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Blanco 4-m Active Optics System Done! (1Mar96) (from CTIO, NOAO Newsletter No. 45, March 1996) The Image Analyzer The "IMAN" Shack-Hartmann image analyzer is now in routine use at the Blanco Telescope. This was the final step in a three-year series of projects to retrofit a modern active-optics system onto this telescope. The active optics system is used to correct static and slowly-varying alignment and support errors. The image analyzer is used to measure the low-order aberrations in the optical system. The analyzer is incorporated into the Cassegrain offset guider probe. To use it, the night assistant clicks on a menu command to move the guide probe to the optical axis, centers a ~10 mag star in the guider field, and then clicks on another menu command to initiate a series of three 30-second measurements. Exposures of this length average out the atmospheric seeing, but are still affected by slowly varying components of dome seeing. The result of these measurements is a table of corrections for spherical, coma, astigmatism, trefoil and quadrafoil aberrations. The Active Optics System The full active optics system also includes the 4-m active primary mirror control system, and the f/8 and f/14 secondary mirror control systems. The active primary system was installed 1 1/2 years ago, and the f/8 secondary system has been in use for over two years. The f/14 secondary control system is finished and installed on the telescope, but the f/14 mirror will not be delivered before February. These various systems are all fully integrated into the 4-m Telescope Control System. The image analyzer writes all of its results into a log file. The night assistant can then click on menu commands to take the desired corrections from this log file and pass them on to the mirror control systems. There are basically three different modes for using the active optics system: (1) Automatic correction of the optics as a function of the telescope's hour angle and declination, using a lookup table. The lookup table is generated about every three months by using the image analyzer to map the aberrations all over the sky (this takes 1/2 night of engineering time). This mode has actually been in use for the past year, using lookup tables generated with a Hartman screen. The advent of the image analyzer just makes this process easier. (2) Routine checking of the optics. The image analyzer will routinely be used at the start of each f/8 and f/14 observing run, to check the collimation. This takes about 15 minutes. (3) Real time correction of the optics, using a mag 10 star near the object to be observed. We call this "tweaking up" the optics. The telescope operators can do this for you on demand. It takes about 10-15 minutes, so we do not expect people to want to use it except on the best nights, where there is the most to be gained. Figure 1 shows an example of the output that the operator saw last November when we used the active optics system to recollimate the telescope, in a test of the tweak correction technique. [Figure not included] Figure 1. Two successive runs with the image analyzer. The first set of three measurements shows large coma, which would contribute 1.77" to the 80% encircled energy diameter. There is also modest astigmatism (0.19" d80). The line starting with "Tweak?" shows the software's recommendations about whether or not the individual aberrations should be corrected. The second block of information shows the result of applying the corrections indicated in the first block. The columns headed "d80 init" in the two blocks show the total 80% encircled image diameter including the low-order aberrations: this dropped from 1.9" in the upper block to 0.6" in the lower block. Overall Results Our goal over the past five years has been to improve the image quality at this 20-year-old telescope in a cost-effective way. Along with the installation of the active optics system, we have had the f/8 secondary repolished and have made many improvements to the thermal environment of the telescope. The thermal improvements have been described in earlier Newsletters; they include dome vent doors, oil cooler, mirror cooler, moving the console room, largely vacating the building, and extensive insulation and air conditioning improvements. The results, in terms of delivered image quality, are shown in Figure 2. The Blanco telescope now gets subarcsecond images on almost 2/3 of the nights. [Figure not included] Figure 2. Comparison of seeing measurements made during the summer of 1992/1993 (summer is CTIO's period of best seeing), and of measurements for the full year ending 16 November 1995. The dashed curve in each panel shows the standard CTIO site seeing results, made several years ago using an image motion monitor at ground level. The total direct costs to CTIO (including payroll and non-payroll) were $310K for the active optics projects, $100K to refigure the f/8 secondary, and $400K for the full package of thermal improvements. The total is $810K in direct costs, or about $1.4M fully burdened. We believe that this is a reasonable price to pay for the results that have been realized. As always in a description of this work, we must convey our great appreciation to a number of people from other institutions who have given us vital help and guidance, especially Ray Wilson, Lothar Noethe and Alain Gilliotte of ESO, and Claude Roddier of the University of Hawaii. Where From Here? During the coming year, we will be working on improvements in three different areas of the Blanco Telescope. These are development and installation of an f/14 tip-tilt secondary, improvements to the telescope control system and servos, and continuing work on the optics. Under Richard Elston's guidance, the new f/14 IR secondary will be commissioned in March, and its tip-tilt capabilities will be gradually brought on line through the year. Ricardo Schmidt, Eduardo Mondaca, and German Schumacher are the CTIO engineers involved in this project. Steve Heathcote will provide the scientific leadership for the work on the 4-m control system, in collaboration with German Schumacher and Rolando Rogers. Our plans have evolved. We now intend to devote a short-term effort to improving the actual performance of the telescope by upgrades to the Telescope Control Software and pulse mixing hardware. This will lead to more accurate offsetting and improved efficiency of use, hopefully by the end of the year. Over a longer term, as a reliability issue, we plan to replace the existing servos with Delta-Tau controllers of the same type that are being used by Gemini. The general program of continuing optics improvements will shift emphasis from the Cassegrain to the prime focus. Alistair Walker will be the scientist responsible for this area plus the overall health of the telescope. Significant improvements were made at prime focus over the past year just by aligning the optics, but it is now time to try using the active optics system to get the optimum possible performance. This will mesh with the arrival of Tony Tyson's Large Area CCD mosaic imager part way through the year. J. Baldwin, G. Schumacher, G. Perez, R. Schmidt, J. Filhaber
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