OTA Focus During SMOV
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Technical Instrument Report WFPC2 97-03 OTA Focus During SMOV S. Casertano, M. Lallo, A. Suchkov and J. Krist December 1, 2008 ABSTRACT Focus measurements taken with WFPC2 after the Second Servicing Mission indicated the need for a slight adjustment of the secondary mirror position, which was successfully carried out on March 18, 1997. The current focus position is about +1 micron, as desired for optimal WFPC2 performance and to facilitate alignment activities for the new instruments. No further focus moves are expected for the next 6-8 months. 1. The data The measurements described in this report derive from WFPC2 observations of a single, isolated bright star placed near the center of the PC, through broad-band, non-UV filters such as F555W (primarily), F439W, F675W and F814W. The primary source of data are the two SMOV proposals 7017, especially designed for focus check and including a series of 8 exposures in each of three orbits, and 7016, primarily designed for contamination monitoring but which also includes two F555W exposures in the PC for each execution. Additional data have been taken from photometric monitoring proposals and the filter sweep. The target of most observations is a hot, bright, isolated white dwarf. Early observations, taken during the Bright Earth Avoidance period, target suitable stars in the Continuous Viewing Zone; after the beginning of March, most observations are of the WFPC2 primary photometric standard, GRW+70d5824. These observations have been supplemented with a few observations of rich star fields in Omega Centauri; in all cases only one well-exposed, isolated star has been singled out for the focus determination. Since there appears to be no systematic dependence of the results on target, filter or proposal, we will not distinguish between different types of measurements in the following. Full information on the exposures used is given in Table 1. 2. Analysis The key focus measurements were obtained using the phase retrieval code developed by John Krist and Chris Burrows (see Krist and Burrows 1995). This code determines the 1 focus position from an observed stellar image by optimizing the match with a model PSF, computed using the known optical characteristics of the OTA and of WFPC2 itself; for details on its use see also Casertano 1995. The optimal focus position is defined as the position that minimizes the rms wavefront error at the image plane, and thus it differs for the different cameras. The present analysis uses exclusively the PC camera, where the effects of defocus are much easier to measure thanks to its better sampling compared to the WF cameras. Note that the PC camera has a slight astigmatism introduced by the corrective optics, and in consequence the PSF is slightly elongated in orthogonal directions depending on whether the focus position is positive or negative; this helps remove the near-degeneracy between (small) positive and negative focus shifts. This method has been used to determine the focus position throughout the life of WFPC2, and the typical errors due to the phase retrieval alone are known to be less than 1 micron. The error in the derived position of the secondary mirror is dominated by OTA terms, such as the wellknown breathing (see Section 3). Each PSF has also been visually inspected for sharpness and asymmetry (due to astigmatism, see above), and the results of the visual inspection were always in excellent agreement with those of the actual fitting - typically we