TECHNICAL
REPORT
Title: Source Acquisition for Coronagraphy
Doc #:
Date:
Rev:
Authors: G. Reike, M.
Meixner, and MIRI Team
Release Date: 19 October 2006
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JWST-STScI-001012, SM-12
19 October 2006
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Introduction
The performance of the MIRI coronagraphs depends critically on the accuracy with
which a star can be centered on the "null" position. This centering is particularly
important for the three phase plate units, because they work at relatively short wavelength
and also because they are designed to allow us to look very close to the star, and bad
centering will undermine this science objective. To center will require that JWST
perform an offset from a known position after measuring the position of the target star at
that position. This "peakup" approach is required because the positioning after a long
slew can be off by up to 3 arcseconds, due to a number of causes within the JWST
telescope. Figure 1 shows the required offsetting performance, which underlies all of the
strategies discussed in this report for centering on the coronagraphs.
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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Figure 1 Offsetting performance requirements for JWST, Sivaramakrishnan et al (2006)
Figure 1 shows that approaching our goal of centering to better than 10mas will require a
two-step peakup process. The target star will first be placed in the coronagraph field of
view after a slew, its position will be determined in the instrument frame, it will then be
moved close to the center of the coronagraph field, a new center position will be
measured, and it will finally be offset to the center of the field. The nominal positions are
that the first field will be 10 arcsec from the center and the second one will be 1 arcsec
away from the center. A consequence of these maneuvers is that the detector array will be
exposed to the bright image of the target star close to the final position for taking science
data. This report examines the resulting issues for instrument performance.
2
Latent Images
Although we do not have direct evidence yet from MIRI prototype arrays, in general
infrared arrays have latent images at a level of just under 1% of the signal in the first read
out after a source has been removed from a pixel. These latent images decay away with
an exponential time constant of the order of 10 seconds initially, but at the level of a few
times 10-4, the decay is significantly slower. This general description applies to the IRAC
and IRS/MIPS Si:As IBC arrays on Spitzer (from Raytheon and DRS Technologies,
respectively) but also to the IRAC InSb array and to the NICMOS HgCdTe array (see,
e.g., Hora et al. 2004; Engelbracht 2005; IRAC Handbook 2006). Because the behavior is
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so universal, it is likely to arise from a common feature of infrared detector architecture,
such as surface traps where the detector crystal comes to an end. In possible
confirmation, the latent images are greatly reduced (to ~ 0.03%) in the Teledyne arrays
being supplied to NIRCam, NIRSpec, and the FGS. This reduction can probably be
traced to the growth of a large band gap HgCdTe layer at the front side of these detectors.
Band bending from this layer provides a potential barrier that reflects the photo-generated
charge carriers away from the physical surface of the detector. In any case, we conclude
that the MIRI arrays are likely to have a similar level of "fast", or "soft", latents as other
arrays that do not employ the current Teledyne strategy.
Brighter sources impose not only stronger latent images, but ones that decay much more
slowly. For example, the IRAC InSb Channel 2 has a "slow", or "hard", latent with a
decay time of more than 2000 seconds. Both the IRAC and IRS/MIPS Si:As IBC arrays
have been observed to have basically permanent negative latent images after exposure to
very bright sources, and until the array is annealed. These behaviors are documented in
the respective instrument handbooks, available at the Spitzer Science Center.
The latent images are a manageable nuisance for most types of science operation.
Multiple dithers and custom flat field frames have been shown to provide photometry at a
repeatability level of better than 1%, despite the variety of potential issues with the latent
images. However, for coronagraphy dithering is not feasible. In addition, the offsetting
performance of JWST necessitates putting a bright source close to the center of the
coronagraph field to measure its position in the instrument frame as part of centering it
accurately on the coronagraph. The resulting latent images may compromise the MIRI
coronagraphic data, unless they are successfully mitigated.
3
Mitigation of Latent Images in Coronagraphy
The necessary mitigation steps can be different in character for the phase plate
coronagraph units vs. the Lyot one. For the Lyot, once a source is behind the central
occulting spot, its light will be substantially attenuated. It is then possible, for example, to
move the filter wheel without blasting the array with strong signals (although thought
needs to be given to minimizing signals even in this case). For the phase plates, even with
a star centered on the mask, until the appropriate pupil mask with band pass filter is
centered on the pupil, strong signals may impinge on the detector array. They can be
basically the full signal from the star within the passband of whatever filter is in the
beam. The impacts of this difference will become apparent as we discuss the overall
issues around mitigations.
