Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/hst/wfc3/documents/ISRs/WFC3-2005-03.pdf Дата изменения: Thu Mar 31 19:51:44 2005 Дата индексирования: Sun Dec 23 01:17:58 2007 Кодировка: Поисковые слова: coma

Instrument Science Report WFC3 2005-03

WFC3 IR Channel Corrector Alignment
G. F. Hartig 31 January 2005 ABSTRACT A direct assessment of the alignment sensitivities of the IR channel corrector, using the flight hardware, was performed in Oct 2004 during the first thermal-vacuum test of the integrated WFC3 instrument. The significant level of cross-coupling between focus and coma-correcting cylinder offsets has been measured and encoded into IDL tools that enable rapid convergence to an optimal image quality configuration of the IR corrector.

Introduction
The WFC3 IR channel incorporates a corrector mechanism to permit adjustments of focus and pupil alignment, to compensate for small internal misalignments as well as those associated with gravity release and installation into the HST OTA. This mechanism, on which the IRM2 mirror is mounted, is very similar to that used on previous HST science instruments, as well as on the WFC3 UVIS channel, but the optomechanical design of the IR channel is relatively compact. (Note that the layout diagram in the WFC3 Instrument Handbook is not to scale; see, e.g., Turner-Valle, 2000.) The large included angle (~24°) in the raypath at the IRM2 results in much greater crosscoupling between the focus adjustment and pupil shear at the refractive corrector plate (RCP). As the focus is adjusted, the pupil image "shears" at the RPC, predominantly in the detector X direction, inducing coma. Also, the substantial focal plane tilt (~24°) results in defocus with field position shift, such as that induced by pupil alignment with the corrector. Furthermore, the mechanism used for pupil alignment is of the nested eccentric cylinder ("Wally Wobbler") design, for which the mapping of pupil image position with cylinder position is not straightforward. This design is intrinsically very stiff and can be used for large mirrors, hence its choice for the ACS WFC and WFC3 UVIS applications, but it incurs some operational complications. Ideally, the pupil image motion produced by rotation of the inner cylinder would be orthogonal to that produced by the outer cylinder, so that a given amount of pupil shear-induced coma can be mapped, without crosscoupling ambiguity, to corrective cylinder rotations. However, this can be true only for


IRM2 mirror pointings falling on a single angular radius from the mechanism center of rotation. The mechanism was optically aligned in the instrument such that its nominal position lies near to this radius (Sullivan, 2002b). The chief ray direction (pupil location) had earlier been mapped to cylinder rotation in a region around this nominal alignment, at the subsystem (mechanism) test level. The interaction of focus adjustment and pupil shear and resulting use of the corrector cylinders away from the region of our experience base became readily apparent when the IR-2 detector was found to be ~ 0.6 mm beyond its nominal focus position. Many adjustment iterations were required to converge on a well-corrected image, contrary to earlier alignment exercises with the surrogate (bare MUX) IR detector, which was wellpositioned in focus. To investigate these cross-coupling effects and provide empirical sensitivity coefficients to permit deterministic alignment adjustments in the future, a small test program was devised and added to the TV#1 procedure.

Measurements & Analysis
A set of IR channel images were obtained on 5 Oct 2004, near the end of the TV1 program, to empirically assess the corrector sensitivities. The CASTLE stimulus provided out of focus point source illumination near field center (point IR-01) at 1.06µ, through filter F105W, so that phase retrieval analysis could be used to measure the coma content and image positions. After successively setting the focus to its nominal, center of travel position, and to ±500 steps from nominal, the cylinders were iteratively adjusted to approximately remove coma. Images were then obtained at settings of ±20 steps from this position on both inner and outer cylinders, using a special CCL script, TIIRALCORTEST, to expedite data acquisition. The step-by-step procedure is attached as Appendix 1. Table 1 lists the measurement database entry numbers, focus setting relative to nominal, focus position sensor (LVDT) reading, and inner and outer cylinder resolver readings, along with relative cylinder step offsets. Phase retrieval analysis was performed using IDL tool wfc3fit on each of the images, which are about 8 mm out of focus at the detector. Only focus, coma and astigmatism were fit, with other aberrations and gaussian pupil apodization fixed at values determined from more complete, through focus, data sets at the IR-01 field point. The image center position (px) and coma (µ, RMS) results are reported in the last four columns of Table 1. The large amount (>0.1 µ) of coma induced as a result of the focus offsets of ±500 steps (~0.48 mm at detector) as well as the large shift in image X-position, are seen in the results for entries 17372 and 17382. Linear fitting of the coma induced as a function of image offset, yields:
Xcoma (µ, RMS) = 0.00160 · X (px), Ycoma (µ, RMS) = 0.00183 · Y (px).


