TIPS-JIM Meeting
15 September 2005, 10am, Auditorium
1.
Update on TGM Performance
Marco Sirianni
2.
3.
HST Optical Behavior and Focus Status
Optimizing JWST Dither Strategies
Matt Lallo
Anton Koekemoer
Next TIPS Meeting will be held on 20 October 2005.
TIPS Sep 15, 2005
Update on TGM operation
Presented by Marco Sirianni
Credits for TGSMOV
Planning:
G. Chapman, M. Reinhart, A. Vick
ACS:
J. Biretta, C. Cox, A. Koekemoer, R. Lucas, J. Mack,
C. Pavlovsky, C. Proffitt, M. Richardson,
K. Sembach, M. Sirianni.
NICMOS:
S. Arribas, R. de Jong S. Malhotra, K.Sahu
A. Schultz, C. Xu,,
TGM transition
• HST successfully transitioned to TGS mode
on August 28th (gyro pair 1-2).
GYRO #
Before 8/28/05
After 9/1/05
1
ON
ON
2
ON
ON
3
Non functioning
Non functioning
4
ON
OFF
5
Non functioning
Non functioning
6
OFF
OFF
Jitter magnitude
• Typical boresight jitter in 3 gyro: 3-5 mas (60 sec)
B.
Clapp
Gyro 1-2: best pair for jitter and disturbance rejection
TGSMOV
Several programs aimed to assess the on-orbit performance:
– Comparison with 3G mode
– Comparison with February 2G test (Gyro pair 2-4)
Instrument
ACS
NICMOS
Program
Goal
Status
10458
HRC PSF- pointing
3 clusters- dither
Completed
10459
HRC PSF- pointing
CVZ Target
Planned
Oct 15, 2005
10460
10461
10464
10462
Coronography
Completed
Moving Target
Completed
PSF
Completed
Coronography
Completed
ACS/HRC PSF
Sirianni, Lucas, Pavlovsky, Mack
NGC 2298 HRC F555W 10 sec
Data:
Multiple exposures of
three rich star clusters
with HRC F555W.
Different exposure time:
10, 100, 500 sec
Bright (V~13) and Faint (v~14) guide star
Analysis:
FWHM measurement for stars
with S/N > 10
ACS/HRC PSF
2 Gyro: August 2005
144 Frames
FWHM (pixels): 2.036+/- 0.039
Min:1.95
Max: 2.19
2 Gyro: February 2005
158 Frames
FWHM (pixels): 2.009+/- 0.026
3 Gyro: Historical (2002-2004)
124 Frames
FWHM (pixels): 2.041+/- 0.034
3 Gyro (Feb 05)
1.987+-/0.016
3 Gyro (Apr-Aug 05) 2.031+/0.040
ACS/HRC PSF
NGC 2298 Aug 29 2005 2G
NGC 416 Aug 12 2005 3G
Images are displayed in logarithmic scale.
Cosmic ray rejection and correction for geometric distortion have not been applied..
Current results agree with previous 2G tests and 3G performance.
HRC/PSF - results
We confirm that, as seen in 3G and in the 2G test in
February:
• The PSF FWHM does not show any dependence on the
brightness of the guide star
• The PSF FWHM in long exposure frames is slightly
broader than in short exposure frames
We also have two minor items under investigation:
•! One of the target show a PSF systematically broader than
the other two. (It will be revisited as CVZ target)
•! The PSF fwhm increases systematically within each orbit
FWHM vs Target
2.026+/- 0.030
2.030+/- 0.033
2.088+/- 0.040
NGC 2298
RA: 06:48:59
DEC:-36:00:19
SunAngle ~ 70
MoonAngle
~64
NGC 1851
RA: 05:14:08
DEC:-40:03:00
SunAngle ~ 89
MoonAngle
~78
NGC 6752
RA: 19:10:52
DEC:-59:59:04
SunAngle ~ 115
MoonAngle
~142
FWHM vs Time
10458 Visit L2:
4x10 sec
4x100 sec
4x10 sec
2x500 sec
10 sec
100 sec
500 sec
The PSF gets broader
with time within any
orbit. The variations
are bigger than those
due to the increased
exposure time.
PSF variation within orbits
• Same pattern observed in
all orbits.
• It does not seem to
change with terminator
crossing.
• Although this pattern was
not as evident in the
February test, we see it in
some recent 3G data.
• It is likely a normal
breathing effect (see next
talk)
Similar comparison with recent 3G data
Three different star clusters: 2x35 sec + 4x300 sec
Apr 05
Jun 05
