TECHNICAL
REPORT
Title: NIRSpec MSA Short Detection and
Mitigation
Authors: J. Tumlinson
Phone: 410338-4553
Doc #:
JWST-STScI-001736, SM-12
Date:
April 23 2009
Rev:
-
Release Date: 22 July 2009
1.0
Abstract
Group on how to proceed with the implementation of command scripts for short detection
and mitigation The Microshutter Array (MSA) is the key technology that enables multiobject spectroscopy for NIRSpec. Each of the four MSA quadrants is a micro-electromechanical (MEMS) device with individually addressable shutters in 365 rows and 171
columns (for a total of 62415 shutters per quadrant). Testing indicates that four types of
electrical shorts may occur: between a row and an adjacent row, between a column and
an adjacent column, between any row and any column, and a “tri-state” short. These
shorts are expected to be rare during flight operations. However, because they prevent the
affected shutters from being commanded open or closed individually, these shorts have a
non-trivial effect on science operations. This document describes these operational
impacts and the decisions of the NIRSpec Ops Working.
2.0
Primer on How the MSA is Addressed
The MSA is a cryogenic micro-electro-mechanical (MEMS) consisting of 365x171 “egg
crate” apertures, in 365 rows and 171 columns. The 365 “back side” row electrodes are
addressed with a large positive voltage +Vpp and the 171 “front side” column electrodes
are addressed with a large negative voltage –Vpp1. For “open” state, the voltages are
commanded to +Vpp for the columns or –Vpp for the rows, while for “closed” they are
commanded to 0V on both sides. When the MSA is ready to be configured for a science
observation with an arbitrary set of “open” and “close” shutters, the voltages are all
commanded to +Vpp or –Vpp, followed by a sweep of the magnet arm across the array to
latch all the shutters open. Then, the shutters that are to be closed during the science
observation are commanded back to 0 V while the magnet is again swept across the array
1
The exact numerical value of the voltage is individually optimized for each array, but typically falls into
the range ±12 – 36 V.
Operated by the Association of Universities for Research in Astronomy, Inc., for the National
Aeronautics and Space Administration under Contract NAS5-03127
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to close them and leave open those shutters still latched to their egg crates where open
apertures are desired.
For more details on this procedure, see the NIRSpec Ops Concept Document §4.2.
Figure 2-1 Schematic MSA layout with row-to-row and column-to-column shorts illustrated
3.0
Short detection and mitigation
Shorts matter for science observations because they will result in a failure of the MSA to
realize the exact MSA configuration that was intended by the observer, with the effect on
the science observations depending on the type of short. Four types can occur on the
MSA: row-to-row shorts that pass current between two adjacent electrodes on the 365
side, column-to-column shorts that pass current between two adjacent electrodes on the
171 side, row-to-column shorts which pass current between a 365 side electrode and a
171 side electrode, and “tri-state” shorts2.
Row-to-row shorts: When these occur adjacent rows will be opened and closed together,
so that when a given shutter is commanded to open a shutter in that same column but one
of the adjacent rows will be open as well. This is quite pernicious since “rows” lie along
the “spatial” or cross-dispersion axis of the MSA. There will therefore be a 2-shutter
aperture open in the dispersion direction where only one was intended, so that the data
obtained in that column will be a mixture of science target and nearby sky. See the left
panel of Figure 1 for the layout.
Column-to-column shorts: When these occur adjacent columns will be opened and closed
together, so that when a given shutter is commanded to open a shutter in that same row
but one of the adjacent columns will be open as well. This type of short is still a problem
for science observations, but less severe than the row-to-row shorts since it works in the
2
As far as we know right now, any column can be shorted to any row, but non-adjacent row and/or
columns cannot short together. In any case, shorts of all types are expected to be rare.
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cross-dispersion direction. However, as shown in the figure it can be a serious problem if
a shutter in the adjacent column but a very different row was commanded open for
another unrelated target. There may then be two spectra on the same detector pixels,
which is undesirable for science observations. See the right panel of Figure 1 for the
layout.
