Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/hst/HST_overview/documents/calworkshop/workshop2005/papers/koekemoer-twogyro.pdf Дата изменения: Wed Apr 5 00:02:25 2006 Дата индексирования: Mon Dec 24 00:00:59 2007 Кодировка: Поисковые слова: omega centauri

The 2005 HST Calibration Workshop Space Telescope Science Institute, 2005 A. M. Koekemoer, P. Goudfrooij, and L. L. Dressel, eds.

The Two-Gyro Pointing Stability of HST, Measured with ACS
A. M. Koekemoer, V. Kozhurina-Platais, M. Sirianni, A. Riess, J. Biretta and C. Pavlovsky Space Telescope Science Institute, 3700 San Martin Dr., Baltimore MD 21218, USA Abstract. We present the results of the p ointing stability tests for HST, as measured with the ACS/HRC during the two-gyro test program conducted in February 2005 and the validation tests p erformed after the transition to two-gyro mode in August 2005. We have measured the shifts of 301 exp osures obtained of the globular clusters NGC1851, NGC2298, NGC6341, NGC6752, and Omega Centauri, obtained over a total of 21 orbits during these two test programs, and compare the measured p ointings to those that were commanded in the observing program. We find in all cases that the measured shifts and rotations have the same level of accuracy as those executed in three-gyro mode. Sp ecifically, the p ointing offsets during an orbit relative to the first exp osure can b e characterized with distributions having a disp ersion of 2.1 - 2.3 milliarcseconds for shifts and 0.0007 - 0.00097 degrees for rotations, thus less than 0.1 HRC pixels, and agree extremely well with similar values measured for comparable exp osures obtained in three-gyro mode. In addition, we successfully processed these two-gyro test data through the MultiDrizzle software which is used in the HST pip eline to p erform automated registration, cosmic ray rejection and image combination for multiple exp osure sequences, and we find good agreement with similar exp osures obtained in three-gyro mode. In summary, we find no significant difference b etween the quality of HST p ointing as measured from these two-gyro test data, relative to the nominal b ehavior of HST in regular three-gyro op erations.

1.

Introduction

As part of the options for extending the lifetime of the Hubble Space Telescop e (HST), a new attitude control system was activated in August 2005 that enables the telescop e to p oint using only two gyroscop es instead of the nominal three by using additional information from the Fine Guidance Sensors, thereby allowing one of the gyroscop es to b e turned off to conserve its life for p ossible future use. The February two-gyro test (F2G) was carried out during 20 - 23 February 2005 to investigate the feasibility of this mode, and based on its results the decision was made to transition to two-gyro mode (TGM) on 28 August 2005, with a second set of validation tests p erformed during 28 - 31 August 2005. One of the principal motivations for these tests was to examine the p ointing stability of HST, since a p ossible concern with TGM op erations is the need to determine whether the p ointing stability is significantly worse than with three gyroscop es. If so, this could p otentially impact the tracking accuracy of HST during an exp osure, as well as the accuracy of dither offset maneuvers from one exp osure to the next, and the accuracy of guide star re-acquisitions from one orbit to the next. In addition to degrading the resolution of the final image, a decrease in p ointing stability can also affect the degree of accuracy with which sub-pixel sampling is achieved with dithering. Therefore, an extensive set of observations was obtained during b oth test p eriods to verify and measure the p ointing stability of HST under TGM. All results from these tests indicate that the two-gyro p ointing p erformance of HST is not significantly different from that under nominal three-gyro op erations. 384 c Copyright 2005 Space Telescop e Science Institute. All rights reserved.


The Two-Gyro Pointing Stability of HST, Measured with ACS 2. Observational Design and Methodology

