the star GRW +70º5824.
The STIS side-2 CCD control electronics do not contain a
working temperature sensor for the CCD detector itself. As a result,
instead of maintaining the detector at a constant temperature of –83 C,
the CCD thermoelectric cooler (TEC) runs at constant current, resulting
in fluctuations of the detector temperature, with corresponding
fluctuations in the detector dark current. The CCD housing temperature,
which is used as a surrogate for the actual detector temperature, now
averages ~2.5 C warmer than in 2004, and ~4.5 C warmer than in
2002. When combined with the accumulated radiation damage, the
result is a detector dark current that is about a factor of two higher than
it was in 2004 (see poster 463.20).
The STIS MAMA detectors are also slightly warmer, averaging
about 1.1 – 1.3 C higher in the fall of 2009 than they had in fall 2003.
FUV MAMA Dark Rate
The FUV MAMA detector’s dark rate is much lower than that of the
NUV detector, but it is also much less predictable. When the detector
high-voltage is first turned on after being off for several hours, the mean
dark rate is as low as 5x10–6 counts/pixel/s, (6 counts/s over the entire
detector); this increases sharply as the detector high voltage remains
powered, increasing by up to a factor of ~20. Between 1997 and 2004,
the upper envelope of the dark rate had been increasing over time (see
Figure 5). The magnitude of the post-SM4 FUV dark current is very
similar to that seen in 2004, but by analogy with past behavior we expect
that the upper envelope will continue to increase over time.
The model of the phosphorescent window glow used prior to the
2004 failure predicted that after SM4, the dark rate would be initially
enhanced by a factor of ~ 2 to 3 above the expected long term
equilibrium mean rate of about 0.0013 counts/pixel/s, but would come
back to near the equilibrium rate within two to three weeks. Instead the
dark rate was found to be enhanced by a factor of about 10, and it seems to be only
slowly decreasing back towards the expected range (see Figure 7).
Figure 7: The predictions of the pre-SM4 NUV dark model are shown by the
orange curve. Filled circles show the measured dark current since the NUV
MAMA recovery after SM4. Adding to the orange curve an extra term
proportional to e–ΔE/kTe–t/τ, with τ =120 days results in the curve shown in red,
and for τ = ∞ the curve in blue.
It appears that the model for the window glow presented in figure 6 is
overly simplified. Multiple states with different time scales appear to be
contributing to the glow, and the longer time scale states appear to be
contributing a much larger fraction of the counts now than they had
prior to SM4.
A ~ 120 day decay time scale for the new component fits the
data well. As of early December 2009, the mean dark rate was about
0.006 counts/pixel/s. If the current trend continues, the mean NUV
MAMA dark count rate should be ~ 0.002 counts/pixel/s by mid-2010.
For programs observing sufficiently bright targets with the NUV
MAMA, the impact will be minimal, but observations of targets with a
count rate comparable to the background can be significantly impacted.
For example, with a dark rate of 0.0013 counts/pixel/s, an E230H
echelle observation at 2300 Å of a source with a flux of 10–13
ergs/cm2/Å/s would require about 4500 s to reach a S/N of 10 per
resolution element, but 14000 s would be required if the dark rate was
0.011 counts/pixel/s.
References
Figure 3: The change in the throughput of the STIS G230L averaged over
100 Å bins is plotted as a function of time over the history of STIS.
Figure 5: The measured values of the STIS FUV MAMA dark rate, integrated
over the whole detector, are shown as a function of observation date. The
size of each symbol scales with the amount of time the detector high voltage
has been powered, with the largest values corresponding to ~ 7.5 orbits. The
orange symbols show the data taken after the switch to the STIS side-2
electronics, and the green symbols show the post-SM4 measurements.
Ferguson, H. & Baum, S. 1999, STIS Instrument Science Report 99-02
(Baltimore: STScI)
Goudfrooij, P., et al. 2009, STIS Instrument Science Report 2009-02
(Baltimore: STScI)
Wolfe, M. et al. 2009, 215th AAS meeting, poster 463.20