TECHNICAL
REPORT
Title: NIRSpec Dithering Strategy Part 3:
The Microshutter Array (MSA)
Authors: J. Tumlinson
1.0
Phone: 410338-4553
Doc #:
JWST-STScI-001769, SM-12
Date:
June 3, 2009
Rev:
-
Release Date: 3 September 2009
Abstract
This study concerns need for dithering of NIRSpec data, the strategies for obtaining
dithered data, the effect of dithering on data calibration, and the effect of dithering on
mechanism lifetimes. For simplicity the study has been broken into three parts for each of
NIRSpec’s major observing modes (Fixed Slits, Part 1; IFU, Part 2; MSA, Part 3).
The very high degree of configurability of the MSA allows the user to realize a slit mask
for an arbitrary set of positions on the sky. Departures from an “ideal MSA” caused by
failed open or closed shutters ensure that a given configuration cannot be shifted by a
fixed offset in either dimension and always recover the same pattern of open apertures.
For the MSA, we conclude that there is only a small set of restricted circumstances in
which predefined dither patterns will be feasible. In particular, if the user has chosen to
place a “slitlet” composed of multiple open shutters on each science target, then dithers
between the open shutters are straightforward and can be standardized in advance for a
specific slitlet shape. This strategy should be effective for a wide range of programs
directed at faint point or marginally resolved sources. Predefined dither patterns will
generally not work for programs that choose to depart from this simple strategy, or for
programs that intend to observe large extended objects. In these cases the burden will fall
on the user and on the MSA planning tool to define separate MSA configurations that
accomplish the purpose of dithers by obtaining data on a given set of targets at different
positions on the array. To capture this general case with a minimum of complexity,
overheads, and mechanism movements, we recommend the implementation of a
capability to observe multiple MSA configurations that refer to the same target
acquisition and guide stars but which are separated by small angle motions (~< 20”).
2.0
Introduction
NIRSpec data needs dithering for a number of important reasons. First, the PSF of the
JWST OTE is undersampled at the FPA, where each 18 µm pixel spans 0.1˝ on the sky
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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compared with a PSF FWHM >~ 0.1 – 0.2˝ over 1-5 µm. Dithering by small spatial
offsets (< 0.1˝) can improve the spatial sampling of the PSF. For many observations
dithering with a larger offset will be needed to cover the gap in wavelength caused by the
~ 3 mm (17.8”) gap between SCA segments. Finally, dithering can reduce noise
associated with pixel-to-pixel sensitivity variations by obtaining measurements of the
same point on the sky at different places on the detector, averaging out the variations.
This report assumes the small angle motion (SAM) accuracies stated in Section 3 of the
fixed-slits and IFU chapters (JWST-STScI-001678 and JWST-STScI-001749
respectively).
3.0
MSA Properties: Slit Losses, the Sweet Spot, and Dithering
The Microshutter Array (MSA) is the key technology that enables multi-object
spectroscopy for NIRSpec. The MSA is a micro-electro-mechanical (MEMS) device with
individually addressable shutters in 365 rows and 171 columns on each of 4 quadrants
(for a total of 249,660 shutters). Each shutter has a rectangular open area that subtends
200 mas in the dispersion direction and 450 mas in the spatial direction. The “walls” that
separate the open areas of the shutters subtend 60 mas in each direction, so the separation
between the shutter centers (the “shutter pitch”) is 260 mas in the dispersion direction and
510 mas in the spatial dimension (this is within 4% of a 2:1 ratio for height:width).
The “slit losses” caused by the MSA shutters are a key factor in planning MSA
observations and in designing their dither patterns. A combination of geometric blocking
and diffraction of light by the MSA causes loss of light that becomes worse near the
edges of each shutter aperture. Slit throughput achieves a maximum value of ~60% near
the center of each shutter and drops off to ~20% at the edges, and depends also on
wavelength as shown in Figure 1.
Figure 1: Slit losses for the MSA shutters as a function of distance from center for the x (dispersion)
and y (spatial) dimensions. The slit losses with respect to the maximum are relatively flat in the
center of the apertures.