The image offset induced by each step of cylinder rotation offsets (about the position of minimal coma) for each of the three studied focus positions is shown in Table 2.

Table 1. IR Corrector Sensitivity Measurements
entry 17362 17363 17364 17366 17367 17368 17369 17370 17371 17372 17373 17374 17375 17376 17377 17378 17379 17380 17381 17382 17383 17384 17385 17386 17387 17388 17389 17390 17391 17392 rel foc 0 0 0 0 0 0 0 0 0 -500 -500 -500 -500 -500 -500 -500 -500 -500 -500 500 500 500 500 500 500 500 500 500 500 500 Lvdt 2243 2243 2243 2243 2243 2243 2243 2243 2243 2074 2074 2074 2074 2074 2074 2074 2074 2074 2074 2411 2411 2411 2411 2411 2411 2411 2411 2411 2411 2411 inner 56091 57229 54898 57229 54898 56083 56084 56087 56084 56085 58117 58110 58458 59579 57294 58427 58428 58426 58426 58427 53729 53724 53147 53145 54234 51937 53149 53150 53165 53164 outer innstep outstep 43311 0 0 43311 20 0 43311 -20 0 43311 20 0 43311 -20 0 43311 0 0 44520 0 20 42099 0 -20 43312 0 0 43312 0 0 43312 35 0 43797 35 8 43796 41 8 43796 61 8 43796 21 8 43796 41 8 45008 41 28 42586 41 -12 43796 41 8 43796 41 8 43796 -39 8 42587 -39 -12 42587 -49 -12 41979 -49 -22 41979 -29 -22 41979 -69 -22 41979 -49 -22 43191 -49 -2 40763 -49 -42 41979 -49 -22 x 650. 637. 662. 637. 662. 650. 653. 648. 650. 661. 638. 639. 634. 621. 647. 634. 638. 632. 634. 613. 663. 660. 666. 665. 655. 675. 666. 667. 665. 666. y 643. 639. 648. 639. 648. 643. 654. 632. 643. 642. 637. 641. 641. 640. 643. 640. 651. 629. 640. 640. 658. 647. 650. 645. 638. 653. 645. 656. 634. 645. xcoma 0.0027 -0.0185 0.0222 -0.0186 0.0222 0.0027 0.0065 0.0008 0.0040 0.0459 0.0089 0.0117 0.0046 -0.0177 0.0256 0.0022 0.0059 0.0030 0.0039 -0.1017 -0.0049 -0.0049 -0.0004 0.0002 -0.0165 0.0160 0.0011 0.0006 0.0045 0.0021 ycoma -0.0066 -0.0099 0.0062 -0.0093 0.0062 -0.0019 0.0157 -0.0243 -0.0075 -0.1250 -0.0175 -0.0065 -0.0087 -0.0109 -0.0097 -0.0091 0.0113 -0.0302 -0.0077 -0.0079 0.0206 0.0007 0.0078 -0.0035 -0.0140 0.0097 -0.0045 0.0192 -0.0244 -0.0053

2 6 4 5 4 5 4 5 2 4 4 2 8 2 8 8 2 4 4 1 1 9 1 6 5 9 1 5 9 1

5 5 5 5 5 5 5 4 4 5 4 5 2 2 4 9 6 6 6 9 8 5 8 5 8 2 6 5 6 4

Table 2. IR image offset per cylinder step at 3 focus settings
Focus: LVDT: x (px) y (px) x (px) y (px) nom 2243 -0.621 -0.225 0.123 0.552 -500 2074 -0.665 -0.08 0.145 0.55 +500 2411 -0.51 -0.36 0.04 0.548 Cylinder INNER INNER OUTER OUTER