Aug 05
Significant variation within an orbit seen also in 3G
ACS/HRC Pointing
Koekemoer
ACS/HRC pointing
Comparison with February 2-gyro test and 3-gyro data obtained
under similar conditions.
Total Shift r.m.s.
(milliarcsec)
Roll Angle r.m.s.
(degrees)
2-Gyro (Feb)
2.29
0.00097
2-Gyro (Aug)
2.08
0.00070
3-Gyro
2.19
0.00093
Pipeline Processing Test - MultiDrizzle:
Combined output for 10 exposures - successful, nominal
Current results agree with previous 2G tests and 3G performance.
ACS/Coronography
Cox
• F775W Coronagraph Images
3-gyro
2-gyro
Normalized
Counts
ACS Coronography
Arcsecond
s
Current results agree with previous 2G tests and 3G performance.
Moving Target Test
C. Proffitt
•
32 x 0.3 S F435W HRC images of
Mars over 1 orbit.
•
Rotation of Mars and movement of
surface features complicates cross
correlation of images to find shifts
There seems to be an apparent shift in
pointing for Mars observations which is roughly
consistent with the effect of a
2.4 s error in HST’s position along its orbit
(~15 km) on the calculated parallax correction.
It could be an effect also present in 3G
Retrospective ephemeris will be used to compare to ephemeris used on orbit.
NICMOS/PSF
Malhotra et al.
•! NIC2 PSF is consistent
with previous
observations
•! NIC1 PSF is broader than
expected. Quite likely the
target is not a point source.
NICMOS/Coronohraphy
Observations of HD 17925, G star, V=6.0
F160W
F110W
Direct image
Coronagraphic
image
Acquisition successful, repeatable
Summary
• TGM since August 28
• HST is operating as good as usual
• Instruments performance is still excellent
•! We are currently tracking few minor items
–! Image shifts in Mars operations
–! Small PSF width changes in one target and within
orbits
•! TGSMOV will be completed in mid October
•! Final summary will be presented at the
calibration workshop at the end of October
TIPS-JIM Meeting
15 September 2005, 10am, Auditorium
1.
Update on TGM Performance
Marco Sirianni
2.
3.
HST Optical Behavior and Focus Status
Optimizing JWST Dither Strategies
Matt Lallo
Anton Koekemoer
Next TIPS Meeting will be held on 20 October 2005.
TIPS
15Sep2005
TELBranch
HSTOpticalBehavior
&FocusStatus
M.Lallo,R.Makidon,S.Casertano
HST Optical Telescope Assembly (OTA)
TIPS
15Sep2005
TELBranch
HST Optical Telescope Assembly (OTA)
,IGHT3HIELD
'RAPHITE%POXY
4ELESCOPE4RUSS
#ENTRAL"AFFLE
3ECONDARY
-IRROR
!SSEMBLY
3PIDER3UPPORT
&OCAL0LANE
!LUMINUM
&ORWARD3HELL
#/
34!
2
34)3
.)#-/3
0RIMARY-IRROR
!#3
7&0#
TIPS
15Sep2005
TELBranch
Timescales & Mechanisms
TIPS
15Sep2005
TELBranch
HST experiences variations in focus, coma, & astigmatism (as
measured through the SIs) on a number of different timescales.
From shortest to longest:
1. Orbital (“breathing”)
2. Medium-Short-Term (“wandering”)
3. Medium-Long-Term (seasonal/annual)
4. Long-Term (desorption)
Orbital Effect on Focus (“breathing”)
TIPS
15Sep2005
TELBranch
HST focus displays a clear orbital period
- Identified in 1993 by P. Bely with FOC data.
- SM despace in µm empirically found = 0.7(LS-MLS)+K
- Variable offset K reflects the “wandering” of the orbital mean focus.
- Scale factor known to vary slightly with SI, however recent (2005) HRC
data shows excellent agreement with above model using original scale
factor.
- Implies aft light shield temperatures are still primary drivers for orbital
focus variations.
- Heating of
radiation.
the SM support structure is driven by IR from earth and solar
TIPS
15Sep2005
TELBranch
Orbital Effect on Focus (“breathing”)