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The observational methodology is describ ed in detail in ACS ISR 2005-07 (Koekemoer et al. 2005) and Sembach et al. (this volume), and is summarized here. Three principal questions need to b e addressed concerning the p ointing stability of HST in two-gyro mode: 1. Maintaining stability within an orbit, so that successive exp osures during an orbit are either located at the same p osition (if no dithering is used), or are dithered accurately according to the commanded offsets, particularly if sub-pixel shifts are required. 2. Maintaining sufficient re-acquisition accuracy from one orbit to the next to enable multi-orbit observations to b e successfully obtained, particularly if sequences of exp osures or dither patterns are spread across multiple orbits. 3. Ensuring that the pip eline processing system, sp ecifically MultiDrizzle image registration, cosmic ray rejection and image combination, can successfully process the data. The observations describ ed here were obtained in programs 10443 and 10458 during February and August 2005, resp ectively. These contained a wide variety of exp osures, using a range of exp osure times, dither offset strategies and different typ es of guide stars, to quantify in detail the b ehavior of the telescop e in TGM. The prop osals used the HRC camera (1024x1024 pixels), which has small pixels (26 milliarcseconds) and can very accurately measure the HST p ointing accuracy. The 10443 observations were spread over 13 orbits, obtaining 155 exp osures of the globular cluster NGC6341 and 32 exp osures of Omega Centauri at a 10 off-nominal roll. The 10458 program covered 8 orbits and obtained 18 exp osures of NGC1851, 18 exp osures of NGC6752, and 78 exp osures of NGC2298. A companion three-gyro program, 10455, was obtained in February 2005 using a subset of the exp osures in 10443 with the same observing configuration, therefore providing useful exp osures that could b e directly compared with those obtained in two-gyro mode. The observations used a range of guide stars with magnitudes V=11, 13, and 14. The exp osure times were 10 seconds, 100 seconds and 500 seconds, and were obtained in a variety of configurations including CRSPLIT sequences of 2, 4 and 5 exp osures (with no dithering) as well as 2-p oint and 4-p oint dither patterns, with some of the 2-p oint patterns containing a 2-exp osure CRSPLIT pair at each location. Most of the observations used the F555W filter, with additional F330W observations obtained in February 2005. 3. 3.1. Analysis and Results Initial Processing and Distortion Correction

All the exp osures were first processed through standard ACS calibration, including gain correction, bias and dark current removal, and flat field correction. The resulting calibrated FLT files were then transformed onto an undistorted output frame using the MultiDrizzle software (Koekemoer et al. 2002), which makes use of Drizzle (Fruchter & Hook 2002) to remove the ACS geometric distortion using the most up-to-date distortion files (IDCTAB) and distortion residual images (DGEOFILES), as sp ecified in the image headers (ACS ISR 2004-15, Anderson & King 2005). This step also accounts for slightly different distortion terms in different filters (F555W and F330W), and removes small additional scale changes due to velocity ab erration resulting from changes in the motion of HST along the line of sight to the target during an orbit. Thus, exp osures from different times and with different filters could b e directly compared with one another. The resulting set of drizzled images, one for each exp osure, were all examined in detail to verify that there were no problems, b efore continuing with the p ointing measurements.


386 3.2.

Koekemoer et al. Catalog Generation

The p ointing accuracy was measured for all the exp osures, regardless of whether they had b een obtained as part of a CRSPLIT or NUMEXP sequence with no dithering, or whether a dither pattern had b een used. The goal was to measure how well each exp osure aligned with the commanded p ointing of the telescop e. This was measured by first creating a catalog for each exp osure using the IRAF DAOFIND software (Stetson 1987), with parameters optimized for centroiding on unresolved stellar sources. For the clusters NGC2298, NGC1851, NGC6341, and NGC6752, the images typically contained 1000 - 2, 500 stars that could b e matched b etween all the exp osures (10, 100 and 500 seconds) while the F330W images contained 800 - 1, 100 stars that could b e matched. For Omega Centauri, only 50 - 60 stars were matched so this dataset served more as a consistency check. Cosmic rays were generally not a problem; most of the exp osures were short enough to have a low numb er of cosmic rays, and the numb er of stars was generally large enough that occasional stars affected by cosmic rays would show up as significant outliers and could b e easily rejected. 3.3. Shift Measurement Results

