TECHNICAL
REPORT
Title: The MIRI LRS Dither Pattern
Authors: Christine H.
Chen1
1.0
Phone: 410338-5087
Doc #:
JWST-STScI-0001634, SM-12
Date:
December 19, 2008
Rev:
-
Release Date: 24 February 2009
Abstract
The MIRI Low Resolution Spectrograph (LRS) will obtain R~100 spectra nominally
between 5 and 10 µm (although some performance out to 14 µm is expected) through a
0.6″x5.5″ (5 pixel x 45 pixel) fixed slit on the direct imaging array. Two modes of
operation are anticipated for LRS observations, depending on the source size. Point
sources will be observed in ‘Point Source/Staring’ mode in which objects are dithered on
the array to mitigate the effect of bad pixels, to sub-sample the PSF, and to provide
background measurements. We propose that these Point Source/Staring mode
observations consist of two dither positions parallel to the slit, one 15 pixels and another
30.5 pixels from the edge of the slit. Extended sources will be observed in ‘Extended
Source/Mapping’ mode in which is the slit is mapped across a region of interest. In
‘Extended Source/Mapping’ mode, observers should be able to (1) specify the amount of
overlap between adjacent slit positions and (2) additional slit positions for background
measurements. The Spitzer IRS has obtained high quality spectroscopic data using this
scheme during the past 5 years.
2.0
Introduction
MIRI LRS observations may be improved by the use of a dither pattern to (1) mitigate the
effect of bad pixels, (2) obtain sub-pixel sampling, and (3) obtain observations of the
background zodiacal emission. Since the LRS spectrum is sampled using 2 – 3 pixels per
resolution element, dithering along the spectral direction should generally not be
necessary; however, dithering along the spatial direction will provide a Nyquist sampled
Point Spread Function (Koekemoer et al. 2005). Dithered observations are typically made
using small angle offsets (<0.5′) that will not require the use of additional JWST guide
stars. It is anticipated that dithering will enhance the majority of LRS observations of
point sources; although, a very small number of scientific programs may prefer
1
We would like to thank K. Gordon and D. Watson (U. Rochester) for their helpful comments and
suggestions.
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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observations made with no dithering. For example, observations of transiting exoplanets
require high precision, time-series photometry that may be complicated by changes in the
sensitivity as a function of location on a pixel. Observations without dithering could be
specified using a 1 slit position ‘Extended Source/Mapping’ mode observation; therefore,
we anticipate that the proposed 2-point dither pattern should be incorporated into all
‘Point Source/Staring’ mode observations.
3.0
The Spitzer IRS Dithering Strategy
The Spitzer Infrared Spectrograph (IRS) has 4 modules: Short-Low (SL, R~60-127, 5.2 –
14.5 µm), Long-Low (LL, R~57-126, 14.0 – 21.7 µm), Short-High (SH, R~600, 9.9 –
19.6 µm), and Long-High (LH, R~600, 18.7 – 37.2 µm). Each module possesses its own
slit. The SL slit is 3.7″x57″ (2.1 pixel x 31.7 pixel); the LL slit is 10.7″x168″ (2.1 pixel x
32.9 pixel); the SH slit is 4.7″x11.3″ (2.0 pixel x 4.9 pixel); and the LH slit is 11.1″x22.2″
(2.5 pixel x 4.9 pixel). With the exception of LL, each module is critically sampled at the
longest wavelength of its spectral range (at 15, 42, 19, and 37 µm for SL, LL, SH, and
LH, respectively), suggesting that MIRI LRS data will be less subject to sub-sampling
issues than Spitzer IRS data.
IRS operates in ‘Staring’ and ‘Mapping’ modes for all four of its modules. In IRS
Staring Mode, observations are made using two nod positions, located at 1/3 and 2/3 of
the way along the length of the slit (with the source centered in the direction
perpendicular to the slit), to provide redundancy again cosmic rays and detector artifacts.
The two IRS nod positions were not implemented to provide spatially Nyquist sampled
data; however, for the low-resolution slits, the slit are sufficiently long that the two nod
positions can be used to subtract background emission. In this case, the 2-dimensional
spectrum from one nod position is subtracted from the other before the 1-dimensional
spectrum is extracted. The high resolution slits are too short to provide background
subtraction; therefore, multiple observations of the adjacent sky are often necessary to
provide adequate background subtraction. In Mapping mode, observations are made with
the source centered in the slit in both the parallel and perpendicular slit directions.
All IRS modules suffer from ‘rogue’ pixels: pixels with variably elevated,
illumination-dependent photoconductive gain and dark current. Rogue pixels are believed
to be the result of massive particle damage. For SL (which possesses a Si:As detector
similar to MIRI), these pixels are fairly minor; however, for LL (which possesses Si:Sb),
these pixels can be severe. Although rogue pixels are not permanently dead, they are
noisy and difficult to calibrate. In Spitzer IRS campaign darks, pixels with large excess
dark current also possess large gain variation; these pixels could not be calibrated and
their values needed to be interpolated from neighboring pixels. Pixels more modestly
affected possess smaller gain variation and can be subtracted out during sky subtraction
(D. Watson, private communication).
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4.0
MIRI LRS Dithering Considerations
Several instrument properties impact the design of dither patterns. For example:
(1) What fraction of the pixels is likely to be affected by cosmic rays? (2) How severe is
the pixel undersampling in the spectral and spatial directions? (3) What is the accuracy
with which the telescope can offset? (4) Will the dithered set of two-dimension spectra be
adequate for background subtraction or will additional background observations be
required?
