TIPS-JIM Meeting
20 May 2004, 10am, Auditorium
1.
JWST FGS SRR
Ed Nelan
2.
NICMOS ∆T test
Tommy Wiklind
3.
ACS polarizers
John Biretta
Next TIPS Meeting will be held on 17 June 2004.
JWST FGS SRR
April 7, 2004
Ed Nelan
TIPS
May 20, 2004
FGS SRR 7-April-04
02-1
System Requirements Review: Objectives
The FGS SRR will be a joint CSA/NASA review.
•
The System Requirements Review (SRR) assesses the
completeness of the baseline System and Instrument
requirements for the FGS and identifies any potential
performance non-compliance’s or marginal design aspects.
•
A successful SRR demonstrates that the Instrument conceptual
design is ready to proceed into preliminary design.
•
Flow-down of requirements will be emphasized.
FGS SRR 7-April-04
02-2
System Requirements Review: Process
•
•
Requirements Overview:
•
•
•
•
JWST System & Requirements Flow
FGS Requirements Flow
Trace of all FGS Requirements from ISIM Requirements
Classification of FGS Requirements
•
•
•
FGS-Guider
FGS-TF
FGS Assemblies:
• Optical Assembly (Structure, Optics, Detectors, Mechanisms)
• FPE/ICE Assemblies (FPE, ICE, TCE)
• C&DH Assembly (SBC, Memory, Interfaces, Software)
SRR Process
•
For each class of requirements those which are design
drivers are identified and will be presented in more detail.
FGS SRR 7-April-04
02-3
JWST Observatory Architecture
Optical Telescope Element (OTE)
Primary Mirror (PM)
• 18 (1.315 m) hex segments
• Semi-rigid WFS&C for phasing
• Deployable chord fold
ISIM
• NIRCam, NIRSpec, MIRI & FGS
Secondary Mirror (SM)
Sunshield
• Passive cooling of ISIM/OTE to
<40K
Spacecraft Bus
FGS SRR 7-April-04
Tower
02-4
FGS Requirements Flow & Status
System
JWST Mission
Requirements
JWST-RQMT-000634 MRD
JWST Science
Requirements Document
JWST-RQMT-002558
JWST Observatory
Specification
OBS
JWST-SPEC-002020
OBS
Baselined
In CM Review
Drafts Available
Allocation Documents
Phase B Documents
JWST Mission Operations
Concept Document
JWST-OPS-002018
Segment
Element
ISIM Requirements
Document
JWST-RQMT-000835
ISIM
ISIM to OTE and
Spacecraft IRD
JWST-IRD-000640
ISIM to OTE and IOS-IR
JWST-ICD-001831
All FGS rqmts flow from ISIM
FGS Science
Rational & Analysis Document
Subsystem
Mission Assurance Requirements
For the JWST Instruments
JWST-RQMT-002363
MAR
IOSC
FGS IRD
JWST-IRD-000783
IF
FGS to ISIM ICD
JWST-ICD-000727
CSA Performance Assurance
Requirements
IFC
FGS Instrument Performance
and Functional Requirements
JWST-SPEC-002069
FGS
FGS Guider
Operations Concept
FGS Tunable Filter
Operations Concept
FGS Optical Assembly
Requirements
FGS SRR 7-April-04
CSA FGS Specification
FGS FPE/ICE/TCE
Requirements
FGS C&DH Requirements
FGS Flight
Software Requirements
02-5
FGS Future Requirements Flow
Phase A Documents
CSA Performance Assurance
Requirements
FGS Tunable Filter
Operations Concept
FGS Instrument Performance
and Functional Requirements
JWST-SPEC-002069 FGS
FGS Interface
Requirements
JWST-IRD-000783 IF
FGS Science
Rationale &
Analysis
Document
FGS Guider
Operations Concept
Phase B Documents
LY
N
O
PT
E
C
CON
FGS Optical Assembly
Requirements
FGS Specification
IFC
FGS FPE/ICE/TCE
Requirements
TMA (Optics)
Specification
OA Structure
Requirements
Filter Wheel & Drive
Specification
Blocking Filter
Requirements
FPE Requirements
Focus Mechanism &
Drive Specification
Tunable Filter
Requirements
ICE Requirements
Detector Specification
Coronagraphic Mask
Requirements
FGS SRR 7-April-04
FGS Interface Control
Document
JWST-ICD-000727
C&DH Specification
Flight Software Requirements
FPE/ICE/TCE Power
Supply Specification
Baselined
In CM Review
Drafts Available
Allocation Documents
Phase B Documents
02-6
FGS Requirements Flow (1 of 6)
FGS SRR 7-April-04
02-7
FGS Requirements Flow
•One ISIM requirement can spawn several FGS requirements,
but each FGS requirement must have just one ISIM “parent”.
