TECHNICAL
REPORT
Title: MIRI Subarrays for Planetary Transits
and Other Bright Targets
Authors: Christine H.
Chen, George H. Rieke,
and Karl D. Gordon
1.0
Phone: 410338-5087
Doc #:
JWST-STScI-001757, SM-12
Date:
March 26, 2010
Rev:
A
Release Date: 13 July 2010
Abstract
The Mid-Infrared Instrument (MIRI) on JWST is designed to obtain 5.6 - 25.5 µm
images with a plate scale of 110 mas pix-1 and 5 - 14 µm low-resolution spectra of the
most distant galaxies in the Universe, to study the formation of the first galaxies. As a
result, MIRI is extremely sensitive and saturates on modestly bright sky backgrounds and
point sources. During the past 5 years, time series photometry and spectroscopy of exoplanets transiting nearby, bright stars have been used to constrain giant planet radii,
orbital and atmospheric properties. The Spitzer Space Telescope measured primary and
secondary transits at mid-infrared wavelengths. It is anticipated that JWST MIRI will
also measure primary and secondary transits of planets around bright stars at mid-infrared
wavelengths to constrain atmospheric properties. To accommodate transiting planet
observations, we propose three new bright object subarrays for direct imaging (with
effective sizes 256x256, 128x128, and 64x64) that can be used in conjunction with any of
the direct imaging filters and a slitless prism subarray (with an effective size 64x512) that
can be used in conjunction with the LRS split-prism.
2.0
Introduction
As of July 2009, radial velocity and transiting planet searches have discovered 291
companions to 251 Solar-type stars with minimum masses <25 MJup
(http://exoplanets.eu), 41 of which transit their host stars. Demographic studies of radial
velocity planets have yielded valuable constraints on planet formation processes such as
the planet-metallicity correlation; however, detailed studies of transiting planets are
providing the opportunity to measure individual planet properties such as radius, orbital
eccentricity, inclination, and spin axis (relative to the host star). HST/STIS spectra of the
transiting planet HD 209458 were (1) collapsed into visual time-series photometry to
measure its light curve with exquisite precision and infer the planet’s radius and orbital
inclination and (2) used to detect Sodium in its atmosphere during primary transit
(Charbonneau et al. 2001). At longer wavelengths, Spitzer IRAC (3.6, 4.5, 5.8, and 8.0
µm), IRS peak-up (16.0 and 22.2 µm), and MIPS (24 µm) observations of secondary
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Aeronautics and Space Administration under Contract NAS5-03127
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transits have been used to characterize the thermal emission from a number of exoplanets. The majority of these studies currently focus on measuring the relative depth of
secondary eclipses as a function of wavelength to constrain the circulation and thermal
heating constant for individual exo-planet atmospheres (Burrows et al. 2006; Laughlin et
al. 2009).
MIRI transiting planet observations will focus on measuring the planet’s thermal
emission by observing the dip in the mid-infrared light curve as the planet passes behind
its host star. Extra-solar giant planets typically emit ~1% of the mid-infrared emission
measured from the aggregate system; therefore, very high signal-to-noise (SNR), timeseries observations are needed to disentangle the secondary transit signal. The MIRI
Low-Resolution Spectrograph (LRS) will provide R~100 spectra to constrain the thermal
emission as a function of wavelength at 5-14 µm. It is anticipated that spectroscopic
observations of planet host stars will be made using the LRS in slitless mode, using the
LRS split-prism without the slit. (i.e. sources will not be positioned behind the LRS slit to
avoid drifts in the position of the target in the slit.) However, many of the currently
known planet host stars are so bright at the shortest MIRI wavelengths that they are
expected to saturate the SCA in the time required to read the full array. Therefore, many
objects must be observed using subarrays, in either LRS or direct imaging mode, with
significantly shorter read (frame) times (a summary of new subarray frame times is given
in Error! Reference source not found.). In summary, subarrays are needed to provide
adequate dynamic range to enable observations of the majority of known host planet stars
in both Imaging and Low Resolution Spectroscopy modes. Although transiting planet
observations will be made without dithering to minimize intra-pixel sensitivity induced
variations in the time series, subarray imaging of other bright targets may benefit from
dithering. Subarray dither patterns will be defined in a revision to the “MIRI Imaging
Dither Patterns” technical report (Chen 2009).