could discern the focus position to within 2-3 micron from visual inspection alone. We have previously shown (Suchkov and Casertano 1997) that the aperture correction for very small apertures (1-2 pixels) correlates strongly with focus position. Therefore we determined the aperture correction for all SMOV focus images, to serve as a sanity check on the focus position determined from the phase retrieval, to test that the camera performance had not changed, and to estimate the observational impact of an out-of-focus status. The results of the aperture check were in excellent agreement with both pre-SMOV measurements and the results of the phase retrieval, and were instrumental in the decision to perform a small focus move (see Section 3). The focus position measured at a specific moment in time differs from the average focus position during that orbit because of the OTA “breathing”. The observer is affected by the actual, instantaneous focus position, but the orbit-averaged position should be used to determine the OTA status and in any trending of the long-term position of the secondary mirror. Hasan and Bely (1994) give a simple formula to determine the breathing correction for any observation, using the temperatures measured at the four light-shield temperature sensors. Typical breathing corrections found during SMOV range from -2 to +2 micron, although larger corrections were found in a few cases. Breathing corrections have been used in all trending plots. 2 Table 1. Focus measurements during SMOV Rootname u3sr0101r u3sr0102r u3sr0201r u3sr0202r u3sr1001r u3sr1002r u3sr1201r u3sr1202r u3sr1401r u3sr1402r u3sa1101r u3sa1102m u3sa1103r u3sa1104r u3sa1105r u3sa1106r u3sa1107r u3sa1108r u3sa1201r u3sa1202r u3sa1203r u3sa1204r u3sa1205r u3sa1206r u3sa1207r u3sa1208r u3sa1301r u3sa1302r u3sa1303r u3sa1304r u3sa1305r u3sa1306r u3sa1307r u3sa1308r u3sr2401r u3sr2402r u3sr2501r u3sr2502r u3sr2601r u3sr2602r u3t9810hr u3sg0101r u3sg0108r u3sg0109r u3sg010ar u3sg0201r u3sg0301m u3sg0401r u3sd0102r u3sd0103r u3sd0104r u3sd0106r u3sd010ar u3sd010br u3sd010cr u3sd010dr u3sd010er u3sd010fr u3sr2801r u3sr2802r u3sr3001r Date 23/02/97 23/02/97 25/02/97 25/02/97 27/02/97 27/02/97 27/02/97 27/02/97 27/02/97 27/02/97 27/02/97 27/02/97 27/02/97 27/02/97 27/02/97 27/02/97 27/02/97 27/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 28/02/97 03/03/97 03/03/97 03/03/97 03/03/97 03/03/97 03/03/97 04/03/97 05/03/97 05/03/97 05/03/97 05/03/97 05/03/97 05/03/97 05/03/97 06/03/97 06/03/97 06/03/97 06/03/97 06/03/97 06/03/97 06/03/97 06/03/97 06/03/97 06/03/97 06/03/97 06/03/97 09/03/97 day:hh:mm 054:20:54 054:20:56 056:16:07 056:16:09 058:10:26 058:10:28 058:14:50 058:14:52 058:19:32 058:19:34 058:23:24 058:23:24 058:23:34 058:23:39 058:23:44 058:23:49 058:23:54 058:23:59 059:02:24 059:02:29 059:02:34 059:02:39 059:02:44 059:02:49 059:02:54 059:02:59 059:08:50 059:08:55 059:09:00 059:09:05 059:09:10 059:09:15 059:09:20 059:09:25 062:01:16 062:01:18 062:08:41 062:08:43 062:22:41 062:22:43 063:15:11 064:05:18 064:05:45 064:05:48 064:05:51 064:06:54 064:08:31 064:10:08 065:20:45 065:20:49 065:20:53 065:21:01 065:22:09 065:22:13 065:22:17 065:22:21 065:22:25 065:22:29 065:04:07 065:04:09 068:03:16 Target Name WD0310-688 WD0310-688 WD0310-688 WD0310-688 WD0310-688 WD0310-688 WD0310-688 WD0310-688 WD0310-688 WD0310-688 S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E S121-E GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 WD0310-688 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 OMEGA-CEN-2 OMEGA-CEN-2 OMEGA-CEN-2 