A number of measures are already included in the instrument design and operations plans
to assist in centering on the coronagraphs without invalidating the data with strong latent
images:
1) We are requesting more than one peakup position for each coronagraph channel
(Lyot and phase plate). There is currently a request for four sweet spot positions,
at least one at roughly 10" radial distance and two at 1" but in different clock
angles around the field center. Multiple peakup positions let us approach the
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2)
3)
4)
5)
coronagraph center from different directions, thus leaving different patterns of
latents and artifacts. The multiple positions should therefore help users judge
whether a given signal is likely to be associated with an astronomical object, or is
an artifact of the centering procedure.
The filter wheel includes a neutral density filter to attenuate the signal from a
bright star during peakup. In many cases, the target stars will be far brighter than
is required for high signal to noise, so attenuating the signal will not impair the
ability to determine a centering position.
We have put the filters and blockers into an optimum order in the filter wheel.
The strategy is illustrated in Figure 2. The star is acquired with the light to the
array blocked off (to avoid latent trails coming to the peakup position). The wheel
is then moved to the neutral density position, the image is centroided, and the
wheel is returned to the opaque position. There, the star is moved to the center of
the field (again avoiding latent trails). At that point, if the centering has put the
star behind the occulting spot in the Lyot channel, the wheel can be moved around
to that position with a reasonable amount of protection from blasting it (we will
discuss another layer of protection under mitigation.
However, if the data are to be obtained in one of the phase plate channels (labeled
with their wavelengths in the figure), then: a.) for the 10.65µm channel, the array
will be blasted until the wheel has centered the filter/pupil mask on the pupil; and
b.) for the other two channels, the array will also be blasted as the wheel advances
over the other filters for which the phase plate is ineffective
While advancing the filter wheel, the array will be read out in "flush" mode. In
this mode, the reset is turned on and the array addresses are advanced at the usual
(maximum) cadence but without digitizing any signals. The entire array can be
reset continuously every 0.02 seconds. Therefore, the signal levels are maintained
at a relatively small level and latent images, particularly hard and slow ones,
should be minimized. Another potential benefit is that there is some anecdotal
evidence that a rapid readout such as in flush mode has a modest beneficial effect
in reducing latent images.
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Figure 2 Basic centering strategy, from top to bottom. The star is acquired with the filter wheel in the
closed or "opaque" position, then the wheel is moved to the neutral density filter and centroided,
then moved back to the opaque position where the star is moved to the center of the field. For
compactness in the figure, we show the wheel fixed rather than rotating.
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These five measures will still leave us with potential latent images for bright stars to be
imaged in the phase plate coronagraphs, and some of these stars will be sufficiently
bright that flush mode cannot completely remove the latents. There are three additional
approaches that have been considered to avoid these artifacts:
1) We could have the telescope do an avoidance maneuver, in which it offsets a
modest amount (~ 20") to take the star out of the coronagraph field while the filter
wheel is moved. The star would then be offset from the avoidance position to the
center of the coronagraph. Although we might be offsetting 20" or a bit more, the
errors will be smaller than the nominal level (Figure 1) because a number of the
error terms such as long term thermal drift will not apply. Nonetheless, we will
have latent trails on the array because it is not protected from the star during the
final centering step. We consider this mitigation as a useful backup mode in case
there is a problem that makes it impossible to use better ones, but will not study it
further at this time.
2) If we anneal the array, it should remove all latent images and give us a clean set
of detectors for the observations. Although such frequent annealing would be
impermissible with the hydrogen dewar, it is not clear that it poses any cryogenic
issue with the cyrocooler. Nonetheless, this statement needs to be evaluated in
terms of any issues with instrument thermal balance. In addition, at present the
nominal anneal duration is 2 hours, at which point the array is supposed to be
back to nominal operating conditions and fully calibrated. It may be possible to
relax some of these conditions for coronagraphy, and thus to speed up the process.
This possibility also needs evaluation.
3) We could close the "contamination control" cover to block all light to the detector
array while the filter wheel is moved into final position for coronagraphy.
Assuming that the mechanism can be engineered and qualified for this use,
closing the cover is the only way identified at present to mitigate bright latent
images that does not either leave trails or impose substantial inefficiencies.
4
Requirements on Contamination Control Cover for Use in Coronagraphy
To support an engineering analysis of the implications for the mechanism use, we have
considered the science applications to derive a number of times the CCC would need to
be cycled (one cycle is closing and then opening).
A crude upper limit can be provided by assuming that the MIRI coronagraph is used for
10% of a total 10 year JWST mission life, and that the CCC must be closed every 2 hours
during such observations. The result is 4400 cycles. It is likely that more realistic science
inputs will provide lower numbers (for example, it might not be necessary to close the
CCC for every observation).
We therefore made an independent estimate by considering the two prime coronagraphic
programs, studies of debris disks and searches for massive planets.
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We first estimated how bright a source would be that would require closing the cover.
