TIPS-JIM Meeting 20 May 2004, 10am, Auditorium

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1 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.

2 JWST FGS SRR April 7, 2004 Ed Nelan TIPS May 20, 2004 FGS SRR 7-April

3 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

4 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

5 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 Tower FGS SRR 7-April

6 FGS Requirements Flow & Status System JWST Science Requirements Document JWST-RQMT JWST Mission Requirements JWST-RQMT MRD JWST Mission Operations Concept Document JWST-OPS JWST Observatory Specification JWST-SPEC OBS OBS Segment Baselined In CM Review Drafts Available Allocation Documents Phase B Documents Element ISIM Requirements Document JWST-RQMT ISIM ISIM to OTE and Spacecraft IRD JWST-IRD ISIM to OTE and IOS-IR Mission Assurance Requirements For the JWST Instruments JWST-RQMT MAR All FGS rqmts flow from ISIM FGS IRD JWST-IRD JWST-ICD IF FGS to ISIM ICD JWST-ICD IOSC IFC CSA Performance Assurance Requirements FGS Science Rational & Analysis Document Subsystem FGS Guider Operations Concept FGS Tunable Filter Operations Concept FGS Instrument Performance and Functional Requirements JWST-SPEC FGS CSA FGS Specification FGS Optical Assembly Requirements FGS FPE/ICE/TCE Requirements FGS C&DH Requirements FGS Flight Software Requirements FGS SRR 7-April

7 FGS Future Requirements Flow Phase A Documents FGS Tunable Filter Operations Concept FGS Guider Operations Concept CSA Performance Assurance Requirements FGS Science Rationale & Analysis Document FGS Instrument Performance and Functional Requirements JWST-SPEC FGS FGS Interface Requirements JWST-IRD IF Phase B Documents CONCEPT ONLY FGS Optical Assembly Requirements FGS Specification FGS FPE/ICE/TCE Requirements C&DH Specification FGS Interface Control Document JWST-ICD IFC Flight Software Requirements TMA (Optics) Specification Filter Wheel & Drive Specification Focus Mechanism & Drive Specification OA Structure Requirements Blocking Filter Requirements Tunable Filter Requirements FPE/ICE/TCE Power Supply Specification FPE Requirements ICE Requirements Baselined In CM Review Drafts Available Allocation Documents Phase B Documents Detector Specification Coronagraphic Mask Requirements FGS SRR 7-April

8 FGS Requirements Flow (1 of 6) FGS SRR 7-April

9 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

10 New ISIM Requirements Number Status Requirement Comment ISIM-xxx ISIM-xxx TBD TBD FGS Optical Distortion 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 ISIM-xxx TBD TBR FGS-Guider Acquisition Performance during Large (<20 ) Slews FGS-Guider Acquisition Performance during Small (<1 ) Slews 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. ISIM-xxx Moving Target 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 Requirements change requests in process for ISIM Requirements FGS SRR 7-April

11 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 J ab ). 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 Requirements change requests in process for IRD Requirements FGS to determine requirements via analysis (operations input) FGS SRR 7-April

12 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

13 Current JWST FOV Layout Coronagraphic Spot Location FGS SRR 7-April

14 JWST FOV Layout Coronagraphic Spot Location FGS SRR 7-April

15 JWST FOV Layout Coronagraphic Spot Location 20 80? 1 2 FGS SRR 7-April

16 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 J AB <19. Nelan & Kriss (STScI-JWST-TM ) show that using GSC-2, ISIM-265 is not achieved for J AB <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 J AB <19.5 FGS will meet ISIM-265 w/o FGS-TF FGS SRR 7-April

17 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

18 Guide Star Recognition FGS images the sky, FGS SRR 7-April

19 Guide Star Recognition overlays a catalog segment, FGS SRR 7-April

20 Guide Star Recognition performs a pattern match, FGS SRR 7-April

21 Guide Star Recognition locates guide star. FGS SRR 7-April

22 Guide Stars in Crowded Fields HST/ACS image of Galactic bulge region from Kailash STScI FGS SRR 7-April

23 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

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). λ λ FOV FGS SRR 7-April

25 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). λ λ Compact object FOV FGS SRR 7-April

26 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. (N settings = R λ/λ) λ 1 λ FOV FGS SRR 7-April