For spectrophotometric measurements loss of light at the slit must be corrected by
dividing out the slit loss function. Because the exact position of the science target within
the slit is only known to within some small positional uncertainty (owing to target
acquisition errors and jitter), this correction carries some uncertainty. This uncertainty
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becomes severe when the target is near the edge of the slit and a small positional error
translates to a large uncertainty in the slit loss correction.
To minimize the uncertainty in the slit-loss correction and to maximize the slit
throughput, NIRSpec’s designers have introduced the concept of a “sweet spot” within
each shutter where the uncertainty in the slit loss correction caused by errors in
knowledge of the target position are constrained to < 10%. The sweet spot occupies the
central 40-60% of the shutter area, depending on wavelength (see Figure 3).
The concept of the sweet spot drives much of NIRSpec’s observing and dither strategies,
since it plays a large role in the overall throughput and photometric accuracy of the data.
4.0
“Slitlet” Dithers for Maximum Throughput
NIRSpec plans to routinely obtain spectra of astronomical
sources near the faint limits of its sensitivity – objects with
fluxes of 100 – 300 nJy, such as faint, high-redshift galaxies
or main-sequence stars in the outer halos of galaxies. For
these science cases maximum total instrument throughput is
required, so it is desirable for every science target to be
observed within the sweet spot of a microshutter. This
requirement entails that the proposal planning system take
into account the geometric filling factor of the sweet spot
within each shutter, and break the input target list into as
many target subsets as are needed to cover the field of the
sky under study.
Even if each science target is observed within a sweet spot,
it is still desirable to place the same science target on
Figure 3: “Sweet spot” for a
different detector pixels for all the usual reasons to dither – single MSA shutter. From
to correct for pixel-to-pixel sensitivity variations and dead NIRSpec OCD.
pixels, and to improve the pixel sampling of the point- and
line-spread functions. To accomplish these purposes and to maintain the high throughput
offered by the sweet spots, the NIRSpec Instrument Science Team has defined a “slitlet”
dithering strategy, illustrated in Figure 4. To implement this strategy, the MSA planning
tool should place a 3x1 shutter “slitlet” for each science target in each subset. Three
separate exposures will be obtained for each target, in the sequence illustrated in the right
panel of Figure 4. For each exposure the two unoccupied shutters will obtain data on the
local sky background. The small-angle motion (SAM) between each shutter will cover a
full shutter pitch in the y-dimension (510 mas). This strategy places the target in the same
point on the shutter in each exposure so that the slit loss is the same for each exposure
(though small changes will result from pointing errors and jitter). Since the target is
always in the sweet spot, the average slit throughput is maximized.
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Figure 4: At left, the MSA plane with five “slitlets”. At right, the observing sequence for the 3-shutter
slitlet. Unoccupied shutters obtain data on the sky background.
The interleaving of the MSA configurations, grating/filter wheel moves, and dithers are
designed to minimize mechanism movements. We intend to conserve MSA
reconfigurations as the highest priority, so all dithers and all gratings will be obtained
before moving to the next MSA configuration for the next target subset. It is also
important to minimize the number of GWA/FWA movements, so each dither position in
the slitlet should be observed with a single GWA/FWA position before changing the
grating (as for the fixed slits). If conceptualized as “nested loops”, the ordering would be:
- MSA configurations (the “outer loop”, executed least frequently)
- Changes of grating / filter
- Small-angle motions to dither (the “inner loop”, executed most frequently)
A variation of this strategy can be used to improve the sampling of the line-spread
function of the NIRSpec gratings. In this variation each target is assigned a 3x2 shutter
window and the slitlet pattern illustrated in Figure 4 is executed twice: once as shown in
Figure 4 with the adjoining 3x1 slitlet closed, followed by an MSA reconfiguration, and
then again in the right 3x1 side of the 3x2 window with the left 3x2 slitlet closed. This is
equivalent to moving the complete 3x1 pattern over by exactly one shutter pitch and
repeating the A-B-C pattern shown in Figure 4. The advantage of this repeated pattern is
that it occurs 260 mas or 2.6 pixels away from the first pattern, and so effectively
resamples the line-spread function of the grating with net 0.6 pixel separation. This
variation is intrinsically less efficient overall since it requires 3x2 shutter clearing in the
MSA planning tool, the number of targets that can be placed in a single configuration
declines accordingly, and so the total number of target subsets must increase to cover the
same field. The number of MSA configurations required to obtain a given target set is
also effectively doubled by the shift of the pattern over by one shutter pitch. However, for
some programs these inefficiencies will be worth the improved sampling1.