Meixner et al. (2006) have calculated that a zero magnitude (Vega system) star will give
us about 2 X 107 e/s on the most strongly illuminated pixel of an array looking though
one of the shorter coronagraph bandpass filters. In flush mode, with an integration of 0.02
s, the collected signal will be 4 X 105 electrons per array-reset interval. We assume that
we want the latent image to decay to 3 X 10-4 of the primary image strength before we
start taking coronagraphic data, and that the tiny offset from the peakup position to the
center of the coronagraph field will require only 100 seconds. It is then straightforward to
show that we would consider closing the CCC for stars brighter than 5th magnitude, to so
that any latent image is significantly less than the array read noise.
The next part of the question is what fraction of the targets in the two key programs will
be brighter than 5th magnitude. I started with the planet search objective. I went through
the listing of nearest stars out to a distance of 6pc, and tried to construct a logical
program. For example, I rejected all stars with companions closer than 20" as confused,
and binary stars with larger separations I treated as two independent targets. I did not
reject very bright stars that passed these criteria, so rather than a strict planet search
program I looked at a "faint companion" search. My final target list had 71 stars. Of
those, 39 were brighter than L = 5 mag. (I used K photometry and appropriate K – L
colors to estimate the L magnitudes). Since virtually all stars of the relevant spectral
types have L - N ~ 0, there will be 39, or just over half the sample, of stars that meet the
criterion for considering closing the CCC. For reference, I found that 58 of the stars were
brighter than L = 7 mag.
I next considered debris disk imaging. Spitzer has resolved only four debris disks, three
of which are no farther than 8pc. For a good yield of successes with JWST, I therefore
assumed a maximum distance of 40pc (giving slightly better physical resolution than
Sptizer obtains on Vega and Fomalhaut). I took targets from the comprehensive survey of
nearby A stars by Su et al. (ApJ, in press). There are 28 stars with detected 24µm
excesses in this sample and within 40pc. All of them are of L = 4.5 mag. or brighter.
From these two examples, I conclude that the MIRI coronagraph will be limited by the
achievable physical resolution before most of the target systems will fall below the
threshold for considering closing the CCC. That is, the CCC would need to be closed for
a large fraction of the targets for both programs.
Next, I considered integrated science programs. For planet searches, I divided the
program into an initial phase, and a confirmation one. Initially, I assumed there were 300
targets (e.g., a complete sample to about 10pc), and that the search strategy would be to
observe each star at 3.6 and 4.5µm with NIRCam and at 15µm with MIRI. Planets should
have a unique signature when observed in these three bands. I assumed that there would
be two CCC cycles for each "bright" target, corresponding to peaking up at two positions
to guard against artifacts. Based on my study, I also assumed that 50% of the targets
would be bright enough to require closing the CCC. This initial search phase then
required 300 CCC cycles.
I also considered a follow up program of 1/4 of the targets that might have something
interesting in the images. Here, we would presumably do a thorough set of observations,
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so I calculated for observing in all three phase plate channels. Since one would also want
to integrate deeply, I assumed 4 CCC cycles per channel, giving a total of 900 CCC
cycles for this phase.
In the case of the debris disk program, Spitzer data can be used for a thorough initial
search, so JWST will be doing nearly purely follow up observations. I assume that each
system we observe will be measured in all four coronagraphic channels, both to
characterize the spectra of the debris systems and to look for planets that might be
shepherding them. Extrapolating from the 28 nearby A stars with 24µm excesses in Su et
al., I took a maximal total program for JWST to be 100 stars (for comparison, this is
about 20% of all the 5th magnitude and brighter A to early F stars, of which most do not
have 24µm excesses). Assuming four CCC cycles per filter, the total for this program is
1200 CCC cycles.
The grand total for these two science programs is thus 2400 CCC cycles. There are no
other science programs that the MIRI Science Team has identified that make nearly such
as intensive use of the coronagraph on bright objects. We can therefore take this estimate
as valid for the overall instrument usage. It is consistent with but below our crude upper
limit, which gives it additional credibility. We conclude that, from the standpoint of
unconstrained science usage, it would be desirable for the CCC to be designed and
qualified for 2400 cycles (where a cycle includes closing the CCC and then opening it)
for coronagraphic observations.
5
References
Anandakrishnan, S. et al. 2006, "JWST Pointing Error Allocation and Performance
Prediction Analysis," NGST, DRD#D36177, Rev. B
Engelbracht, C. W. 2005, MIPS Si:As Detector Artifacts," technical memo
Hora, J., et al. 2004, "In-flight Performance and Calibration of the Infrared Array Camera
(IRAC) for the Spitzer Space Telescope," Proc. SPIE, 5487, 77
IRAC Handbook, version 3.0, Jan. 20, 2006, issued by Spitzer Science Center, edited by
Seppo Laine and Bill Reach
Meixner, M. et al. 2006, "MIRI Coronagraphy Operations and Mitigation Strategies for
Detector Persistence," JWST-STScI-000900
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