HRC measured focus (blue) and Bely “breathing” model:
ACS/HRC Focus Measurements & Lightshield Breathing Model
0.0
Focus (in microns @ Secondary Mirror)
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
17.00
17.25
17.50
17.75
18.00
18.25
18.50
18.75
19.00
Hours (Day 2005.142)
19.25
19.50
19.75
20.00
20.25
20.50
Orbital Effect on Coma & Astigmatism
TIPS
15Sep2005
TELBranch
Coma & Astigmatism show an orbital signature
- Thermally induced motion of
the SM is likely to also include tilts and
decenters.
- With HRC (well-sampled & off-axis) we can accurately measure coma and
astigmatism which could be induced by such motions.
- We identify orbital signatures in coma and astigmatism; behavior appears
repeatable but more complex than focus (no model yet).
- Aberrations can be mapped back to tilts & decenters of
optical elements
using ZEMAX model of OTA+HRC
- Ongoing analysis suggests relatively large motions if
induced only by HST
SM.
- More data will be obtained via Cycle 14 Optical Monitor Program, 10752.
TIPS
15Sep2005
TELBranch
Orbital Effect on Coma & Astigmatism
Coma & astigmatism in HRC over 2 consecutive orbits
ACS/HRC AOA Test: Y-Coma v. Time
ACS/HRC AOA Test: X-Coma v. Time
0.003
0.000
0.002
-0.001
0.001
-0.002
0.000
-0.003
X-Coma
-0.004
-0.002
-0.003
-0.005
-0.007
AOA Test (Orbit 1)
AOA Test (Orbit 2)
-0.008
-0.006
-0.007
17.00
-0.005
-0.006
-0.004
-0.009
17.25
17.50
17.75
18.00
18.25
18.50
18.75
19.00
19.25
19.50
19.75
20.00
20.25
-0.010
17.00
20.50
AOA Test (Orbit 1)
AOA Test (Orbit 2)
17.25
17.50
17.75
18.00
18.25
0.012
0.011
0.011
0.010
0.010
0.009
0.009
0.008
0.008
0.007
0.007
0.006
0.006
0.005
0.005
Y-Astigmatism
0.012
0.004
0.003
0.002
19.25
19.50
19.75
20.00
20.25
20.50
AOA Test (Orbit 1)
AOA Test (Orbit 2)
0.004
0.003
0.002
0.000
0.000
-0.001
-0.001
-0.002
-0.002
-0.003
AOA Test (Orbit 1)
AOA Test (Orbit 2)
-0.004
-0.005
-0.005
-0.006
17.00
19.00
0.001
0.001
-0.004
18.75
ACS/HRC AOA Test: Y-Astigmatism v. Time
ACS/HRC AOA Test: X-Astigmatism v. Time
-0.003
18.50
Hours (Day 2005.142)
Hours (Day 2005.142)
X-Astigmatism
Y-Coma
-0.001
17.25
17.50
17.75
18.00
18.25
18.50
18.75
19.00
Hours (Day 2005.142)
19.25
19.50
19.75
20.00
20.25
20.50
-0.006
17.00
17.25
17.50
17.75
18.00
18.25
18.50
18.75
19.00
Hours (Day 2005.142)
19.25
19.50
19.75
20.00
20.25
20.50
Medium-Short-Term “Wandering”
TIPS
15Sep2005
TELBranch
Mean orbital focus varies over timescales of many orbits
- Identified early in HST mission as a trend toward negative focus (SM
closer to PM) during near anti-sun pointings (e.g. Mars opposition).
- focus wandering seen in science programs observing stellar targets
over days.
- Results in an unknown offset to the orbital breathing model for each
orbit.
- An empirical model (Hershey 1997) fitted measured focus with spacecraft
attitude and sun/earth aspect. Model broke down as long-term baseline
became more complex.
Coma and astigmatism have not been as well sampled over this
mid-frequency but may experience similar behavior.
Medium-Short-Term “Wandering”
TIPS
15Sep2005
TELBranch
Focus behavior over one week as predicted by the Hershey
attitude-based model. Trends can be significantly larger than the
orbital variation (high frequency structure):
Medium-Long-Term (seasonal/annual)
TIPS
15Sep2005
TELBranch
Annual period
- We see some suggestion of negative focus (smaller truss, cooler temperatures)
in autumn months. Magnitude ~3 µm SM.
- Possibly related to the annual variation in solar intensity and seasonal effects.