We present a proposal for three direct imaging and one LRS slitless subarrays to
enable secondary transit observations. In general, we select subarray locations on the
SCA such that they (a) do not overlap with any of the coronagraph subarrays or the
detector region on which the LRS spectrum falls (b) are adjacent to reference pixels that
provide critical detector bias data that is important for data reduction, and (c) minimize
the read (frame) times. We propose to place the two smallest subarrays (64x64 and
128x128) within the MASKLYOT subarray and the larger subarrays adjacent to the LRS
mask to provide minimum frame times. Subarrays smaller than 64x64 violate the MIRI
requirements and can not be allowed. We also suggest that observers be allowed to use
any of the readout modes for visits shorter than 3 hours and be limited to using
SLOWMode, FASTINTAVG, and FASTGRPAVG for visits longer than 3 hours to help
control data volume. (i.e. FASTMode observations without co-addition will not be
allowed for visits lasting longer than 3 hours.)
3.0
Existing Subarrays
The MIRI Operations Concept Document currently envisions 6 subarrays to be used in
conjunction with the Imaging/Low-Resolution Spectrograph (LRS) SCA: The FULL
subarray is a 1024x1290 array (with 1024x1024 active pixels and 1024x258 reference
outputs) that encompasses the entire Imaging/LRS SCA and is the default subarray to be
used to for Imaging/LRS observations. The MASK1065, MASK1140, and MASK1550
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are 256x256 subarrays whose locations are defined by the MIRI Imager optics for the
10.65, 11.4, and 15.5 µm 4-quadrant phase mask coronagraphs. The MASKLYOT is a
320x320 subarray whose location is defined by the MIRI Imager optics for the Lyot
coronagraph. The BRIGHTSKY subarray is currently a 512x512 subarray for
observations of bright, diffuse astronomical objects (such as the Orion star forming
region) that may saturate the MIRI FULL subarray at 25.5 µm during integrations as
short as 6 s (Meixner 2007). The subarray positions on the SCA and their overall
properties are shown in Error! Reference source not found. and summarized in Error!
Reference source not found..
Figure 1. The locations of the current subarrays, including coordinate system, for the MIRI direct
imaging and LRS SCA as viewed from the telescope looking down on the detector. The
coronagraphic subarray locations are defined by the MIRI Imager optics and are approximately
256x256 in size. (MIRI Operations Concept Document.)
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Table 1 Current MIRI Imager Subarray Locations and Sizes
Subarray
ROWS COLUMNS FIRST ROW
CORNER
FIRST COL
CORNER
FULL
1024
1032
1
1
LIGHT
SENSITIVE
COLUMNS
1024
MASK1065
MASK1140
MASK1550
MASKLYOT
BRIGHTSKY
256
256
256
320
512
256
256
256
320
868
1
229
452
705
1
1
1
1
1
1
224
224
224
275
512
4.0
Science Drivers for Additional Subarrays
4.1
Solar System Observations
We anticipate observations of bright, extended objects in our Solar System such as the
giant planets, if feasible. The proposed SUB64 subarray (with a frame time 0.065 sec)
provides improved capability to observe bright targets compared with the FULL array
(with a frame time 2.775 sec). We list the predicted saturation limits for MIRI direct
Imaging using the smallest, assuming that the full well is 250,000 e- and that the quantum
efficiency is midway between the specification and 100% (e.g., 70% at 5.6 µm, 80%
around 12 µm, 75% around 20 µm).