•An ISIM requirement can be flowed to FGS from higher level
(i.e., 95% guide star availability is a mission level requirement
that must be met by the FGS.)
FGS SRR 7-April-04
02-8
New ISIM Requirements
Number
Status
Requirement
Comment
ISIM-xxx
TBD
FGS Optical Distortion
ISIM-xxx
TBD
FGS-TF Optical Distortion
(Astrometric Calibration)
Initial flowdown of OBS-93 (SI+OTE field distortion correction)
was removed, although this is partly a ground allocation, there
should be a requirement for the SI ‘s and FGS-Guider to collect
data to support the ground calibration.
Note that in the case of the FGS-Guider there is a related
requirement ISIM-589 Pointing Accuracy but its purpose is
different.
ISIM-xxx
TBD
FGS-Guider Acquisition Performance
during Large (<20”) Slews
ISIM-xxx
TBR
FGS-Guider Acquisition Performance
during Small (<1”) Slews
ISIM-xxx
Moving Target
The Observatory will slew to a target during Acquisition or Fine
Guiding, and a performance should be specified during the
slew. Also there is the potential Observatory The slew
parameters will be described in the FGS IRD Section 3.8.
A moving target requirement has been discussed and is
assumed to imply 30 mas / s rates where the FGS-Guider NEA
can relax to >4.3 mas
Key
Requirements change requests in process for
Mission / Observatory Requirements
FGS SRR 7-April-04
Requirements change requests in process
for ISIM Requirements
02-9
FGS (IPFR) TBD/TBR Requirements
Number
Status
Requirement
Comment
FGS-104
TBD
Operating Modes, Updates
Define which parameters can be updated “on the fly” (while guiding), and which
should be restricted to updating on subsequent command: For ISIM Requirements
FGS-117
TBD
Identification Image Collection
IRD needs to specify maximum allowable motion rate (low slew rate mode) for
entry into Identification Mode. Estimated to be ~1”/50 s.
FGS-124
TBD
Acquisition Mode Entry
IRD needs to specify maximum allowable motion rate (low slew rate mode) for
entry into Acquisition Mode. Estimated to be 0.5”/s.
FGS-231
TBR
Identification Mode, Sensitivity
Derive dimmest identifiable star (min Jab).
FGS–239
TBR
Identification Mode, Pattern Recognition
Derive max. # of objects to be processed by recognition algorithm.
FGS–242
TBD
Identification Mode, Single Stars
Specify min. angular separation of binary star systems that need to be resolvable.
FGS–262
TBD
Acquisition Mode, Single Stars
Specify min. angular separation of binary star systems that need to be resolvable.
FGS–268
TBR
Image Motion Rate, Large Angle Slews
Specify max. motion rate for Acquisition Mode. (See “New ISIM Reqs.”)
FGS–270
TBR
Image Motion Rate, Small Angle Slews
Specify max. motion rate for Acquisition Mode. (See “New ISIM Reqs.”)
FGS-277
TBR
NEA Magnitude Limit
ISIM Spec TBR dim magnitude limit.
FGS–393
TBD
TF Wavefront Error, Long Term Stability
ISIM / IRD needs to specify allowable I/F temp. rate of change.
