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Altair Undergoes Successful Performance Upgrade

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The recent addition of a field lens in the Altair adaptive optics system has resulted in a significant improvement in near-infrared imaging capabilities at Gemini North.  Altair with the field lens will begin regular science use in semester 2006A.

Figure 1: H band mosaic images of the core of M33 with field lens out (top) and field lens in (bottom). These images obtained on August 18, 2005 under favorable turbulence conditions within a period of 30 minutes. Field of view is 38.0 x 6.5 arcseconds at f/32 using the core of M33 (approximate R magnitude of 14.5) as a guide source.

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Figure 2: Radial and tangential full-width-half-maximum (FWHM) versus off-axis distance (in arcseconds) based on H band images shown in Figure 1. Black and red crosses are radial and tangential FWHM measurements (respectively) without the field lens and blue and green crosses are radial and tangential FWHM measurements (respectively) with the field lens engaged.

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The lens was installed based on recent turbulence monitoring that demonstrated that about 60% of the turbulence on Mauna Kea occurs close to the ground. Altair was originally designed based on the assumption of a dominant turbulence layer at 6.5 kilometers. According to Gemini’s Associate Project Scientist for Adaptive Optics, Dr. François Rigaut, “These results give us a taste of what is coming with our Multi-Conjugate Adaptive Optics [MCAO] system at Gemini South, in terms of accessible field of view and uniformity of image quality. In addition, MCAO will provide this uniformity over 10 times the field of the M33 images in nearly all seeing conditions.”

Figure 2 illustrates the relationship between the full-width-half-maximum (FWHM) of stellar images before and after the use of the field lens. With the field lens inserted into the optical path, the point-spread function of the stellar images is extremely uniform out to at least 30 arcseconds from the guide source, as indicated by the flatness of the blue and green crosses in Figure 2. The data for this plot came from the images presented in Figure 1 of the core of M33 in H-band. Figure 3 shows a color composite from additional data in J, H and K bands also obtained with Altair using the field lens.

All images were obtained with Altair feeding the Gemini Near Infrared Imager at f/32 during field lens commissioning in August and September 2005. A known limitation is an 8% throughput loss using the temporary silica lens, but this will be reduced soon when it is replaced with a CaF2 lens.

Figure 3: Color mosaic composite of J, H and K-band images of M33 core region using NIRI and Altair with field lens at f/32. Note the uniformity of stellar images to the edge despite wide field (for AO) of 39 x 21 arcseconds.

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Commission at f/14 is also underway as illustrated by the K’, K+H2, FeII (iron II) color composite image of the planetary nebula M2-9 (Figure 4, below). A comparison with the HST image of the same object (in the optical) is provided for comparison.

Figure 4: Color composite (center) of the planetary nebula M2-9 using Altair AO images in the following bands: K’ (green), K+H2=1-0 (violet) and Fell (orange). Field of view is 38.5 x 42.5 arcseconds with NIRI at f/14. Inset: HST WFPC2 optical image (1997).

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