TIPS/JIM
April 16, 2009
Agenda:
INS Division News (Jerry Kriss)
MIRI Dither Patterns (Christine Chen)
Observing Exoplanets with JWST (Kailash Sahu)
NICMOS Grism Wavelength Calibration (Nor Pirzkal)
Next TIPS/JIM: May 21, 2009 (maybe)
1
Instruments Division News
4/16/2009
•
•
•
•
•
Welcome to our newest staff members: Kevin Volk joins INS as a Canadian Space
Agency (CSA) Scientist working on the JWST Tunable Filters Imager (TFI). Also
new as a CSA Scientist is André Martel, although we know André already for his
work over the past two years on the WFC3 team.
Farewell, too, to some staff whom we will miss: Jessica Kim, Helene McGlaughlin,
and Katya Verner will all be leaving the institute over the next week. We greatly
appreciate your contributions, wish you well in your new endeavors, and hope to see
you back again some day.
HST news:
o Atlantis is on the pad and still preparing for flight to HST on May 12. 26
days to go!
o Ground system software freeze is in place. Please consult the HST
Mission Office if you have any questions.
o The SM4 delta-readiness review is coming up next Tuesday, April 21, in
the auditorium.
JWST news:
o The WIT team hosted a productive calibration summit. They made lots of
progress on identifying common elements among the instruments and
laying groundwork for calibrations during I&T.
o Planning is underway for the next partner’s workshop, May 21-23, in
Ottowa.
Security changes: I know that the flurry of new procedures for ITAR, computer
security, and physical security have made many of us uncomfortable. I want to thank
the vast majority of you who have taken these changes in stride with civility and
equanimity. This is exactly what we have been working to achieve over the past
couple of years. However, there were a number of unpleasant confrontations between
a few staff members and the personnel at the front desk. One of the lessons we
learned from our consultations with Ivan Rosenberg two years ago is that it only takes
a few incidents like this to poison the atmosphere of trust and respect among the staff.
Failure by managers to take corrective action then exacerbates the problem. I want
you to know that we have not ignored these interactions nor do we deem them
acceptable. We have taken steps to rectify them. Please remember that it is paramount
that we maintain our civility and respect for each other, even when confronted with
procedures that we dislike. And we should especially refrain from venting our
frustrations on staff who are responsible for enforcing the procedures. I and all the
management staff welcome feedback and suggestions. Many of these rules result
from government or NASA regulations over which we have little control. They often
have short deadlines for implementation that leave little time for either investigating
exceptions or developing a full panoply of workarounds. We are and will continue to
work on ways to make STScI both more secure and procedures less intrusive or
complex for our staff. Please work with us in a civil way to do so.
•
•
The next INS lunch is next Thursday, April 23, in the Boardroom, from 12:00-1:30.
Again, volunteers are needed for coordinating this!
The next TIPS meeting may be Thursday, May 21, 2009, depending on how busy we
all are.
TIPS/JIM
April 16, 2009
Agenda:
INS Division News (Jerry Kriss)
MIRI Dither Patterns (Christine Chen)
Observing Exoplanets with JWST (Kailash Sahu)
NICMOS Grism Wavelength Calibration (Nor Pirzkal)
Next TIPS/JIM: May 21, 2009 (maybe)
MIRI Dither Patterns
Christine H Chen
Dithering Goals
1. Mitigate the effect of bad pixels
2. Obtain sub-pixel sampling
3. Self-calibrate data if changing scattered
light and/or thermal emission background is
significant
⇒ It is anticipated that dithering will enhance
the majority of science observations
(although some programs will require no
dithering)
MIRI Observing Modes
• Direct Imaging
Full array
– Subarray
× Coronagraphic Imaging
Low Resolution Spectrograph (LRS)
• Medium Resolution Spectrograph
(MRS)
MIRI Direct Imaging Specifications
• Available Filters: 5.6, 7.7,
10.0, 11.3, 12.8 15, 18, 21,
and 25.5 µm
• Plate Scale: 0.11″/pixel
• Critically sampled at 7 µm
• Field of View: 75″x112″
(680x1024 pixels)
• Geometric Distortion: <0.9%
at array corners
Gordon & Meixner 2008
Time-Variable Thermal Background
•
•
•
•
Telescope thermal emission is
expected to dominate the
background for λ >15 µm
Thermal background is expected
to change due to variable
telescope illumination as
telescope is slewed
Self-calibration of deep fields
with time-variable pedestals has
been demonstrated using
NICMOS HDF-N and NDF-S
data (Arendt, Fixsen, & Mosley
2002)
Propose using 12-point
Reuleaux and 311-point random