FGS–403
TBD
TF PSF Anisotropy
ISIM Spec TBR low order asymmetric wave front error.
FGS–405
TBD
TF PSF Anisotropy Stability
Specify low order asymmetric wave front error variation.
FGS–436
TBR
Power-Up Duration
ISIM Spec TBR power-up duration.
FGS–567
TBD
Guider Temporal Efficiency
ISIM Spec TBD Guider efficiency.
FGS–569
TBR
TF Temporal Efficiency
ISIM Spec TBR TF efficiency.
Requirements change requests in
process for ISIM Requirements
FGS SRR 7-April-04
Requirements change requests in
process for IRD Requirements
FGS to determine requirements
via analysis (operations input)
02-10
FGS-Guider Operations Concept
Requirements Driving the Design and Operation of the FGS-Guider
•
ISIM-264 (MR-171) Guide Star Availability
• 95% guide star availability (with both FGS channels)
•
ISIM-265 (MR-365) FGS Single-Point Failure
• 90% guide star availability (single channel)
•
ISIM-591 (OBS-1677) FGS Noise Equivalent Angle
• 3.5 mas/axis RMS
determines sensitivity & total FOV for guide function, limiting
magnitude of candidate guide stars, catalog performance
FGS SRR 7-April-04
02-11
Current JWST FOV Layout
Coronagraphic Spot Location
20”
80”
1
FGS SRR 7-April-04
2
02-12
JWST FOV Layout
Coronagraphic Spot Location
20”
80”
1
FGS SRR 7-April-04
2
02-13
JWST FOV Layout
Coronagraphic Spot Location
20”
80”
?
FGS SRR 7-April-04
1
2
02-14
Loss of a Single Channel (FOV) in FGS
•
ISIM-265 requires 90% probability of a guide star being available
following the loss of 1 FGS FOV.
•
•
•
•
trade off between instrument sensitivity & remaining FOV that can
be used to access guide stars (is FGS-TF available for guiding?).
CSA has proposed sensitivity to access guide stars with JAB<19.
Nelan & Kriss (STScI-JWST-TM-2004-0008) show that using GSC-2,
ISIM-265 is not achieved for JAB<19 unless FGS-TF can be used
as a guider.*
We registered our concern that FGS-TF is not required to be
capable of guide function. We also tried unsuccessfully to
add a requirement that GSC-2 be the JWST GSC.
*On going study of GSC-2 suggests with JAB<19.5 FGS will meet ISIM-265
w/o FGS-TF
FGS SRR 7-April-04
02-15
Guide Star Identification Issues
•
During SRR there was an extended discussion about
the guide star identification process as this drive cost
for FSW and the FGS C&DH.
•
FGS-Guider OCD points out that FGS will be
confronted with both sparse and crowded fields.
Same guide star identification should be applied in
both cases.
FGS SRR 7-April-04
02-16
Guide Star Recognition
FGS images the sky,
FGS SRR 7-April-04
02-17
Guide Star Recognition
overlays a “catalog segment”,
FGS SRR 7-April-04
02-18
Guide Star Recognition
performs a pattern match,
FGS SRR 7-April-04
02-19
Guide Star Recognition
locates guide star.
FGS SRR 7-April-04
02-20
Guide Stars in Crowded Fields
HST/ACS image of Galactic bulge region from Kailash Sahu @ STScI
FGS SRR 7-April-04
02-21
FGS SRR, FGS-TF
•
Most significant FGS-TF issue is the size of the FGS
partition on the solid state recorder.
•
FGS-TF data will not be stored on same partition as
other SIs (NIRCam, NIRSpec, MIRI).
•
Vicki’s day-in-the-life assessment of FGS-TF data
volume suggests that it will fill its partition in ~1/2 day!
•
Changing partition size on orbit requires complete
erase of SSR -> not going to happen. We do not
expect FGS-TF observing “campaigns”.
FGS SRR 7-April-04
02-22
Spectral/Spatial Variation of TF:
Data Volume
•
Central wavelength of pass band will vary across the FGS-TF
FOV for a given etalon setting (Fabry-Perot design).
