Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/hst/HST_overview/documents/calworkshop/workshop2005/papers/makidon.pdf Дата изменения: Wed Apr 5 00:02:25 2006 Дата индексирования: Sun Dec 23 23:25:13 2007 Кодировка: Поисковые слова: coma

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

HST Temporal Optical Behavior: Models and Measurements with ACS
R. B. Makidon, S. Casertano, and M. Lallo Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Abstract. While HST provides a stable stellar image relative to ground-based observatories, it features its own characteristic changes in the PSF it delivers to the Science Instruments (Lallo et al. 2005). HST focus has b een monitored and adjusted throughout the life of the observatory. More recently, the resolution and off-axis location of ACS/HRC has allowed us to accurately measure changes in coma and astigmatism as well. The aim of this current work is to relate the accurate and reliable phase retrieval measurements of wavefront error back to characterizations more common to science data analysis (e.g. encircled energy, FWHM, ellipticity). We encourage the further examination of these effects on HST science observations.

1.

Introduction

It has b een known since early in the Mission that the focal length of HST varies on b oth orbital and longer time scales (Bґly 1993). These changes have generally b een attributed to e a physical motion of the secondary mirror (SM) resulting from variations in the metering truss structure that supp orts it. HST focus has always b een monitored on at least a monthly basis (Figure 1). The primary purp ose for this monitoring is to characterize the focus state of the observatory and accurately plan the time and amount of the occasional SM move for focus maintenance. ACS/HRC has enhanced our ability to measure the additional image ab errations of coma and astigmatism. Like focus, these ab errations vary over the HST orbit, suggesting the p ossibility of a more complex motion of the SM, or other sources of misalignment. As part of our routine monitoring program in recent years, phase retrieval analysis (Krist & Burrows 1995) is p erformed on stellar targets in HRC and values of focus, 0 and 45 degree astigmatism, and x and y coma (Zernike coefficients 4 through 8) are obtained over an orbit at roughly monthly intervals. The plots in Figure 2 show these data. Units are in microns rms wavefront error for coma and astigmatism, and microns of SM despace 2. PSF Morphology in ACS

It is well known that the morphology of the ACS PSF in b oth WFC and HRC exhibits appreciable field dep endence (see Krist 2003). However, the understanding of how that PSF morphology varies over time and as a function of e.g. focus has not b een well understood. Understanding the ACS PSF and the factors that affect it has b ecome increasingly imp ortant as science observations in fields such as weak lensing and circumstellar environments continue to push the limits of what is observable.

405 c Copyright 2005 Space Telescop e Science Institute. All rights reserved.


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ACS/HRC Focus Measured Over ACS Life
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HST refocus 2 Dec 2002 +3.6 microns HST refocus 22 Dec 2004 +4.2 microns

Microns of Secondary Mirror Despace

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Figure 1: Secondary mirror (SM) despace (in microns) determined from a monthly series of observations of isolated bright stars in the ACS/HRC. Phase Retrieval was used to first calculate the Zernike focus term (z4) for the star in each observation, which can then b e related to microns of motion at the SM. The filled circle represents the orbital mean of the focus observations, while the bar represents the range of measurements obtained for that orbit.
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Figure 2: Coma (top observations used to the Zernike terms z5 within an orbit, and time.

row) and astigmatism (b ottom row) measured from the monthly set of monitor ACS and HST focus. Phase Retrieval was used to determine through z8 from these data. Each p oint corresp onds to an observation are plotted in units of microns rms of wavefront error as a function of


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Figure 3: PSF ellipticities in the ACS/WFC measured from Tiny Tim PSF models. At left: ellipticities measured from models at nominal focus. At right: ellipticities measured from models at -5µm of SM despace.

Figure 4: PSF ellipticities of stellar sources in 47 T uc observed in the ACS/WFC with F 814W while H S T was known to b e in a negative focus state of -5µm 2.1. Observed and Modeled PSF Ellipticities in WFC

Using Tiny Tim, we generated model PSFs on a finely-sampled grid of field p ositions on the WFC (Figure 3). Here, measured PSF ellipticities across the WFC field are shown for nominal focus (left) and a Secondary Mirror despace of -5µm or 30nm wavefront error (middle). In Figure 4 we show PSF ellipticity measurements using stellar data from observations of 47 T uc observed with F 814W while H S T was known to b e in a negative focus state of -5µm (Sept 2004). The ellipticities measured in the 47 T uc data are in good agreement with those in the -5µm model. It is clear that even such a modest despace (typical of orbital focus variation) results in an observable change in ellipticity over the WFC field.