1
As noted in Part 1 (Fixed Slits), curvature of the iso-wavelength lines caused by optical distortion across
the field mimics the effect of small spatial and/or spectral dithers. Thus two spectra taken with identical
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Since the “slitlet” approach uses three shutters in the spatial direction, and moves each 3shutter slitlet by two shutters in each direction, five shutters worth of sky in the spatial
direction will be observed. This may present problems for background subtraction, if a
bright object or another target of interest falls within this 5-shutter region. Unintended
light in the background shutters is not necessarily fatal to the background subtraction, but
would present problems. To deal with this problem the planning software could either
explicitly clear the 5x1 shutter region of the sky, or just mark it so that the user could
check for problems. In most fields a couple of contaminated background shutters may not
be prohibitive, but severe crowding of sources may force the user to define MSA
configurations that depart from this simple slitlet scheme and use the more general
schemes outlines in Section 7.
5.0
“Subshutter” Dithers for Efficient Multiplexing
The “sweet spot” observing strategy and the simple dither pattern that goes along with it
excel at obtaining data with maximum average throughput. However, since this strategy
uses only a portion of each shutter’s area it is less efficient at covering a field of targets
than a strategy that uses the full open area of the shutters. The cost is that a larger number
of MSA configurations are needed to complete a given set of targets in a field. Observers
who can tolerate a loss of average throughput or who desire greater photometric accuracy
may instead opt to pursue a different strategy for configuration and dithering, which we
will call “subshutter” dithering (Regan 2005).
In this strategy, targets are placed by the MSA planning tool on any available shutter
whether they fall into the “sweet spot” region or not. Then a set of exposures is obtained
using dithers on a regular grid (4 x 4, 3 x 3, etc.) with total extent of one shutter pitch in
each direction (see Figure 5). In principle this approach should lead to lower errors in the
slit loss correction to the flux calibration, since the target will be observed on both sides
of the slit loss curve. This strategy also in principle reduces the number of target subsets
that are needed to cover a given field by using the full open extent of each shutter.
The regular pattern of subshutter moves suggests that this observing strategy can be
executed with a fixed and pre-programmed dither pattern. Simple modeling of this
approach is adequate to show that this is not so. Figure 5 illustrates a simple field
observed with this strategy. The initial location of each target within its shutter is at
position 1. Each of the sixteen positions would be observed in turn using small angle
dither motions between each exposure. Since some targets will move into adjoining
shutters for some subset of the 16 positions, more than one MSA configuration is needed
to realize this strategy. If only one open shutter is assigned to each target, then up to 16
different configurations may be needed. The number of unique configurations can be
reduced if each target is assigned a 2x1 shutter open aperture – this allows positions in a
single column (i.e. positions 1 – 4) to use the same configuration and leaves open a “sky
shutter” adjoining the target shutter for each of the 16 positions. For the illustrated case, 4
conditions but at different y-positions will have slightly different pixel sampling of the PSF and LSF.
However, the magnitude of these effects are not known for small offsets and may not be sufficient to
accomplish the resampling that deliberate moves can achieve.
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unique configurations are needed to cover the 16 positions with 2x1 shutter aperture
assigned to each target. The four configurations cover positions 1-4 (first configuration),
5-8 (second configuration), 9-12 (third configuration), and 13-16 (fourth configuration).
Depending on the configuration of targets on the sky, the smallest, most efficient number
of unique configurations can range from 1 to 16 just for the simple choice of 1x1 shutter
apertures and a 4 x 4 pattern.
The upshot of this exercise is that for the subshutter strategy there is no pre-programmed
optimal set of positions and configurations that will work for every program. The exact
minimum set of configurations and their correspondence to the offset positions can be
worked out only when the exact set of targets is known – that is, at the time the
observation is planned. The range of possibilities is driven by the user choice to adopt a
specific pattern (3 x 3 or 4 x 4 moves to cover a shutter pitch) and by the actual positions
of the targets on the sky.
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Figure 5 An example of subshutter dithering for a field of six targets observed with a 4 x 4 subshutter
dither pattern. The sixteen dither positions for each of the six targets are numbered as shown in the
inset. This set of targets and dither positions requires four configurations; more general fields could
require up to 16.