- We also observe an annual sinusoid in astigmatism.
- Shorter-term variations (~2 months) have also been seen in NICMOS and
WFPC2 data (Suchkov & Hershey 1998)
Focus data obtained with HRC since
early 2002 folded to 1 year. Units are
expressed in µm at SM
Medium-Long-Term (seasonal/annual)
TIPS
15Sep2005
TELBranch
Astigmatism component (Z5) observed in HRC. Blue bars
represent peak to peak spread of astigmatism values due to orbital
effects. Units are µm rms wavefront error.
0.015
0o-Astigmatism measured at HRC
0.010
0o-Astigmatism (in microns)
0.005
0.000
-0.005
-0.010
-0.015
Jan-04 Feb-04 Mar-04 Apr-04 May-04 Jun-04
Jul-04 Aug-04 Sep-04 Oct-04 Nov-04 Dec-04 Jan-05 Feb-05 Mar-05 Apr-05 May-05 Jun-05
Jul-05 Aug-05 Sep-05 Oct-05
Long-Term Desorption
TIPS
15Sep2005
TELBranch
Secular behavior is a persistent shrinking of OTA
- Since HST deployment in April 1990, the separation between the SM & PM
has decreased by over 150 µm (0.003% of the 5 meters separating them).
- There have been 21 documented SM despace adjustments to maintain
observatory focus.
- early in the mission, refocusings were frequent and of large magnitude (~20 µm)
- adjustments are currently rare (two since January 2001, <5µm each)
- Shrinkage followed an exponential until late 1998 when the trend, though
shallow, became more erratic.
- exponential shrinkage understood to be due to desorption of the graphite epoxy truss in
vacuum.
- behavior in current epoch not well understood. There appears to be little publicly
available data on graphite epoxy structures in space for 15 years.
Long-Term Desorption
Shrinkage of OTA Metering Truss over Mission Life
TIPS
15Sep2005
TELBranch
TIPS
15Sep2005
TELBranch
Current Focus State
Observed means & orbital spreads of focus monitor data from HRC
approx. monthly since
2002. Refocusings are indicated.
ACS/HRC Focus Measured Over ACS Life
Microns of Secondary Mirror Despace
15
10
HST refocus 2 Dec 2002
+3.6 microns
HST refocus 22 Dec 2004
+4.2 microns
5
0
-5
-10
-15
01/1/02
01/1/03
01/1/04
01/1/05
01/1/06
Challenges
TIPS
15Sep2005
TELBranch
The main challenges are:
- to maintain best overall observatory focus.
- to model PSF variations at an appropriate level of effort.
- to understand their impacts to science and operations.
Maintaining average focus close to optimal on timescales of months benefits
science.
- PC & ACS are nearly parfocal (~1µm); focus variations by ~3µm alter the PSF
noticeably.
- NIC1 & 2 are brought to optimal focus with a consistent PAM setting.
- short-term variations preclude focus control to better than ~3µm.
Modeling PSF variations is incomplete and ongoing.
-effects induced at the SIs are known to contribute though the amounts in
most cases are not well understood.
Understanding the PSF variations can help assess impacts to science in areas
such as weak lensing, and other programs relying on a well characterized or
stable PSF.
TIPS
15Sep2005
TELBranch
Instrument Science Report, ISR-TEL-2005-03 “in press”
will be available at
http://www.stsci.edu/hst/observatory/documents/isrs
TIPS-JIM Meeting
15 September 2005, 10am, Auditorium
1.
Update on TGM Performance
Marco Sirianni
2.
3.
HST Optical Behavior and Focus Status
Optimizing JWST Dither Strategies
Matt Lallo
Anton Koekemoer
Next TIPS Meeting will be held on 20 October 2005.
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
42
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
with assistance from Kevin Lindsay (DAB)
Goals of the study:




Present the principal scientific drivers for different dither patterns, in the
context of the science goals outlined in the JWST DRM
Define metrics to quantify advantages/disadvantages of dither patterns
Carry out simulations of different dither patterns and calculate metrics
Discuss optimization of dither patterns, including overhead issues
Methodology:

Concentrate on NIRCam with the most detailed simulations:
– numerically simulated exposures with different dither patterns
– analytic calculations of overhead times associated with various dither patterns


Extend results to FGS-TF, MIRI imaging modes
Consider spectroscopic issues related to MIRI/IFU,LRS and NIRSpec
Reference:

Koekemoer & Lindsay, “An Investigation of Optimal Dither Strategies for
JWST”, STScI-JWST-R-2005-0002-A, July 2005 [circulated for approval]
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
43
Science Drivers for Dithering
Improve sub-pixel PSF sampling:


Several DRM programs require high spatial resolution: e.g., faint white
dwarfs in globular clusters; AGN/galaxy connection; weak lensing, ...
Most severely undersampled instruments are:
– FGS-TF (1 - 5 μm: 65 mas/pix) - only Nyquist sampled above ~4 μm
– NIRCam with two wavelength channels:
• 0.6 - 2.3 μm: 31 mas/pix - only Nyquist sampled above ~2 μm
• 2.4 - 5 μm: 65 mas/pix - only Nyquist sampled above ~4 μm
– NIRSpec has two different types of undersampling:
• 0.6 - 5 μm: 100 mas/pix - undersampled across entire wavelength range
• MSA shutters: 200x450 mas, spaced on a 250x500mas grid

Note that MIRI is relatively well sampled: 5 - 27 μm, 110 mas/pix
Note - sampling vs. resolution:


Sub-pixel dither and small output scale (1/2, 1/3) provides good sampling
Removing extra convolution by pixel size (“pixfrac”) improves resolution
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
44
Science Drivers (cont’d)
Improved astrometry and photometry:



Precise photometry is required by some DRM programs, including halo
population studies and globular cluster stellar photometry
Intra-pixel sensitivity variations cause photometric errors, hence also
affecting astrometry, in single exposures - effect can be up to ~0.4 mags in
HST/NIC3 (Lauer 1999)
Mitigated by dither patterns to sample sub-pixel sensitivity structure: the
additional variance from this effect decreases as ~1/N dither points
Improved background subtraction:



Many DRM programs are aimed at faint sources, often a few orders of
magnitude fainter than the background
Variations in background (time-dependent, or intrinsic limitations in
flatfield calibration accuracy) propagate into background subtraction
Mitigated by dither patterns that sample the scales of background variations
across the detector
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
45
Science Drivers (cont’d)
Dithering to avoid detector gaps, blemishes etc:


All these represent pixels with lost information that needs to be recovered
Even programs that don’t require contiguous imaging of large regions will
likely still want to cover the gaps between the 2x2 NIRCam detectors
(similar to the 2” chip gap in ACS)
Large-scale mosaic dithering:



Some DRM programs require contiguous imaging of regions much larger
than the detector (~100 or more pointings): e.g. weak lensing; rich clusters
Large-scale dithers can be combined with smaller-scale patterns to mitigate
other effects previously discussed
Requirements are:
– good acquisition accuracy to minimize overlap required between pointings
– maintaining a similar orient for all pointings, to minimize overlap required

Also need to consider overhead time due to dither slews / filter changes:
this requires specific trade-off studies for various observing schemes
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
46
Metrics for Optimizing Dither Patterns
Philosophy:

In order to determine which patterns are “optimal”, define metrics that
provide a quantitative description for a given dither pattern:
– “Positive” metrics that show how well a given measurable quantity can be
recovered from the data, for a given dither pattern
– “Negative” metrics that reflect various costs associated with carrying out a given
dither pattern

The trade-offs may be different, depending on the type of science
“Positive” metrics:



Morphological properties (incl. FWHM, etc.)
Astrometric centroiding precision
Photometric precision
“Negative” metrics:


Overhead time from dithering maneuvers, filter changes, etc.
Area coverage lost near the edges of the field
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
47
Measuring Metrics for Simulated Data
NIRCam - properties:


NIRCam will likely be the dominant imager at short wavelengths (<5 μm)
and is also relatively undersampled
Two optical trains, each with five 20482 sensor chip assemblies (SCAs):
– 0.6 - 2.3 μm: 31 mas/pix, 2x2 array of SCAs, 5” gap, undersampled < 2 μm
– 2.4 - 5 μm: 65 mas/pix, one SCA, undersampled < 4 μm


Assume read-noise = 15 e-/pix; dark current = 0.01 e-/s/pix
Filters: F070W, F110W, F150W, F200W, F270W, F357W, F444W
Simulations:

Concentrate initially on 5 sub-pixel dither patterns:
– 1/2-pixel dithers: 2pt-line; 4pt-box, 4pt-box with short & long λ matched
– 1/3-pixel dithers: 3pt-line; 9pt-box

For each dither pattern, create a set of dithered exposures for each filter:
– populated with 1024 stars (32x32 grid), offset randomly in sub-pixel space
– nominal JWST PSF (obtained from S. Casertano / A. Sivaramakrishnan)
– count-rate uniformly sampled from 0.01 - 10 counts/s
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
Example Simulated NIRCam Image
Images created:






For each of the 5 dither patterns
For each of the 7 filters
Total of 154 dithered 20482 images
Each image contains 1024 stars,
separated by 64 pixels, with random
shifts added up to 1 pixel
Image simulation software provided by
S. Casertano / E. Morse
Example image (right): F270W
Processing:



Images were combined using
MultiDrizzle (Koekemoer et al. 2002)
Final drizzled image has pixel
scale = 0.5x input pixel size
Combined drizzled images used in
subsequent analysis
48
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
Close-up of a simulated star:




F270W
4-point dither pattern:
(1.0,0.5) (1.5,1.5)
(0,0) (0.5,1.0)
Includes detector-related readnoise, dark current and photon
statistics
Output drizzled image (lower
2 panels):
– 1/2 the input pixel scale
– two different greyscale stretches:
• extended large-scale PSF
• improved PSF core sampling
49
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
50
Analysis of Dithered Images
Properties measured:


Centroid and photometry of each star, using DAOphot
FWHM of each star, using SExtractor
Verification checks performed:

measurements compared between input values, dithered exposures, and
output drizzled images
Metrics calculated:



Astrometry: measured position of each star, relative to its input position:
Photometry: measured count-rate of each star, relative to input values
For both astrometry and photometry:
– compare width of the resulting distribution with the theoretical width expected if
the only sources of error were due to Poisson statistics, and not undersampling
– Goal is to quantify additional error introduced by undersampling