λ
∆λ
FOV
FGS SRR 7-April-04
02-23
Spectral/Spatial Variation of TF:
Data Volume
•
•
Central wavelength of pass band will vary across the FGS-TF
FOV for a given etalon setting (Fabry-Perot design).
This will not be an issue for observations of compact objects (or
blind surveys for emission lines sources).
Compact object
λ
∆λ
FOV
FGS SRR 7-April-04
02-24
Spectral/Spatial Variation of TF:
Data Volume
•
•
Central wavelength of pass band will vary across the FGS-TF
FOV for a given etalon setting (Fabry-Perot design).
To sample the entire field at a given wavelength, multiple
exposures at different etalon settings will be necessary.
(Nsettings = R ∆λ/λ)
5
4
3
2
λ
1
∆λ
FOV
FGS SRR 7-April-04
02-25
JWST FGS SRR Summary:
•
•
•
•
•
CSA gave the SRR a passing grade.
We (STScI, GSFC) do not know what RFAs were
submitted, or how they were addressed.
The RFA list and CSA’s SRR assessment are to be
made available (don’t know when).
Other news:
FGS RFP submission deadline was 5/19/04. CSA
expects to chose a vendor by mid-June.
We are curious to see how/if the FGS-TF survives
descope (if any).
FGS SRR 7-April-04
02-26
NICMOS ∆T-TEST
TIPS May 20 2004
Tommy Wiklind
• NICMOS Cryo-History
• NICMOS temperatures
• ∆-T Test
• Temperature lag
May 20, 2004
TIPS NICMOS Temperature tests
NICMOS CRYO-HISTORY
I. The cold period (1997-1999)
• NICMOS was installed on the HST February 1997
• Solid nitrogen coolant maintained the temperature at 61K
• The dewar temperature slowly increased by 1.5K during 1997-1998
• A thermal short lead to a faster-than-projected sublimation of the nitrogen
• Nitrogen was exhausted on January 4 1999
• NICMOS stopped obtaining scientific data on January 9 1999
II. The warm period (1999-2002)
• NICMOS warmed up to 260K and was useless for scientific applications
III. The cool period (2002- )
• The NICMOS Cooling System (NCS) was installed on March 8 2002
• NCS is a mechanical cooler using neon gas in a closed-loop reverse-Brayton cycle
• The temperature is controlled through the compressor speed
• The dewar temperature is maintained at ~77.1K
• Temperature control is ‘manual’
• The NCS has stopped once since March 2002 (August 2003)
May 20, 2004
TIPS NICMOS Temperature tests
NICMOS Dewar Temperature
The NICMOS detectors show a number of subtle effects sensitive to temperature.
Both the actual temperature and the temperature stability are important
• Detector quantum efficiency (DQE). Higher temperature gives higher sensitivity
• Reset level (the count level immediately following a detector reset) influences the
saturation levels (15% lower for NIC1 and NIC2, 7% lower for NIC3 compared to
pre-NCS era)
• The shading. A noiseless signal gradient (pixel dependent bias) depending on the
amplifier temperature
• The linear dark current
Non-temperature dependent effects
• Read-out noise
• Pedestal
May 20, 2004
TIPS NICMOS Temperature tests
The NICMOS dark signal consists of four components:
• Amplifier glow
Radiation from the read-out amplifiers. In a given read-out, the amount
of signal in each pixel scales directly with the number of read-outs since
the last detector reset.
• Shading
Time-dependent bias that changes across a quadrant as the pixels are
sequentially read out. The amplitude of the signal depends on the time
since previous read-out and read out direction (subtract the median value
of each column/row perpendicular to the fast read-out direction).
• Pedestal
A DC offset, or bias, leftover in an image after it has the dark reference
file subtracted from it. Simple solution: subtract the median value of each
quadrant before flat-fielding.