cycling patterns to optimize selfcalibration
Reuleaux Triangle
•
•
•
Reuleaux polygon is a curve of
constant width; the distance
between two opposite, parallel,
tangent lines to its boundary is
constant
The Reuleaux triangle optimizes
the figure of merit (Arendt
Fixsen, & Mosley 2000),
samples a wide range of spatial
frequencies in a uniform
manner, and is therefore wellsuited to the Fixsen leastsquares flat field technique
The 36-point Reuleaux triangle
has been use in detailed
characterization of the IRAC
PSF (Marengo et al. 2008)
The Random Cycling Pattern
•
•
•
•
Predetermined table of
311 dither positions
The x- and y- offsets
from the array center
are randomly drawn
from a Gaussian
distribution with a
specified FWHM
Observer specifies
beginning position and
end position in dither
pattern
Every contiguous 4
offset positions contain
1/2 pixel offsets in each
direction
Subpixel Sampling
•
•
•
•
A. Fruchter
Since MIRI is not badly
undersampled, 0.5 pixel
subsampling should be adequate
for the majority of science
observations
Reuleaux and Cycling patterns
have 0.5 pixel offsets built-in to
provide some subpixel sampling
The measured geometric distortion
(<0.9% in the corners) implies that
10 pixel offsets in the center of the
array will correspond to 10.1 pixel
offsets in the corners of the array
A 4-point box pattern
(0,0),(0,2.5),(2.5,0),(2.5,2.5) will be
offered that can be used alone or
in conjunction with either the
Reuleaux or Cycling Patterns
JWST Observatory Offsetting Accuracy
•
•
•
Anandakrishnan et al. 2006
Offsets smaller than 0.5′ (270
pixels) do not require use of
new guide stars
Commanded offsets <10 pixels
will have adequate source
placement precision (11 mas)
for interlacing from 1/2 pixel
sub-sampled images at the
center of the array
Observatory will possess 7 mas
jitter while pointed at a fixed
position
Proposed Direct Imaging Dither Patterns
Pattern
4-Pt Box
Cycling
12-Pt Reuleaux
Scale
N/A
Small
Medium
Large
Small
Medium
Large
Max Offset
3.5 pix
11 pix
119 pix
161 pix
13 pix
27 pix
55 pix
Median Offset
2.5 pix
10.5 pix
53 pix
97 pix
15 pix
30 pix
59 pix
Sub-Pixel
pix
pix
pix
pix
pix
pix
pix
MIRI LRS Specifications
• Wavelength range: 5-10 µm
nominal (2-14 µm expected)
• Slit Dimensions: 0.6″×5.5″
(5x45 pixels)
• Spectral Resolution: R=100
at 7.5 µm
• Spatial Plate Scale: 0.11″
/pixel
• Spectral Plate Scale: 2
pixels/resolution element
• Critically sampled (spatially)
at 7 µm
Gordon & Meixner 2008
Background Subtraction
• Simultaneous
measurements of the sky
are needed to perform
background subtraction
• PSF size: (1.22λ/D=) 0.54″
at 14 µm, ~1/10th slit
length, suggesting that 2
dither positions separated
by 1/3 of the slit length
should be adequate for
background subtraction
Proposed LRS Observing Modes
• Point Source/Staring Mode
• Two dither positions with source near the center of the slit
• Extended Source/Mapping Mode
• Observer specified dither pattern
• Number of slit positions parallel and perpendicular to the slit
• The size of the offset in each direction
JWST Observatory Offsetting Accuracy
•
•
•
Anandakrishnan et al. 2006
Offsets smaller than 0.5′
(270 pixels) do not require
use of new guide stars
Observatory will possess 7
mas jitter while pointed at a
fixed position
Commanded dither offsets
of 1/3 slit length will place
the source onto the detector
with 17.1 mas precision
(20% precision) adequate
for 1/2 pixel subsampling
Summary
• Direct Imaging (full array)
– Subpixel sampling: 4 point box
– Self-Calibration: 12 point Reuleaux triangle
and random cycling
• LRS
– Extended Source/Mapping mode
– Point Source/Staring Mode
Observatory Pointing Efficiency
Slew Performance: [Max Accel, Max Rate, T1] =0.0001453 8.19e -005, 0.0539
2
0.036, 60
10
slew capability (6 rwas)
slew capability (4 rwas)
slew requirement
1
Time To Complete Slew, min
10
0
10
Mitchell 2008
-1
10 -6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
Angle (degrees)
•
•
The slew time for offsets up to 3.6″ (33 pixels) will be 10 sec independent of
slew size (4-point box, 12-point Reuleaux, and small Cycling patterns)
Larger slews will take exponentially longer times (medium and large Cycling
patterns)
TIPS/JIM
April 16, 2009
Agenda:
INS Division News (Jerry Kriss)
MIRI Dither Patterns (Christine Chen)
Observing Exoplanets with JWST (Kailash Sahu)
NICMOS Grism Wavelength Calibration (Nor Pirzkal)
Next TIPS/JIM: May 21, 2009 (maybe)
OBSERVING TRANSITS WITH JWST:
SOME OPERATIONAL ISSUES
Kailash C. Sahu
STScI
4/20/09
1
OUTLINE
 Science Cases for Transit Observations
 Observation scenarios (NIRCam, NIRSPEC and
MIRI)
 Saturations/Expected data volumes…
 Possible solutions
SCIENCE CASES
I.
Science Case - I: An Earth-like planet
around a nearby sun-like star