Table 2 Extended Source Saturation Limits
Saturation Level
Saturation Level
(Jy/″)
(mJy/pix)
5.6
15
180
7.7
9
110
10
13
155
11.3
40
500
12.8
15
180
15
15
175
18
17
200
21
15
180
25.5
22
270
Wavelength
(µm)
The surface brightnesses for Mars, Jupiter, and Saturn in selected MIRI filters are shown
in Error! Reference source not found. extrapolated from ISO measurements (Meixner
et al. 2007). The neutral density filter, FND, has an effective wavelength of 13.5 µm and
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the LRS estimates are made for 7.5 µm. We calculate the amount of time that it takes to
saturate the imaging SCA, assuming that the bright disk of each planet generates 200,000
e- (obtained in 2 reads), approximately 80% the value expected for array saturation. We
conclude that (1) Mars is too bright to be observed using any subarray with direct
imaging or the LRS; (2) Jupiter can only be observed using the 5.6 µm and FND filters
with direct imaging (e.g. Jupiter saturates the direct imaging SCA if any filter other than
the 5.6 µm or the FND is used), (3) Saturn can be directly imaged at 5.6 – 18 µm and
observed using the LRS, and (4) Neptune and Uranus can be directly imaged using any of
the direct imaging filters and observed using the LRS.
The full disks of Jupiter and Saturn are sufficiently large (with diameters of 38″
and 17″, respectively) that they may overfill the subarrays needed to observe them. For
example, we estimate (based on the Saturn saturation times in Error! Reference source
not found. and the subarray properties in Error! Reference source not found.) that
Saturn can be observed with the F560W and F1000W filters using the SUB256 subarray
(28″FOV), the F1800W filter using the SUB128 subarray (14″ FOV), and the FND and
LRS using the FULL frame; therefore, only the F1800W observations will not cover the
full Saturn disk. Pixels in rows outside of the subarray are continuously reset; however,
all pixels in the subarray rows are reset together regardless of whether or not they are part
of the subarray.
Table 3 MIRI Saturation Times for Mars, Jupiter and Saturn
Filter
F560W
F1000W
F1800W
FND
LRS
4.2
kJy
12
90
150
120
47
Mars
Jy/″ Tsat [s]
375
0.002
2810 4×10-5
4680 0.0002
3630
0.08
1430
0.01
kJy
20
40
126
78
28
Jupiter
Jy/″
18
35
112
69
25
Tsat [s]
0.04
0.003
0.007
4.4
0.6
Jy
280
330
1100
690
330
Saturn
Jy/″ Tsat [s]
1.3
0.5
0.8
1.5
4.6
0.2
2.9
110
1.4
11
Extra-Solar Giant Planet Transit Observations
We also anticipate observations of extra-solar planets around bright, nearby host
stars using the full suite of direct imaging filters and the Low-Resolution Spectrograph in
slitless mode. Error! Reference source not found., Error! Reference source not
found. and, Error! Reference source not found. show the brightness distribution of
known transiting planet host stars overlaid on a plot of estimated SNR obtained in 10 min
of on source integration as a function of stellar flux with direct imaging at 5.6 and 25.5
µm and the LRS at 7.5 µm for the currently implemented subarrays (FULL and
BRIGHTSKY). We estimate the expected SNR using ramp measurements of illuminated
flight arrays obtained by M. Ressler at JPL, scaled to the theoretical instrument
performance, estimated by G. Rieke’s MIRI sensitivity calculator (MIRIradmodeldb.xls
spreadsheet). The ramp appears linear to first order until pixels reach a value of ~40,000
ADU (~250,000 e-), at which point the array saturates. We infer the mid-infrared
brightness of planet host stars from their 2MASS Ks-band fluxes, assuming that stellar
atmospheres are Rayleigh-Jeans (Fν ∝ ν2) at wavelengths longer than 2.2 µm. For
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reference, we estimate Ks magnitudes and 5.6, 7.5, and 25.5 µm fluxes for F0V, G0V,
K0V, and M0V stars with a V-band magnitude of 8 mag (see Error! Reference source
not found.), using intrinsic Johnson-2MASS V-Ks colors estimated by E. Mamajek
(http://www.pas.rochester.edu/~emamajek/memo_vk.html) and assuming that stellar
atmospheres are Rayleigh-Jeans (Fν ∝ ν2) at wavelengths longer than 2.2 µm.