OMEGA-CEN-2 OMEGA-CEN-2 OMEGA-CEN-2 OMEGA-CEN-2 OMEGA-CEN-2 OMEGA-CEN-2 OMEGA-CEN-2 GRW+70D5824 GRW+70D5824 GRW+70D5824 Prop 7016 7016 7016 7016 7016 7016 7016 7016 7016 7016 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7017 7016 7016 7016 7016 7016 7016 7018 7020 7020 7020 7020 7020 7020 7020 7021 7021 7021 7021 7021 7021 7021 7021 7021 7021 7016 7016 7016 MJD 50502.87100 50502.87238 50504.67169 50504.67308 50506.43488 50506.43627 50506.61822 50506.61961 50506.81405 50506.81544 50506.97516 50506.97863 50506.98211 50506.98558 50506.98905 50506.99252 50506.99600 50506.99947 50507.10016 50507.10363 50507.10711 50507.11058 50507.11405 50507.11752 50507.12100 50507.12447 50507.36822 50507.37169 50507.37516 50507.37863 50507.38211 50507.38558 50507.38905 50507.39252 50510.05294 50510.05433 50510.36266 50510.36405 50510.94530 50510.94669 50511.63280 50512.22100 50512.23975 50512.24183 50512.24391 50512.28766 50512.35502 50512.42238 50513.86405 50513.86683 50513.86961 50513.87516 50513.92238 50513.92516 50513.92794 50513.93072 50513.93350 50513.93627 50513.17169 50513.17308 50516.13627 3 Focus 4.99 4.80 4.64 4.33 6.14 5.41 4.56 4.80 4.14 5.44 4.48 4.80 4.87 5.34 5.05 4.90 4.21 4.32 4.01 5.92 5.76 5.81 6.03 6.64 6.89 5.81 3.42 4.04 4.52 4.91 4.78 6.10 5.80 5.58 5.75 5.44 3.13 2.99 6.89 6.69 5.04 5.46 4.95 4.65 4.06 5.43 5.40 5.68 3.55 3.22 2.24 1.20 5.53 3.67 2.93 3.85 2.68 2.88 4.19 4.17 0.39 Focus(BC) 5.19 5.00 4.64 4.33 5.94 5.21 5.26 5.50 4.54 5.94 3.68 3.90 3.77 4.04 3.55 3.60 3.51 3.82 3.96 5.62 4.66 4.31 4.33 4.74 4.89 3.81 3.12 3.64 3.52 3.81 2.98 4.10 3.80 3.58 6.35 5.94 3.23 2.94 4.69 4.69 2.44 4.86 4.55 4.55 4.16 4.03 3.70 3.68 3.05 3.72 3.24 3.00 3.53 2.17 2.13 3.35 2.98 3.88 2.59 2.47 0.29 x coma 0.0075 0.0074 0.0178 0.0138 0.0166 0.0152 0.0163 0.0117 0.0199 0.0159 0.0124 0.0169 0.0178 0.0147 0.0149 0.0127 0.0126 0.0156 0.0147 0.0138 0.0170 0.0149 0.0154 0.0129 0.0137 0.0138 0.0141 0.0184 0.0189 0.0161 0.0170 0.0145 0.0151 0.0166 0.0105 0.0133 0.0111 0.0127 0.0163 0.0155 0.0112 0.0124 0.0163 0.0154 0.0136 0.0127 0.0124 0.0172 0.0181 0.0113 0.0107 0.0130 0.0126 0.0131 0.0153 0.0085 0.0095 0.0097 0.0119 0.0120 0.0115 y coma -0.0024 -0.0045 -0.0019 -0.0066 -0.0041 -0.0046 -0.0126 -0.0012 -0.0088 -0.0098 0.0004 -0.0034 -0.0009 -0.0071 -0.0023 -0.0028 -0.0021 -0.0034 -0.0014 -0.0007 -0.0011 0.0008 -0.0048 0.0004 -0.0011 -0.0024 -0.0004 -0.0037 0.0008 -0.0069 -0.0029 -0.0019 -0.0008 -0.0057 -0.0055 0.0012 -0.0048 -0.0026 -0.0038 -0.0034 -0.0024 -0.0002 0.0016 -0.0066 -0.0023 -0.0009 -0.0002 -0.0030 0.0006 -0.0015 -0.0049 -0.0041 0.0108 -0.0165 0.0153 -0.0048 0.0034 -0.0009 -0.0024 -0.0015 -0.0037 Filter F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F439W F675W F814W F555W F555W F555W F439W F439W F439W F439W F439W F439W F439W F439W F439W F439W F555W F555W F555W Rootname Date u3sr3002r u3sr3201r u3sr3202r u3sr3401r u3sr3402r u3sr3601r u3sr3602r u3sr3801r u3sr3802r u3sr3901r u3sr3902r u3sr4001r u3sr4002r u3sr4101r u3sr4102r u3dy1901r u3sr4301r u3sr4302r u3dy2001r 09/03/97 13/03/97 13/03/97 16/03/97 16/03/97 20/03/97 20/03/97 21/03/97 21/03/97 24/03/97 24/03/97 27/03/97 27/03/97 30/03/97 30/03/97 31/03/97 04/04/97 04/04/97 05/04/97 day:hh:mm 068:03:18 072:00:10 072:00:12 075:01:57 075:01:59 079:01:07 079:01:09 080:07:35 080:07:37 083:01:56 083:01:58 086:02:28 086:02:30 089:01:43 089:01:45 090:22:26 094:16:50 094:16:52 095:18:31 Target Name GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 Prop 7016 7016 7016 7016 7016 7016 7016 7016 7016 7016 7016 7016 7016 7016 7016 6902 7016 7016 6902 MJD 50516.13766 