27 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

28 NICMOS T-TEST TIPS May Tommy Wiklind NICMOS Cryo-History NICMOS temperatures -T Test Temperature lag May 20, 2004 TIPS NICMOS Temperature tests

29 NICMOS CRYO-HISTORY I. The cold period ( ) 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 A thermal short lead to a faster-than-projected sublimation of the nitrogen Nitrogen was exhausted on January NICMOS stopped obtaining scientific data on January II. The warm period ( ) 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 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

30 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

31 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

32 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

33 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

34 May 20, 2004 TIPS NICMOS Temperature tests

35 May 20, 2004 TIPS NICMOS Temperature tests

36 May 20, 2004 TIPS NICMOS Temperature tests

37 May 20, 2004 TIPS NICMOS Temperature tests

38 May 20, 2004 TIPS NICMOS Temperature tests

39 May 20, 2004 TIPS NICMOS Temperature tests

40 May 20, 2004 TIPS NICMOS Temperature tests

41 May 20, 2004 TIPS NICMOS Temperature tests

42 May 20, 2004 TIPS NICMOS Temperature tests

43 -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

44 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

45 May 20, 2004 TIPS NICMOS Temperature tests

46 Hot pixel linear dark current as function of temperature May 20, 2004 TIPS NICMOS Temperature tests

47 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

48 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

49 Time lag for NICMOS detector temperature increase: Similar plot for temperature increase during delta-t test. May 20, 2004 TIPS NICMOS Temperature tests

50 Time lag for NICMOS detector temperature increase: Similar plot for temperature increase during delta-t test. May 20, 2004 TIPS NICMOS Temperature tests

51 May 20, 2004 TIPS NICMOS Temperature tests

52 May 20, 2004 TIPS NICMOS Temperature tests

53 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 1 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

54 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 2 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 Q 2 + U 2 P = fraction of I in linear polarization P = I θ = angle of linear pol. E-vector Three unknowns requiring three independent observations of target -- observer needs three independent observations of target to solve. θ = Tan Q U

55 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 3 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!)

56 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 4 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

57 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 5 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.)

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59 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 6 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, unpolarized and polarized standard stars, star cluster 47 Tuc, reflection nebula.

60 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 7 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.

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62 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 8 Results: Polarizer angles on the sky (WFC) Filter POL0V + F625W POL60V + F625W POL120V + F625W POL0UV + F814W POL60UV + F814W POL120UV + F814W E-vector angle on sky (PA_V3 +...) Derived from design RAS/Cal test 38.2± ± ± ± ± ± ± ± ± ± ± ± 0.4 Nice agreement for POLUV+F814W... but... Poor agreement for POLV+F625W -- problem with test or F625W filter(?).

63 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 9 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.

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65 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 10 Results: Instrumental Polarization (HRC) RAS/HOMS test bad -- modeling of RAS/HOMS optics indicates ~6% internal polarization data on GD319 questionable -- double star, saturated images. Model for M3 mirror cannot account for observations in UV... other sources of instrumental polarization... CCD?

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69 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 11 Results: Instrumental Polarization (HRC&WFC) New on-orbit data & 9661 for GD319 (double star) & G191B2B (single star, used for WFPC2) are all in good agreement. Including CCD effects (Si / SiO 2 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

70 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 12 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)

71 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 13 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 (T perp 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)

72 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 14 Preliminary Polarization Calibration for GO Data Math: Apply correction C to observed count rate r obs for each polarizer filter (n=0, 60, 120). Example: r(n) = C(n, spectral filter, detector) r obs (n) Compute Stokes vector I, Q, U 2 I = -- 3 [ r ( 0 ) + r( 60) + r( 120) ] 2 Q = -- 3 [ 2r ( 0 ) r( 60) r( 120) ] 2 U = [ r( 120) r( 60) ] 3

73 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 15 Compute fractional polarization of target: Q 2 + U 2 P = Tpar + Tperp I Tpar Tperp Correct angles for rotation of POL0 filter on sky (PA_V3 and camera specs); target polarization E-vector is at PA: PA = Tan Q + ( PAV3 ) 69.4 U (HRC) PA = Tan 1 U --- Q + ( PAV3 ) 38.2 (WFC)

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76 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 16 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

77 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 17 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.

78 SPACE TELESCOPE SCIENCE INSTITUTE TIPS / ACS Polarization 20 May 2004 John Biretta 18 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 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.).