Table 4 Typical Fluxes for Main Sequence Stars
Spectral Type
V
Ks
5.6 µm
7.5 µm
25.5 µm
(mag)
(mag)
(mJy)
(mJy)
(mJy)
F0V
8
7.20
130
74
6.4
G0V
8
6.58
230
130
11
K0V
8
6.03
390
220
19
M0V
8
4.53
1500
860
74
The majority of the currently known planet host stars (214/251=85%) are too
bright to be observed using the MIRI FULL (1024x1024) and/or BRIGHTSKY
(512x512) subarrays at 5.6 µm. Therefore, smaller subarrays with faster readout times
(and therefore smaller minimum integration times) are needed to observe the majority of
these stars. Error! Reference source not found. shows the estimated SNR obtained in
10 min as a function of stellar flux for the faster proposed SUB256 (256x256), SUB128
(128x128), and SUB64 (64x64) direct imaging subarrays at 5.6 µm. These subarrays
increase the number of observable planet host stars at 5.6 µm from 28 (11%) and 38
(15%), using the FULL and BRIGHTSKY subarrays, respectively, to 43 (17%), 139
(55%), and 171 (68%), respectively.
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Figure 2. The estimated SNR for 10 min integration of bright sources plotted as a function of 5.6 µm
stellar flux. The solid curves show the expected SNR for subarrays of various sizes. The histogram
shows the magnitude distribution of the 251 planet host stars (http://exoplanets.eu), together with
the number of host stars that can be observed using each subarray (e.g. 28/251 host stars can be
observed with the FULL array).
While smaller subarrays are urgently needed to observe exoplanet host stars at the
shortest MIRI wavelengths, they are not required to observe the majority of these objects
at the longest wavelength (25.5 µm) if the thermal background is modest. Error!
Reference source not found. shows the estimated SNR obtained in 10 min as a function
of stellar flux for the proposed SUB256 (256x256), SUB128 (128x128), and SUB64
(64x64) direct imaging subarrays at 25.5 µm. The majority of the planet host stars
(240/251=96%) can be observed using the FULL array at 25.5 µm because the stellar
photospheres are significantly fainter at 25.5 µm than at 5.6 µm. The 25.5 µm SNR
estimates shown in Error! Reference source not found. assume an average telescope
thermal background of 1700 MJy sr-1 (or 4100 e- s-1, G. Rieke private communication),
significantly larger than the sky background. (The telescope thermal emission dominates
the background at wavelengths longer than 20 µm.) For stars fainter than 60 mJy, the
thermal background is substantially brighter than the target star at 25.5 µm and is
responsible for the reduced SNR. However, the telescope thermal emission may saturate
the array at 25.5 µm, depending on the telescope orientation with respect to the Sun;
therefore, 25.5 µm direct imaging observations may require use of the BRIGHTSKY
subarray independent of target flux.
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Figure 3. Same as Figure 2 except for 25.5 µm rather than 5.6 µm.
Since telescope drifts over time may change the location of a star in the LRS slit,
observers will prefer to monitor very small variations in exo-planet host star brightness
using LRS in slitless mode. Observers can currently select the LRS split-prism from the
filter wheel, place their target star in the direct imaging field of view, and record the
spectrum using the FULL subarray. Since the host star’s light is dispersed with an R=100
using the LRS split-prism, the majority (195/251=78%) of exo-planet host stars can be
observed using the FULL array. However, 30% of all known exo-planet host stars are
anticipated to be too bright to be imaged directly using MIRI at 5.6 µm; therefore, a
SLITLESSPRISM subarray is needed to provide LRS observations of these host stars.
Error! Reference source not found. shows the estimated SNR obtained in 10 min as a
function of stellar flux for the FULL and proposed SLITLESSPRISM (64x512) subarrays
at 7.5 µm. A SLITLESSPRISM subarray will make almost all of the currently known
exo-planet host stars (236/251=94%) observable with MIRI at the shortest wavelengths.
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Figure 4. Same as Figure 2 except for LRS in slitless mode at 7.5 µm.
Gordon et al. (2009) have drafted a plan to determine the absolute calibration of the
JWST instruments using standards that have been well characterized at both visual and
infrared wavelengths using HST and Spitzer. While the majority of the currently
proposed white dwarf, A-, and G-type main sequence calibrators are sufficiently faint that
they can be imaged using the MIRI FULL subarray, the A-type star HD 165459 and the
G-type star HD 209458 are more than an order of magnitude too bright at 5.6 µm and
require the use of a bright object subarray. In addition, implementation of the bright
object subarrays discussed here will enable observations of the Spitzer IRAC and MIPS
calibrators.