50520.00780 50520.00919 50523.08141 50523.08280 50527.04669 50527.04808 50528.31614 50528.31752 50531.08072 50531.08211 50534.10294 50534.10433 50537.07169 50537.07308 50538.93489 50542.70155 50542.70294 50543.77169 Focus 0.15 0.08 0.38 1.10 0.78 0.74 0.38 0.85 0.52 -0.62 -0.43 -1.37 -1.79 -4.74 -4.87 -1.89 2.24 1.70 2.41 Focus(BC) 0.05 0.78 1.08 2.00 1.68 1.84 1.68 0.05 -0.18 -0.02 0.07 -0.37 -0.89 -1.94 -2.07 -1.69 0.44 0.20 -0.09 x coma 0.0106 0.0120 0.0126 0.0088 0.0048 0.0111 0.0101 0.0126 0.0032 0.0127 0.0129 0.0104 -0.0109 0.0119 0.0131 0.0096 0.0112 0.0121 0.0120 y coma -0.0038 -0.0029 -0.0036 -0.0077 -0.0040 -0.0059 -0.0068 -0.0024 -0.0020 -0.0034 -0.0043 -0.0040 -0.0109 -0.0054 -0.0052 -0.0045 -0.0060 -0.0045 -0.0023 Filter F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W F555W 3. Results Table 1 gives a complete list of all observations used to measure the WFPC2 and OTA focus during SMOV. The columns are: 1) the data set name, 2) the date and 3) the time of the observation, 4) the target of the observation (all target stars were single, hot white dwarfs, except for the omega Cen observation) 5) the proposal number, 6) the approximate Modified Julian Date of the observation, 7) the raw focus position measured with the phase retrieval method, in microns of secondary mirror displacement (positive if the secondary was too far away from the primary), 8) the focus position, after correction for OTA breathing, 9) and 10) the x and y component of the image coma, in micron rms at the focal plane, and 11) the filter used. All focus measurements since the last pre-SMOV mirror move (October 30, 1996) are plotted in Figure 1. Shortly after the resumption of WFPC2 activities, it became apparent that the focus position was systematically positive by about 3-4 micron (see Fig. 1), in agreement with the rather sparse pre-SMOV measurements since the previous focus move (October 30, 1996). Obviously, SM activities had no significant impact on the position of the secondary mirror. The focus position remained mostly around 3-4 micron for the following two weeks, and a secondary mirror move of -2.4 micron was executed on March 18, 1997, bringing the average focus position to about 1 micron. The focus measurements after that date reflect this new position, and it is expected that there will be no need for additional moves of the secondary mirror over the next 6-8 months, or longer depending on the desorption rate (see Section 4). The results of the aperture correction measurements largely confirm those of the phase retrieval. Aperture corrections are closely related to the magnitude of the focus positions (see Suchkov and Casertano 1997). For PC observations in filters such as F555W, a focus displacement of 4 micron corresponds to a 9% change in the fraction of the flux within 1 4 pixel radius, or a 0.10 mag change in the aperture correction for that aperture. This is easily measurable in individual images. Figure 2 shows how the 1-pixel aperture corrections correlate with the focus position measured with the phase retrieval method for a subset of the SMOV data, with the solid line indicating the quadratic relationship determined from pre-SM data (see Suchkov and Casertano 1997). The pre- and post-SM data are in excellent agreement with one another, indicating that the encircled energy in small apertures is essentially unchanged compared to pre-SM measurements. Figure 1: Measured focus position since October 30, 1996, including all SMOV data. 5 Figure 2: Small-radius aperture corrections vs. focus position. The solid