FWHM: measured for each star, compared with theoretical value expected
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
Astrometry: Comparison of x,y values
51
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
Astrometry as a function of count-rate
52
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
Photometry Comparison
53
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
FWHM Comparison
54
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
55
Summary of Results
Individual Dither Patterns / Filters:


Produced a set of 35 sets of plots like the preceding ones, for each dither
pattern and filter
For each quantity (astrometry, photometry, FWHM), extracted one
quantity:
– Astrometry: dispersion of measured values, relative to theoretical dispersion
expected if undersampling did not contribute any error
– Photometry: dispersion of measured values, relative to theoretical dispersion
expected if undersampling did not contribute any error
– FWHM: Average width of bright stars, relative to theoretical FWHM for each filter

All these quantities can be summarized on two plots:
– short-wavelength filters F070W, F110W, F150W, F200W
– long-wavelength filters F270W, F357W, F444W
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
NIRCam Short-Wavelength: Results
56
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
NIRCam Long-Wavelength: Results
57
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
58
NIRCam Dither Patterns: Overall Results
FWHM:



significant improvement generally obtained from 2 to 3-point
further improvement from 3 to 4-point, with near-optimal results
9-point pattern (1/3-pixel 3x3) does not yield significant further gains likely because 1/2-pixel 2x2 dither already provides Nyquist sampling
Photometry:


Improvements are more gradual than for FWHM; for several filters,
optimal results are obtained only for the 9-point (1/3-pixel 3x3) pattern
Most likely related to the fact that photometric fidelity is driven more by
pixel-to-pixel effects
Astrometry:


Generally, optimal astrometry is only obtained with a 9-point dither pattern
Likely related to a combination of pixel-to-pixel effects as well as Nyquist
sampling
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
Overhead Time Evaluation
Consider 3 types of program (following NIRCam OCD):



short programs, representative of bright star surveys
medium programs, eg moderately faint extragalactic programs
long programs, aimed at deep imaging
Scenario A: N dithers, cycle filters at each dither pattern
Scenario B: N dithers, cycle filters only once
59
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
Final Results: Cost/Benefit Metrics
60
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
61
Summary & Conclusions (NIRCam)
For short programs:



9-point patterns are generally not optimal
for 3- or 4-point patterns, infrequent filter changes are significantly more
cost-effective
for 2-point patterns, filters could be changed at each dither without too
much penalty
For medium programs:


for 2,3,4-point patterns, overhead difference is less when changing filters at
each dither point, so this might be desirable to produce more uniform data
9-point patterns are probably generally not worth the additional overhead
For long programs:

Relative cost increase for 9-point pattern is less, reaching a maximum of
only 3% of total observing time, even when changing filters at each dither
point, so this will likely be preferred in most cases
STScI TIPS/JIM
15 September 2005
Optimizing JWST Dither Strategies
Anton Koekemoer (INS)
62
Future Work
NIRCam:


include effects of intra-pixel sensitivity variations
include geometric distortion, which produces different subsampling across
chip
Other instruments:





FGS-TF: can adapt current code relatively easily, since detectors are the
same
MIRI imaging mode: code can be run with some modifications
MIRI/IFU - study already conducted by A. Glasse, combining instrument
design with dither pattern optimization
MIRI/LRS - code will need work to adapt to spectroscopic observations
NIRSpec: preliminary study by M. Regan
Broadening the science applications:


Include different galaxy types (exponential disks, r1/4 profiles, etc)
Simulate galaxy luminosity functions and possible selection effects
TIPS-JIM Meeting
15 September 2005, 10am, Auditorium
1.
Update on TGM Performance
Marco Sirianni
2.
3.
HST Optical Behavior and Focus Status
Optimizing JWST Dither Strategies
Matt Lallo
Anton Koekemoer
Next TIPS Meeting will be held on 20 October 2005.