• Linear dark current
The ‘real’ dark component
May 20, 2004
TIPS NICMOS Temperature tests
NICMOS Dewar Temperature
The aim with the ∆-T test is to quantify the temperature dependence
of the linear dark current
Re-enable temperature dependent dark calibrations in the pipeline
NCS was commanded to change the neon temperature set-point
to three test positions at +0.5K, -0.5K and -1.0 and relative to the
current set-point of 72.4K
Provides data for evaluation of leaving NCS idle during orbit night
May 20, 2004
TIPS NICMOS Temperature tests
NCS
Ne in- and
outlet temp
sensors
(~1.5m)
NIC3 mounting cup
NIC1 mounting cup
NIC2 mounting cup
Dewar = NIC1 mounting cup
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
∆-T test results:
Comparison of dark
components from largest ∆T pairs are shown for each
camera.
Ampglow is largely
independent of temperature.
The linear dark component
shows significant structure
in NIC3, perhaps due to
physical imperfections in
the detector.
The shading is a thermal
effect that is a complex
function of ∆-time and
pedestal is an essentially
random electronic DC offset.
May 20, 2004
TIPS NICMOS Temperature tests
Linear dark current as
function of
temperature: The
linear dark current is
shown for each
NICMOS camera at two
temperatures.
As expected, the median
dark current in NIC1
and NIC2 increases with
temperature. The
behavior of NIC3 is
anomalous because of a
large pedestal
contribution.
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
Hot pixel linear dark current as function of temperature
May 20, 2004
TIPS NICMOS Temperature tests
Time lag for NICMOS detector temperature increase: Following the NCS safing event in
August 2003, the thermal response of the NIC1 mounting cup thermal sensor NDWTMP11 is
shown. For almost 2 hours the temperature at the mounting cup is unchanged. During this time
the neon in the cooling loop has been increasing its temperature. The lag in the dewar
temperature response may be related to the fact that the neon average temperature is some 5K
lower than the mounting cup.
May 20, 2004
TIPS NICMOS Temperature tests
Summary
• Linear darks are in the normal range
expected based on dark monitoring in cycles 11 and 12
• Data sufficient for re-enabling of temperature-dependent
darks to calibration pipeline
• No unexpected anomalies where found
• Characterization of the temperature variations continue
exploring options for future NCS operations
May 20, 2004
TIPS NICMOS Temperature tests
Time lag for NICMOS detector temperature increase: Similar plot for
temperature increase during delta-T test.
May 20, 2004
TIPS NICMOS Temperature tests
Time lag for NICMOS detector temperature increase: Similar plot for
temperature increase during delta-T test.
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
May 20, 2004
TIPS NICMOS Temperature tests
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
ACS Polarization Calibration:
Introduction and Progress Report
J. Biretta, V. Platais, F. Boffi, W. Sparks, J. Walsh
Introduction: Theory / ACS Polarizers / Supported Modes
Potential Issues for ACS Polarization Calibration
Calibration Programs / Results
Preliminary Calibration for GO Data
Future
1
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Introduction: Theory
Polarization of target usually expressed as a Stokes Vector -- (I, Q, U, V)
• I = total intensity
• Q = linear polarized intensity with E-vector along principle axes
• U = linear pol intensity with E-vector along 45 degrees or both axes
• V = circular pol intensity (usually ignored)
Alternate expression -- (I, P, θ )
• I = total intensity
2
2
Q +U
• P = fraction of I in linear polarization P = ------------------------I
• θ = angle of linear pol. E-vector
1
–1 U
θ =  --- Tan  ----
 Q
 2
Three unknowns requiring three independent observations of target -observer needs three independent observations of target to solve.
2
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Introduction: ACS Polarizer Filter Sets
•
•
•
•
•
•
Visible Polarizer set (wheel 2) -- POL0V, POL60V, POL120V
UV Polarizer set (wheel 1) -- POL0UV, POL60UV, POL120UV
Polarization E-vectors set at nominal 60 degree angles
Use with either HRC or WFC detectors
“Small” filters -- illuminate full HRC or ~ quadrant of WFC
Designed to be used with spectral filter (include weak lens -- distortion!)