line represents the pre-SM relationship. Dots and crosses represent data for different stars: dots for GRWd70+5824, and crosses for S121-E. The outlying point at (3.4, -1.1) is from an image heavily affected by a cosmic ray. 4. Long-term focus trends and secondary mirror moves The focus monitoring carried out over the first three years of WFPC2 operations has indicated a continuing shrinkage of the OTA, at a rate estimated at about 0.7-0.8 micron/ month. In order to compensate for this shrinkage, the secondary mirror has been moved at approximately 7-month intervals, by an amount of 5-6 micron each time. This long-term trend is illustrated in Figure 3, where the amount of focus moves has been added back to the focus position in order to indicate the relative shrinkage of the OTA. For example, the focus position of +17 micron given for December 1994 does not imply that the telescope 6 was that far out of focus; it is the result of the focus position measured at that time, -3 micron, plus the 20 micron of focus moves applied since then. Figure 3: Long-term trending of focus position after the first servicing mission. All measurements based on PC observations. The dotted and dashed lines represent the best-fit linear regressions to all data respectively before and after the focus adjustment of March 14, 1996. It is now evident that the shrinkage of the OTA has slowed significantly over the last few months, possibly going back to March 1996. The best-fit linear regression since March 1996 (dashed line in Fig. 2) has a slope of only 0.28 micron/month, drastically different from the slope measured until that date (0.75 micron/month, dotted line). The change in slope may have been gradual, but - because of the variation in individual focus measurements - it was not recognized at the time of the last pre-SM secondary mirror 7 move (October 30, 1996), which consequently overshot what, in retrospect, would have been the optimal focus position at that time. The focus position has been systematically high over the last 6 months, during which, however, relatively few WFPC2 observations have ben carried out. The focus move of March 18 has corrected this situation. The current value of the OTA focus drift rate due to desorption is difficult to determine unambiguously. The average drift rate since March 1996, 0.28 micron/month, is probably an upper limit. However, more recent data - since October 1996 - are entirely consistent with an average desorption rate of zero, that is, the telescope could have stopped shrinking altogether. At the moment we do not have enough long-term trending to discriminate between these two extremes, and we can only conclude that the current OTA focus drift rate is somewhere between 0.3 and 0 micron/month. The current target focus position of +1.0 micron has been chosen with this uncertainty in mind. If high desorption continues to produce a focus drift rate of 0.3 micron/month, the telescope will drift through best focus and reach -1 micron in about 6-8 months. On the other hand, if desorption has truly stopped, then the telescope could stay in its present configuration indefinitely, as a focus position of +1 micron is acceptable for WFPC2 observations. 5. References • Casertano, S., 1995, Instrument Science Report OTA-18 • Hasan, H., and Bely, P. Y., 1994, in “The Restoration of HST Images and Spectra II”, eds. R. J. Hanisch and R. L. White (Baltimore, STScI), p. 157 • Krist, J., and Burrows, C. J., Appl. Opt. 34, 4951 • Suchkov, A., and Casertano, S.,1997, Instrument Science Report WFPC2 97-01 8