4.3
Extra-Solar Earth-Like Planet Transit Observations
The Kepler Mission is expected to discover transiting extra-solar Earth-like planets
during the next few years. Detecting the transits of Earth analogs is even more
challenging because their smaller radii produce smaller transit signals. For example, a
3R⊕ planet is expected to produce a transit signal ~0.1%, significantly smaller than the
few percent signals measured toward transiting giant planets. Therefore, observations of
transiting Earth-like planets will be most efficient if they are made around the brightest
stars. Sahu et al. (2009) estimate that the probability of a transit for an Earth at 1 AU
around a G-type star is ~0.5%, suggesting a search of the closest 200 G-type stars may
yield the first Earth-like planet around a Sun-like star. By examining the distribution of
G-type stars near the Sun, they estimate that the brightness for the host planet of the first
extra-solar Earth-like planet will be V ~ 6 mag.
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Observing such bright host stars will be challenging for MIRI direct imaging even
using the smallest subarrays. A Sun-like star with a V ~ 6 mag will possess [V-K] = 1.45
mag (Kenyon & Hartmann 1995), corresponding to Fν(2.2 µm) = 10.1 Jy and Fν(5.6 µm)
= 1.5 Jy assuming Fν(*) ∝ ν2. Such bright stars will be observable using MIRI LRS in
conjunction with the SLITLESSPRISM subarray and using direct imaging using various
combinations of the subarrays at long wavelengths. However, direct imaging
observations will be marginal at 5.6 µm using the SUB64 subarray. Unfortunately, the
overheads associated with small subarrays are non-trivial; therefore, subarrays a factor of
4 smaller than SUB64 will only provide a 10-20% improvement in frame time. Since the
frame time improvements for subarrays smaller than SUB64 are marginal, the MIRI team
does not plan to support subarrays smaller than SUB64.
5.0
Proposed MIRI Subarrays
The minimum integration time for a given subarray is determined by its readout time. For
MIRI, the readout time for a subarray can be approximated using the following
expression:
 x

n
 
t frame =  + 4  t d +  x + 5t d n y + (1024 − n y )t d

4
 
 4
(MIRI Operations Concept Document) where x is the starting column, nx and ny are the
number of columns and rows in the subarray, td is the read time for pixels (10 µs);
however, more
€ accurate estimates can be calculated using M. Ressler’s Exposure Time
Calculator. The origin for the SCA (1,1) is the bottom, left corner adjacent to the 15.5 µm
4 Quadrant Phase Mask subarray (see Error! Reference source not found.). Therefore,
the smallest subarrays (128x128 and 64x64) will possess the fastest frame time if they are
located in the bottom-left corner of the MASKLYOT subarray. Larger subarrays
beginning at position (353,1), adjacent to the 4 Quadrant Phase Mask subarrays and the
low-resolution 2-dimensional spectrum, are expected to possess the fastest frame times or
minimum possible integration times without overlapping either the coronagraph fields-ofview or the LRS spectrum. Subarray frame times are independent of starting y position.
The MIRI direct imaging SCA possesses 4 reference pixels with no light
sensitivity at the beginning and end of each row (x=1-4 and x=1029-1032). It is currently
anticipated that references pixels will be used to measure and remove bias drifts and other
artifacts with timescales longer than twice the row period, whenever the data from the
full SCA is obtained. A subarray must be adjacent to the left or right edge of the SCA if
any reference pixels are to be readout in conjunction with the subarray. In the current
proposal, the SUB128, SUB64, and SLITLESSPRISM subarrays will be adjacent to
reference pixels, making collection of reference pixel data straight-forward; however, the
BRIGHTSKY and SUB256 subarrays are located to the right of the LRS spectrum area.
