TECHNICAL
REPORT
Title: NIRSpec Dithering Strategy Part 1:
The Fixed Slit Apertures
Authors: J. Tumlinson
Phone: 410338-4553
Doc #:
JWST-STScI-001678, SM-12
Date:
March 6, 2009
Rev:
-
Release Date: 12 May 2009
1.0
Abstract
This study concerns the need for dithering of NIRSpec data, the strategies for obtaining
dithered data, 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 (Part 1:
Fixed Slits; Part 2: IFU; Part 3:MSA).
2.0
Introduction
NIRSpec data needs dithering for a number of important reasons. First, the point-spread
function of the JWST OTE is undersampled at the NIRSpec Focal Plane Array (FPA),
where each 18 µm pixel spans 100 mas on the sky compared with a PSF FWHM ~ 38 –
163 mas over 1-5 µm. Dithering by small spatial offsets (< 100 mas) 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
devices. 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.
For NIRSpec, finding the optimal dithering strategy is complicated by the complexity of
operating the instrument, particularly the MSA. Also, the usage of the mechanisms for
positioning the filter wheel (FWA) and grating wheel (GWA) can increase depending on
the number of dither positions that will be observed per visit (see Section 8.2 of the
NIRSpec Ops Concept Document). It may not be possible to optimize data quality from
dithering while also minimizing the number of instrument mechanism moves, so we will
need to find a balance between these competing desires.
3.0
Accuracy of Small Angle Motions
From the mission Interface Requirements Document, the JWST slew accuracy
requirements for small-angle motions are (per axis): 5 mas up to 0.5”, linearly increasing
between 5 mas and 20 mas from 0.5 – 2” (1% of the motion), 20 mas from 2 – 20”, and
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90 mas at 20-30”. Informal guidance from Ed Nelan indicates that actual performance
will exceed these requirements, so that small angle maneuver accuracy will be 5 mas for
moves up to 0.5”, linearly increasing from 5 to 10 mas from 0.5 to 2”, and then 10 mas
from 2 – 30” (all quoted accuracies are per axis). Thus positioning error associated with
small angle maneuvers within slits of 0.2” – 0.4” width and 3.5” length should be small,
of order 10 mas or less, and present no impediment to executing small dithers. The exact
sequence of events by which the SI FOV will be moved with respect to the sky (e.g. a
small slew or just a fine steering mirror motion) is not important here as long as the offset
Figure
1: Small
motion accuracies from mission IRD requirements.
accuracies
meet
theseangle
requirements.
4.0
NIRSpec Slit Geometry
Because they present the simplest case of dithers and bear the closest resemblance to
other more conventional spectrographs (STIS is the best example), we start by
considering the fixed slits. Dithers involving the fixed slits are perhaps the easiest to
define, since they involve only small motions to mitigate the effects of detector nonuniformity (hot pixels, cosmic rays, etc.)1 and/or to improve spectral sampling (except for
one special-purpose dither). NIRSpec has five fixed apertures and an IFU (Table 1).
Table 1: NIRSpec Fixed Apertures
Slit name
Width [µm]
Length [µm]
Width ["]
Length ["]
A_200_1
78.6 ± 0.7
1350 ± 10
0.200 ± 0.002
3.494 ± 0.026
A_200_2
78.6 ± 0.7
1350 ± 10
0.200 ± 0.002
3.494 ± 0.026
A_400
157.3 ± 0.7
1540 ± 10
0.400 ± 0.002
3.986 ± 0.026
1
HST spectrographs with long slits, such as STIS, also enable dithers on the sky in the spectral dimension
to map extended objects. We anticipate that NIRSpec observers will use the IFU for this purpose, so we do
not define this capability for the fixed slits.
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A_100*
“Large Square”†
B_200
IFU window
39.3 ± 0.7
628.8
78.6 ± 0.7
770 ± 10
618.2
1350 ± 10
Circular with diameter 2.0 ± 0.1
0.100 ± 0.002
1.993 ± 0.026
1.6
1.6
0.200 ± 0.002
3.494 ± 0.026
Elliptical with axes 5.176"x5.084"
* will be replaced with the “Large Square” aperture; † dimensions are approximate
Figure 2: A schematic showing the 16 possible choices of dither patter in the 0.2" x 3.5" fixed slits.
The circle represents the position of the target with the PSF of NIRSpec at the MSA plane (FWHM =
100 mas at 3 µm). There are four primary patterns with 1, 2, 3, or 5 positions, and three possible
combinations of secondary patterns. The letters at bottom indicate the proposed user choices in APT
that are described in Section 5.5 – the numeral indicates the number of primary positions, and the
y/n whether to include spatial and/or spectral secondary patterns.
5.0
Fixed Slit Dither Patterns
5.1 The Primary Pattern: spatial dither patterns along the fixed slits
Spatial dithers along the “long slit” will place the spectra of the target at different ypositions on the SCA to mitigate the effects of pixel-to-pixel sensitivity on the detectors.
The selectable spatial patterns should consist of 5-position, 3-position, and 2-position
dithers. The option to not dither along the slit will need to be implemented as a 1-position
pattern in the same way, so that secondary dither patterns can be executed at a single
primary position (the slit center), if desired. These secondary patterns are described
below.