Row-to-column shorts: These shorts are also called “front-to-back shorts” since they
connect one of the 171 “front side” electrodes to one of the 365 “back side” electrodes.
Because it involves electrodes from two different sides, the voltage difference between
the row and column can get as high as 72 V during MSA addressing, which may generate
enough current to be detected by the MCE and generate an error and place NIRSpec in
preferred safe. This outcome is very undesirable, so row-to-column shorts will need to be
investigated in flight and masked even though their science impact on the usability of
shutters is no greater than for the shorting of adjacent rows or columns. Row-to-column
shorts can be isolated using the same algorithm as the previous two cases.
“Tri-state” shorts: These are shorts that do not fall into the simple category of either
adjacent 365-side, 171-side shorts or front to back (row to column shorts). They may
result from shorts between surfaces other than the 171 or 365-side electrodes, and so they
may not be of clear origin. These shorts can be mitigated by a “tri-state” mask that
disconnects the offending electrode from the driver by placing it in a high impedance
state.
4.0
Operational aspects of short detection and mitigation
The NIRSpec Ops Working Group (OWG) discussed the nature and impact of shorts at
length and arrived at a consensus procedure for their detection and mitigation. The
microampere (µA) currents that indicate shorts on the array are generally too low to
trigger automatic health and safety warnings and instrument safing. The impact of shorts
is therefore operational, and it is not absolutely necessary to identify and mask them to
continue with normal operations. The decision to actively search for and mitigate shorts
will need to be made on the ground by instrument scientists weighing the science impact
of a short against the impact on the planning and scheduling of MSA observations due to
the loss of shorted rows and/or columns and the time required to find and mask them.
Engineering advice in April 2009 indicates that shorts of all types should be uncommon
(perhaps a few to 10 over years of operation) and should become more so as the device
matures (i.e., the shorts that can occur on a given device will occur early and the rate of
change will decline over time as the device matures). We now lay out the considerations
that should be taken into account when operational decisions about shorts are made.
Short notification: The first indication of a problem will usually be the appearance of an
anomaly either in science data with unanticipated open shutters or in trended currents
delivered by telemetry. Since the downlinked and processed data will not automatically
contain the information needed to compare the actual MSA configuration with the
intended one, science data will reveal shorts only if each exposure is examined carefully.
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The random probability of using a particular row is of order 1 / 365, so hundreds of
science observations will need to be searched or monitored for a single row-to-row short
to be noticed. Approximately half of all columns may be used in a random configuration,
so the chance of serendipitously discovering a column-to-column short is somewhat
higher, but still science observations would have to be continually and closely monitored
for shorts to be detected in this manner. Because of the randomness in this process it will
not necessarily discover shorts efficiently, so we recommend that an efficient scheme for
trending the relevant currents (from the MSA driver telemetry) be devised. Once it is
suspected from telemetry that a new short has occurred, recent science observations can
be checked for corroborating evidence and for an indication of the type of short and its
location.
Short Detection: Once a short is suspected, a visit to execute the short detection and
mitigation scripts can be scheduled (see §2.2.4 of the NIRSpec Operations Requirements
Document). This comprehensive script has the goal of detecting shorts of all types and of
automatically applying “zero-potential” masks to 365 and 171 side 0 V shorts, for a
single quadrant at a time3. The script will also detect the more difficult “tri-state” shorts
that arise from shorting other than adjacent rows/columns or simple back-front, rowcolumn shorts. The inner details of the script are described in the requirements document,
only the algorithm and its effects are described here.