Since there is no time penalty associated with reading out the pixels in between the
effective subarray area and the reference pixels, we propose to extend the remaining
subarrays from the SCA origin, past the LRS spectrum, to the bright object field desired
(i.e. Bright object areas of interest are 512x512, and 256x256 subarrays that begin at
(353,1)). Subarrays should encompass these areas of interest but also extend to the SCA
origin to include one set of reference pixels). For example, instead of defining SUB256 as
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a 256x256 array beginning at (353,1), we choose to define SUB256 as a 608x256 array
beginning at the origin (1,1). This rectangular array includes the region of interest and is
adjacent to one set of reference pixels, allowing reference pixels to be read out with the
subarray. Although the rectangular subarrays are larger than the square subarrays, their
frame times are identical to those of square subarrays that begin at (353,1). Reading out
rectangular arrays will generate larger data files.
We propose three new direct imaging subarrays [SUB256, a 608x256 subarray
with a starting position of (1,1); SUB128, a 132x128 subarray with a starting position of
(897,1); and SUB64, a 68x64 subarray with a starting position of (897,1)] and a LRS
subarray to be used to conjunction with the LRS split-prism but without the LRS slit
[SLITLESSPRISM, a 68x512 subarray with a starting position of (1,348)] with the fastest
possible frame times to make observations of bright sources feasible. The current and
proposed subarray properties, including the frame times and maximum fluxes at 5.6 and
25.5 µm are shown schematically in Error! Reference source not found. and listed in
Error! Reference source not found., assuming that the maximum flux allowed for each
subarray corresponds to 200,000 e- (obtained in 2 reads) in the brightest pixel,
approximately 80% the value expected for array saturation. We assume the instrument
performance estimated by M. Ressler’s MIRI Exposure Time Calculator
(miri_exposure_time.py) and G. Rieke’s MIRI Sensitivity Calculator
(MIRIradmodeldb.xls spreadsheet).
Table 5 Current and Proposed MIRI Imager Subarray Locations and Sizes1
Subarray
Size
Columns
by Rows
Start
Pos
FAST
Frame
Time
Max Flux
F560W
[mJy]
Max Flux
F2550W2
[mJy]
FULL
1024
(1,1)
2.775
17
360
BRIGHTSKY
864x512
(1,1)
1.183
38
870
SUB256
608x256
(1,1)
0.453
100
2400
SUB128
132x128
(1,897)
0.100
440
10000
SUB64
68x64
(1,897)
0.065
680
16000
(1,348)
0.164
2900 using P750L at 7.5 µm
SLITLESSPRISM 68x512
We design the SLITLESSPRISM subarray assuming that any LRS observation
should (1) include the 5-14 µm spectral range and (2) provide adequate background
observations to reliably background-subtract acquired data. The LRS prism (P750L) has a
resolving power of ~100 at 7.5 µm with 2 pixels per resolving element, suggesting that
~369 pixels in the spectral direction will cover the 5-14 µm spectrum. At the longest
wavelengths anticipated (14 µm), a PSF will possess a size (2*1.22λ/D =) ~1.1″ or 10
pixels. If the SLITLESSPRISM subarray is 6 times larger than the PSF at the longest
1
Pixel coordinates are given excluding reference outputs
The 25.5 µm max flux estimates are calculated assuming telescope thermal emission consistent with
requirements (1700 MJy sr-1 or 4100 e- s-1).
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wavelength in the spatial direction, then there should be adequate background
measurements for background subtraction. Therefore, we suggest that the
SLITLESSPRISM subarray should possess a size of 64x512 pixels. We note that the
SLITLESSPRISM subarray is designed to obtain the spectrum of a single unresolved
source at a time. For reference, the saturation limit for LRS-Slit spectroscopy using the
FULL subarray is 650 mJy at 7.5 µm.
Figure 5. (Top) Effective areas for proposed subarrays. The SUB128, and SUB64 subarrays are
labeled using the numbers 128 and 64. The boundaries of BRIGHTSKY, SUB128, and SUB64 are
shown using a solid black line, while those of SUB256 and SLITLESSPRISM are shown with dashed
and dotted lines, respectively. (Bottom) Actual proposed subarrays.
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Note: The left edge of the BRIGHTSKY and SUB256 subarrays have been extended to include the
reference pixels on the left-hand side of the detector, without increasing the frame time.