3
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Introduction: Supported / Unsupported Modes
Supported (already in use by GOs; 39 combinations):
• WFC x POLV(0,60,120) x F475W, F606W, F775W
• HRC x POLV(0,60,120) x F475W, F606W, F625W, F658W, F775W
• HRC x POLUV(0,60,120) x F220W, F250W, F330W, F435W, F814W
Unsupported but Available:
• WFC x POLV(0,60,120) x F625W, F658W
• WFC x POLUV(0,60,120) x any
• either detector x POLV(0,60,120) x F555W, F550M, F502N, G800L
• either detector x POLUV(0,60,120) x F660N, FR388N, FR656N, PR200L,
F344N, FR459M, FR914M, FR505N
4
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Potential Issues for ACS Polarization Calibration
Polarizer Filters
• Perpendicular transmissions are high for UV polarizers.
• Polarization angles of the filters on the sky not known.
• Non-uniformities in polarization properties across filters.
• Spurious distortion due to extra lens in the pol filters, polarizing films.
ACS Optics
• Tilted components modify pol. properties of wavefront....
• Mirrors (especially IM3, M3) -- reflectance varies with position angle of
wavefront -- phase retardance ∆ converts linear pol to elliptical pol
• CCD detectors have effects similar to mirrors
• Spectral filter anomalies (birefringence, etc.)
5
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Calibration Programs
•
•
•
•
•
Lab measurements on polarizer filters.
Lab measurements on M3 and IM3 mirrors.
ACS RAS/HOMS test at Ball (2 March 2001) -- instrumental pol.
ACS RAS/Cal test at Ball (15-22 August 2001) -- polarizer angles.
On-orbit programs 9586, 9661, 10055 -- unpolarized and polarized standard
stars, star cluster 47 Tuc, reflection nebula.
6
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Results: Polarizer Filters (Leviton)
• Lab measurements with unpolarized light source -- throughputs of single
polarizer filters and crossed pairs (parallel and perpendicular axes).
• Throughputs appear identical for 0, 60, and 120 degree filters in each set.
• POLV - excellent rejection of cross-polarized light (low leakage).
• POLUV - 5% leakage in UV, 20% leakage in far-red.
7
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Results: Polarizer angles on the sky (WFC)
Filter
E-vector angle on sky (PA_V3 + ...)
Derived from design
RAS/Cal test
POL0V + F625W
– 38.2 ± 1.0
– 39.5 ± 0.2
POL60V + F625W
21.8 ± 1.0
28.3 ± 0.4
POL120V + F625W
81.8 ± 1.0
78.1 ± 0.3
POL0UV + F814W
– 38.2 ± 1.0
– 38.4 ± 0.4
POL60UV + F814W
21.8 ± 1.0
22.6 ± 0.4
POL120UV + F814W
81.8 ± 1.0
81.8 ± 0.4
Nice agreement for POLUV+F814W.... but ....
Poor agreement for POLV+F625W -- problem with test or F625W filter(?).
8
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Results: Instrumental Polarization
•
•
•
•
Define as fractional polarization “P” seen when observing unpolarized target.
Provides a measure of spurious polarization within the instrument.
Ideally should be zero.
Design goal 5% HRC, 1% WFC.
Data:
• RAS/HOMS test at Ball using flatfields.
• Program 9586 using unpolarized star GD319 (turns out to be a double star).
• Lab data and models for M3 and IM3 mirrors.
9
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Results: Instrumental Polarization (HRC)
• RAS/HOMS test bad -- modeling of RAS/HOMS optics indicates ~6% internal polarization.
• 9586 data on GD319 questionable -- double star, saturated images.
• Model for M3 mirror cannot account for observations in UV.... other sources
of instrumental polarization.... CCD?
10
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Results: Instrumental Polarization (HRC&WFC)
• New on-orbit data -- 9586 & 9661 for GD319 (double star) & G191B2B (single star, used for WFPC2) are all in good agreement.