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Figure 5: PSF ellipticities in the ACS/HRC measured from Tiny Tim models at nominal focus. The length of the vector represents 10e, where e is the ellipticity of the PSF. 2.2. Modeled Ellipticities in HRC

Using Tiny Tim, we generated model PSFs with the F555W filter on a 7 в 7 grid of p ositions across the HRC field. We examined the variation of the PSF ellipticity as a function of focus (holding coma and astigmatism constant), and as a function of coma and of astigmatism at nominal focus. We show the measured ellipticities (calculated from image moments) for the nominal focus case (Fig.5), and the difference in ellipticities at 5µm on either side of nominal focus (Fig.6). Fig.7 illustrates the change in ellipticities over typical ranges of coma (left pane) and astigmatism (right) at nominal focus. It is clear that as with WFC, changes in focus induce a measurable effect on ellipticity, presumably due to astigmatic changes in the PSF due to the combined effects of the HST OTA and the ACS optics. However, neither coma nor astigmatism variations alone at a constant focus (as might b e due to slight motions of optics within the HRC instrument itself ) have shown a comparably significant effect on the observed PSF morphology. 3. Conclusion

Changes in the field-dep endent morphology of the PSF in b oth HRC and WFC app ears more strongly sensitive to focus variations due to HST SM despace than to the observed range of coma and astigmatism. While these ab errations have b een shown to exhibit b ehavior on orbital and longer timescales, the causes of these variations are not yet well-understood. Though the use of phase retrieval can precisely quantify these variations, more familiar-- though less sensitive--"real world" characterizations of the observed PSF (e.g. encircled energy, FWHM, and ellipticity) are far less sensitive to the range of coma and astigmatism observed than to focus. Understanding the state of the HST focus at the time of one's observations is necessary to achieve the photometric and astrometric precisions required of many current HST programs (see Gilliland et al. 2002; Suchkov & Casertano 1997), and to accurately describ e the morphologies of barely resolved ob jects.


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Figure 6: Measured changes in PSF ellipticity in the ACS/HRC from Tiny Tim models at focus p ositions ±5µm on either side of nominal focus. Changes are shown relative to ellipticities measured from Tiny Tim models at nominal focus, with the length of each vector given by 10(e), where e is the difference in measured ellipticity relative to nominal focus at that field p osition.

Figure 7: Measured changes in PSF ellipticity in the ACS/HRC from Tiny Tim models at nominal focus over typical ranges of coma (left) and astigmatism (right). Changes are shown relative to ellipticities measured from Tiny Tim models at nominal focus for default coma and astigmatism values. Here, the length of each vector is given by 10(e), where e is the difference in measured ellipticity relative to default coma and astigmatism values at zero focus.


410 References

Makidon, Casertano, & Lallo

Bґly, P., Hasan, H., & Miebach, M., 1993, "Orbital Focus Variations in the Hubble Space e Telescop e", SESD-93-16 Gilliland, R.L. et al., 2000, ApJ, 545, L47 Krist, J. E. & Burrows, C. J., 1995, Appl. Opt. 34, 495 Krist, J. E., 2003, "ACS WFC & HRC field-dep endent PSF variations due to optical and charge diffusion effects", Instrument Science Report ACS 2003-06 (Baltimore: STScI), available through http://www.stsci.edu/hst/acs Lallo, M., Makidon, R. B., Casertano, S., Gilliland, R., & Stys, J., 2005, "HST Temp oral Optical Behavior & Current Focus Status", Instrument Science Report TEL 2005-03 (Baltimore: STScI), available through http://www.stsci.edu/hst/ Suchkov, A. & Casertano, S., 1997, "Impact of Focus Drift on Ap erture Photometry", Instrument Science Report WFPC2 97-01 (Baltimore: STScI), available through http://www.stsci.edu/hst/wfp c2