6.0
Readout Patterns
MIRI currently supports 4 readout patterns: SLOWMode, FASTMode,
FASTINTAVG, and FASTGRPAVG. In SLOWMode, each pixel is sampled 10 times at
10 µs intervals, corresponding to a FULL frame time of 27.75 s; the first two samples are
dropped and the last eight are averaged together. In FASTMode, each pixel is sampled
once, corresponding to a FULL frame time of 2.775 sec. For each integration, the
overhead time associated with a reset-read of the detector is 1 frame time. We currently
anticipate that observers will use SLOWMode and/or FASTMode in conjunction with the
FULL and BRIGHTSKY subarrays to make observations of sources that do not saturate
the FULL detector in 2.775 s. In these cases, the Astronomical Proposal Tool (APT) will
select the readout mode needed and calculate the number of reads needed to obtain a
scientifically justified SNR.
Exo-planet transit observations require host star brightness monitoring with high
cadence (on the order of minutes) for long periods (similar to a day). To first order, the
data rate for continuous observing with FASTMode is expected to be constant as a
function of subarray size because the pixel clock time (10 µs) is independent of subarray
size. However, the overheads associated with subarray observing are disproportionately
large for smaller subarrays. The amount of data obtained by the MIRI FULL array
running FASTMode continuously in a day,

s 
86400
 16 bits 
day 

Data Rate = ( n x n y pix)

 pix  t frame 




where nx = ny = 1024 and tframe = 2.775 s, is 543 Gbits, significantly larger than the data
recorder’s storage capacity, 229 Gbits/day; therefore, two FASTMode co-addition
€ readout patterns have been implemented to reduce the data rate (Friedman & Meixner
2006). In FASTINTAVG, one ramp with four FASTMode reads (read-read-read-readreset) is averaged using flight software to reduce the data rate for bright objects. In
FASTGRPAVG, four separate FASTMode read-reset ramps are averaged using flight
software to reduce the data rate for very bright objects. These bright object readout
patterns reduce the data rate by factors of 5 and 8, respectively. Since the subarrays
possess fewer pixels, their data rates are smaller than those of the FULL array (see
Error! Reference source not found.). In fact, only continuous FASTMode observations
using the FULL and BRIGHTSKY subarrays are prohibitively high.
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Table 6 Current and Proposed Subarray Data Rates
Subarray
SLOWMode
Data Rate
[Gbits/day]
FASTINTAVG
Data Rate
[Gbits/day]
FASTGRPAVG MAX
Data Rate
FASTMode
[Gbits/day]
Data Rate
[Gbits/day]
FULL
52
104
65
522
BRIGHTSKY
52
104
66
524
SUB256
23
49
29
233
SUB128
18
37
23
184
SUB64
9
17
10
93
35
55
276
SLITLESSPRISM 28
We believe that exo-planet transit observers will not favor the highest data rate
subarrays because their observations will demand the highest cadence reads up the ramp
until the array saturates, to obtain the highest SNR photometry in the shortest possible
time. Since host stars will be point sources and the SUB64 subarray has the shortest
frame time, exo-planet observers should prefer to observe all of their targets in
FASTMode, repeatedly reading out the SUB64 subarray until it nears saturation. Error!
Reference source not found. shows the estimated SNR for a 17 mJy source at 5.6 µm
using FASTMode in conjunction with the FULL (magenta), BRIGHTSKY (blue),
SUB256 (green), SUB128 (red), and SUB64 (black) subarrays. This source is at the
proposed bright limit for the FULL array but could theoretically be observed with any of
the subarrays. The SUB64 obtains the highest SNR photometry at any given clock time
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JWST-STScI-001757
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(see Error! Reference source not found.).
Figure 6. The estimated SNR of a 17 mJy star at 5.6 µm plotted as a function of clock time for
FASTMode observations using the FULL (magenta), BRIGHTSKY (blue), SUB256 (green), SUB128
(red), and SUB64 (black) subarrays. The SNR obtained at discrete reads are overlaid using
diamonds. BRIGHTOBJ3 observations using the FASTMode Co-addition patterns FASTINTAVG
and FASTGRPAVG are shown with squares and triangles for comparision.