• Including CCD effects (Si / SiO2 model) improves model predictions.... exact
CCD details are proprietary however....
• F625W sticks out from general trend.
Bottom line:
• HRC ~ 5% instrumental pol. in red; 8 - 14% instrumental pol. UV
• WFC ~ 2% instrumental pol. F475W, F606W, F775W
• Design goal met only for HRC in far-red
11
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Results: Geometric Distortion (Platais)
• Compare observations of 47 Tuc with / without polarizers
• Large scale distortion due to filter power well-corrected (HRC F606W)
• Unexpected small-scale distortion caused by ripples in polaroid material (+/0.3 pixel)
12
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Preliminary Polarization Calibration for GO Data
•
•
•
•
•
Method:
Calibrate polarization “zero point” using results for standard star G191B2B.
Assume POL filter angles derived from ACS design specs.
Correct for cross-polarization leakage (Tperp in POLUV filters)
Ignore all complex effects in mirrors, detectors (phase retardance, etc.)
Test: compare “known” properties of polarized standard stars Vela I and
BD+64D106 with those measured on-orbit (programs 9586, 9661)
13
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Preliminary Polarization Calibration for GO Data
Math:
Apply correction “C” to observed count rate robs for each polarizer filter
(n=0, 60, 120). Example:
r(n) = C(n, spectral filter, detector) robs(n)
Compute Stokes vector I, Q, U
2
I =  --- [ r ( 0 ) + r ( 60 ) + r ( 120 ) ]
 3
2
Q =  --- [ 2r ( 0 ) – r ( 60 ) – r ( 120 ) ]
 3
2
U =  ------- [ r ( 120 ) – r ( 60 ) ]
 3
14
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Compute fractional polarization of target:
2
2
Q +U
T par + T perpP = ------------------------- × --------------------------------------------I
T par – T perp
Correct angles for rotation of POL0 filter on sky (PA_V3 and camera specs);
target polarization E-vector is at PA:
1
–1 U
PA =  --- Tan  ---- + ( PAV 3 ) – 69.4°
 Q
 2
(HRC)
1
–1 U
PA =  --- Tan  ---- + ( PAV 3 ) – 38.2°
 2
 Q
(WFC)
15
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Preliminary Polarization Calibration for GO Data
Results of test on polarized standard stars:
• Good accuracy from 300nm to 700nm:
Fractional pol +/-1% (i.e. 5% +/- 1% pol) and PA +/- 2 degrees
• Larger errors for F220W, F250W, F775W, and F814W.
Remaining uncalibrated systematics errors (phase retardance, etc.):
• Detailed modeling of HRC optics and calibration process...
• Fractional pol has systematics of ~1 part in 10 (i.e. 40% +/- 4% pol)
• PA have systematics +/- 3 degrees
16
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Advice for Observers
• Most accurate modes are likely to be HRC + POLV + visible filter (e.g.
F606W).
• Poor calibration for some modes: F220W, F250W, F330W (no lab data, effects
in CCD), F625W (anomalies), F775W, F814W (larger systematics, no lab data
for IM3 mirror).
• WFC is somewhat risky until more calibration data (IM3 mirror phase retardance is unknown).
• Impacts on non-polarization data: if the target is significantly polarized the
high instrumental polarization for HRC (especially in UV) will decrease photometric accuracy.
17
TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Future
• Better modeling of mirrors and detectors (diattenuation, phase retardance,
etc.) -- proprietary coatings on IM3 and CCDs are an issue.
• Calibrate higher-order terms that depend on HST roll angle (10% effects) -polarized std target at many ORIENTs (program 10055 in progress, etc.).
• Generate model-based calibration with full mirror & detector effects included
(similar to WFPC2 calibration).
• Full calibration planned for only F330W and F606W.... but filter anomalies
(e.g. F625W) are a concern.... what about other filters?
• Distortion: corrections for small-scale ripples in polarizer filters.
• Field dependence: polarimetric cal. as function of field position (improved
flats from 47 Tuc data, dither standard star (10055, etc.).
18