The subshutter dithering strategy therefore places the burden of constructing dither
patterns onto the proposal planning system. Each program will want to observe a unique
set of targets, which will require a unique set of MSA configurations and the
corresponding sequence of small-angle motions that can only be worked out by the MSA
planning tool. The tool may be able to speed the process significantly by, for example,
automatically generating the set of configurations and dithers given the targets and user
choices for the size of the subshutter grid.
6.0
The “Wavelength Gap”
The ~19” gap between detector segments causes a gap in wavelength coverage for spectra
obtained with the R = 2700 grating. A simple remedy for this gap is to shift the pattern of
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targets on the MSA by 20” (approximately 100 shutters) to shift the missing wavelengths
onto the detector. At first blush this dither appears so simple that if anything could be
implemented as a pre-programmed dither, it would be this one. However, two features of
the MSA prevent even this simple move from being defined uniformly for all cases. First,
optical distortion of the field at the MSA plane ensures that while some targets would
move 100 shutters over on the array for a move of 20”, others would move 99 or 101 or
100.54 shutters. Some target might move from the sweet spot of an open shutter to a
failed closed shutter, or to a point behind an MSA wall. The cost is especially high if it is
a high-priority target that is lost. Thus even for a simple move we cannot guarantee that
all targets will be observed at both positions if the optimization is derived for only one of
them, and that the wavelength gap will be filled for all.
In this case the only recourse may be to perform two separate optimizations of shutter
placement with different center positions and for the user, perhaps assisted by the
software, to ensure that the missing wavelengths are obtained at one or more of the
positions. Thus we reach the same conclusions as for the “subshutter” dithers: unique
MSA configurations linked together with small angle motions in between is required to
achieve the purpose of dithers in light of a non-ideal MSA.
7.0
Generic Dithers for Extended Objects and Difficult Cases
The configurability of the MSA will enable new types of spectroscopic observations that
have never been achievable in space. The diversity of astrophysical targets – from
crowded fields of point sources to extended sources of arbitrary shape – means that we
cannot plan in advance a canned set of dither patterns that will serve the need of every
user in every case. For users with typical fields and standard requirements one of the two
major strategies above will suffice, but even these will not admit one-size-fits-all dither
patterns. For “hard cases”, we must enable another approach that still uses small-angle
motions (as dithers) to combine multiple MSA configurations within a single visit.
To implement this capability, we envision the MSA planning tool providing the option of
linking together uniquely defined configurations into a custom pattern of configurations
separated by small-angle motions, provided the whole pattern satisfies some constraint on
the maximum angular separation. This approach is illustrated with a “hard case” in Figure
5, where we use the Cas A supernova remnant as an example of a complex extended
source. In this schematic, three unique MSA configurations have been defined separately
to cover bright knots of emission in the remnant. Because the center point of the MSA
(the yellow stars) lie within a 20” circle (that is, their maximum separation < 20”), these
three settings can be observed with the same guide stars and only small angle motions
between them. This particular set of knots cannot be observed with a single configuration
because it would require use of two shutters in the same column (along the dispersion
direction), which is disallowed because the spectra would overlap. For example,
observing the knots marked A, C, and E requires a different configuration from B, C, and
D, since having, e.g. shutters for knots A and B open simultaneously would yield
overlapping spectra if observed in the same configuration. In addition to this, small angle
motions may be required to place the desired set of bright knots optimally in their
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corresponding shutter, that is, a single position cannot capture their positions optimally,
even with different configurations. In this general case, multiple configurations at
multiple positions would be required. If for some reason the user desires additional
exposures with small motions of order one shutter at each one of the marked center
positions (for instance, those described as “subshutter” dithers above) these count as
separate positions as well. Once the desire and/or need for custom sets of configurations
and center points is established, it can easily accommodate nested patterns (analogous to
“primary” and “secondary” motions from the FS and IFU chapters) within the generic
scheme.
The alternative is for these three configurations to be executed as separate visits, with the
additional overhead of a new target acquisition and definite loss of efficiency. The
baseline observing scheme already allows for multiple MSA configurations centered at
the same point on the sky, within a single visit. This flexible dithering plan enables an
arbitrary set of targets over a small area and adds only a small degree of complexity
associated with the small-angle motions between the center points.