First, the script begins phase 1 by checking that there are elevated currents associated
with the 365-side electrodes (rows). If so, the script looks for row-to-row shorts by
stepping through each row individually and testing the current when each row is
commanded individually. When current is associated with a particular row, that row is
flagged for later masking and the next row is tested in a loop until all are tested. If more
than 10 new rows are flagged, NIRSpec has likely experienced a serious problem and the
script will exit with instructions for NIRSpec to enter preferred safe mode. If less than 10
new shorts are detected, they will be masked with the “zero-potential” mask and the
script will move on to looking for column-side shorts. In this phase 2, as before, the first
step in this search checks for elevated current associated with the 171-side (columns). If
there is elevated current, an analogous column-by-column search will be executed and
shorted columns will be masked. The same exit criterion of ten new shorts applies. At the
end of phases 1 and 2, new 365-side and 171-side shorts (including row-to-column, With
this mask, the shorted rows and/or columns will be commanded closed and will be
unavailable for science use.
At the end of phases 1 and 2, the row and column currents are measured again. If they are
below the threshold, the script exits safely with the shorts masked out and normal
operations can continue. If the current is still elevated, the script will move on to search
for tri-state shorts.
3
The MSA quadrant to check is an input parameter, and multiple quadrants can be searched in separate
visits, but the need for this should be rare.
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In phases 3 and 4, the script applies the tri-state masks to the 365 side and searches for
shorts row-by-row, and reports the offending column if one is found. Phase 4
immediately follows and performs the same search for the 171 columns. These shorts
may indicate more serious problems, and disconnection of the shorted electrode can
subject it to charging from its environment, so automatic masking is ill-advised. Instead
of automatically masking these shorts as in Phases 1 and 2, the tri-state searches end with
a return to the previously operational tri-state mask and the location of the shorts and the
associated currents are reported in telemetry for investigation on the ground. If the shorts
are not detected and masked by phases 1 and 2, the script will run both 3 and 4 and report
the results. At the >10 news shorts exit point from phases 1 and 2 and at the end of phase
4, NIRSpec will either return to nominal operations or enter preferred safe mode,
depending on the value of an input parameter. This parameter will need to be set
individually for the visit based on the judgment of instrument scientists and engineers
about the severity of the problem. The sense of the OWG in April 2009 is that this
parameter will typically be set to force NIRSpec into safe mode since more than 10 new
shorts or any undetected tri-state short likely indicates a potentially serious problem that
deserves ground intervention and a careful diagnosis. For some ground testing purposes
and for other unanticipated purposes it is desirable to retain some control over the safing
of the instrument at these exit points, even though the decision will usually be to enter
safe mode.
As pointed out in Section 3 above, column-to-column shorts are tolerable provided they
are known and can be planned around. Row-to-row shorts are more compromising to
science data, but present little trouble to, e.g. science targets that are much brighter than
the sky. Since these shorts represent no safety concern, whether we mitigate them or not
is mainly a judgment about science impact that will need to be made in the moment.
Whether shorts are masked or not, their presence will need to be incorporated into the
“failed shutters” maps used by the proposal planning systems to help users plan
configurations around imperfections in the MSA. If short diagnosis is desired for
incorporation into the planning system, but masking is not, then the zero-potential
masking applied by the main script described above can be undone immediately after in
the same visit by uploading the “old” mask with a separate script (NIRSpec Ops Req.
Document §2.2.5). This same script can be used to upload a new “tri-state” mask when
one is needed. This script can be used together with and immediately after the detection
script, or on its own. This scheme was adopted to enable separate diagnosis and masking
of all types of shorts, to retain flexibility in scheduling these visits, and to minimize their
impact on observatory operations.
5.0
Conclusion
NIRSpec MSA shorts are expected to occur, but not very often. Testing indicates that
they will come less frequently for a given quadrant over time. Because they represent no
immediate health-and-safety issue, many shorts can be tolerated and their existence can
be included in the APT MSA planning tool. Others must be masked and the loss of their
shutters fed back into the PPS system. The NIRSpec command scripts are being
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implemented as described here to provide the necessary diagnostics and repair tools with
a reasonable amount of flexibility and minimal impact on science observations.
6.0
References
Böker and Valenti, NIRSpec Operations Concept Document, JWST-OPS-003212, v5.0
NIRSpec Operations Requirement Document
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