Since long periods of FASTMode observations with the FULL and/or
BRIGHTSKY subarrays will overfill the data recorder, we propose that observers be
limited to using the SLOWMode, FASTINTAVG, and FASTGRPAVG readout patterns
if they plan continuous observations (with any of the subarrays) lasting longer than 3
hours. If their visit lasts less than 3 hours, then FASTMode should also be allowed since
the amount of data collected will not be prohibitively high. The current time division
between when FASTMode is and is not allowed may be revised in the future depending
on the data recorder demands of the other instruments. We propose that the readout mode
selected for long-term monitoring be based on the target’s intrinsic brightness. The
brightest targets should be observed using FASTGRPAVG; the next brightest targets
should be observed using FASTINTAVG; and the faintest targets should be observed
using SLOWMode. Long-term monitoring of all targets below one-quarter the
SLOWMode saturation limit should be done using SLOWMode (see Error! Reference
source not found.). Long term monitoring of brighter targets that are fainter than onequarter the FASTGRPAVG saturation limit should be done using FASTINTAVG (see
Error! Reference source not found.). All long term monitoring of the brightest targets
should be done using FASTGRPAVG.
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JWST-STScI-001757
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Table 7 SLOWMode Long Term Monitoring Maximum Fluxes
Subarray
MAX
Flux 5.6
µm
[mJy]
MAX
Flux
11.3 µm
[mJy]
MAX
Flux
12.8 µm
[mJy]
MAX
Flux 15
µm
[mJy]
MAX
Flux 18
µm
[mJy]
MAX
Flux 21
µm
[mJy]
MAX
Flux
25.5 µm
[mJy]
FULL
0.8
2.6
3.7
4.5
7.9
7.4
9
BRIGHTSKY 1.9
6.0
8.5
11
19
19
35
SUB256
5.0
16
22
28
50
52
110
SUB128
22
71
99
120
220
240
530
SUB64
34
109
150
190
340
370
820
Table 8 FASTINTAVG Long Term Monitoring Maximum Fluxes
Subarray
MAX
Flux 5.6
µm
[mJy]
MAX
Flux
11.3 µm
[mJy]
MAX
Flux
12.8 µm
[mJy]
MAX
Flux 15
µm
[mJy]
MAX
Flux 18
µm
[mJy]
MAX
Flux 21
µm
[mJy]
MAX
Flux
25.5 µm
[mJy]
FULL
2.1
6.6
9.2
11
20
21
40
BRIGHTSKY 4.7
15
21
26
47
50
100
SUB256
12
40
56
70
120
130
300
SUB128
55
180
250
310
560
590
1300
SUB64
85
270
380
480
850
920
2000
7.0
Conclusions
At the current time, we do not believe that reference pixels will substantially aid data
reduction for bright sources; however, we recommend implementing the three direct
imaging subarrays and slitless LRS subarray with reference pixels. Acquisition of
reference pixels does not interfere with obtaining the fastest frames times to enable
observations of the closest, brightest transiting planet host stars. SUB256 is a 608x256
subarray with a starting position of (1,1); SUB128, and SUB64 are 132x128, and 68x64
subarrays with starting positions of (897,1) that will provide unsaturated images of point
sources as bright as 0.68 Jy at 5.6 µm and 16 Jy at 25.5 µm. The SLITLESSPRISM is a
68x512 subarray with a starting position of (1,1) that will provide unsaturated spectra of
point sources as bright as 2.9 Jy at 7.5 µm. All of the subarrays discussed in this report
will include reference pixels. We additionally recommend that observers be allowed to
use any of the readout patterns for visits lasting less than 3 hours and limited to the
SLOWMode, FASTINTAVG, FASTGRPAVG readout patterns, depending on object
brightness, for visits lasting longer than 3 hours.
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We thank M. Cracraft for providing the flight array data measurements shown here.
8.0
References
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Check with the JWST SOCCER Database at: http://soccer.stsci.edu/DmsProdAgile/PLMServlet
To verify that this is the current version.
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