SPIE Astronomical Telescopes and Instrumentation, San Diego, CA, 27th June – 2nd July 2010

Characterization of the mid-IR image quality at Gemini SouthDan Li, Charles M. Telesco, and Frank Varosi

Department of Astronomy, University of Florida, 311 Bryant Space Science Center, P. O. Box 112055, Gainesville, FL-32611, USA

Poster 7737-92

ABSTRACTTo help the prospective observer take full advantage of the mid-IR capability of Gemini South, we characterize a key aspect of the mid-IR performance of the 8-meter telescope at Gemini-S, namely, the appearance and stability of its delivered mid-IR image profiles, with the goal of demonstrating that it can be used with a level of precision not used before. About 2000 images obtained with T-ReCS (a facility mid-IR camera at Gemini-S) between late 2003 and early 2009 were used for our image quality (IQ) analysis. All targets are flux standards and recorded at one or more of the four bands Si-2 (8.74 μm), N (10.36 μm), Si-5 (11.66 μm), and Qa (18.3 μm). A non-linear least squares fitting of three profile models (Lorentzian, Gaussian, and Moffat) was performed on each image, and the FWHM, ellipticity, position angle and Strehl-ratio (SR) were measured from the fitted profile. We find that the long-time-scale image quality is quite stable in terms of profile width or ellipticity, though short-time-scale variation is evident. We also examined the correlation between image quality and many ambient parameters and confirmed the interdependence between the image quality in the Qa band and the ambient humidity. The ellipticity of the profile was analyzed statistically as well. The average profiles for different filters can be used as important references in the future when a high-quality profile reference is not available during an observation.

Figure 1 (left). An example of how critical the PSF reference is in the detection of a barely resolved emitting region of ζ Lep. The azimuthally averaged intensity profiles at 18.3 μm of ζ Lep (diamonds) and the reference PSF star (dots) show a small difference in their FWHM, which is indicated by the two vertical lines. The intrinsic half-width of the deconvolved disk is 0.14”±0.02”, or 3.0±0.4 AU (Moerchen et al. 2007).

Figure 2 & 3 (above). The temporal variation of the FWHM and Strehl ratio (SR). The vertical dashed lines indicate the beginning of each calendar year, whereas the horizontal dashed lines represent the diffraction-limited FWHM. For most images, the image quality is close to the diffraction-limit, and the overall IQ for all four filters has been quite stable since 2006, despite the significant short-time-scale variances. We checked many ambient parameters found in the FITS headers to see if any of them could account for the large dispersion in these IQ measurements, and confirmed only one correlation between IQ and humidity in Qa band (see Fig. 5.).

Figure 4 (above). A direct comparison between FWHM and SR, which are both used to evaluate the IQ in this study, shows that SR is a more sensitive measure of the IQ than FWHM when one is nearly diffraction-limited. The FWHM does not change as noticeably as the SR does when the IQ is approaching the theoretical limit.

Dan Li June, 2010

Figure 5 (above). A weak correlation between the ambient humidity and IQ (in terms of SR) observed in Qa images. This suggestion that the IQ in Qa band is more sensitive to the high humidity than other filter is consistent with the fact that there is strong water absorption within the Qa passband.

Figure 6 & 7 (above). A statistical correlation between ellipticity and SR, which can help us evaluate the reliability of an ellipticity measurement (i.e., whether a measured, non-zero ellipticity is due to the real physical extension of the celestial object, or due to an instrumental imperfection). For example, we can look at the images with a small range of SR (e.g., Si5 images with 0.35<SR<0.45, as shown in Fig. 6) and plot their ellipticity histogram (Fig. 7), which fits a Gaussian very well.Assuming that there is a new Si5 image with SR=0.4, and ellipticity=0.2, barely outside the 3σ according to Fig. 7, one can conclude that it probably reflects the real elongation of the object, since we seldom see an image with a similar SR having such a high ellipticity in our image database of flux standards. Similarly, the “standard” ellipticities of other filters and SR ranges can be computed as summarized in Table 2 (above right).

Figure 8 (above). Averaged Moffat (left panels) and Gaussian (right panels) profiles. The dotted profiles are the best PSFs (i.e., with the smallest FWHM) in the database. Observers can use these profiles as empirical references for planning or if there is no high-quality PSF star available during an observation. They give us a general idea of the actual resolution that the Gemini-South and TReCS can achieve routinely.ACKNOWLEDGMENTSThis study is supported by NSF grant AST-0738883 to C. Telesco, and based on observations obtained at the Gemini Observatory (acquired through the Gemini Science Archive), which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciėncia e Tecnologia (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). The authors wish to acknowledge the kind assistance of Gemini staff astronomers Scott Fisher, Tom Hayward, and Rachel Mason.

SUMMARY1) In terms of PSF size and SR, the image quality of Gemini-S with T-ReCS has been quite stable since 2006;2) SR is a better measure of image quality than is FWHM, especially when one is nearly diffraction-limited;3) The ambient humidity is weakly, but noticeably, correlated with the quality of the Qa (18.3 μm) images;4) There is a statistical correlation between the SR and the ellipticity of the image profile, and observers should be cautious with non-zero ellipticities measured from the mid-IR images;5) The average PSF profiles, which can be used as artificial references for future observations, are computed for four filters and two PSF models.

SUMMARY1) In terms of PSF size and SR, the image quality of Gemini-S with T-ReCS has been quite stable since 2006;2) SR is a better measure of image quality than is FWHM, especially when one is nearly diffraction-limited;3) The ambient humidity is weakly, but noticeably, correlated with the quality of the Qa (18.3 μm) images;4) There is a statistical correlation between the SR and the ellipticity of the image profile, and observers should be cautious with non-zero ellipticities measured from the mid-IR images;5) The average PSF profiles, which can be used as artificial references for future observations, are computed for four filters and two PSF models.