The various features of a five-position along-the-slit dither pattern are illustrated in
Figure 1. Because the default target acquisition algorithm for the fixed slits will place the
requested target in the x-y center of the slit, the primary pattern starts there. These
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positions are chosen to place the NIRSpec PSF at widely separated y-positions so that the
spectra at each position fall on different parts of the detector with negligible overlap. For
instance, the 5-position dither for A_200 will fall at the center of the slit, above and
below the center by 0.55" and 1.1" while the 3-position pattern will use the center and the
positions 1.1" above and below the center. The 2-position pattern will use the extreme
end of the slit for maximal coverage and efficiency for covering extended objects.
Figure 3: A possible sequence for the five-position primary pattern with combined primary and
secondary patterns. An exposure is obtained at each of the 20 positions before changing the
grating/filter combination. The positions 1-20 correspond to the time sequence only for the first set –
a possible ordering for the other sets is described in Section 5.4.
5.2 Secondary Pattern 1: subpixel dithers in the spectral dimension
For NIRSpec’s R = 100 and R = 1000 modes, the spectral resolution element is sampled
by more 2-4 pixels across the band, so further sampling of the line-spread function will
only add noise. However, the R = 2700 spectral resolution element spans only ~0.6 pixel,
so it may be desirable for some programs to execute small dithers in the spectral
dimension to improve the sampling of the resolution element, at least at short
wavelengths. These spectral dithers can be implemented as “secondary patterns” on top
of each primary dither pattern position (1, 2, 3, or 5 positions). This secondary pattern is
illustrated in the second set of primary positions in Figure 1, where dashed circles show
+/- 0.5 pixel offsets in the spectral dimension. Data at the flanking secondary positions,
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+/- 0.5 pixels from the slit center line, are subject to increased slit losses. However, 0.5
pixel shifts should still place the peak of the PSF at short wavelengths within the region
where the slit transmission is relatively flat and where photometric errors from exact
target placement are less than 10% (this is akin to the “sweet spot” for the microshutters).
Of course, at wavelengths longer than 3 µm where the FWHM of the PSF exceeds 100
mas, slit losses will be more severe and so the photometric losses and uncertainty may
become too severe for these types of dithering to be practical. However, some users with
relatively bright targets where optimal sampling of the spectral resolution element is
desired may choose to use these spectral dithers even at the longer wavelengths.
5.3 Secondary Pattern 2: subpixel dithers in the spatial dimension
Subpixel dithers in the spatial dimension improve the sampling of the PSF. This
secondary pattern is illustrated in the third set of primary positions in Figure 1, where the
dashed circles show +/-0.5 pixel (50 mas) offsets. Since these positions remain in the xcenter of the slit the slit losses do not increase for each position in the secondary pattern.
5.4 Combining Primary and Secondary Patterns
The simplest fixed-slit observation will observe the target in the x-y center of the slit and
nowhere else (a “1-position primary dither pattern”). From there, the user may choose to
increase the number of primary positions, and to execute either spectral subpixel dithers,
or spatial subpixel dithers, or both, at each primary position (see the right set of slits in
Figure 2). A combination of both secondary patterns would observe at four secondary
positions for each primary position, with the nominal primary position in the center. We
envision that these patterns will be made available to the user as a menu, with choices
that express each reasonable combination of primary and secondary pattern, leaving little
if any room for user error in the definition of the pattern.
To some extend the exact sequence of events in these patterns depends on the details of
implementation, but one recommended time sequence appears in Figure 3, which
illustrates the 5-position primary pattern with a combined spatial and spectral secondary
pattern (5,yes,yes). Each of the 20 positions is labeled and these labels are retained from
panel to panel. To save mechanism moves, each of the 20 positions is observed before
GWA/FWA movements. For Band I, the first set, the pointings might be observed in their
numerical order, 1 through 20, beginning in the slit center since that is where the default
target acquisition algorithm places it. Once these 20 positions are observed, the target is
at position 20. The next step is to change the grating and filter to Band II, and then walk
back through the 20 positions in the following sequence: 20, 19, 18, 17, 16, 15, 14, 13, 4,
3, 2, 1, 12, 11, 10, 9, 8, 7, 6, 5. After another grating change, the sequence would be 5, 6,
7, 8, 9, 10, 11, 12, 1, 2, 3, 4, 13, 14, 15, 16, 17, 18, 19, 20. These sequences within the
four-point secondary patterns could be changed – the key points are that (1) each of the
four positions is visited before moving to the next primary position, (2) for the three
bands, the primary positions are covered from top to bottom and then back to top, but
starting in the center after a target acquisition.
The user will need to choose among these patterns carefully to optimally balance
improved sampling against S/N goals, since faint targets near the read-noise limit will
suffer large S/N reductions from the many secondary positions.