Assume: every star has an Earth-like planet
•
The probability of transit for an Earth at 1 AU
around a G-type star: ~ R⨀/a ~ 7 x 1010/1.5
x 1013 ~ 0.5%
The optimal sample size needed to observe
the first earth-like planet around a sunlike star ~200
Expected brightness of that the first sun-like
host of an earth-like planet: V ~6.
Science Case I: Observations


Transit duration for an earth analogue ~ 12
hours
Expected Science observation:
•
Continuous monitoring of the star
before, during, and after transit (total of
36 hours)
— Imaging with NIRCam: to get very
high S/N for (i) accurate radius
determination, (ii) determination of
inclination angle…
— Spectroscopy with NIRSpec: high
S/N spectra to detect possible
atmospheric features
— Imaging and spectroscopy with
MIRI
— Imaging with FGS/TFI.
P ~ 1yr
Ttr~12 hrs
NIRCam
 2 Modules
 Each module has two channels
(SW:0.6 to 2.3 µm & LW:2.4 to 5 µm)
 Total of 10 detectors, 8 for SW
and 2 for LW
 Each detector has 2048x2048
pixels
Pixel scale:
SW: 0.032”/pix; LW: 0.064”/pix
 Image size: 2.2’ x 4.4’. SW and
LW channels observe the same
field simultaneously
Module B

Module A
Short wavelength channel
2.2’
Long wavelength channel
SCIENCE CASE-I: Expected Data Rate
Can we observe such a bright star?
•
•
•
Saturation occurs at V~17, for the minimum ‘exp time’ of 10.6sec.
Fortunately, NIRCam has defocusing mirrors, which allow observations of stars up to V ~4.
Subarrays can also be used which allow shorter integrations, and allow observations of
brighter stars.
Courtesy John Krist
In Focus F210M
12λ Defocus x10
SCIENCE CASE-I: Expected Data Rate
Expected observation cadence:
NIRCam: 10.6 sec + 10.6 for readout, 2 detectors (1 SW and 1 LW)
(MULTIACCUM pattern:
TGROUP=10.6 s, NGROUP=1 to 2, NFRAME=1, NSKIP=0)
(Data volume is roughly the same if subarrays are used for brighter stars)
Expected Data Volume per day:
2(channels)x2048x2048(pixels)x16(bits per pixel read) x86400/20.6 = 5.6e11 = 563 Gbits/day.


This exceeds the data volume limit by a factor of ~2.
Compression algorithm will help, but may not completely solve the problem.

For NIRSPEC (which has 2 detectors), the data volume constraints are
similar.
For MIRI (one detector), constraints are smaller by a factor of 2.

SCIENCE CASES
Science Case - II: Determining the
frequency of hot earths
(Or, to detect the first extragalactic exoplanets)
 The goal is to determine the
frequency of hot earths
 Expected transit signal ~ 0.1% (R ~
3 REarth), transit duration ~ 3 hours,
orbital period ~ 1 to 5 days.
 A reasonable way to achieve this is
to monitor a rich stellar field, similar
to the SWEEPS program towards
the Galactic bulge.
HST Image of
the SWEEPS Field
2.3’ x 2.3’
~200,000 stars
Determining the frequency of
hot earths
POSSIBLE TARGET:

Monitor a nearby, rich, high-metallicity cluster, such as NGC 6791
([Fe/H] ~+0.4).

