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4-m Image Quality Improvements (1Mar95) (from CTIO, NOAO Newsletter No. 41, 1 March 1995) After several years of work on a whole suite of upgrades to the CTIO 4-m telescope, we are now starting to see substantial improvements in the image quality there, and the telescope is now regularly delivering sub-arcsecond images. Figure 1 shows the improvement in image quality over the past two years. The solid curves are results measured at zenith using the seeing monitor at the f/8 focus. These are compared in each panel to the site seeing data taken several years ago in the free air at ground level (dashed curves). Obviously, something is still systematically limiting the performance at the 0.7" level. However, since the time of the commissioning of the active optics system on 21 October 1994, the median seeing has been 0.96" (lower panel), an improvement of about 0.3" over the case two years earlier (upper panel). This means that background-limited objects can be observed in approximately 70% less time. We attribute this improvement to the combined effects of the whole package of upgrades to the optics and thermal environment that has been carried out over the past several years. [Figures not included] The last major project in the 4-m active optics upgrade, the image analyzer for the 4-m Cassegrain focus, is now being worked on at high priority. It consists of installing a Shack-Hartmann lenslet array and a CCD on a spare port in the Cassegrain offset guider system. Engineering tests on the telescope are taking place as this is written. Once it is finished, the image analyzer will be used to regularly check the collimation of the f/8 secondary (especially after top-end flips) and to update the maps of active optics corrections a few times per year. It will also be available to tune up the active optics in real time for critical observations. Also a CCD-TV camera has been installed on the autoguider at the 4-m Cassegrain focus. This is a standard CTIO camera using autoguiding software written by Steve Shectman. The result is a great improvement in the accuracy of the guiding; during tests in 0.8" FWHM seeing, there was no difference between the image diameters obtained in 30 second unguided exposures and 30 minute guided exposures. In the f/8 configuration, the guider works reliably on stars in the magnitude range 14 < V < 16. We are grateful to Steve Shectman for providing us with his elegant software. J. Baldwin
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