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Note on overheads: from informal guidance from the Line-of-Sight Working Group (via
Ed Nelan), it appears that generically, loss of Fine Guidance Sensor Lock will be
necessary for moves larger than one FGS pixel (69 mas). Thus the moves in our
secondary patterns (~1 NIRSpec pixel or 100 mas) will require FGS to re-lock on the
guide stars for each position in the combined pattern. Each move+reacquisition should
require ~80 sec (more or less) total. These overheads will need to be accounted for in
proposed planning and it will be the observer’s responsibility to balance need against
time, and the TAC’s responsibility to judge that balance.
5.5 Possible Implementation in APT
From the user perspective, these dithers can be very simple. We can envision that APT
will present only three choices to the user: (1) how many primary positions to obtain (1,
2, 3, or 5), (2) whether to perform +/- 0.5 pixel offsets in the spatial dimension, and (3)
whether to perform +/- 0.5 pixel offsets in the spectral dimension. From these three
choices the actual dither pattern can be specified behind the scenes. For example, choices
of [5, no, no] would result in the fourth pattern in the first group in Figure 2, [5, no, yes]
would result in the fourth pattern in the third group, [5,yes,no] would give the fourth
pattern in the second group, and [5,yes,yes] would give the fourth pattern in the last
group. With four possible choices for the primary pattern and four combinations of
primary and secondary patterns for each of those, there are 16 possible combinations, all
of which appear in Figure 2.
To minimize mechanism usage we recommend that exposures be obtained at each dither
position in the pattern before moving the grating and/or filter wheel to another band.
Thus, the typical observing sequence will be TA, 3-5 exposures separated with dithers,
then a grating/filter change, followed by a backtrack through the dither patterns. In cases
where a secondary pattern has been selected, all secondary positions at a single primary
position should be observed before moving to the next primary position. In fact, when
implemented this way these dithers do not increase the number of mechanism motions
triggered by this observation because all the dithers occur between GWA/FWA motions
that would occur anyway if the user has chosen to observe in more than one band (e.g.
G140M/F070LP at 1.3 µm, G235M/F170LP at 2.2 µm, G395M/F290LP at 3.7 µm).
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Figure 4: The SCA plane with the positions of the spectra marked for the R = 2700 grating. The
spectra from the fixed slits lie in the middle. The special-purpose dither between A_200_1 and
A_200_2 moves along the red arrow. The wavelengths that are missing in one spectrum are covered
by the other (both marked with the short red line segments in the middle).
5.6 Dithers between A_200_1 and A_200_2
In addition to dithers along the spatial dimension of the slit, another special type of dither
is required for data obtained from the fixed slits. For high-resolution spectra obtained
through A_200_1, A_200_2, A_400, or A_100, there will be a gap in wavelength
coverage of approximately 0.04 – 0.12 µm because of the gap between SCA devices2.
Observers who choose to use A_400 or A_100 will have to do without these
wavelengths, but for data obtained with either A_200_1 or A_200_2 the gap can be filled
with a dither to the other slit, followed by an identical exposure. These two slits are
positioned approximately 19.5” apart for this purpose. This special purpose dither should
take the form of a single choice by the user to request the dither or not, and this choice
will be available only if they are using the R = 2700 grating. The complete dither pattern
executed in the first slit will be duplicated in the second, and the motion between the slits
should occur after the complete (primary and secondary) pattern has been fully executed
in the first slit to minimize mechanism motions. See Figure 4.
6.0
Science and Calibration Needs for Complex Dithers
The proposed dither patterns provide a flexible scheme for an individual observer to
choose dithers to optimize the quality of their data. The absolute simplest possible use of
NIRSpec is to obtain a single spectrum at a single position within a single slit, that is, to
use [1,no,no]. This method will not optimally sample the point-spread function or the
2
The exact gap varies with the fixed slit being used and will not be determined exactly until the flight
model is built. Approximate values for the gaps are Δλ = 0.04 µm at 1.3 µm for G140H, Δλ =0.07 µm at
2.2 µm for G235H, and Δλ = 0.12 µm at 3.7 µm for G395H. PRISM and M-grating spectra fall entirely on
one segment for all bands and slits.
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line-spread-function and it won’t mitigate bad pixels. To do all these things, some
combination of multiple primary positions and secondary patterns are needed, but the
exact choices must be left up to the individual investigator who knows their own
requirements best.
The complexity of the dithers and the time required to execute them depend linearly on
the number of primary positions. We have proposed 5 primary positions as a reasonable
upper limit to what an investigator will need to use. It may turn out that given actual
detector performance, five primary positions offer little if any improvement over 3
(though three will always beat one). Or it may turn out that 5 positions does improve
performance and calibration, but the additional overheads (80 sec per move at the
smallest) require a strong motivation on science grounds. Since dithers will be defined
before the TAC process (in the current PPS plan), the additional overheads will be judged
along with the scientific justification. We may also decide later to ration the use of the
more complex patterns to improve overall efficiency. We cannot make such policy
decisions now, in part because such tradeoffs are properly located with the TAC process
and in part because they involve tradeoffs that are ill-defined without more detailed
knowledge of actual instrument performance.
We can anticipate that the more complex proposed dither patterns will be very useful for
instrument calibration proposals to map out the slit-loss function, detector properties, and
to test improvements to data quality as a function of dither complexity. Without
implementing such dithers, it will be impossible to make such measurements in flight.
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