Saturation will be just avoided for solar-like star with V ~17.

This coincides with the turn-off magnitude for this cluster, making this
an ideal target.

Hot-earths can be detected with 10-sigma detection.

Monitoring of a 2000 to 5000 stars can lead to detection of ~20 hot
earths, further boosted by metallicity.
SCIENCE CASE-II: Observations

Expected Observations:

NIRCam imaging using all the 10
detectors

Continuous monitoring for 8 to 10 days
similar to SWEEPS and 47-TUC HST
observations.

Filters to be used: F115W and F150W
for the SW channel; F277W and
F356W for the LW channel

Extra-galactic planets:


Stars in LMC are ~3 magnitudes
fainter than the bulge stars.
NIRCAM/JWST is more sensitive
by 2 to 3 mag. compared to
ACS/HST.
There are 100,000 stars in the
NIRCAM/JWST calibration field,
which is ideal for such a study.
JWST Calibration Field
Courtesy: Jay Anderson
SCIENCE CASE-II:
Expected Data Rate
Expected observation cadence:
10.6 sec + 10.6 for readout
(MULTIACCUM pattern:
TGROUP=10.6 s, NGROUP=1 to 2, NFRAME=1, NSKIP=0)
Expected Data Volume per day:
10(channels)x2048x2048(pixels)x16(bits per pixel read) x86400/20.6 =
2.8e12 = 2,815 Gbits/day.