We used the catalogs to iteratively solve for shifts, rotations and p ossible scale changes. All scale changes from velocity ab erration were successfully removed, and no significant additional time-dep endent scale changes were present. The only remaining scale change is b etween short and long exp osures, along the y-axis to the level of a few times 10-5 . This is accounted for by a known effect related to CTE (charge transfer efficiency) which produces slight changes in the centroids of stars. However, this scale change is not timedep endent, and comparisons b etween exp osures of the same length showed no significant scale changes during the observations. No other geometric changes were found and the final analysis was conducted by solving for shifts and rotations, keeping other terms fixed. The resulting measurements for each exp osure represent the difference b etween the commanded HST p ointing and the actual p ointing obtained. All of these can b e presented relative to the first exp osure in each orbit, and are shown in Figure 1. The results show that the stability of the telescop e is generally very good, with each exp osure aligning to within a few milliarcseconds of the commanded offsets. While some orbits show a slight gradual change with time up to 4 - 5 milliarcseconds, this is also found to occur in three-gyro mode and is interpreted as the result of thermal changes within the telescop e. It should b e recalled that the orbits during each test were not contiguous but obtained over several days, during which other targets at different sun angles were also observed, therefore slight changes in tracking due to thermal effects might b e exp ected. However, the principal result is that the translational and rotational stability of the telescop e remains good and shows no dramatically different b ehavior to three-gyro mode. We also compared these results to three-gyro data from program 10455 and Cycle 12 program 9750 (PI: K. Sahu), which used ACS/WFC over 105 orbits to observe the galactic bulge (TEL ISR 2005-02, Gilliland et al. 2005). Since the telescop e b ecomes thermally stable after ab out a day, we chose the first 20 orbits of this program as b eing representative of three-gyro data. In Figure 2 we show a histogram summarizing the relative offsets that were presented in Figure 1 for two-gyro mode, along with the similar measurements obtained for the comparable data from program 9750. In all cases, the shifts and rotations represent the differences from the commanded p ointings for all exp osures within an orbit, relative to the first exp osure in the orbit. From the figure it is apparent that the distributions are not significantly different, and this is also b orne out by the quantitative comparison b etween the two distributions when we characterize each distributions in terms of its r.m.s. disp ersion. This was verified indep endently by A. Riess and the other team memb ers. The results are presented in Table 1. We also find that the p ointing rep eatability for guide star re-acquisitions b etween orbits does not app ear significantly worse than in three-gyro mode, although we were only able to verify this for a few orbits.


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Two-Gyro Pointing Stability Within Each Orbit: Program 10443 (ACS/HRC)
X (mas) Tot. Shift (mas) Roll angle (deg) Y (mas)

Orbit 1 Orbit 2 Orbit 3 Orbit 4 Orbit 5 Orbit 6 Orbit 7 Orbit 8 Orbit 9 Orbit 10 Orbit 11 Orbit 12 Orbit 13
20 15 10 5 0 10 5 0 -5 -10 10 5 0 -5 -10

0.005 0.000

-0.005 0 20 40 0 20 40 0 20 40 0 20 40 0 20 40 0 20 40 0 20 40 0 20 40 0 20 40 0 20 40 0 20 40 0 20 40 0 20 40 minutes minutes minutes minutes minutes minutes minutes minutes minutes minutes minutes minutes minutes

Two-Gyro Pointing Stability: Program 10458 (ACS/HRC)
Orbit 1 Orbit 2 Orbit 3 Orbit 4 Orbit 5

Anton Koekemoer, 2 September 2005

V3 (mas) V2 (mas) Tot. Shift (mas)

Orbit 6

Orbit 7

Orbit 8

15 10 5 0 5 0 -5 5 0 -5

Roll angle (deg)

0.005 0.000 -0.005 0 20 40 0 minutes 20 40 0 20 40 0 minutes minutes 20 40 0 20 40 0 20 40 0 20 40 0 20 40 minutes minutes minutes minutes minutes

Figure 1: Relative p ointing accuracy of HST for the observations obtained in February 2005 (top) and August 2005 (b ottom), for a total of 301 exp osures, including dither patterns. The offsets represent the difference b etween the commanded and actual p ointing, to quantify the stability of HST during an orbit. It can b e seen that the stability is very good to the level of a few milliarcseconds, which is comparable to three-gyro p erformance.


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Koekemoer et al.
Two-Gyro Pointing Stability: Program 10458 (ACS/HRC)
Anton Koekemoer, 2 September 2005

80 Exposures 60 40 20 0 0 2 4 6 8 10 Total Shift (milliarcseconds) 12

Total Shift 2-Gyro: r.m.s. = 2.08 milliarcsec 3-Gyro: r.m.s. = 2.19 milliarcsec

14

80 Exposures 60 40 20 0 0.0000

Roll Angle Change 2-Gyro: r.m.s. = 0.00070 degrees 3-Gyro: r.m.s. = 0.00093 degrees

0.0010

0.0020 0.0030 Roll Angle Change (degrees)

0.0040

0.0050

Figure 2: A comparison of the tween the two-gyro test data an differences b etween commanded no significant difference b etween

histogram distribution of shifts and rotational offsets b ed comparable three-gyro data. These offsets represent the and measured p ointings, as plotted in Figure 1. There is two-gyro and three-gyro data for either shifts or rotations.