Sub-Pixel Sampling: MIRI LRS spectra will be obtained using the direct
imaging SCA which has a 0.11″/pixel plate scale. Point sources will be critically sampled
at 7 µm; therefore, MIRI LRS will be spatially undersampled at 5 – 7 µm. At these
shorter wavelengths, sub-pixel sampling will be necessary to Nyquist sample unresolved
point sources. Since MIRI is not badly under-sampled, ½ pixel sub-sampling will be
adequate for most science programs (Koekemoer et al. 2005). To obtain spatially Nyquist
sampled spectra, we recommend 2 ‘Point Source/Staring’ mode dither positions,
separated by N+ ½ pixels.
Cosmic Rays: Cosmic rays are expected to affect ~10% of pixels in 1000 sec. At
solar maximum, cosmic rays and solar particles may affect ~30% of pixels in 1000 sec
(see JWST Mission Ops Concept Document MO-81). However, it is currently anticipated
that cosmic rays will be identified and removed in the course of the up-the-ramp slopefitting procedure and will therefore not directly drive our dithering considerations.
However, energetic nuclei hits on the detectors may produce “rogue” or “dead” pixels
that are too bright or saturated compared to the incident flux. Preliminary analysis of VM
test data indicates that there are at least a handful of pixels with elevated gain in the nonflight detectors. In these cases, small angle maneuvers may be used to recover
information from areas sampled by bad pixels. To mitigate the effects of bad pixels, we
recommend that the two ‘Point Source/Staring’ mode dither positions be located at
approximately 1/3 and 2/3 (15 and 30.5 pixels) of the way along the slit, similar to IRS
Staring mode. In addition, subtraction of a nod position is expected to mitigate the effects
of rogue pixels.
Background Subtraction: For wavelengths between 5 and 10 µm, thermal
emission from zodiacal dust is expected to dominate the background. The background
may vary substantially depending on the slit position relative to the ecliptic; therefore,
observations of the sky are needed to subtract the background from the source. At the
longest wavelength (10 µm), the PSF is expected to have a size 2*1.22*λ/D = 0.39″ (3.5
times the PSF size at 10 µm and 2.5 times the PSF size at 14 µm); therefore, the two
proposed nod positions will be separated by ~4 times the PSF size. This separation
should be adequate for subtracting the background for point sources; however, this
separation is too small for extended sources. Extended sources will require a separate
pointing to measure the background that should be specified by the observer. For Spitzer
IRS, some high resolution observations required 2 - 4 independent measurements of the
nearby sky to ensure adequate background subtraction.
If the target or the sky background is bright, then persistence may affect
background subtraction. Current VM test data indicates that latent images from nonsaturating bright objects decay on timescales of seconds (T. Greene, private
communication), similar to the small angle slewing timescale (10 sec for slews < 3.6″;
Mitchell et al. 2008), suggesting that latent images from non-saturating sources may not
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severely impact background subtraction in ’Point Source/Staring’ mode. However,
saturation and latent image effects should be examined in more detail in the future.
Offsetting Error Budget: The accuracy with which the telescope can be offset
from one dither position to another is determined by the uncertainty in position
introduced by image motion and the uncertainty in position introduced by offsetting the
telescope. Currently, JWST is expected to introduce 7 mas jitter while pointed at a fixed
target. The observatory offsetting uncertainty has been calculated as a function of offset
position (see Figure 1 from Anandakrishnan et al. 2006) and is expected to be between 5
and 20 mas for offsets between 0.5″ and 2″ (4 and 18 pixels). Commanded dithers of 15.5
pixels are expected to place the source on the detector with 17.1 mas precision (or 20%
precision) that should be adequate for ½ pixel sub-sampling, Offsets smaller than 0.5′
(~270 pixels) do not require guide star changes.
Telescope Constraints: No ‘Point Source/Staring’ Mode observation or
‘Extended Source/Mapping’ mode observation should (1) include dither positions that
require additional guide stars and (2) violate visiting timing constraints.
Figure 1. JWST observatory offset accuracy as a function of offset distance
(Anandakrishnan et al. 2006).
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Figure 2. Proposed MIRI LRS dither patterns. In ‘Extended Source/Mapping’
Mode, the source is centered in the slit and the observer can specify a regular grid of
slit positions (including the number of positions and offsets between them) around
the central pointing shown above. Bottom: In ‘Point Source/Staring’ Mode, the
source is observed at two positions in the slit, one 15 pixels and another 30.5 pixels
from the edge of the slit.
5.0
Conclusions
We propose that MIRI LRS ‘Point Source/Staring’ mode observations be obtained using
two nod positions, located 15 pixels and 30.5 pixels along the length of the slit. The use
of these two nod positions will (1) mitigate the effect of bad pixels, (2) provide spatially
Nyquist sampled data, and (3) provide observations of the background zodiacal emission.
LRS observations of extended sources should be made using ‘Extended Source/Mapping’
mode in which the slit is stepped across the source. Since the observer will be able to
define the number and magnitude of the offset slit positions in the parallel and
perpendicular directions, they will be able to design their extended source observations to
ensure that sufficient background data is obtained.
6.0
References
Anandakrishnan, S. et al. 2006, “JWST Pointing Error Allocation and Performance
Prediction Analysis,” NGST, DRD#D36177, Rev. B
Koekemoer, A. M., Kriss, G., Long, K., Casertano, S., & Whitman, R. 2005, “An
Investigation of Optimal Dither Strategies for JWST”, JWST-STScI-000647
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