This exceeds the data volume limit by an order of magnitude!
One way to solve this impasse would be to require for this type of
observation using exposures 10 times as long, or stars 2.5 magnitudes
fainter. This, however, results in a less interesting experiment. Being able to
reach to a few Earth radii as the limit for planet size would certainly be
advantageous. And important spectroscopic follow-up observations are also
possible at V ~ 17, but impractical at V > 20.
Science Case I: Expected Data Rate
Possible solution:
Fortunately, the transits typically last 1 to 12
hours.
So it would be scientifically acceptable to
average, or sum the individual 10s
exposures to 10 minute cadence
onboard, which provides a clean
solution.
FPAP has the capability to do such onboard averaging from 2 to 16, in
powers of 2. It can handle full frames
from all the 10 NIRCam detectors.
The plan is to take advantage of this
capability, which will facilitate these
transit observations.
P ~ 1yr
Ttr~12 hrs
NIRSPEC
(with thanks to: Jason Tumlinson)
 Wavelength range: 0.6 to 5 microns.
 3 observing modes: R ~ 100 prism mode, R
~ 1000 multi-object mode, and R ~ 3000
integral field and long-slit spectroscopy
mode.
 Two 2048 x 2048 detectors
 A 1.6x1.6 arcsec slit has been specially
introduced in the MSA for exoplanet transit
observations.
NIRSPEC
(with thanks to: Jason Tumlinson)
SATURATION:
 >85% of the planet-hosting stars are too bight in full-frame mode.
 Subarrays allow observations of ~99% of the planet hosts.
 Subarrays restricted to spectral features will further facilitate such observations.
 On-board averaging capability can solve any data-volume problems.
MIRI
thanks to: Scott Friedman
 Wavelength range: 5 to 27 microns.
 Imager: broad and narrow-band
imaging, phase-mask
coronagraphy, Lyot
coronagraphy, and prism lowresolution (R ~ 100) slit
spectroscopy from 5 to 10
microns, 1024 x 1024 detector
 Spectrograph: R~300, over 5 to
27 microns, 1024 x 1024 detector.
 Maximum data volumes: ~2 times
larger than the data volume limit,
which can be solved by on-board
averaging.
TIPS/JIM
April 16, 2009
Agenda:
INS Division News (Jerry Kriss)
MIRI Dither Patterns (Christine Chen)
Observing Exoplanets with JWST (Kailash Sahu)
NICMOS Grism Wavelength Calibration (Nor Pirzkal)
Next TIPS/JIM: May 21, 2009 (maybe)
NICMOS grism wavelength calibration
TIPS - April 16, 2009
N. Pirzkal
R. Bohlin
D. Thatte
TIPS - April 16, 2009
NICMOS Grism Mode
• 3 grisms
• NIC3
• Low resolution (200 angstrom/pixel)
• G096: 0.8μ < λ < 1.2μ
• G141: 1.1μ < λ < 1.9μ
• G206: 1.4μ < λ < 2.5μ
TIPS - April 16, 2009
Previous Calibration
• Mainly Cycle 7 and 8 programs. i.e. ~10 years ago
• Calibration done using PNe observations of VY-22 and HB12
λ = m ∗ ∆x + b
∆x = (x − x0 )
• No field dependence calibration
• No verification of repeatability
• Previous calibration done in column space
and not along the tilted trace
• No field dependence information
TIPS - April 16, 2009
m
b
G096
-0.00536
0.9487
G141
-0.00799
1.401
G206
-0.01152
2.045
PNe as calibrator in the NIR
• Bright emission lines
• Provides a few good lines in the range of each of the NIC3 grisms
• Some are blends, but these can be measured in smoothed reference spectrum
G096
G141
G206
TIPS - April 16, 2009
Program 11331 Strategy
• 2 PNe
• 4 positions
• 2 dithers
• Total of 16 independent
spectra
• Aim for ~4 lines per spectra
• HB12 and VY22 observed 4
months apart
TIPS - April 16, 2009
NICMOS Trace
• Measure positions of lines on the grism/slitless images
• Compute the equation of the trace:
(y-y0 ) = a1 ∗ (x − x0 ) + a2 ∗ (x − x0 )2 + ... + an ∗ (x − x0 )n
• NICMOS is linear within the measurement uncertainties
• Slope varies randomly (due to grism/filter positioning errors)
200
Grism
Slope (Deg)
150
G096
3.05 +/-0.19
100
G141
0.95 +/-0.20
50
G206
1.37 +/- 0.36
100
150
200
TIPS - April 16, 2009
Parametrizing the dispersion relation
TIPS - April 16, 2009
Allowing for field dependence
• m and b can be made to vary as a function of the position of the source (x,y) on
the detector
λ = m ∗ δl + b
λ = m(x0 , y0 ) ∗ δl + b(x0 , y0 )
m(x, y) = m0 + m1 ∗ x + m2 ∗ y + ...
b(x, y) = b0 + b1 ∗ x + b2 ∗ y + ...
0.943
!0.00563
!0.00564
0.942
!0.00565
0.941
200
!0.00566
!0.00567
200
0.940
0
0
100
100
100
100
200
200
0
0
TIPS - April 16, 2009
NICMOS grism variation over field
• Very small variation over the field
m
b
G096
-0.0056
+/- 0.3%
0.941
+/- 1%
G141
-0.0080
1.396
+/- 2% +/- 0.2%
G206
-0.0114
2.039
+/- 2% +/- 0.2%
• No significant change over a period of
4 months
• Once the varying spectral slope is
accounted, all 3 grisms are well
calibrated
• 1st order trace
• 1st order wavelength dispersion
• 1st order field dependence
TIPS - April 16, 2009
Example: G096 - Small field variation
200
150
0.004
m
0.002
0.000
200
!0.002
100
!0.004
0
100
100
50
200
0
60
80
100
120
140
160
180
200
150
0.002
b
0.001
0.000
200
!0.001
!0.002
100
0
100
100
50
200
0
60
80
100
120
140
160
180
New values confirm previous calibration
New
m
b
Old
G096
-0.0056
+/- 0.3%
0.941
+/- 1%
G096
-0.0054 0.9487
G141
-0.0080
1.396
+/- 2% +/- 0.2%
G141
-0.008
1.401
G206
-0.0114
2.039
+/- 2% +/- 0.2%
G206
-0.0115
2.045
TIPS - April 16, 2009
m
b
Putting it all together: Consistent solution
over all 3 grisms
TIPS - April 16, 2009
G096 - Old
TIPS - April 16, 2009
G096 - New
TIPS - April 16, 2009
G141 - Old
TIPS - April 16, 2009
G141 - New
TIPS - April 16, 2009
G206 - Old
TIPS - April 16, 2009
G206 - Old
TIPS - April 16, 2009
All 3 grisms - Old
TIPS - April 16, 2009
All 3 grisms - New
TIPS - April 16, 2009
Conclusion
• Wavelength calibration of all 3 NICMOS grism show no large change from
previous values (Cycle 7). (This is to be further tested after SM4)
• 16 independent observations, 4 months apart show no time variation
• New wavelength solution is consistent over all three grisms
• Calibration is done *along* the trace and not just by counting the number of
columns between source and pixel in the spectrum. This is how it is done for
ACS and WFC3
• Field dependence has been calibrated (but it is only a ~ 1-2% effect)