Using the above results, for a given focus (LVDT) setting in the measured range, the sensitivity matrix coefficients relating coma to cylinder offset can be interpolated. Parabolic fits are used for the inner cylinder sensitivities; linear fits suffice for the outer cylinder. The inverse sensitivities can then be computed by inverting the matrix to determine the cylinder offsets required to correct a measured amount of coma. This process has been encoded in the IDL procedure wfc3_ir_corr, which is called by wfc3fit when performing IR phase retrieval analysis. The cylinder offsets (steps) and resulting image position offset required to compensate 1 nm (RMS) of coma in each axis, at the nominal focus setting, is shown in Table 3. The cross-coupling between the cylinders is evident. Table 3. IR corrector compensation per 1 nm (RMS) of coma at nominal focus
INNER 1.09 -0.21 OUTER 0.44 -1.08 x (px) -0.63 0 y (px) 0 -0.55

Xcoma Ycoma

Conclusion
The significant cross-coupling between focus and coma-correcting cylinder offsets in the WFC3 IR channel has been studied for the range of focus offset that is expected in future alignment operations. The appropriate sensitivity coefficients have been determined and encoded into IDL tools to enable rapid convergence to an optimal image quality configuration of the IR corrector.

Acknowledgements
Special thanks to Dave Hickey for rapidly producing the CCL script used for efficient data collection and to the entire WFC3 TV team for accommodating this unscheduled investigation.

References
Turner-Valle, J. "Primary Imaging Optic Axes Clocking Relative to CAD Model", Ball SER OPT-046, 21 Aug 2000. Sullivan, J.F. "IRM2 and Corrector Mechanism Integration and Alignment in the Optical Bench", Ball SER OAT-019, 25 May 2002.


Appendix 1.
TV1 IR Alignment Investigation Procedure Part 1. Determine IR-2 alignment offsets. 1. Set W FC3 IR corrector to FOCUS=2244±2 (nominal ctr of travel), INNER=56400±40, OUTER=43066±40. This is best current estimate for coma minimization at nominal corrector focus. 2. Move CASTLE LD1060 fiber to IRN01, nominal focus, 6.5 mA. 3. Configure W FC3: CSM at IR, filter=F105W , GAIN=2.5 4. Obtain RAPID, NREADS=2, 512SQ image. 5. Move CASTLE to ­35 mm focus, LD1060 current 20mA 6. Obtain RAPID, NREADS=2, 512SQ image. 7. Run wfc3fit to determine possible corrector adjustments. Optimal setting leaves (0.005,-0.003) in (X,Y) coma at IRN01, corresponding to ~ (+3,+3) step indicated correction. 8. Apply corrector adjustments, if required. Repeat steps 6-8, as required. 9. Run SMSs IRAL1S4B,5A,6A, which obtain EE focus data centered on current best focus position (~+2.5 mm at CASTLE fiber). Part 2. Determine Corrector Sensitivities and Cross-coupling Effects at Nominal Focus W ith correctors set at nominal focus (2244) and coma minimized, per Part 1: Run CCL TIIRALCORTEST to obtain IRN01 images (RAPID/2/512SQ) at ±20 steps on both INNER and OUTER cylinders, with LD1060 at 20mA and ­35mm focus. Part 3. Determine Corrector Sensitivities and Cross-coupling Effects at -500 Steps Focus Apply -500 steps focus (move ­600, then +100 steps) Obtain ­35 mm image Adjust cylinders to minimize coma, per Part 1 steps 2-8, above Run CCL TIIRALCORTEST Part 4. Determine Corrector Sensitivities and Cross-coupling Effects at +500 Steps Focus Apply +1000 (to +500 from nominal) steps focus (watch focus motor temperature!) Obtain ­35 mm image Adjust cylinders to minimize coma, per Part 1 steps 2-8, above Run CCL TIIRALCORTEST Part 5. Return IR Corrector to settings at start of this procedure, to ensure IR images in following tests are in focus: Set IR corrector focus to LVDT=2409 (should already be very close) Set IR corrector INNER cylinder to resolver=53154 Set IR corrector OUTER cylinder to resolver=42407

Note: The IDL programs and data required are publicly accessible in subdirectories of /wfc3/data20/intdata/pro/hartig, which can be placed in your IDL path.