Though these closely spaced MSA positions and their corresponding configurations arise
from considering dithering strategies, and may prove to be very similar to dithers
operationally, they differ in many respects from the typical usage of the term “dither”.
For example, the data reduction pipeline will not always be asked to combine data from
these different positions / configurations as is often done for dithered data. The different
positions may request different exposure times, while traditional dithers use the same
exposure at each position. For these reasons it is probably best to devise a different term,
(“cluster target” is one possibility) for these patterns to be used in APT and in the user
documentation, to avoid confusion.
While this approach to extended sources by dithering does require flexibility in the
planning of the observation, it need not add additional risk to the program. The exact
pattern of moves and even their sequence can be derived uniquely and correctly by the
planning software and passed downstream after checking for errors and constraints. There
is no need for the user to specify the size, direction, and sequence of moves, if APT can
derive these from the desired set of MSA configurations and center positions.
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Figure 6: A schematic view of how an observer might use the MSA to explore an extended source of
arbitrary shape. There are three MSA configurations shown that have unique shutter open/closed
patterns and different central positions. Provided the central positions of the MSA lie within a circle
of diameter somewhat than the FGS field (say, less than 20” as a rule of thumb) the three positions
can be treated as a custom dither pattern, derived by the software based on the users MSA setups.
Note: the real Cas A is very large with respect to the microshutters, but the principle holds.
8.0
Conclusions and Recommendations
Conceptual development of the various dithering strategies proposed for NIRSpec
observations has revealed that even simple dither patterns are always difficult and often
impossible to design in a predetermined fashion. It does not appear possible to pre-define
dither patterns that will work for all programs, or even a representative subset of
programs, while also ensuring that each target of interest will be observed at each and
every dither position. Even if such canned patterns were possible, the desire of many
users to observe extended objects or use long slits drives us toward a different
conclusion: namely, that dither patterns for MSA observations should be constructed by
the MSA planning tool in tandem with the configuration themselves. Furthermore, once
this conclusion has been adopted it is probably most efficient and simpler to
conceptualize and implement all of the strategies in this fashion. Put another way, there is
no reason for the downstream ground system and flight software to know that it is being
asked to execute the 3x1 slitlet strategy or the 4x4 subshutter strategy provided it receives
a list of MSA configurations and their corresponding positional offsets and knows how to
act on these in the proper sequence. This approach places the hard work of defining
patterns into the proposal planning system. It is the effectively infinite complexity of the
MSA as a device and the idiosyncrasy of each device as built that forces these
conclusions.
Our recommendations for NIRSpec MSA dithering strategies and implementation are:
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1) We recommend that NIRSpec MSA dither patterns be defined within the
MSA planning tool as linked sets of MSA configurations and the
corresponding offset positions.
2) We recommend that the linked sets of MSA configurations and offsets be
passed downstream in a fixed formation that is common to all of the dithering
strategies here, since they are all cases of a generic strategy.
3) We recommend that APT include the specific strategies and special-purpose
dithers presented here as “shortcuts” or tools to assist the user in defining the
necessary set of configurations. For many strategies the exact pattern can be
derived deterministically for a given set of targets and so the software can
relieve the user of the need to define completely unique and separate
configurations once the basic strategy is chosen and the targets are known.
The exact strategies and how they will be implemented require further study
and quantitative modeling with the ongoing MSA optimization study.
4) We recommend that downstream systems and scripts adopt a common
approach to receiving MSA configurations and offsets from the planning
system in a generic way, so that a single algorithm can implement these
various strategies based on the received configuration files and offsets without
knowing the exact strategy (slitlet, subshutter, etc.) that they correspond to.
9.0
References
Böker, T. and Valenti, J. 2008, “NIRSpec Operations Concept Document”, v5.0
Regan, M., 2005 “An Alternative Observing Strategy for NIRSpec and its Effect on
NIRSpec Target Acquisition”, JWST-STScI-000674
Tumlinson, J. 2009 “NIRSpec Dithering Strategies, Part I: The Fixed Slits”, JWSTSTScI-001678
Tumlinson, J. 2009 “NIRSpec Dithering Strategies, Part II: The Integral Field Unit”,
JWST-001749
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