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The 4-m Blanco Telescope continues to deliver excellent images. Median seeing since the beginning of the year has been 0.9" FWHM as measured by the f/7.8 south port camera using George Jacoby's software. The Georgia State University speckle camera was on the telescope 4-7 March and showed 0.5" speckles. On Friday, 8 March, the optical performance and operating conditions of the of the Blanco Telescope were measured during a period of excellent seeing. The results of these tests and previous performance data show that the 4-m telescope is capable of delivering V images as good as 0.56" FWHM.
On the evening of the optical tests the environmental conditions were as follows: The primary mirror was within 0.5 degrees C cooler than the dome air. The dome air temperature was approximately equal to the outside air temperature (15 degrees C). The dome vent doors were open. The wind speed was about 5 mph from the north. The site seeing monitoring tower was reporting 0.3" seeing.
The chip used was a Tek 2048 with 0.157" pixels. Fifteen second focus frames showed images with 0.56" FWHM. Five minute guided images were 0.79" FWHM. Out of focus movies were taken with an intensified CCD guide camera at 60 frames per second. In these movies, air currents affecting the image quality can be plainly seen. The outside wind and "seeing waves" are clearly visible as well as some air currents emanating from the center of the "donut."
The new primary mirror cell air extraction system was also tested while taking data with the south port seeing camera as well as out of focus movies. The seeing camera showed no degradation in image quality; the out of focus movies showed a slight modification in the flow pattern. During these tests the south port camera was reporting 0.66" FWHM. That the air extractor did not improve the image quality is not surprising since the primary mirror temperature was already within 0.5 degrees C of the dome air temperature. The air extractor should give the best improvements when the change in outside air temperature is greater than what the primary mirror cooling system can handle. It should also help with the hot air problem in the chimney. The chimney will be modified during the July shutdown so that air may be extracted more efficiently from the chimney and focal plane area.
>The Shack-Hartman wavefront analyzer results showed 0.45" residuals in a 30s exposure. This coupled with the 15s V image at 0.56" images and the site seeing monitor data suggest that the telescope optics in their present state are contributing 0.45"-0.50" to the image quality budget. The tracking and guiding systems in the telescope are contributing an additional 0.2" increase in delivered image diameter. Identifying and reducing the tracking and guiding errors are a central goal in the upcoming July engineering shutdown.
John Filhaber