Table 1: HST Pointing Stability Summary: ACS/HRC Total Shift r.m.s. (milliarcseconds) 2.29 2.08 2.19 Roll Angle r.m.s. (degrees) 0.00097 0.00070 0.00093

Two-gyro (February 2005) Two-gyro (August 2005) Three-gyro (nominal)

3.4.

Processing Two-Gyro Data through MultiDrizzle

As a final test, we processed the two-gyro exp osure sequences through the MultiDrizzle software (Koekemoer et al. 2002), which is used in the HST pip eline to automatically register images based on their headers, p erform cosmic ray cleaning and create a final combined image using Drizzle. A variety of multiple exp osure sequences were obtained in the two-gyro test, including those sp ecified using CRSPLIT, NUMEXP, and dither patterns. As an initial test, the images were processed through MultiDrizzle using only their header astrometry as a basis for registration, thus ignoring the offsets of a few milliarcseconds that were shown in Figure 1. This is the current b ehavior of the pip eline, in the sense that the images are registered based on their commanded offsets. The results from these tests showed that there is no significant degradation in the quality b etween the images, which was confirmed quantitatively by measuring the PSF of stars in the images in each case. This agreement is consistent with the fact that the offsets of the exp osures as shown in Figure 1 were found to b e only a few milliarcseconds, or less than ab out 0.1 HRC


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pixels. Therefore, these two-gyro images can generally b e combined directly to the same degree of accuracy as three-gyro data, using the astrometric headers information. If it is ever necessary to incorp orate measured shifts, as demanded by certain typ es of scientific programs, then this can b e done by means of the "Tweakshifts" script which uses the techniques describ ed here to solve for shifts and apply them to the image headers, prior to image combination. 4. Summary

We have presented the results of the p ointing stability tests for HST, as measured with the ACS/HRC during the Two-Gyro test program conducted in February 2005 as well as the verification observations in August 2005. We have measured the shifts of 301 exp osures of the globular clusters NGC1851, NGC2298, NGC6341, NGC6752, and Omega Centauri, obtained over a total of 21 orbits, and have compared the measured p ointings to those that were commanded in the observing program. We find in all cases that the measured offsets of shifts and rotations agree with those that were commanded to the same level of accuracy as in three-gyro mode. Sp ecifically, the differences b etween commanded and actual p ointings during an orbit relative to the first exp osure can b e characterized with distributions having disp ersions of 2.1 - 2.3 milliarcseconds for shifts and 7.0 - 9.7 в 10-4 degrees for rotations, thus less than 0.1 HRC pixels, and agree extremely well with similar values measured for comparable exp osures obtained in three-gyro mode. In addition, we successfully processed these two-gyro test data through the MultiDrizzle software which is used in the HST pip eline to p erform automated registration, cosmic ray rejection and image combination for multiple exp osure sequences, and we find excellent agreement with similar exp osures obtained in three-gyro mode. In summary, we find no significant difference b etween the quality of HST p ointing as measured from these two-gyro test data, relative to the nominal b ehavior of HST in regular three-gyro op erations. Acknowledgments. We are grateful to the ACS group and the op erations teams at STScI and GSFC who worked to implement two-gyro op eration, and to Ron Gilliland for kindly sharing the data from his ISR in order to facilitate a comparison with three-gyro observations. References Anderson, J., & King, I., 2004, "Multi-filter PSFs and Distortion Corrections for the HRC", Instrument Science Report ACS 2004-15 (Baltimore: STScI), available through http://www.stsci.edu/hst/acs Fruchter, A. S. & Hook, R. N., 2002, PASP, 114, 144 Gilliland, R. 2005, "Guiding Errors in 3-Gyro: Exp erience from WF/PC, WFPC2, STIS, NICMOS, and ACS", Instrument Science Report TEL 2005-02 (Baltimore: STScI) Koekemoer, A. M., Fruchter, A. S., Hook, R. N., & Hack, W.,2003, in Proc. 2002 HST Calibration Workshop, ed. S. Arribas, A. Koekemoer, & B. Whitmore (Baltimore: STScI), p. 337 Koekemoer, A. M., Kozhurina-Platais, V., Riess, A., Sirianni, M., Biretta, J., & Pavlovsky, C. 2005, "Two-Gyro Pointing Stability of HST Measured with ACS", Instrument Science Report ACS 2005-07 (Baltimore: STScI) Stetson, P. B. 1987, PASP, 99, 191